CA2057373A1 - Tia1 intermetallic articles and method of making same - Google Patents
Tia1 intermetallic articles and method of making sameInfo
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- CA2057373A1 CA2057373A1 CA 2057373 CA2057373A CA2057373A1 CA 2057373 A1 CA2057373 A1 CA 2057373A1 CA 2057373 CA2057373 CA 2057373 CA 2057373 A CA2057373 A CA 2057373A CA 2057373 A1 CA2057373 A1 CA 2057373A1
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Abstract
TiAl INTERMETALLIC ARTICLES AND
METHOD OF MAKING SAME
Abstract of the Disclosure A TiAl alloy base melt including at least one of Cr, C, Ga, Mo, Mn, Nb, Ni Si, Ta, V and W and at least about .5 volume % boride dispersoids is investment cast to form a crack-free, net or near-net shape article having a gamma TiAl intermetallic-containing matrix with a grain size of about 10 to about 250 microns as a result of the presence of the boride dispersoids in the melt. As hot isostatically pressed and heat treated to provide an equiaxed grain structure, the article exhibits improved strength.
METHOD OF MAKING SAME
Abstract of the Disclosure A TiAl alloy base melt including at least one of Cr, C, Ga, Mo, Mn, Nb, Ni Si, Ta, V and W and at least about .5 volume % boride dispersoids is investment cast to form a crack-free, net or near-net shape article having a gamma TiAl intermetallic-containing matrix with a grain size of about 10 to about 250 microns as a result of the presence of the boride dispersoids in the melt. As hot isostatically pressed and heat treated to provide an equiaxed grain structure, the article exhibits improved strength.
Description
TiAl INTERNETALLIC ARTICLES AND
METHOD OF MAXING SAME
Field of the Invention The present invention relates to a method of maXing articles based on TiAl intermetallic materials and, more particularly, to TiAl intermetallic base articles having a net or near-net shape for an intended service application and having improved strength.
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~ 15 Background of the Invention : :, For the past several years, extensive research has been devoted to the development of . ~.. intermetallio materials, suoh as titanium aluminides, for u e in the manufacture of light weight structural comp~nents capable of withstanding high temperatures/stresses. Suchl components are represented, for example, by blades, vanesj disks, sha~ts, casings and other components of the turbine ~: 25 s~ction of modern gas turbine engines where higher gas : and resultant~component temperatures are desired to incr~ase engine thrust/efficiency and oth~r applications reguiring light weight, high temperature materials.
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P 310 Howmet 2 Intermetallic materials, such as gamma titanium aluminide, exhibit improved high temperature mechanical properties, including high strength-to-weight ratios, and oxidation resistance relative to conven~ional hi.gh temperature titanium alloys.
However, general exploitation of these intermetallic materials has been limited by the lack o~ strength, room temperature ductility, and toughness, as well as the technical challenges associated with processing and fabricating the material into the complex end-use shapes that are exemplified, for example, by the : aforementioned turbine componen~s.
The Kampe et al U.S. Patent 4,915,905 issued April 10, 1990 describes in detail the development of various metallurgical processing techniques for ~ improving the lou (room) temperaturP duc~ility and toughness of intermetallic material~ and increasing their high temperature strength. The Kampe et al '905 patent relates to the rapid solidification of metalIic matrix composites. In particular, in this patent, an intermetallic-second phase composite is formed; for example, by reacting second phase-forming constituents in the presence of a solvent metal, to form in~situ : ~
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20~73~3 P 310 Howmet 3 precipitated second phase paxticles, such as boride dispersoids, within an inter~etallic-containing matrix, such as titanium aluminide. The intermetallic-second phase composite is then subjected to rapid solidification to produce a rapidly solidified composite. Thus, for example, a composite comprising in-situ precipitated TiB2 particles within a : titanium aluminide matrix may be formed and then rapidly solidlfled to producs a rapidly solidified : 10 powder o the composite. The powder is then consolidated by such consolidation techniques as hot isostatic pressing, hot extrusion and superplastic .. ~. ....... .. ~orging to provide near~inal (i.e., neax-net) shapes.
U.S. Patent 4,836~g82 to Brupbacher et al also relates to the rapid solidification of metal matrix composites wherein second phase-forming : con~tituents are reacted in the presence of a solvent metal to ~orm in-situ precipitated second phase particles, such as TiB2 or TiC, within the solvent me~al, such as aluminum. .
U.S. Patents 4,774,052 and 4,916,029 to Nagle et al are specifically directed toward the : .
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' : ' P-310 Howmet 4 2 ~ ~ 7 3 !1 3 production of metal matrix second phase composites in which the metallic matrix compris~s an intermetallic material, such as titanium aluminide. In one embodiment, a first composite is formed which comprises a disparsion of second phase particles, such as TiBz, within a metal or alloy matrix, such as Al~
This composite i9 then introduced into an additional .
metal which is reactive with the matrix to form an intermetallic matrix. For example, a first composite comprising a dispersion of TiB2 particl~s within an Al matrix may be introduced into molten titanium to form a final composite compri~ing TiB2 dispersed within a _ titanium aluminide matrix. U.S. Patent 4,9~5,903 to Brupbacher et al describes a modification of the method~ tau~ht in the aforemen~ioned Nagle et al pat~nts.
' An attempt to improve room temperature : ductility by alloying intermetallic materials with one or more metals in combination with certain plastic forming techniques is disclosed in the Blackburn U.S.
Patent 4,294,615 wherein vanadium was added to a TiAl composition to yield a modified composition of Ti-31 to 36% Al-0 to 4% V. The modified composition was ,~
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P-310 Howmet 5 2~7373 ; melted and isothermally forged to shape in a heated die at a slow deforma~ion rate necessitated by the dependency of ductility of the intermetallic material : on strain rate. The isothermal forging process is carried out at above 1000C such that special die material~ (e.g., a Mo alloy known as TZM) must be used. Generally, it is extremely difficult to process TiAl intermetallic materlals in this way as a result :of their high strength, high temperature nature and the dependence of their ductility on strain rate.
A series of U.S. patents comprising U.S.
. . Patents 4,836,983; 4,842,817; 4,842,819; 4,842,820;
4,857,268; 4,879,092; 4,897,127; 4,~902,474; and 4,916,028, have described a1:tempts to make gamma TiAl iDtermetalli materials having both a modified stoichiometri~c ratio of Ti/Al and one or more alloyant additions to improve room temperature strength and ; ductility. In making cylindrical shapes from these modified compositions, the alloy was typically first made into an in~ot by electro-arc melting. The ingot was melted and melt spun to form rapidly solidified ~:
ribbon. The ribbon was placed in a suitable container and hot isostatically pressed (HIP'ped) to form a ' , . , ~ . - :
P-310 Howmet ~ 3~3 consolidated cylindrical plug. The plug was placed axially into a central opening of a billet and sealed therein. The billet was heated to 975C for 3 hours and extruded through a die to provide a reduction of about 7 to 1. Samples ~rom the extruded plug were removed from the billet and heat treated and aged.
U.S. Patent 4,916,028 (included in the series of patents listed above) also refers to . 10 processing the TiAl basa alloys as modi~ied to include : C, Cr and Nb additions by ingot metallurgy to achieve ~: desirable combinations of ductility, ætrength and . other properties at a lower processing cost than the aforementioned rapid solidification approach. In particular, the ingot metallurgy approach described in the '02~ paten involves melting the modified alloy and~solidifying it into a hockey puck-shaped ingot of simple geometry and small size (e.g., 2 inches in diameter and .5 inch thick), homogenizing the ingot at 20 1250C for 2 hours, enclosing the ingot in a steel annulus, and then hot forging the annulus/ring assembly to provide a 50% reduction in ingot thickness. Tensile specimens cut from the ingot were annealed at various temperatures above 1225C prior to . .
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P-310 Howmet 7 2~r~`7373 t~nsile testing. Tensile specimens prepared by this ingot metallurgy approach exhibited lower yield strengths but greater ductility than specimens prepared by the rapid solidification approach.
Despite the improvements described hereabove ~ in the ductility and strength of intermetallic : materials, there is a continuing desire and need in the high performance material-using industries, ~: 10 especially in the ga~ turbine engine industry, for intermetallic materials with improved properties or combinations of properties and also ~or manufacturing -- technology that will allow t:he fabrication of such intermetallic materials into usable, complex engineered end-use shapes on a relatively high volume basis at much lower cost. It is an object o~ the ~ ~ present invention to satisfy these desires and needs.
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Summary of the Invention The present invention involves a method of making titanium aluminide base intermetallic articles having a net or near-net shape for intended service application and having improved strength. The method ', .~
P-310 Howmet 8 ~7373 of the present invention involves forming a titanium-aluminum melt comprising (in atomic %) Ti in an amount of about 40% to about 52%, Al in an amount of about 44% to about 52~, and one or more of Cr, C, Ga, Mo, Mn, Nb, Ni~ Si, Ta, V, and W each in an amount o~ about 0.05% to about 8~. Boride dispersoids are provided in the melt in an amount of at least about .5 volume % of the melt. Preferably, a low volume % of boride dispersoids in the range of abou~ . 5 to about 2.0 volume % is provided in the malt.
The dispersoid-containing melt i5 cast and ; , . solidified-in a mold cavity of a ceramic investment mold wherein the mold cavity is configured in the net or near-net shape o~ the article to ~e cast. The melt is solidified in a manner to yield a cra~k-free, net or near net shape cast article compriRinq a titanium aluminide-containing matrix (e.g., gamma TiAl) havlng :
a grain size of about 50 to about 250 microns as a result of grain ref;nement from the boride dispersoids being-distributed throughout the melt during solidification. The melt is solidified in the mold at a cooling rate sufficiently fast to avoid migration of the boride dispersoids to the grain boundaries during P-310 Howmet 9 2~7~7~
solidification and yet sufficiently slow to avoid cracking of th~ article. A cooling rate in the range of about 102 tG about 103F/second is preferred to this end. Following solidification, the net or near-net shape, investment cast article may be subjected to a consolidation operation to olose any porosity in the as-cast condition. The consolidated article may then be heat treated to provide at least a partially equiaxed grain morphology.
In one embodiment of the invention, the ; boride dispersoids are provided in the melt by - -introducin~ a preformed ~oride master material to the melt. In another embodiment of the invention, the boride dispersoids are provided in the melt by introducing an effective amount of elemental boron in the melt to form the desired volume % of borides in situ therein. Regardless of how the boride dispersoids are~provided in the melt, the melt is maintained at a selected superheat temperature for a given melt hold time prior to casting to avoid deleterious coarsening (growth) of the boride particles ~dispersoids) present in the melt.
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P-310 Howmet 10 The present invention also invulves a titanium aluminide base article having a net or near-net investment cast shape for intended service application and a titanium aluminide-containing matrix (e.g., gamma TiAl) consisting essentially of (in atomic %) about 40% to about 52% Ti, about 44% to about 52% Al and one or more of Cr, C, Ga, Mo, Mn, Nb, 5i, Ta, V and W each included in an amount of about ~ 0.05% to about 8%. The matrix includes at least about : 10 .5 volume % boride dispersoids distributed uniformly throughout and a fine, equiaxed, grain structure have : a grain size of about 10 to about 250 microns.
~- Preferably, the article, as consolidated and heat treated to provide:the partially equiaxed grain structure, exhibits a yield strength at room ~: temperature ~70F) of at least about 55 ksi and a tensile ductility at room temperatura of at least ~.
~ bout 0.5% (measured by the ASTM E8M test procedure).
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~, 20: Thus, the present invention has as a : particular purpose to provide net or near-net shape :
articles o~ a TiAl base intermetallic material modified by the addition of selected alloyant(s)/dispersoids and~formed to shape by ,. .
P-310 Howmet 11 2~7373 investment casting in a crack-free condition treatable by consolidation/heat treatment to exhibit improved strength and ductility at room temperature. The method of the invention provides an alternative to much more costly techniques heretofore used to fabricate TiAl base intermetallics.
' The advantag4s of the present invention will become more readily understood by consideration of the following detailed description and examples.
BrieP Description of the Drawinqs :
Figure 1 is a flow sheet illustrating one embodiment of the method of the invention.
Figures 2A through 2F are photomicrographs of in~estment castings of Alloys A through E, respectively, illustrating the effect of increasing ':: :
~ 20 boron in the melt on grain refinement.
' Figures 3A-3B are photomicrographs of the microstructures of investment castings illustrating the affect of heat treatment under different P-310 Howmet 12 ~7~73 conditions on grain morphology.
Figures 4A-4C are photomicrographs illustrating ths boride dispersoids present in a particular Alloy D investment casting.
Figures 5A-SC are photomicrographs illustrating the boride dispersoids present in a particular Alloy E investment casting.
Figures 6A-6C are photomicrographs illustratinq the boride dispersoids present in a - - particular Alloy F investment casting.
' -lS Figures 7A-7F are photomicrographs of investment castings illustrating the effect of increasiny borides (added by ma~t~r borid~ material) in the melt on grain refinement.
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: ~ 20 Figure 8A 8F are photomicrographs of the as-cast microstructures of the investment castings of Figs. 6A-6F.
Figure 9A-9F are photomicrographs of the hot .
2~ 7 3 P-310 Howmet 13 isostatically pressed microstructures of the investment castings of Figs. 7A-7F.
Figures lOA-lOB are photomicrographs o~ the microstructures of investment castings illustrating the effect of heat treatment under different conditions on grain morphology.
: Figures llA-llC are photomicrographs illustrating the boride dispersoids present in a particular Alloy lXD invsstment casting (as-cast).
': ' Figures 12A-12C are photomicrographs illustrating the boride dispersoids present in a particular Alloy 2XD investment casting (as-cast).
Figures 13A-13C are photomicrographs illustrating the boride dispersoids present in a particular Alloy 3XD investment casting (as-cast).
Figures 14A-14C are photomicrographs illustrating the boride ~ispersoids present in a particular Alloy 5Xd investment casting (as-cast).
~73~3 P-310 Howmet 1~
Figures 15A-15C are photomicrographs illustrating boride particle~ extracted from the Alloy 2XD investment casting (as-cast~.
Figures 16A-16C are photomicrographs illustrating boride particles extracted from the Alloy 3XD investment casting (as-cast).
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: ~ Figure 17 is a schematic illustration of boride particles o~ various morphology that occur in the investment castings.
~ ~ - Detailed Description of the Invention .: .
The present invention relates to net or near-net shape articles comprised of a titanium aluminide base intermetallic material modified by the :~ addition of selected alloyant(s)/dispersoids and ~
~ormed to shape by investment casting in a crack-free, . .
fine grained condition treatable by consolidationtheat treatment to exhibit improved strength at room temperature. Titanium-aluminum base alloys employed in practicing the present invention consist essentially of, by atomic %, about 40% to about 52%
P~310 Howmet 15 2 ~ ~ 7 3 7 3 Ti, about 4~% to about 52% Al and one or more of the alloyants Cr, C, ~a, Mo, Mn, Nb, Ni, Si, Ta, V,and W
each in an amount of about 0.05% to about 8%. The listed alloyants are provided in the base composition as a result of their beneficial effect on ductility when present in certain combinations and/or concen~rations.
preferred base alloy ~or use in practicing the present invention consists essentially of, by .
atomic %, about 44% to about 50% Ti, about 46% to about 49% Al and at least one of Cr, C, &a, Mo, Mn, ~ Nb, Ni, si, Ta, V,and W wherein Cr, Ga, Mo, Mn, Nb, ;~ : Ta, V and W, when present, are each included in an amount of about 1% to about 5% and wherein C, Ni and Si, when present, are each 1ncluded in an amount of about O 05% to about 1.0%. Two or more of the ;~ ; alloyants~Cr, C, Ga, Mo, Mn, Nb, Si, Ta, V and W are : : present in an even more preferred embodiment within the concentration ranges given. Although the present : invention is~not limited to a particular base composition within the ranges set forth hereabove, certain specific preferred base compositions are described in the Examples set forth hereinbelow.
, P-310 Howmet 16 ~37373 Referring to Figure 1, the various steps involved in practicing one embodiment of the method of the invention are illus rated. In this embodiment, a melt of the TiAl base alloy is formed in a suitable container, such as a crucible, by a variety of melting techniques including, but not limited to, vacuum arc melting (VAR), vacuum induction melting (VIM~, induction skull melting (ISR~, electron beam melting ~: (EB?, and plasma arc melting (PAM). In the vacuum arc : 1~ melting technique, an electrode .is fabricated of the base alloy composition and is melted by direct electrical arc heating (i.e., an arc astablished between the electrode and the crucible) into an undexlying non-reactive crucible. An actively cooled copper crucible is useful in this regardO Vacuum induction melting involves heating and melting a charge of the base alloy in a non-reactive,~ refractory ::crucibla by induction heating th charge usin~ a surrounding electrically energized induction coil.
Induction skull melting involves inductively heating and melting a charge o~ the base alloy in a water-cooled, segmented, non contaminating copper crucible surrounded by a suitable induction coil.
Electron beam melting and plasma melting involve ;
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P-310 Howmet 17 ~73~3 melting using a configuration of electron beam(s) or a plasma plume directed on a charge in an actively : cooled copper crucible. These melting techniques are known genarally in the art of melting o~ metals and alloys.
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Although the present invention is not limited to any particular melting process, certain ;~ specific melting processes are described in the : 10 Examples set forth hereinbelow.
~: Referring again to Figure l, the melt of the TiAl base alloy in the container (crucible) is provided with boride disper~;oids in an amount o~ at least about 0.5 volume % prior to casting of the melt in an investment mold to be described in detail herebelow. Typically, the boride dispersoids comprise simple titanium borides (TiB2) and/or complex borides such as (Ti,M)XBy where M is Nb, W, Ta or other al}oyant. Although varying amounts of the boride disparsoids may be used depending upon the end-use properties desired ~or the cast article, relatively low boride dispersoids levels of about 0.5 to about 20.0 volume % are useful in practicing the invention P-310 ~owmet 18 ~ 3 7 to achieve the desired grain refinement effects in the casting as well as s~rength and ductility improvements upon further treatment of the casting. Boride dispersoid levels above the upper limit set ~orth tend ; 5 to reduce ductility and thus are not preferxed. In accordance with the invention, optimum strength and ductility are achieved when the boride dispersoid level is preferably about 0.5 to about 2.0 volume % of the melt or cast article.
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The TiAl base alloy melt described hereabove can be provided with the desired level of boride dispersoids in a variety of ways including the addition of a boride mastar material to the melt in accordance with U.S. Patents 4,751,048 and 4,916,030, the teachings of which are incorporated herein by reference. In particular, a porous sponge having a relatively high concentration of boride particles (e.g., ~iB~) is introduced and incorporated in the TiAl base melt to provide a lower concentration of boride particles therein~ Of course, the concentration of boride particles in the sponge is chosen to yleld a selectad lower concentration of the particles in the melt; for example, at least about .5 volume ~ boride .
:, P-310 ~owmet 19 20~7373 dispersoids in the melt. Boride mastPr materials (i.e. sponges) useful in practicing the present invention are available from Martin Marietta Corporation, Bethasda, Md and its licensees.
The TiAl base alloy melt also can be provided with the desired level of boride dispersoids by providing an effective amount o~ elemental boron in the melt to form and precipitate the aforementioned simple and/or complex titanium boride particles in-situ therein. When using the VAR melting process to form the TiAl base melt, elemental boron can be : -provided-in the melt by dispersing elemental boron in the VAR electrode with the other alloyants as described in the Examples hierebelow. When the electrode is melted into thle underlying crucible, the TiAl base composition and the boron are brought together in the melt so that the boron can react with metals in the melt to precipitatP simple borides (e.g., TiB2) and/or complex borid s (e.g., Ti,Nb)XBy in the melt. When using the vacuum induction, induction skull, electron beam and plasma mel~ing processes referred to hereabove, the elemental boron an be provided in the melt hy blending with the :: `
,, P-310 Howmet 20 ~7373 ~ initial alloyants of the charge to be melted or by : addition to the already melted alloy charge.
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, Other methods of providing the desired level - 5 of boride dispersoids in the melt are described in U.
~: S. Patent ~,915,052 and 4,916,029, although the present invention is not limited to any particular technique in this regard.
Importantly, the dispersoid-containing TiAl base alloy melt is maintained at a selected superheat temperature (~or a given melt hold time prior to :~ -- casting) to avoid growth of th~ boride particles present in the melt to a harmful size. Namely, the ~ 15 superheat of the melt is mai.ntained sufficienkly low ; so as to avoid formation of deleterious TiB needles (whiskers) having a length ~reater than about 50 microns~ These TiB needles form from the existing TiB2 particles in the melt by particle growth pxocesses and are quité harmful to the properties,:especially the ductility, of the casting. In general, the superheat temperature of the melt is maintained at the melting temperature of the TiAl base composition plus about 25 to 200F thereabove to this end. Temperature . .
P-310 Howmet 21 ~37373 maintenance in this manner ~osters the presence of blocky (e.g., equiaxed), lacey and/or small needles (less than about 50 microns length) of TiB2 in the melt~ Such boride particles are illustrated schematically in Fi~. 17.
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Pre~erably, the dispersoid-containing TiAl base alloy melt is stirred in the crucible prior to casting. When tha aforementioned VAR, VIM, ISR and other melting techniques are used, the melt is stirred in the crucible by the action o~ an induction heating coil on the melt. Melt stirring in this manner aintains a homogenous ~elt with the boride d~spersoids distributed uniformly throughout.
Melting and casting o~ the TiAl base alloy containing the boride dispersoids is conducted under relative vacuum (e.g., 1 micron vacuum) or under inert : atmosphere (e.g., .5 atmosphere Ar) to minimize ~:
70 contamination of the melt.
The dispersoid-containing TiAl base alloy melt is cast into a non-reactive, ceramic investment mold having one or more mold cavities configured in .
P-310 Howmet 22 2~7373 the net or near-net shape of the article to be cast.
- Net shape castings require no machining to achieve final print dimensions/tolerances~ Near-net shape castings may require only a minor machining operation of the casting, or portion thereof, to provide final print dimensions/tolerances. Investment molds used in ~ practicing the invention are made in accordance with ;~ conventional mold forming processes wherein a ~ugative pattern (e.g., a wax pattern) having the near-net shape to be cast is repeatedly dipped in a ceramic ~ slurry, stuccoed with ceramic particulate and then ; dried to build up a suitable shell mold about the pattern. After the desired thickness of the shell mold is formed, the pattern is removed ~rom the mold, leaving one or more mold cavities therein. When wax patterns axe used, the patterns can be removed by known dewaxing techniques, such as steam autoclave dewaxing, ~lash dewaxing in a furnace and the like.
After pattern removal, the shall mold is treated at elevated temperatures to remove absorbed water and gases there~rom. Although the invention 1s not limited to any particular mold formation process, certain specific mold formation processes are set forth in the Examples herebelow.
, P-310 Howmet 23 2~737~ :
The investment mold is made from ceramic materials which will be substantially nonreactive with the TiAl base alloy melt so as not react with and contaminate the melt. In particular, the mold facecoat that contacts the melt typically comprises a ceramic material selected Prom zirconia, yttria and the like to this end~ The mold coats subsequently applied to the facecoat (i.e., the backup coats) may be selected from a variety of ceramic materials depending upon the particular casting application involved. The investment mold may be made in various configurations as needed for a particular casting application.
Referring to Figure 1, the dispersoid-containing TiAl base alloy mslt at the appropriate superheat temperature is cast (e.g., poured) from the melting crucible into a preheated investment mold and ; solidified therein to form a net or near-net shape, cast article whose microstructure will be describad in detail herebelow~ The melt may be gravity or countergravity cast into an investment mold that is stationary or that is rotat~d as, ~or example, in centri~ugal casting processes. Regardless of the , - ' ' P-310 Howmet 24 2~3~3 ~- casting me~hod employed, the cooling (freezing) rate of the melt and cooling rate of the casting are controlled so as to be fast enough to prevent migration and segregation of the boride dispersoids to the grain boundaries and yet slow enough to avoid cracking of the solidified casting. The cooling rate employed will depend upon the melt superheat, the section size of the casting to be produced, the configuration of the casting to be produced, the particular TiAl base alloy composition, the loading level o~ dispersoids in the melt as well as other factors. In general, cooling rates of about 102 to - about 1030F per second are employed to this end. Such cooling rates are typically achieved by placing the melt-filled investment mold in a bed of refractory material (e.g., Al~03) and allowing the melt to ~solidify to ambient temperature. Once the casting has cooled to ambient temperature ~or other demold temperature~, the casting and the investment mold are separated in usual manner, such as by vibration.
: , Referring again to Figure 1, following separation o~ the mold and~the casting, the casting may be subjected to a consolidation operation to close ' , i ~., ~ . , , ",~,:
P-310 Howmet 25 ~7373 any porosity in the casting. Preferably, the casting is hot isostatically pressed at, for example, 2100-2400F and a pressure o~ 10-45 ksi for 1-10 hours depending on the size of the casting, to close any : 5 porosity present in casting. Thereafter, the HIP'ped casting i~ heat treated to provide at least a partially equiaxed grain structure in lieu of the lamellar grain structure present in the as-cast microstructure. Heat treat parameters of 1600-2500F
for 1-75 hours may be used. Of course, other consolidation processes/parameters and heat treatment procasses/parameters can be employed in practicing the invention.
The titanlum alum:inide base casting produced in accordance with the present invention is characterized as having a net or near-net shape for the intended servic~ application and a predominantly gamma TiAl intermetallic matrix corresponding in composition to that of the base composition. The matrix exhibits a fine, as-cast grain structure of lamellar morphology and a grain siz~ within the range of about 10 to about 250 microns, preferably about 50 to about 150 microns. The matrix may include other .:
P-310 Howmet 26 2~7373 titanium aluminide phases (e.g., Ti3A1 or TiAl3) in minor amounts such as up to about 15.0 volume %. The as-ca~t lamellar grain structure is changed to a partially equiaxed grain structure by the subsequent heat treatment operation.
~s will become apparent from the Examples ~ set forth herebelow, a certain minimu= level of boride ; dispersoids, such as at least about 0.5 volume %
dispersoids, must be uniformly distributed throughout the melt during solidification in order to achieve a grain refinement effect that yields as-cast and heat treated grain sizes in the aforementioned ranges for strength enhancement purposes. Dispersoid levels below the minimum level are .ineffective to produce the fine as-cast grain sizes required for improved strength. The dispersoids are distributed generally uniformly throughout the as-cast matrix (as shown in ~: Figs. 5, 6, 13 and 14) and are not segregated at the grain boundaries.
As will also becoma apparent from the Examples set for~h herebelow, the boride dispersoids are present in the matrix in various morphologies including a) ribbon shapes generally 0.1-2.0 microns ,, . ;
~, .
P-310 Howmet 27 20~7373 thick, 0.2-5.0 microns wide and 5.0-1000 microns long, b) blacky (equiaxed) shapes generally of 0.1-50O0 microns average size (major particle dimension), c) needle shapes generally 0.1-5.0 microns wide and 5.0-50.0 microns long, and d) acicular shapes generally 1.0-10.0 microns wide and 5.0-30.0 microns long. These various dispersoids particle shapes are illustrated schematically in Fig. 17. As mentioned hereabove, large TiB needles having a length greater than about 50 microns are to be avoided in the matrix so as not to adversely affect the ductility of the casting.
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Consolidated and heat treated TiAl intermetallic base investment castings in accordance with the invention typically exhibit a yield strength at room temperature (70F) of at lea~t about 55 ksi and a ductility~at room temperature o~ at least 0.5%
as measured by the ASTM E8M test procedure.
Consolida ed and heat treated TiAl intermetallic base investment castings of the invention having the aforementioned even more pre~erred composition typically exhibit a yield strength at room temperature (70F) of at least about 60 ksi and a ductility at ` ' ,. ..
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P-310 Howmet 28 2~5~73 room temperature of at least about 1.0% as measured by the same ~STM test procedure. These room temperature properties represent a substantial improvement over the room temperature properties demonstrated heretofore by investment cast TiAl intermetallic materials which have not been modified by addition of borides or boron.
- ~ The following Examples are offered to illustrate the invention in further de~ail without limiting the scope thereof.
- EXA~PLE 1 This example illustrates practice of one embodiment of the invention wherein elemental boron is provided in the TiAl base alloy melt in order to form boride di~persoids in-situ therein. Various amounts of elemental boron were provided in the TiAl base melt to determine the dependence of grain refinement on the amount of boride dispersoids present in the melt. The following melt compositions were prepared by the VAR
melting process referred to hereabove:
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P-310 Howmet 29 2~7373 Alloy A--- Ti 47.1% Al-2.1% Nb-1.6~ Mn-0.047% B
(0.04 v/o borides3 Alloy B -- Ti-47.8% Al-2.1% Nb-2.4% Mn-0.11% B
(0~07 v/o borides) Alloy C--- Ti-46.9~ Al-2.0% Nb-1.7~ Mn 0.17% B
(0.13 v/o borides) 10 Alloy D--- Ti-47.2% Al-2.0% Nb-1.5% Mn-0.3~ B
(0.27 v/o borides or 0.30 atomic % B) - Alloy E--- Ti-48.4~ Al-2.0~ Nb-1.5% Mn-1.0% B
(0.70 v/o borides or 1.0 atomic ~ B~
Alloy F~-- Ti-45 . 3~ Al-l . 9% Nb~ % Mn-2.49% B
(1.94 v/o borides or 2.5 atomic % Bj : ~ :
~ : A cylindrical electrode of each of these TiAl base ; 20 alloy compositions was prepared by cold pressing Ti sponge, Al pellets~ A1/Nb master alloy chunks, Al/Mn master alloy chunks and elemental boron powder in the appropriate amounts in a Ti tube. The cold pressed body was subjected to a ~irst meltiny operation to ' ' :' ' ;
~ "~ ., ' ~ P-~10 Howmet 30 2~57373 produce an ingot. The ingot was grit blasted and then remelted again to produce the electrode~ Each electrode was then VAR melted into a copper crucible to form a TiAl base alloy melt in which elemental boron was present.
Each TiAl alloy melt was maintained at a superheat temperatuxe of about 25F above the melting point by VAR melting prior to casting. Agitation during VAR melting also acted to stir the melt prior to casting. Each melt was poured from the crucible into a preheated (600F) cexamic investment mold compri~ing a Zr203 mold facecoat for contacting the melt and nine backup coats of Al203. Each mold included five mold cavities in the shape of cylinders having the following dimensions: 0.625 inch diameter x 8;inches long. Each melt was melted and cast into the:
mold under 7 microns vacuum. Each melt-filled molds was placad in~a bed o~ Al203 (to a depth of about 8 inches) and allowed to cool to ambient temperature :
over a period of about Z hours. Each mold and the cylindrical-shaped casting were then separated.
Figures 2A-2F illustrate the effect of boron :: . :' ~, :
P-310 Howmet 31 2~737'~
concentration (expressed in atomic %) of the base alloy composition and of volume % boride dispersoids in the castings on the as-cast grain structure. It is evident that little or no grain refinement was observed in Figs. 2A through Fig. 2D for the Alloy A, B, C and D castings. on the other hand, dramatic grain refinement was present in the Alloy E and F
castings as shown in Fig. 2E and Fig. 2F. The transition from no observed grain re~inement to dramatic grain refinement occurred between Alloy D
(0.3 atomic % B) and Alloy E (1.0 atomic % B). The grain size of Alloy E casting and Alloy F casting were about 50 t~ about 150 microns, respectively.
.
Alloy E castings were hot isostatically pressed at 2300F and 25 ksi. for 4 hour~ and then subjected to different heat treatments to determine : response of the as-cast lamellar grain struc~ure to dif~erent temperatures. Figures 3A and 3B illustrate 20~ the change in grain structure from lamellar to partially equiaxed after heat treatments at 2100F and 1850F with the same time-at-temperature and gradual ~urnace cool (GFC). The change from lamellar to partially equiaxed grain structure is evident in both " . ' ,.
,~ . ~ - :.':
, ~7~73 P-310 Howmet 32 Figs. 3A,3B.
Figures 4A-4C, 5A-5C, and 6A-6C illustrate the effects of boron concentration on the appearance of boride dispersoids in Alloys D, E and F, respectively, as consolidated/heat treated. Three different known electron microprobe techniques were used to vi w the dispersoids; namely, the secondary technique, the back sca~ter technique and the boron : 10 dot map. Based upon these Figures, the solubility of borcn in the Ti-Al-Nb-Mn compositions set forth above appears to be less than 0.05 atomic % B.
Table 1 sets forth strength and ductility properties of tho Alloy A, B, D, and E castin~s after HIP'ing at 2300F and 25 ksi for 4 hours followed by heat treatment at 1850F for 50 hours in an inert ~atmosphere. Included for comparison purposes in Table i9 a base alloy (T1-48~Al-2%Nb-2%Mn-0%~) HIP'ed ~: 20 using the same parameters and heat treated to a qimilar microstructure. Tensile t~sts were conducted at room (70F) temperatuxe in accordance with ASTM E8M
test procedure and at 1500F in accordance with ~STM
E21 test procedure.
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37~3 P-310 Howmet 33 Table 1 TEST TEMP. I~S Y~ ELONG.
(F) (KSI) (KSI) (%) Base Alloy 70 58.0 40.01.7 1500 50.0 37.030.0 Alloy A 70 62.2 52 8l 0 1500 54.4 4~ 644 7 Alloy ~ 70 52.2 46.10.6 1500 62.~ 45.26.8 Alloy D 70 54.3 50 00 5 1500 61.3 39 717 1 Alloy E 70 69.4 59.20.7 15~0 66.1 45.220.7 This combination strength and ductility properties represent significa~t improvements over those obtainable heretofore in the casting of gamma titanium aluminide (TiAl).
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:
: : This example illustrates practice of another :: : embodiment of the invention wherein preformed boride dispersoids (TiB2) are provided in the TiAl base alloy melt by adding a master boride material thereto. The master boride material comprised a porous sponge having 70 weight % of borides (TiB2) in an Al matrix .
' 3 ~ 3 P-310 Howmet 34 metal. Various amounts o~ the sponge material were added to tha TiAl base alloy melt so as to determine the dependence of grain refinement on the amount (volume %) of boride dispersoids present in the melt.
The following melt compositions were prepared by the VAR melting process referred to hereabove:
Alloy OXD--~ Ti-45.4~ Al-1.9% Nb-1.4% Mn-O vol.%
TiB2 ( at.% ~) ; Alloy lXD--- Ti-45.4% A1-1,9% Nb-1.4~ Mn- 0.1 vol.%
TiB2 (.17 at.% B or 0.1 volume % borides) .
Alloy 2XD--- Ti-46.1% Al-1.8% Nb-1,6% Mn-0.4 vol.% TiB2 (.50 at.% B or 0.4 volume % borides) .
~: Alloy 3XD--- Ti-47.7% A1-2.0% Nb-2.0% Mn-l.O vol.% TiB2 (1.40 at. % B or 1.0 volume % borides) 20 Alloy 4XD--- Ti-44.2~ Al-2.0~ N~-1.4% Mn-2.0 vol.% TiB2 (2.59 at. % B) :
Alloy 5XD--- Ti-45.4% Al-1.9% Nb 1.6% Mn-~.6 vol.% TiB2 (5.97 at.% B or 4.6 volume % borides) , ', ~7373 P-310 Howmet 35 Interstitial concentrations in these alloys are set forth below:
INTERSTITIALS (ppm wt%) O N H
Alloy OXD--- 716 42 6 :Alloy lXD--- 632 58 9 Alloy 2XD -~ 684 68 14 Alloy 3XD--- 538 47 10 Alloy 4XD -- 795 90 10 Alloy 5XD--- 654 48 13 Each of these TiAl base alloy compositions was fabricated into a cylindrical alectrode by the procedure described hereina]bove for Example 1. After double melti~g as described above, each electrode was subjected to a surface treatment operation using a SiC
grinding tool, grit blasting (or alternatively chemical milling operatlon using 10~ HF aqueous solution as an etchant) to remove surface oxidation therefrom. About a .020 inch depth was removed from the electrode. Each electrode was then VAR melted by direct electric arc heating into a copper crucible to form a TiAl base alloy melt to which the preformed ~ .
2 ~ 7 3 P-310 Howmet 36 ma~ter sponge was added.
:
Each TiAl alloy melt was maintained at a superheat temperature of about 25 F above the alloy melting point by electric arc melting prior to castinq. Each melt was poured from the crucible into a preheated (600F) ceramic investment mold comprising a Zr2o3 mold facecoat for contacting the melt and nine ~backup coats of Al2030 Each mold included five mold cavities in the shape of cylinder~ haviny the following dimensions: 0.625 inch diameter x 8 inches long. Each melt was melted and cast into the mold - under a 7-micron vacuum. Each melt-filled mold was placed in a bed of Al203 (to a depth of about 8 inches) and allowed to cool to ambient temperature over a period of about 2 hours. Each mold and the cylindrical-shaped castings were then separated.
: Figures 7A-7F illustrate the effect of 2Q :boride loading (volume %) on ~he as-cast grain structure of ~lloys lXD through 5XD, respectively. It is evident from Figs. 7A through 7C, that little or no grain refinement was observed for the Alloy OXD, lXD
and 2XD castings. on the other hand, dramatic grain :
,: ;; :
2~737~
P-310 Howmet 37 refinement was present in the Alloy 3XD, 4XD and 5XD
castings as shown in Figs. 7D through 7F. The transition from no observed grain refinement to dramatic grain refinement oscurred between Alloy 2XD
(0.4 vol. % TiB2) and Alloy 3XD ~1.0 vol.% TiB2). The grain size of Al.loy 3XD, 4XD and 5XD castings was about 50 to about 150 microns.
: Figures 8A-8F illustrate the as-cast microstructures of the castings OXD-5XD, respectively.
Figures 9A-9F illustrate the as-HIP'ped ~: - microstructures of the castings OXD-5XD, respectively.
, Alloy 3XD castings were hot isostatically pressed at 2300F and 25 ksi for 4 hours and then : subjected to different heat treatments to determine response of the as-cast lamellar grain structure to different temperatures. Figures lOA and lOB
illustrat- the change in grain structure from lamellar to partially equiaxed after heat treatments at 2100F
and 1850F with the same time-at-temperature and gradual furnace cool. The change from lamellar to equiaxed grain structure is evident in both Figs.
P-310 Howmet 38 2~7~7~
lOA, 1OB.
: , Figures llA-llC, 12A-12C, 13A-13C and 14A-14C illustrate the effects of boron concentration on the appearance of boride dispersoids in Alloys lXD, 2XD, 3XD, and 5XD, respectively, as-cast. Three different known electron microprobe techniques were used to view the dispersoids; namely, the secondary techniqu~, the back scatter technique and the boron ~: 10 dot map.
: `
Figures 15A-15C and 16A-16C illustrate various TiB2 particle shape~; extracted from Alloy 2XD
and 3XD, respectively.
Table 2 sets forth strength and ductility ~: : : propertie~ of the Alloy 2XD and 3XD castin~s after ~ :
HIP'ing:at 2300F and 25 ksi ~or 4 hours followed by ~ ~ ~ : heat treatment at 1850~F for 50 hours in an inert (Ar~
:~ :atmosphere. Tensile tests were conducted at room - ~
. (70F) temperature and at 1500F in accordance with : ASTM E8M and E21 test procedures, respectlvely.
.; .
P-310 Howmet 39 ~737`
Table 2 TEST TEMP. UTS YS ELONG. AVERAGE
tF~(KSI) (KSI) (%) GRAIN SIZE
Alloy 2XD 7062.2 51.0 1.0 1000 um 150065.8 45.2 1~.0 Alloy 3XD 7084.4 78.2 0.7 75 um 150060.6 48.~ 8.9 This combination of strength and ductility properties represent significant improvements over : those heretofore obtainable in the prior art cast gamma (TiAl) titanium a1uminide alloys.
EX~MPLE 3 ~ ..
This example illustrates practice of still another embodiment o~ the invention wherein a charge of Ti sponge, Al:pellets, AllMn master alloy chunks, Al/Nb master alloy chunks and elemental boron powder are melted using:the induction skull melting procedure. In particular, the charge was melted in a ~egmented, water-cooled copper crucible such that a ~solidified metal skull ~ormed on the crucible surfaces shortly after melting of the meltlng of the charge.
The charge was melted~by energization of an induction coil positioned about the crucible (see U.S. Patent , 2~373 P-310 Howmet 40 4,923,508) and was maintained at a superheat temperature of about 50F above the alloy melting point by induction heatiny. The melt was stirred as a result of the induction heating.
The melt was poured from the crucible into a preheated (600F) ceramic investment mold comprising a Zr203 mold facecoat for contacting the melt and nine back up coats of~Al2O3. Each mold included 5 mold cavities in the shape of cylinders having the following dimensions: 0.652 inch diameter x 8 inches long. Each melt was melted under .5 atmosphere Ar and - ~ast into ~he mold under 200 microns vacuum. Each melt-~illed mold was placed in a bed of Al203 (to a depth of about 8 inches) and allowed ~o cool to ambient temperature over a period of about 2 hours.
Each moId and the cylindrical-shaped castings were then separated.
The following melt compositions (in atomic %) were ISR melted and investment cast as described above:
. ~ .
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P-310 Howmet 41 2~737~
Alloy 1--- Ti-45.6~ Al-1.9% Nb-2.3% Mn-1.10% B
Alloy 2--- Ti-45.1% Al 1.9~ Nb-2.2% Mn-2.4% B
Fox comparison purposes, two alloys (XD0 and XD7) were prepared in accordance with EXample 2 to include 0 volume % and 7 volume % titanium borides.
Table 3 sets forth room temperature strength : 10 and cluctility properties o~ Alloys 1-2 after HIP'ing at 2300F and 25 ksi for 4 hours followed by heat treatment at 1650F for 24 houxs in lnert (Ar) at~o~phere. Alloys XD0 and XD7 (Ti-48%Al-2%Nb-2~Mn : with 0 volume % and 7 volume~ % borides, respectively) were HIP' ed using the same parameters and heat treated to a similar microstructure. The room temperature tensile tests were conducted pursuant to ASTM ~8M test procedure.
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, P-310 Howmet 42 2~7373 : :.
Table 3 ROOM TEMPERATURE TENSILE RESULTS
BORIDE/BORON YIELD ULTIMATE PLASTIC
AMOUNT STRENGTH STRENGTH ELONGATION
XD0 0 40.0 58.0 1.7 :
XD7 7 Vol.% 65.0 79.0 0.5 :
~: Alloy 1 1.10 At%B 74.0 89.0 1.3 : Alloy 2 2.40 At%B 75.0 86~0 0.9 ::
While the invention has been described in terms of specif ic em~odiments thereof, it is not : intended to be :limited thereto but rather only to the - extent set forth in the following claims.
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: : :
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, . ; ' ~
METHOD OF MAXING SAME
Field of the Invention The present invention relates to a method of maXing articles based on TiAl intermetallic materials and, more particularly, to TiAl intermetallic base articles having a net or near-net shape for an intended service application and having improved strength.
:
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~ 15 Background of the Invention : :, For the past several years, extensive research has been devoted to the development of . ~.. intermetallio materials, suoh as titanium aluminides, for u e in the manufacture of light weight structural comp~nents capable of withstanding high temperatures/stresses. Suchl components are represented, for example, by blades, vanesj disks, sha~ts, casings and other components of the turbine ~: 25 s~ction of modern gas turbine engines where higher gas : and resultant~component temperatures are desired to incr~ase engine thrust/efficiency and oth~r applications reguiring light weight, high temperature materials.
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, : ~ :
Pt ~
P 310 Howmet 2 Intermetallic materials, such as gamma titanium aluminide, exhibit improved high temperature mechanical properties, including high strength-to-weight ratios, and oxidation resistance relative to conven~ional hi.gh temperature titanium alloys.
However, general exploitation of these intermetallic materials has been limited by the lack o~ strength, room temperature ductility, and toughness, as well as the technical challenges associated with processing and fabricating the material into the complex end-use shapes that are exemplified, for example, by the : aforementioned turbine componen~s.
The Kampe et al U.S. Patent 4,915,905 issued April 10, 1990 describes in detail the development of various metallurgical processing techniques for ~ improving the lou (room) temperaturP duc~ility and toughness of intermetallic material~ and increasing their high temperature strength. The Kampe et al '905 patent relates to the rapid solidification of metalIic matrix composites. In particular, in this patent, an intermetallic-second phase composite is formed; for example, by reacting second phase-forming constituents in the presence of a solvent metal, to form in~situ : ~
~ ' .
20~73~3 P 310 Howmet 3 precipitated second phase paxticles, such as boride dispersoids, within an inter~etallic-containing matrix, such as titanium aluminide. The intermetallic-second phase composite is then subjected to rapid solidification to produce a rapidly solidified composite. Thus, for example, a composite comprising in-situ precipitated TiB2 particles within a : titanium aluminide matrix may be formed and then rapidly solidlfled to producs a rapidly solidified : 10 powder o the composite. The powder is then consolidated by such consolidation techniques as hot isostatic pressing, hot extrusion and superplastic .. ~. ....... .. ~orging to provide near~inal (i.e., neax-net) shapes.
U.S. Patent 4,836~g82 to Brupbacher et al also relates to the rapid solidification of metal matrix composites wherein second phase-forming : con~tituents are reacted in the presence of a solvent metal to ~orm in-situ precipitated second phase particles, such as TiB2 or TiC, within the solvent me~al, such as aluminum. .
U.S. Patents 4,774,052 and 4,916,029 to Nagle et al are specifically directed toward the : .
- .
' : ' P-310 Howmet 4 2 ~ ~ 7 3 !1 3 production of metal matrix second phase composites in which the metallic matrix compris~s an intermetallic material, such as titanium aluminide. In one embodiment, a first composite is formed which comprises a disparsion of second phase particles, such as TiBz, within a metal or alloy matrix, such as Al~
This composite i9 then introduced into an additional .
metal which is reactive with the matrix to form an intermetallic matrix. For example, a first composite comprising a dispersion of TiB2 particl~s within an Al matrix may be introduced into molten titanium to form a final composite compri~ing TiB2 dispersed within a _ titanium aluminide matrix. U.S. Patent 4,9~5,903 to Brupbacher et al describes a modification of the method~ tau~ht in the aforemen~ioned Nagle et al pat~nts.
' An attempt to improve room temperature : ductility by alloying intermetallic materials with one or more metals in combination with certain plastic forming techniques is disclosed in the Blackburn U.S.
Patent 4,294,615 wherein vanadium was added to a TiAl composition to yield a modified composition of Ti-31 to 36% Al-0 to 4% V. The modified composition was ,~
:.
P-310 Howmet 5 2~7373 ; melted and isothermally forged to shape in a heated die at a slow deforma~ion rate necessitated by the dependency of ductility of the intermetallic material : on strain rate. The isothermal forging process is carried out at above 1000C such that special die material~ (e.g., a Mo alloy known as TZM) must be used. Generally, it is extremely difficult to process TiAl intermetallic materlals in this way as a result :of their high strength, high temperature nature and the dependence of their ductility on strain rate.
A series of U.S. patents comprising U.S.
. . Patents 4,836,983; 4,842,817; 4,842,819; 4,842,820;
4,857,268; 4,879,092; 4,897,127; 4,~902,474; and 4,916,028, have described a1:tempts to make gamma TiAl iDtermetalli materials having both a modified stoichiometri~c ratio of Ti/Al and one or more alloyant additions to improve room temperature strength and ; ductility. In making cylindrical shapes from these modified compositions, the alloy was typically first made into an in~ot by electro-arc melting. The ingot was melted and melt spun to form rapidly solidified ~:
ribbon. The ribbon was placed in a suitable container and hot isostatically pressed (HIP'ped) to form a ' , . , ~ . - :
P-310 Howmet ~ 3~3 consolidated cylindrical plug. The plug was placed axially into a central opening of a billet and sealed therein. The billet was heated to 975C for 3 hours and extruded through a die to provide a reduction of about 7 to 1. Samples ~rom the extruded plug were removed from the billet and heat treated and aged.
U.S. Patent 4,916,028 (included in the series of patents listed above) also refers to . 10 processing the TiAl basa alloys as modi~ied to include : C, Cr and Nb additions by ingot metallurgy to achieve ~: desirable combinations of ductility, ætrength and . other properties at a lower processing cost than the aforementioned rapid solidification approach. In particular, the ingot metallurgy approach described in the '02~ paten involves melting the modified alloy and~solidifying it into a hockey puck-shaped ingot of simple geometry and small size (e.g., 2 inches in diameter and .5 inch thick), homogenizing the ingot at 20 1250C for 2 hours, enclosing the ingot in a steel annulus, and then hot forging the annulus/ring assembly to provide a 50% reduction in ingot thickness. Tensile specimens cut from the ingot were annealed at various temperatures above 1225C prior to . .
: ", ; ,~ .
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P-310 Howmet 7 2~r~`7373 t~nsile testing. Tensile specimens prepared by this ingot metallurgy approach exhibited lower yield strengths but greater ductility than specimens prepared by the rapid solidification approach.
Despite the improvements described hereabove ~ in the ductility and strength of intermetallic : materials, there is a continuing desire and need in the high performance material-using industries, ~: 10 especially in the ga~ turbine engine industry, for intermetallic materials with improved properties or combinations of properties and also ~or manufacturing -- technology that will allow t:he fabrication of such intermetallic materials into usable, complex engineered end-use shapes on a relatively high volume basis at much lower cost. It is an object o~ the ~ ~ present invention to satisfy these desires and needs.
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Summary of the Invention The present invention involves a method of making titanium aluminide base intermetallic articles having a net or near-net shape for intended service application and having improved strength. The method ', .~
P-310 Howmet 8 ~7373 of the present invention involves forming a titanium-aluminum melt comprising (in atomic %) Ti in an amount of about 40% to about 52%, Al in an amount of about 44% to about 52~, and one or more of Cr, C, Ga, Mo, Mn, Nb, Ni~ Si, Ta, V, and W each in an amount o~ about 0.05% to about 8~. Boride dispersoids are provided in the melt in an amount of at least about .5 volume % of the melt. Preferably, a low volume % of boride dispersoids in the range of abou~ . 5 to about 2.0 volume % is provided in the malt.
The dispersoid-containing melt i5 cast and ; , . solidified-in a mold cavity of a ceramic investment mold wherein the mold cavity is configured in the net or near-net shape o~ the article to ~e cast. The melt is solidified in a manner to yield a cra~k-free, net or near net shape cast article compriRinq a titanium aluminide-containing matrix (e.g., gamma TiAl) havlng :
a grain size of about 50 to about 250 microns as a result of grain ref;nement from the boride dispersoids being-distributed throughout the melt during solidification. The melt is solidified in the mold at a cooling rate sufficiently fast to avoid migration of the boride dispersoids to the grain boundaries during P-310 Howmet 9 2~7~7~
solidification and yet sufficiently slow to avoid cracking of th~ article. A cooling rate in the range of about 102 tG about 103F/second is preferred to this end. Following solidification, the net or near-net shape, investment cast article may be subjected to a consolidation operation to olose any porosity in the as-cast condition. The consolidated article may then be heat treated to provide at least a partially equiaxed grain morphology.
In one embodiment of the invention, the ; boride dispersoids are provided in the melt by - -introducin~ a preformed ~oride master material to the melt. In another embodiment of the invention, the boride dispersoids are provided in the melt by introducing an effective amount of elemental boron in the melt to form the desired volume % of borides in situ therein. Regardless of how the boride dispersoids are~provided in the melt, the melt is maintained at a selected superheat temperature for a given melt hold time prior to casting to avoid deleterious coarsening (growth) of the boride particles ~dispersoids) present in the melt.
7 3 ~ ~
P-310 Howmet 10 The present invention also invulves a titanium aluminide base article having a net or near-net investment cast shape for intended service application and a titanium aluminide-containing matrix (e.g., gamma TiAl) consisting essentially of (in atomic %) about 40% to about 52% Ti, about 44% to about 52% Al and one or more of Cr, C, Ga, Mo, Mn, Nb, 5i, Ta, V and W each included in an amount of about ~ 0.05% to about 8%. The matrix includes at least about : 10 .5 volume % boride dispersoids distributed uniformly throughout and a fine, equiaxed, grain structure have : a grain size of about 10 to about 250 microns.
~- Preferably, the article, as consolidated and heat treated to provide:the partially equiaxed grain structure, exhibits a yield strength at room ~: temperature ~70F) of at least about 55 ksi and a tensile ductility at room temperatura of at least ~.
~ bout 0.5% (measured by the ASTM E8M test procedure).
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~, 20: Thus, the present invention has as a : particular purpose to provide net or near-net shape :
articles o~ a TiAl base intermetallic material modified by the addition of selected alloyant(s)/dispersoids and~formed to shape by ,. .
P-310 Howmet 11 2~7373 investment casting in a crack-free condition treatable by consolidation/heat treatment to exhibit improved strength and ductility at room temperature. The method of the invention provides an alternative to much more costly techniques heretofore used to fabricate TiAl base intermetallics.
' The advantag4s of the present invention will become more readily understood by consideration of the following detailed description and examples.
BrieP Description of the Drawinqs :
Figure 1 is a flow sheet illustrating one embodiment of the method of the invention.
Figures 2A through 2F are photomicrographs of in~estment castings of Alloys A through E, respectively, illustrating the effect of increasing ':: :
~ 20 boron in the melt on grain refinement.
' Figures 3A-3B are photomicrographs of the microstructures of investment castings illustrating the affect of heat treatment under different P-310 Howmet 12 ~7~73 conditions on grain morphology.
Figures 4A-4C are photomicrographs illustrating ths boride dispersoids present in a particular Alloy D investment casting.
Figures 5A-SC are photomicrographs illustrating the boride dispersoids present in a particular Alloy E investment casting.
Figures 6A-6C are photomicrographs illustratinq the boride dispersoids present in a - - particular Alloy F investment casting.
' -lS Figures 7A-7F are photomicrographs of investment castings illustrating the effect of increasiny borides (added by ma~t~r borid~ material) in the melt on grain refinement.
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: ~ 20 Figure 8A 8F are photomicrographs of the as-cast microstructures of the investment castings of Figs. 6A-6F.
Figure 9A-9F are photomicrographs of the hot .
2~ 7 3 P-310 Howmet 13 isostatically pressed microstructures of the investment castings of Figs. 7A-7F.
Figures lOA-lOB are photomicrographs o~ the microstructures of investment castings illustrating the effect of heat treatment under different conditions on grain morphology.
: Figures llA-llC are photomicrographs illustrating the boride dispersoids present in a particular Alloy lXD invsstment casting (as-cast).
': ' Figures 12A-12C are photomicrographs illustrating the boride dispersoids present in a particular Alloy 2XD investment casting (as-cast).
Figures 13A-13C are photomicrographs illustrating the boride dispersoids present in a particular Alloy 3XD investment casting (as-cast).
Figures 14A-14C are photomicrographs illustrating the boride ~ispersoids present in a particular Alloy 5Xd investment casting (as-cast).
~73~3 P-310 Howmet 1~
Figures 15A-15C are photomicrographs illustrating boride particle~ extracted from the Alloy 2XD investment casting (as-cast~.
Figures 16A-16C are photomicrographs illustrating boride particles extracted from the Alloy 3XD investment casting (as-cast).
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: ~ Figure 17 is a schematic illustration of boride particles o~ various morphology that occur in the investment castings.
~ ~ - Detailed Description of the Invention .: .
The present invention relates to net or near-net shape articles comprised of a titanium aluminide base intermetallic material modified by the :~ addition of selected alloyant(s)/dispersoids and ~
~ormed to shape by investment casting in a crack-free, . .
fine grained condition treatable by consolidationtheat treatment to exhibit improved strength at room temperature. Titanium-aluminum base alloys employed in practicing the present invention consist essentially of, by atomic %, about 40% to about 52%
P~310 Howmet 15 2 ~ ~ 7 3 7 3 Ti, about 4~% to about 52% Al and one or more of the alloyants Cr, C, ~a, Mo, Mn, Nb, Ni, Si, Ta, V,and W
each in an amount of about 0.05% to about 8%. The listed alloyants are provided in the base composition as a result of their beneficial effect on ductility when present in certain combinations and/or concen~rations.
preferred base alloy ~or use in practicing the present invention consists essentially of, by .
atomic %, about 44% to about 50% Ti, about 46% to about 49% Al and at least one of Cr, C, &a, Mo, Mn, ~ Nb, Ni, si, Ta, V,and W wherein Cr, Ga, Mo, Mn, Nb, ;~ : Ta, V and W, when present, are each included in an amount of about 1% to about 5% and wherein C, Ni and Si, when present, are each 1ncluded in an amount of about O 05% to about 1.0%. Two or more of the ;~ ; alloyants~Cr, C, Ga, Mo, Mn, Nb, Si, Ta, V and W are : : present in an even more preferred embodiment within the concentration ranges given. Although the present : invention is~not limited to a particular base composition within the ranges set forth hereabove, certain specific preferred base compositions are described in the Examples set forth hereinbelow.
, P-310 Howmet 16 ~37373 Referring to Figure 1, the various steps involved in practicing one embodiment of the method of the invention are illus rated. In this embodiment, a melt of the TiAl base alloy is formed in a suitable container, such as a crucible, by a variety of melting techniques including, but not limited to, vacuum arc melting (VAR), vacuum induction melting (VIM~, induction skull melting (ISR~, electron beam melting ~: (EB?, and plasma arc melting (PAM). In the vacuum arc : 1~ melting technique, an electrode .is fabricated of the base alloy composition and is melted by direct electrical arc heating (i.e., an arc astablished between the electrode and the crucible) into an undexlying non-reactive crucible. An actively cooled copper crucible is useful in this regardO Vacuum induction melting involves heating and melting a charge of the base alloy in a non-reactive,~ refractory ::crucibla by induction heating th charge usin~ a surrounding electrically energized induction coil.
Induction skull melting involves inductively heating and melting a charge o~ the base alloy in a water-cooled, segmented, non contaminating copper crucible surrounded by a suitable induction coil.
Electron beam melting and plasma melting involve ;
.
P-310 Howmet 17 ~73~3 melting using a configuration of electron beam(s) or a plasma plume directed on a charge in an actively : cooled copper crucible. These melting techniques are known genarally in the art of melting o~ metals and alloys.
. ' :
Although the present invention is not limited to any particular melting process, certain ;~ specific melting processes are described in the : 10 Examples set forth hereinbelow.
~: Referring again to Figure l, the melt of the TiAl base alloy in the container (crucible) is provided with boride disper~;oids in an amount o~ at least about 0.5 volume % prior to casting of the melt in an investment mold to be described in detail herebelow. Typically, the boride dispersoids comprise simple titanium borides (TiB2) and/or complex borides such as (Ti,M)XBy where M is Nb, W, Ta or other al}oyant. Although varying amounts of the boride disparsoids may be used depending upon the end-use properties desired ~or the cast article, relatively low boride dispersoids levels of about 0.5 to about 20.0 volume % are useful in practicing the invention P-310 ~owmet 18 ~ 3 7 to achieve the desired grain refinement effects in the casting as well as s~rength and ductility improvements upon further treatment of the casting. Boride dispersoid levels above the upper limit set ~orth tend ; 5 to reduce ductility and thus are not preferxed. In accordance with the invention, optimum strength and ductility are achieved when the boride dispersoid level is preferably about 0.5 to about 2.0 volume % of the melt or cast article.
}O
The TiAl base alloy melt described hereabove can be provided with the desired level of boride dispersoids in a variety of ways including the addition of a boride mastar material to the melt in accordance with U.S. Patents 4,751,048 and 4,916,030, the teachings of which are incorporated herein by reference. In particular, a porous sponge having a relatively high concentration of boride particles (e.g., ~iB~) is introduced and incorporated in the TiAl base melt to provide a lower concentration of boride particles therein~ Of course, the concentration of boride particles in the sponge is chosen to yleld a selectad lower concentration of the particles in the melt; for example, at least about .5 volume ~ boride .
:, P-310 ~owmet 19 20~7373 dispersoids in the melt. Boride mastPr materials (i.e. sponges) useful in practicing the present invention are available from Martin Marietta Corporation, Bethasda, Md and its licensees.
The TiAl base alloy melt also can be provided with the desired level of boride dispersoids by providing an effective amount o~ elemental boron in the melt to form and precipitate the aforementioned simple and/or complex titanium boride particles in-situ therein. When using the VAR melting process to form the TiAl base melt, elemental boron can be : -provided-in the melt by dispersing elemental boron in the VAR electrode with the other alloyants as described in the Examples hierebelow. When the electrode is melted into thle underlying crucible, the TiAl base composition and the boron are brought together in the melt so that the boron can react with metals in the melt to precipitatP simple borides (e.g., TiB2) and/or complex borid s (e.g., Ti,Nb)XBy in the melt. When using the vacuum induction, induction skull, electron beam and plasma mel~ing processes referred to hereabove, the elemental boron an be provided in the melt hy blending with the :: `
,, P-310 Howmet 20 ~7373 ~ initial alloyants of the charge to be melted or by : addition to the already melted alloy charge.
~
, Other methods of providing the desired level - 5 of boride dispersoids in the melt are described in U.
~: S. Patent ~,915,052 and 4,916,029, although the present invention is not limited to any particular technique in this regard.
Importantly, the dispersoid-containing TiAl base alloy melt is maintained at a selected superheat temperature (~or a given melt hold time prior to :~ -- casting) to avoid growth of th~ boride particles present in the melt to a harmful size. Namely, the ~ 15 superheat of the melt is mai.ntained sufficienkly low ; so as to avoid formation of deleterious TiB needles (whiskers) having a length ~reater than about 50 microns~ These TiB needles form from the existing TiB2 particles in the melt by particle growth pxocesses and are quité harmful to the properties,:especially the ductility, of the casting. In general, the superheat temperature of the melt is maintained at the melting temperature of the TiAl base composition plus about 25 to 200F thereabove to this end. Temperature . .
P-310 Howmet 21 ~37373 maintenance in this manner ~osters the presence of blocky (e.g., equiaxed), lacey and/or small needles (less than about 50 microns length) of TiB2 in the melt~ Such boride particles are illustrated schematically in Fi~. 17.
.
Pre~erably, the dispersoid-containing TiAl base alloy melt is stirred in the crucible prior to casting. When tha aforementioned VAR, VIM, ISR and other melting techniques are used, the melt is stirred in the crucible by the action o~ an induction heating coil on the melt. Melt stirring in this manner aintains a homogenous ~elt with the boride d~spersoids distributed uniformly throughout.
Melting and casting o~ the TiAl base alloy containing the boride dispersoids is conducted under relative vacuum (e.g., 1 micron vacuum) or under inert : atmosphere (e.g., .5 atmosphere Ar) to minimize ~:
70 contamination of the melt.
The dispersoid-containing TiAl base alloy melt is cast into a non-reactive, ceramic investment mold having one or more mold cavities configured in .
P-310 Howmet 22 2~7373 the net or near-net shape of the article to be cast.
- Net shape castings require no machining to achieve final print dimensions/tolerances~ Near-net shape castings may require only a minor machining operation of the casting, or portion thereof, to provide final print dimensions/tolerances. Investment molds used in ~ practicing the invention are made in accordance with ;~ conventional mold forming processes wherein a ~ugative pattern (e.g., a wax pattern) having the near-net shape to be cast is repeatedly dipped in a ceramic ~ slurry, stuccoed with ceramic particulate and then ; dried to build up a suitable shell mold about the pattern. After the desired thickness of the shell mold is formed, the pattern is removed ~rom the mold, leaving one or more mold cavities therein. When wax patterns axe used, the patterns can be removed by known dewaxing techniques, such as steam autoclave dewaxing, ~lash dewaxing in a furnace and the like.
After pattern removal, the shall mold is treated at elevated temperatures to remove absorbed water and gases there~rom. Although the invention 1s not limited to any particular mold formation process, certain specific mold formation processes are set forth in the Examples herebelow.
, P-310 Howmet 23 2~737~ :
The investment mold is made from ceramic materials which will be substantially nonreactive with the TiAl base alloy melt so as not react with and contaminate the melt. In particular, the mold facecoat that contacts the melt typically comprises a ceramic material selected Prom zirconia, yttria and the like to this end~ The mold coats subsequently applied to the facecoat (i.e., the backup coats) may be selected from a variety of ceramic materials depending upon the particular casting application involved. The investment mold may be made in various configurations as needed for a particular casting application.
Referring to Figure 1, the dispersoid-containing TiAl base alloy mslt at the appropriate superheat temperature is cast (e.g., poured) from the melting crucible into a preheated investment mold and ; solidified therein to form a net or near-net shape, cast article whose microstructure will be describad in detail herebelow~ The melt may be gravity or countergravity cast into an investment mold that is stationary or that is rotat~d as, ~or example, in centri~ugal casting processes. Regardless of the , - ' ' P-310 Howmet 24 2~3~3 ~- casting me~hod employed, the cooling (freezing) rate of the melt and cooling rate of the casting are controlled so as to be fast enough to prevent migration and segregation of the boride dispersoids to the grain boundaries and yet slow enough to avoid cracking of the solidified casting. The cooling rate employed will depend upon the melt superheat, the section size of the casting to be produced, the configuration of the casting to be produced, the particular TiAl base alloy composition, the loading level o~ dispersoids in the melt as well as other factors. In general, cooling rates of about 102 to - about 1030F per second are employed to this end. Such cooling rates are typically achieved by placing the melt-filled investment mold in a bed of refractory material (e.g., Al~03) and allowing the melt to ~solidify to ambient temperature. Once the casting has cooled to ambient temperature ~or other demold temperature~, the casting and the investment mold are separated in usual manner, such as by vibration.
: , Referring again to Figure 1, following separation o~ the mold and~the casting, the casting may be subjected to a consolidation operation to close ' , i ~., ~ . , , ",~,:
P-310 Howmet 25 ~7373 any porosity in the casting. Preferably, the casting is hot isostatically pressed at, for example, 2100-2400F and a pressure o~ 10-45 ksi for 1-10 hours depending on the size of the casting, to close any : 5 porosity present in casting. Thereafter, the HIP'ped casting i~ heat treated to provide at least a partially equiaxed grain structure in lieu of the lamellar grain structure present in the as-cast microstructure. Heat treat parameters of 1600-2500F
for 1-75 hours may be used. Of course, other consolidation processes/parameters and heat treatment procasses/parameters can be employed in practicing the invention.
The titanlum alum:inide base casting produced in accordance with the present invention is characterized as having a net or near-net shape for the intended servic~ application and a predominantly gamma TiAl intermetallic matrix corresponding in composition to that of the base composition. The matrix exhibits a fine, as-cast grain structure of lamellar morphology and a grain siz~ within the range of about 10 to about 250 microns, preferably about 50 to about 150 microns. The matrix may include other .:
P-310 Howmet 26 2~7373 titanium aluminide phases (e.g., Ti3A1 or TiAl3) in minor amounts such as up to about 15.0 volume %. The as-ca~t lamellar grain structure is changed to a partially equiaxed grain structure by the subsequent heat treatment operation.
~s will become apparent from the Examples ~ set forth herebelow, a certain minimu= level of boride ; dispersoids, such as at least about 0.5 volume %
dispersoids, must be uniformly distributed throughout the melt during solidification in order to achieve a grain refinement effect that yields as-cast and heat treated grain sizes in the aforementioned ranges for strength enhancement purposes. Dispersoid levels below the minimum level are .ineffective to produce the fine as-cast grain sizes required for improved strength. The dispersoids are distributed generally uniformly throughout the as-cast matrix (as shown in ~: Figs. 5, 6, 13 and 14) and are not segregated at the grain boundaries.
As will also becoma apparent from the Examples set for~h herebelow, the boride dispersoids are present in the matrix in various morphologies including a) ribbon shapes generally 0.1-2.0 microns ,, . ;
~, .
P-310 Howmet 27 20~7373 thick, 0.2-5.0 microns wide and 5.0-1000 microns long, b) blacky (equiaxed) shapes generally of 0.1-50O0 microns average size (major particle dimension), c) needle shapes generally 0.1-5.0 microns wide and 5.0-50.0 microns long, and d) acicular shapes generally 1.0-10.0 microns wide and 5.0-30.0 microns long. These various dispersoids particle shapes are illustrated schematically in Fig. 17. As mentioned hereabove, large TiB needles having a length greater than about 50 microns are to be avoided in the matrix so as not to adversely affect the ductility of the casting.
;:
Consolidated and heat treated TiAl intermetallic base investment castings in accordance with the invention typically exhibit a yield strength at room temperature (70F) of at lea~t about 55 ksi and a ductility~at room temperature o~ at least 0.5%
as measured by the ASTM E8M test procedure.
Consolida ed and heat treated TiAl intermetallic base investment castings of the invention having the aforementioned even more pre~erred composition typically exhibit a yield strength at room temperature (70F) of at least about 60 ksi and a ductility at ` ' ,. ..
:
.
P-310 Howmet 28 2~5~73 room temperature of at least about 1.0% as measured by the same ~STM test procedure. These room temperature properties represent a substantial improvement over the room temperature properties demonstrated heretofore by investment cast TiAl intermetallic materials which have not been modified by addition of borides or boron.
- ~ The following Examples are offered to illustrate the invention in further de~ail without limiting the scope thereof.
- EXA~PLE 1 This example illustrates practice of one embodiment of the invention wherein elemental boron is provided in the TiAl base alloy melt in order to form boride di~persoids in-situ therein. Various amounts of elemental boron were provided in the TiAl base melt to determine the dependence of grain refinement on the amount of boride dispersoids present in the melt. The following melt compositions were prepared by the VAR
melting process referred to hereabove:
~ . . .
P-310 Howmet 29 2~7373 Alloy A--- Ti 47.1% Al-2.1% Nb-1.6~ Mn-0.047% B
(0.04 v/o borides3 Alloy B -- Ti-47.8% Al-2.1% Nb-2.4% Mn-0.11% B
(0~07 v/o borides) Alloy C--- Ti-46.9~ Al-2.0% Nb-1.7~ Mn 0.17% B
(0.13 v/o borides) 10 Alloy D--- Ti-47.2% Al-2.0% Nb-1.5% Mn-0.3~ B
(0.27 v/o borides or 0.30 atomic % B) - Alloy E--- Ti-48.4~ Al-2.0~ Nb-1.5% Mn-1.0% B
(0.70 v/o borides or 1.0 atomic ~ B~
Alloy F~-- Ti-45 . 3~ Al-l . 9% Nb~ % Mn-2.49% B
(1.94 v/o borides or 2.5 atomic % Bj : ~ :
~ : A cylindrical electrode of each of these TiAl base ; 20 alloy compositions was prepared by cold pressing Ti sponge, Al pellets~ A1/Nb master alloy chunks, Al/Mn master alloy chunks and elemental boron powder in the appropriate amounts in a Ti tube. The cold pressed body was subjected to a ~irst meltiny operation to ' ' :' ' ;
~ "~ ., ' ~ P-~10 Howmet 30 2~57373 produce an ingot. The ingot was grit blasted and then remelted again to produce the electrode~ Each electrode was then VAR melted into a copper crucible to form a TiAl base alloy melt in which elemental boron was present.
Each TiAl alloy melt was maintained at a superheat temperatuxe of about 25F above the melting point by VAR melting prior to casting. Agitation during VAR melting also acted to stir the melt prior to casting. Each melt was poured from the crucible into a preheated (600F) cexamic investment mold compri~ing a Zr203 mold facecoat for contacting the melt and nine backup coats of Al203. Each mold included five mold cavities in the shape of cylinders having the following dimensions: 0.625 inch diameter x 8;inches long. Each melt was melted and cast into the:
mold under 7 microns vacuum. Each melt-filled molds was placad in~a bed o~ Al203 (to a depth of about 8 inches) and allowed to cool to ambient temperature :
over a period of about Z hours. Each mold and the cylindrical-shaped casting were then separated.
Figures 2A-2F illustrate the effect of boron :: . :' ~, :
P-310 Howmet 31 2~737'~
concentration (expressed in atomic %) of the base alloy composition and of volume % boride dispersoids in the castings on the as-cast grain structure. It is evident that little or no grain refinement was observed in Figs. 2A through Fig. 2D for the Alloy A, B, C and D castings. on the other hand, dramatic grain refinement was present in the Alloy E and F
castings as shown in Fig. 2E and Fig. 2F. The transition from no observed grain re~inement to dramatic grain refinement occurred between Alloy D
(0.3 atomic % B) and Alloy E (1.0 atomic % B). The grain size of Alloy E casting and Alloy F casting were about 50 t~ about 150 microns, respectively.
.
Alloy E castings were hot isostatically pressed at 2300F and 25 ksi. for 4 hour~ and then subjected to different heat treatments to determine : response of the as-cast lamellar grain struc~ure to dif~erent temperatures. Figures 3A and 3B illustrate 20~ the change in grain structure from lamellar to partially equiaxed after heat treatments at 2100F and 1850F with the same time-at-temperature and gradual ~urnace cool (GFC). The change from lamellar to partially equiaxed grain structure is evident in both " . ' ,.
,~ . ~ - :.':
, ~7~73 P-310 Howmet 32 Figs. 3A,3B.
Figures 4A-4C, 5A-5C, and 6A-6C illustrate the effects of boron concentration on the appearance of boride dispersoids in Alloys D, E and F, respectively, as consolidated/heat treated. Three different known electron microprobe techniques were used to vi w the dispersoids; namely, the secondary technique, the back sca~ter technique and the boron : 10 dot map. Based upon these Figures, the solubility of borcn in the Ti-Al-Nb-Mn compositions set forth above appears to be less than 0.05 atomic % B.
Table 1 sets forth strength and ductility properties of tho Alloy A, B, D, and E castin~s after HIP'ing at 2300F and 25 ksi for 4 hours followed by heat treatment at 1850F for 50 hours in an inert ~atmosphere. Included for comparison purposes in Table i9 a base alloy (T1-48~Al-2%Nb-2%Mn-0%~) HIP'ed ~: 20 using the same parameters and heat treated to a qimilar microstructure. Tensile t~sts were conducted at room (70F) temperatuxe in accordance with ASTM E8M
test procedure and at 1500F in accordance with ~STM
E21 test procedure.
::
37~3 P-310 Howmet 33 Table 1 TEST TEMP. I~S Y~ ELONG.
(F) (KSI) (KSI) (%) Base Alloy 70 58.0 40.01.7 1500 50.0 37.030.0 Alloy A 70 62.2 52 8l 0 1500 54.4 4~ 644 7 Alloy ~ 70 52.2 46.10.6 1500 62.~ 45.26.8 Alloy D 70 54.3 50 00 5 1500 61.3 39 717 1 Alloy E 70 69.4 59.20.7 15~0 66.1 45.220.7 This combination strength and ductility properties represent significa~t improvements over those obtainable heretofore in the casting of gamma titanium aluminide (TiAl).
:
:
: : This example illustrates practice of another :: : embodiment of the invention wherein preformed boride dispersoids (TiB2) are provided in the TiAl base alloy melt by adding a master boride material thereto. The master boride material comprised a porous sponge having 70 weight % of borides (TiB2) in an Al matrix .
' 3 ~ 3 P-310 Howmet 34 metal. Various amounts o~ the sponge material were added to tha TiAl base alloy melt so as to determine the dependence of grain refinement on the amount (volume %) of boride dispersoids present in the melt.
The following melt compositions were prepared by the VAR melting process referred to hereabove:
Alloy OXD--~ Ti-45.4~ Al-1.9% Nb-1.4% Mn-O vol.%
TiB2 ( at.% ~) ; Alloy lXD--- Ti-45.4% A1-1,9% Nb-1.4~ Mn- 0.1 vol.%
TiB2 (.17 at.% B or 0.1 volume % borides) .
Alloy 2XD--- Ti-46.1% Al-1.8% Nb-1,6% Mn-0.4 vol.% TiB2 (.50 at.% B or 0.4 volume % borides) .
~: Alloy 3XD--- Ti-47.7% A1-2.0% Nb-2.0% Mn-l.O vol.% TiB2 (1.40 at. % B or 1.0 volume % borides) 20 Alloy 4XD--- Ti-44.2~ Al-2.0~ N~-1.4% Mn-2.0 vol.% TiB2 (2.59 at. % B) :
Alloy 5XD--- Ti-45.4% Al-1.9% Nb 1.6% Mn-~.6 vol.% TiB2 (5.97 at.% B or 4.6 volume % borides) , ', ~7373 P-310 Howmet 35 Interstitial concentrations in these alloys are set forth below:
INTERSTITIALS (ppm wt%) O N H
Alloy OXD--- 716 42 6 :Alloy lXD--- 632 58 9 Alloy 2XD -~ 684 68 14 Alloy 3XD--- 538 47 10 Alloy 4XD -- 795 90 10 Alloy 5XD--- 654 48 13 Each of these TiAl base alloy compositions was fabricated into a cylindrical alectrode by the procedure described hereina]bove for Example 1. After double melti~g as described above, each electrode was subjected to a surface treatment operation using a SiC
grinding tool, grit blasting (or alternatively chemical milling operatlon using 10~ HF aqueous solution as an etchant) to remove surface oxidation therefrom. About a .020 inch depth was removed from the electrode. Each electrode was then VAR melted by direct electric arc heating into a copper crucible to form a TiAl base alloy melt to which the preformed ~ .
2 ~ 7 3 P-310 Howmet 36 ma~ter sponge was added.
:
Each TiAl alloy melt was maintained at a superheat temperature of about 25 F above the alloy melting point by electric arc melting prior to castinq. Each melt was poured from the crucible into a preheated (600F) ceramic investment mold comprising a Zr2o3 mold facecoat for contacting the melt and nine ~backup coats of Al2030 Each mold included five mold cavities in the shape of cylinder~ haviny the following dimensions: 0.625 inch diameter x 8 inches long. Each melt was melted and cast into the mold - under a 7-micron vacuum. Each melt-filled mold was placed in a bed of Al203 (to a depth of about 8 inches) and allowed to cool to ambient temperature over a period of about 2 hours. Each mold and the cylindrical-shaped castings were then separated.
: Figures 7A-7F illustrate the effect of 2Q :boride loading (volume %) on ~he as-cast grain structure of ~lloys lXD through 5XD, respectively. It is evident from Figs. 7A through 7C, that little or no grain refinement was observed for the Alloy OXD, lXD
and 2XD castings. on the other hand, dramatic grain :
,: ;; :
2~737~
P-310 Howmet 37 refinement was present in the Alloy 3XD, 4XD and 5XD
castings as shown in Figs. 7D through 7F. The transition from no observed grain refinement to dramatic grain refinement oscurred between Alloy 2XD
(0.4 vol. % TiB2) and Alloy 3XD ~1.0 vol.% TiB2). The grain size of Al.loy 3XD, 4XD and 5XD castings was about 50 to about 150 microns.
: Figures 8A-8F illustrate the as-cast microstructures of the castings OXD-5XD, respectively.
Figures 9A-9F illustrate the as-HIP'ped ~: - microstructures of the castings OXD-5XD, respectively.
, Alloy 3XD castings were hot isostatically pressed at 2300F and 25 ksi for 4 hours and then : subjected to different heat treatments to determine response of the as-cast lamellar grain structure to different temperatures. Figures lOA and lOB
illustrat- the change in grain structure from lamellar to partially equiaxed after heat treatments at 2100F
and 1850F with the same time-at-temperature and gradual furnace cool. The change from lamellar to equiaxed grain structure is evident in both Figs.
P-310 Howmet 38 2~7~7~
lOA, 1OB.
: , Figures llA-llC, 12A-12C, 13A-13C and 14A-14C illustrate the effects of boron concentration on the appearance of boride dispersoids in Alloys lXD, 2XD, 3XD, and 5XD, respectively, as-cast. Three different known electron microprobe techniques were used to view the dispersoids; namely, the secondary techniqu~, the back scatter technique and the boron ~: 10 dot map.
: `
Figures 15A-15C and 16A-16C illustrate various TiB2 particle shape~; extracted from Alloy 2XD
and 3XD, respectively.
Table 2 sets forth strength and ductility ~: : : propertie~ of the Alloy 2XD and 3XD castin~s after ~ :
HIP'ing:at 2300F and 25 ksi ~or 4 hours followed by ~ ~ ~ : heat treatment at 1850~F for 50 hours in an inert (Ar~
:~ :atmosphere. Tensile tests were conducted at room - ~
. (70F) temperature and at 1500F in accordance with : ASTM E8M and E21 test procedures, respectlvely.
.; .
P-310 Howmet 39 ~737`
Table 2 TEST TEMP. UTS YS ELONG. AVERAGE
tF~(KSI) (KSI) (%) GRAIN SIZE
Alloy 2XD 7062.2 51.0 1.0 1000 um 150065.8 45.2 1~.0 Alloy 3XD 7084.4 78.2 0.7 75 um 150060.6 48.~ 8.9 This combination of strength and ductility properties represent significant improvements over : those heretofore obtainable in the prior art cast gamma (TiAl) titanium a1uminide alloys.
EX~MPLE 3 ~ ..
This example illustrates practice of still another embodiment o~ the invention wherein a charge of Ti sponge, Al:pellets, AllMn master alloy chunks, Al/Nb master alloy chunks and elemental boron powder are melted using:the induction skull melting procedure. In particular, the charge was melted in a ~egmented, water-cooled copper crucible such that a ~solidified metal skull ~ormed on the crucible surfaces shortly after melting of the meltlng of the charge.
The charge was melted~by energization of an induction coil positioned about the crucible (see U.S. Patent , 2~373 P-310 Howmet 40 4,923,508) and was maintained at a superheat temperature of about 50F above the alloy melting point by induction heatiny. The melt was stirred as a result of the induction heating.
The melt was poured from the crucible into a preheated (600F) ceramic investment mold comprising a Zr203 mold facecoat for contacting the melt and nine back up coats of~Al2O3. Each mold included 5 mold cavities in the shape of cylinders having the following dimensions: 0.652 inch diameter x 8 inches long. Each melt was melted under .5 atmosphere Ar and - ~ast into ~he mold under 200 microns vacuum. Each melt-~illed mold was placed in a bed of Al203 (to a depth of about 8 inches) and allowed ~o cool to ambient temperature over a period of about 2 hours.
Each moId and the cylindrical-shaped castings were then separated.
The following melt compositions (in atomic %) were ISR melted and investment cast as described above:
. ~ .
' .
P-310 Howmet 41 2~737~
Alloy 1--- Ti-45.6~ Al-1.9% Nb-2.3% Mn-1.10% B
Alloy 2--- Ti-45.1% Al 1.9~ Nb-2.2% Mn-2.4% B
Fox comparison purposes, two alloys (XD0 and XD7) were prepared in accordance with EXample 2 to include 0 volume % and 7 volume % titanium borides.
Table 3 sets forth room temperature strength : 10 and cluctility properties o~ Alloys 1-2 after HIP'ing at 2300F and 25 ksi for 4 hours followed by heat treatment at 1650F for 24 houxs in lnert (Ar) at~o~phere. Alloys XD0 and XD7 (Ti-48%Al-2%Nb-2~Mn : with 0 volume % and 7 volume~ % borides, respectively) were HIP' ed using the same parameters and heat treated to a similar microstructure. The room temperature tensile tests were conducted pursuant to ASTM ~8M test procedure.
' ~ -. .
, P-310 Howmet 42 2~7373 : :.
Table 3 ROOM TEMPERATURE TENSILE RESULTS
BORIDE/BORON YIELD ULTIMATE PLASTIC
AMOUNT STRENGTH STRENGTH ELONGATION
XD0 0 40.0 58.0 1.7 :
XD7 7 Vol.% 65.0 79.0 0.5 :
~: Alloy 1 1.10 At%B 74.0 89.0 1.3 : Alloy 2 2.40 At%B 75.0 86~0 0.9 ::
While the invention has been described in terms of specif ic em~odiments thereof, it is not : intended to be :limited thereto but rather only to the - extent set forth in the following claims.
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:
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: : :
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, . ; ' ~
Claims (21)
1. A method of making a titanium aluminide base article having improved strength and a net or near-net shape for intended service application, comprising the steps of:
a) forming a titanium-aluminum melt comprising titanium in an amount of about 40 to about 52 atomic %, aluminum in an amount of about 44 to about 52 atomic %, and one or more of Cr, C, Ga, Mo, Mn, Nb, Ni, Si, Ta, V and W each in an amount of about 0.05 to about 8 atomic %, b) providing boride dispersoids in the melt in an amount of at least about .5 volume % of said melt, c) casting the melt into a mold cavity of an investment mold, said mold cavity being configured in the net or near-net shape for the intended service application, and d) solidifying the melt in the mold cavity to form a crack-free, solidified article, said solidified P-310 Howmet 44 article having a titanium aluminide-containing matrix with said boride dispersoids distributed throughout the matrix, said matrix having a grain size of about 10 to about 250 microns as a result of the presence of said dispersoids in said melt.
a) forming a titanium-aluminum melt comprising titanium in an amount of about 40 to about 52 atomic %, aluminum in an amount of about 44 to about 52 atomic %, and one or more of Cr, C, Ga, Mo, Mn, Nb, Ni, Si, Ta, V and W each in an amount of about 0.05 to about 8 atomic %, b) providing boride dispersoids in the melt in an amount of at least about .5 volume % of said melt, c) casting the melt into a mold cavity of an investment mold, said mold cavity being configured in the net or near-net shape for the intended service application, and d) solidifying the melt in the mold cavity to form a crack-free, solidified article, said solidified P-310 Howmet 44 article having a titanium aluminide-containing matrix with said boride dispersoids distributed throughout the matrix, said matrix having a grain size of about 10 to about 250 microns as a result of the presence of said dispersoids in said melt.
2. The method of claim 1 including the additional step of consolidating the solidified article.
3. The method of claims 1 or 2 including the further step of heat treating the solidified article to provide at least a partially equiaxed grain-structure.
4. The method of claim 1 wherein the boride dispersoids are present in an amount of about .5 to about 2 volume % .
5. The method of claim 1 wherein the grain size of the matrix is about 50 microns to about 150 microns.
6. The method of claim 1 wherein the P-310 Howmet 45 solidified article is consolidated by hot isostatic pressing.
7. The method of claim 1 wherein the melt is subjected to a cooling rate of less than about 102°F/second during the solidification step.
8. A method of making a titanium aluminide base article having improved strength and a net or near-net shape for intended service application, comprising the steps of:
a) forming a titanium-aluminum melt comprising titanium in an amount of about 44 to about 50 atomic %, aluminum in an amount of about 46 to about 49 atomic %, and one or more of Cr, C, Ga, Mo, Mn, Nb, Ni, Si, Ta, V and W, said Cr, Ga, Mo, Mn, Nb, Ta, V
and W, when present, being in an amount of about l to about 5 atomic %, and said Ni, Si and C, when present, being in an amount of about 0.05 to about 1 atomic %.
b) providing an effective amount of boron in the melt to form at least about .5 volume % of boride dispersoids in-situ in the melt, P-310 Howmet 46 c) casting the melt in a mold cavity of an investment mold, said mold cavity being configured in the net or near-net shape for the intended service application, and d) solidifying the melt in the mold cavity to form a crack-free, solidified article, said solidified article having a titanium aluminide-containing matrix with said boride dispersoids distributed throughout the matrix, said matrix having a grain size of about 10 microns to about 250 microns as a result of the presence of said dispersoids in said melt.
a) forming a titanium-aluminum melt comprising titanium in an amount of about 44 to about 50 atomic %, aluminum in an amount of about 46 to about 49 atomic %, and one or more of Cr, C, Ga, Mo, Mn, Nb, Ni, Si, Ta, V and W, said Cr, Ga, Mo, Mn, Nb, Ta, V
and W, when present, being in an amount of about l to about 5 atomic %, and said Ni, Si and C, when present, being in an amount of about 0.05 to about 1 atomic %.
b) providing an effective amount of boron in the melt to form at least about .5 volume % of boride dispersoids in-situ in the melt, P-310 Howmet 46 c) casting the melt in a mold cavity of an investment mold, said mold cavity being configured in the net or near-net shape for the intended service application, and d) solidifying the melt in the mold cavity to form a crack-free, solidified article, said solidified article having a titanium aluminide-containing matrix with said boride dispersoids distributed throughout the matrix, said matrix having a grain size of about 10 microns to about 250 microns as a result of the presence of said dispersoids in said melt.
9. The method of claim 8 including the additional step of consolidating the solidified article.
10. The method of claims 8 or 9 including the further step of heat treating the solidified article to provide at least a partially equiaxed grain structure.
11. The method of claim 8 wherein boron is provided in the melt in an amount effective to form P-310 Howmet 47 from about .5 to about 2 volume % boride dispersoids.
12. The method of claim 8 wherein the boron is provided in the melt by incorporating boron into a body comprising a titanium-aluminum alloy and melting the body to form said melt.
13. The method of claim 12 wherein the body is an electrode that is melted to form said melt.
14. The method of claim 8 wherein the grain size of the matrix is about 50 microns to about 150 microns.
15. The method of claim 8 wherein the solidified article is consolidated by hot isostatic pressing.
16. The method of claim 8 wherein the melt is subjected to a cooling rate of about 102 or less during the solidification step.
17. A titanium aluminide base article having a cast net or near-net shape for intended P-310 Howmet 48 service application, said article having a titanium aluminide containing matrix consisting essentially of about 44 to about 52 atomic % Ti, about 44 to about 52 atomic % Al and one or more of Cr, C, Ga, Mo, Mn, Nb, Ni, Si, Ta, V and W each in an amount of about 0.05 to about 8 atomic %, said article having at least about .5 volume % boride dispersoids distributed throughout the matrix and having a yield strength of at least about 55 ksi and a ductility of a least about 0.5%.
18. A titanium aluminide base article having a cast net or near-net shape for intended service. application, said article having a titanium aluminide containing matrix consisting essentially of about 44 to about 50 atomic Ti, about 46 to about 49 atomic % Al, and one or more of Cr, C, Ga, Mo, Mn, Nb, Si, Ta, V, and W, said Cr, Ga, Mo, Mn, Nb, Ta, V and W, when present, being in an amount of about 1 to about 5 atomic %, and said C, Ni and Si, when present, being in an amount of about 0.05 to about 1 atomic %, said article having at least about .5 volume % boride dispersoids distributed throughout the matrix and having a yield strength of at least about 60 ksi and a ductility of at least about 1.0%.
P-310 Howmet 49
P-310 Howmet 49
19. The article of claims 17 or 18 wherein the matrix includes at least two of Cr, C, Ga, Mo, Mn, Nb, Ni, Si, Ta, V and W.
20. The article of claims 17 or 18 wherein the matrix ha an equiaxed grain structure having a grain size of about 10 to about 250 microns.
21. The article of claims 17 or 18 wherein the boride dispersoids are present from about .5 to about 2 volume %.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US69618491A | 1991-05-06 | 1991-05-06 | |
| US696,184 | 1991-05-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2057373A1 true CA2057373A1 (en) | 1992-11-07 |
Family
ID=24796043
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2057373 Abandoned CA2057373A1 (en) | 1991-05-06 | 1991-12-10 | Tia1 intermetallic articles and method of making same |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2057373A1 (en) |
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-
1991
- 1991-12-10 CA CA 2057373 patent/CA2057373A1/en not_active Abandoned
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