Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
  • B22F3/23—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces involving a self-propagating high-temperature synthesis or reaction sintering step
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/04—Making non-ferrous alloys by powder metallurgy
  • C22C1/047—Making non-ferrous alloys by powder metallurgy comprising intermetallic compounds
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C14/00—Alloys based on titanium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C29/00—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides
  • C22C29/005—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides comprising a particular metallic binder
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
  • C22F1/18—High-melting or refractory metals or alloys based thereon
  • C22F1/183—High-melting or refractory metals or alloys based thereon of titanium or alloys based thereon
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
  • B22F2998/10—Processes characterised by the sequence of their steps

Definitions

• the invention relates generally to two-phase titanium alurninide alloy compositions useful for resistive heating and other applications such as structural applications.
• Titanium alurninide alloys are the subject of numerous patents and publications including U.S. Patent Nos. 4,842,819; 4,917,858; 5,232,661; 5,348,702; 5,350,466; 5,370,839; 5,429,796; 5,503,794; 5,634,992; and 5,746,846, Japanese Patent Publication Nos. 63-171862; 1-259139; and 1-42539;
• 5,489,411 discloses a powder metallurgical technique for preparing titanium alurninide foil by plasma spraying a coilable strip, heat treating the strip to relieve residual stresses, placing the rough sides of two such strips together and squeezing the strips together between pressure bonding rolls, followed by solution annealing, cold rolling and intermediate anneals.
• U.S. Patent No. 4,917,858 discloses a powder metallurgical technique for making titanium alurninide foil using elemental titanium, aluminum and other alloying elements.
• 5,634,992 discloses a method of processing a gamma titanium alurninide by consolidating a casting and heat treating the consolidated casting above the eutectoid to form gamma grains plus lamellar colonies of alpha and gamma phase, heat treating below the eutectoid to grow gamma grains within the colony strucmre and heat treating below the alpha transus to reform any remaining colony structure a strucmre having c ⁇ laths within gamma grains.
• the invention provides a two-phase titanium aluminum alloy having a lamellar microstructure controlled by colony size.
• the alloy can be provided in various forms such as in the as-cast, hot extruded, cold and hot worked, or heat treated condition.
• the alloy can be fabricated into an electrical resistance heating element having a resistivity of 60 to 200 ⁇ -cm.
• the alloy can include additional elements which provide fine particles such as second-phase or boride particles at colony boundaries.
• the alloy can include grain-boundary equiaxed structures.
• the additional alloying elements can include, for example, up to 10 at% W, Nb and/or
• the alloy can be processed into a thin sheet having a yield strength of more than 80 ksi (560 MPa), an ultimate tensile strength of more than 90 ksi (630 MPa), and/or tensile elongation of at least 1.5%.
• the aluminum can be present in an amount of 40 to 50 at% , preferably about 46 at% .
• the titanium can be present in the amount of at least 45 at% , preferably at least 50 at% .
• the alloy can include 45 to 55 at% Ti, 40 to 50 at% Al, 1 to 5 at% Nb, 0.5 to 2 at% W, and 0.1 to 0.3 at% B.
• the alloy is preferably free of Cr, V, Mn and/or Ni.
• Figures la-d are optical micrographs at 200X of PMTA TiAl alloys hot extruded at 1400°C and annealed for 2 hours at 1000°C.
• Figure la shows the microstructure of PMTA-1
• Figure lb shows the microstructure of PMTA-2
• Figure lc shows the microstructure of PMTA-3
• Figure Id shows the microstructure of PMTA-4;
• Figures 2a-d show optical micrographs at 500X of PMTA alloys hot extruded at 1400°C and annealed for 2 hours at 1000°C.
• Figure 2a shows the microstructure of PMTA-1
• Figure 2b shows the microstructure of PMAT-2
• Figure 2c shows the microstructure of PMAT-3
• Figure 2d shows the microstructure of PMTA-4;
• Figure 3 shows ghost-pattern bands observed in a back-scattered image of PMTA-2 hot extruded at 1400°C and annealed for 2 hours at 1000°C wherein the non-uniform distribution of W is shown;
• Figure 4 shows a back-scattered image of PMTA-2 hot extruded at 1400 °C and annealed for 2 hours at 1000°C;
• Figure 5a is a micrograph at 200X of PMTA-3 hot extruded at 1400°C and annealed for one day at 1000°C and Figure 5b shows the same microstructure at 500X;
• Figure 6a shows the microstructure at 200X of PMTA-2 hot extruded at 1400°C and annealed for 3 days at 1000°C and Figure 6b shows the same microstructure at 500X;
• Figure 7a is an optical micrograph of TiAl sheet (Ti-45Al-5Cr, at%) in the as-received condition and Figure 7b shows the same microstructure after annealing for 3 days at 1000°C, both micrographs at 500X;
• Figure 8a shows a micrograph of PMTA-6 and Figure 8b shows a micrograph of PMTA-7, both of which were hot extruded at 1380°C (magnification 200X);
• Figure 9a is a micrograph of PMTA-6 and
• Figure 9b is a micrograph of
• Figure 10 is micrograph showing abnormal grain growth in PMTA hot extruded at 1380°C;
• Figures lla-d are micrographs of PMTA-8 heat treated at different conditions after hot extrusion at 1335 °C, the heat treatments being two hours at
• Figure 12 is a graph of resistivity in microhms versus temperature for samples 1 and 2 cut from an ingot having a PMTA-4 nominal composition
• Figure 13 is a graph of hemispherical total emissivity versus temperature for samples 1 and 2;
• Figure 14 is a graph of diffusivity versus temperature for samples 80259-1,
• 80259-2 and 80259-3 cut from the same ingot as samples 1 and 2;
• Figure 15 is a graph of specific heat versus temperature for titanium alurninide in accordance with the invention.
• Figure 16 is a graph of thermal expansion versus temperature for samples 80259-1H, 80259-1C, 80259-2H, 80259-3H, and 80259-3C cut from the same ingot as samples 1 and 2.
• the invention provides two-phase TiAl alloys with thermo-physical and mechanical properties useful for various applications such as resistance heater elements.
• the alloys exhibit useful mechanical properties and corrosion resistance at elevated temperatures up to 1000°C and above.
• the TiAl alloys have extremely low material density (about 4.0 g/cm 3 ), a desirable combination of tensile ductility and strength at room and elevated temperatures, high electrical resistance, and/or can be fabricated into sheet material with thickness ⁇ 10 mil.
• One use of such sheet material is for resistive heating elements of devices such as cigarette lighters.
• the sheet can be formed into a tubular heating element having a series of heating strips which are individually powered for lighting portions of a cigarette in an electrical smoking device of the type disclosed in U.S. Patent Nos. 5,591,368 and 5,530,225, the disclosures of which are hereby incorporated by reference.
• the alloys can be free of elements such as Cr, V, Mn and/or Ni.
• tensile ductility of dual-phase TiAl alloys with lamellar structures can be mainly controlled by colony size, rather than such alloying elements.
• the invention thus provides high strength TiAl alloys which can be free of Cr, V, Mn and/or Ni.
• Table 1 lists nominal compositions of alloys investigated wherein the base alloy contains 46.5 at% Al, balance Ti. Small amounts of alloying additions were added for investigating effects on mechanical and metallurgical properties of the two-phase TiAl alloys. Nb in amounts up to 4% was examined for possible effects on oxidation resistance, W in amounts of up to 1.0% was examined for effects on microstructural stability and creep resistance, and Mo in amounts of up to 0.5% was examined for effects on hot fabrication. Boron in amounts up to 0.18% was added for refinement of lamellar structures in the dual-phase TiAl alloys. Eight alloys identified as PMTA-1 to 9, having the compositions listed in
• Table 1 were prepared by arc melting and drop casting into a 1 " diameter x 5" long copper mold, using commercially-pure metals. All the alloys were successfully cast without casting defects. Seven alloy ingots (PMTA -1 to 4 and 6 to 9) were then canned in Mo cans and hot extruded at 1335 to 1400°C with an extrusion ratio of 5: 1 to 6: 1. The extrusion conditions are listed in Table 2. The cooling rate after extrusion was controlled by air cooling and quenching the extruded rods in water for a short time. The alloy rods extruded at 1365 to 1400 °C showed an irregular shape whereas PMTA-8 hot-extruded at 1335° C exhibited much smoother surfaces without surface irregularities. However, no cracks were observed in any of the hot-extruded alloy rods.
• microstructures of the alloys were examined in the as-cast and heat treated conditions (listed in Table 2) by optical metallography and electron superprobe analyses.
• the as-cast condition all the alloys showed lamellar strucmre with some degree of segregation and coring.
• Figures 1 and 2 show the optical micrographs, with a magnification of 200X and 500X, respectively, for hot extruded alloys PMTA-1 to 4 stress-relieved for 2 hours at 1000°C. All the alloys showed fully lamellar structures, with a small amount of equiaxed grain structures at colony boundaries. Some fine particles were observed at colony boundaries, which are identified as borides by electron microprobe analyses. Also, there is no apparent difference in microstructural features among these four PMTA alloys.
• Electron microprobe analyses reveal that tungsten is not uniformly distributed even in the hot extruded alloys. As shown in Figure 3, the ghost- pattern bands in a darker contrast are found to be depleted with about 0.33 at% W.
• Figure 4 is a back-scattered image of PMTA-2, showing the formation of second- phase particles (borides) in a bright contrast at colony boundaries.
• the composition of the borides was determined and listed in Table 3 together with that of the lamellar matrix.
• the second-phase particles are essentially (Ti,W,Nb) borides, which are decorated and pinned lamellar colony boundaries.
• Figures 5 and 6 show the optical microstructures of hot extruded PMTA-3 and 2 annealed for 1 day and 3 days at 1000 °C, respectively. Grain-boundary equiaxed structures are clearly observed in these long-term annealed specimens, and the amount increases with the annealing time at 1000°C. A significant amount of equiaxed grain structures exists in the specimen annealed for 3 days at 1000 °C.
• Ti-45Al-5Cr 9-mil thick TiAl sheet (Ti-45Al-5Cr, at%) was evaluated.
• Figure 7 shows the optical microstructures of the TiAlCr sheet in both as-received and annealed (3 days at 1000°C) conditions.
• the TiAlCr sheet has a duplex structure, and its grain strucmre shows no significant coarsening at
• Table 4 summarizes the tensile test results.
• alloys stress-relieved for 2 hours at 1000 °C exhibited 1 % or more tensile elongation at room temperamre in air.
• the tensile elongation was not affected when the specimen thickness varied from 9 to 20 mils.
• alloy PMTA-4 appears to have the best tensile ductility. It should be noted that a tensile elongation of 1.6% obtained from a 20- mil thick sheet specimen is equivalent to 4% elongation obtained from rod specimens with a gage diameter of 0.12 in.
• the tensile elongation appears to increase somewhat with annealing time at 1000 °C, and the maximum ductility is obtained in the specimen annealed for 1 day at 1000°C.
• All the alloys are exceptionally strong, with a yield strength of more than 100 ksi (700 MPa) and ultimate tensile strength more than 115 ksi (800 MPa) at room temperamre.
• the high strength is due to the refined fully lamellar strucmres produced in these TiAl alloys.
• the TiAlCr sheet material has a yield strength of only 61 ksi (420 MPa) at room temperamre.
• the PMTA alloys are stronger that the TiAlCr sheet by as much as 67 % .
• the PMTA alloys including 0.5% Mo exhibited significantly increased strengths, but slightly lower tensile elongation at room temperamre.
• Figures 8a-b and 9a-b show the optical micrographs of PMTA-6 and 7 hot extruded at 1380°C and 1365 °C, respectively. Both alloys showed lamellar grain strucmres with little intercolony structures. Large colony grains (see Figure 10) were observed in both alloys hot extruded at 1380°C and 1365°C, which probably resulted from abnormal grain growth in the alloys containing low levels of boron after hot extrusion. There is no significant difference in microstructural features in these two PMTA alloys.
• Figures lla-d show the effect of heat treatment on microstructures of PMTA-8 hot extruded at 1335°C.
• the alloy extruded at 1335°C showed much finer colony size and much more intercolony structures, as compared with those hot extruded at 1380°C and 1365°C.
• Heat treatment for 2 h at 1000°C did not produce any significant change in the as-extruded structure ( Figure 11a).
• heat treatment for 30 mins at 1340°C resulted in a substantially larger colony strucmre (Figure lib).
• Lowering the heat-treatment temperature from 1340°C to 1320-1315°C (a difference by 20-25°C) produced a sharp decrease in colony size, as indicated by Figures lie and lid.
• the annealing at 1320-1315 °C also appears to produce more intercolony strucmres in PMTA-8.
• the abnormal grain growth is almost completely eliminated by hot extrusion at 1335 °C.
• the heat treatment at 1320-1315°C resulted in higher tensile elongation, but lower strength at the test temperatures.
• PMTA-8 hot extruded at 1335 °C and annealed for 20 min at 1315 °C exhibited the best tensile ductility at room and elevated temperamres. This alloy showed a tensile ductility of 3.3% and 11.7% at room temperature and 800°C, respectively.
• PMTA-8 heat treated at 1315°C appears to be substantially stronger than known TiAl alloys.
• Figure 12 is a graph of resistivity in microhms versus temperamre for samples 1 and 2 which were cut from an ingot having a nominal composition of PMTA-4, i.e. 30.8 wt% Al, 7.1 wt% Nb, 2.4 wt% W, and 0.045 wt% B.;
• Figure 13 is a graph of hemispherical total emissivity versus temperature for samples 1 and 2;
• Figure 14 is a graph of diffusivity versus temperature for samples 80259-1, 80259-2 and 80259-3 cut from the same ingot as samples 1 and 2;
• Figure 15 is a graph of specific heat versus temperamre for titanium alurninide in accordance with the invention;
• Figure 16 is a graph of thermal expansion versus temperature for samples 80259-1H, 80259-1C, 80259-2H, 80259-3H, and 80259- 3C cut from the same ingot as samples 1 and 2.
• the hot PMTA alloys extruded at 1365 to 1400°C exhibited mainly lamellar strucmres with little intercolony structures while PMTA-8 extruded at 1335 °C showed much finer colony strucmres and more intercolony strucmres.
• the heat treatment of PMTA-8 at 1315-1320°C for 20 min. resulted in fine lamellar strucmres.
• the alloys may include (Ti,W,Nb) borides formed at colony boundaries.
• mngsten in the hot-extruded alloys is not uniformly distributed, suggesting the possibility of high electrical resistance of TiAl alloys containing W additions. The inclusion of 0.5 at.
• % Mo significantly increases the yield and ultimate tensile strengths of the TiAl alloys, but lowers the tensile elongation to a certain extent at room temperamre.
• PMTA 1-4 PMTA-4 with the alloy composition Ti-46.5 Al-3 Nb-0.5 W- 0.2 B (at%) has the best combination of tensile ductility and strength at room temperamre.
• TiAlCr sheet material Ti-45 Al-5Cr
• PMTA-4 is stronger than the TiAlCr sheet by 67% .
• the TiAlCr sheet showed no bend ductility at room temperamre while PMTA-4 has an elongation of 1.4% .
• the tensile elongation of TiAl alloys is independent of sheet thickness in the range of 9 to 20 mils.
• the alloys PMTA 4, 6 and 7 heat treated at 1000°C for 2h showed excellent strength at all temperatures up to 800 °C, independent of hot extrusion temperamre.
• Hot extrusion temperamres of 1400-1365 °C however, provides lower tensile ductilities ( ⁇ 4%) at room and elevated temperatures.
• the foregoing titanium alurninide can be manufactured into various shapes or products such as electrical resistance heating elements.
• the compositions disclosed herein can be used for other purposes such as in thermal spray applications wherein the compositions could be used as coatings having oxidation and corrosion resistance.
• the compositions could be used as oxidation and corrosion resistant electrodes, furnace components, chemical reactors, sulfidization resistant materials, corrosion resistant materials for use in the chemical industry, pipe for conveying coal slurry or coal tar, substrate materials for catalytic converters, exhaust walls and turbocharger rotors for automotive and diesel engines, porous filters, etc.
• the resistivity of the heater material can be varied by changes in composition such as adjusting the aluminum content of the heater material, processing or by incorporation of alloying additions. For instance, the resistivity can be significantly increased by incorporating particles of alumina in the heater material.
• the heater material can optionally include ceramic particles to enhance creep resistance and/or thermal conductivity.
• the heater material can include particles or fibers of electrically conductive material such as nitrides of transition metals (Zr, Ti, Hf), carbides of transition metals, borides of transition metals and MoSi 2 for purposes of providing good high temperature creep resistance up to 1200°C and also excellent oxidation resistance.
• the heater material may also incorporate particles of electrically insulating material such as Al 2 O 3 , Y 2 O 3 , Si 3 N 4 , ZrO 2 for purposes of making the heater material creep resistant at high temperamre and also improving thermal conductivity and/or reducing the thermal coefficient of expansion of the heater material.
• the electrically insulating/conductive particles/fibers can be added to a powder mixture of Fe, Al, Ti or iron alurninide or such particles/fibers can be formed by reaction synthesis of elemental powders which react exothermically during manufacture of the heater element.

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• 1999
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