CA1255929A - Method for imparting strength to intermetallic phases - Google Patents

Abstract

METHOD FOR IMPARTING STRENGTH TO INTERMETALLIC PHASES

ABSTRACT OF THE DISCLOSURE

A substantial increase in strength of a boron doped nickel aluminide is achieved by employing a substituent metal in the Ni3Al composition to replace a part of the aluminum. Vanadium and silicon are successfully substituted for a portion of the aluminum to provide a composition:
(Ni0.75Al0.20X0.05)99B1 where X is selected from the group consisting of vanadium or silicon.

Description

L2~

RD 15,123 METHOD FOR IMPARTI~G SIRENGIFl TO INIERMElALLIC_EHAS~S
BACKGROUND OF TFlE INV~NTIQN
By a previous U. S. Patent No. 4,478,791, issued October 23, 1984, the inventors disclosed and claimed a set of alloys having a boron additive whic~
made possibie the achievement ot a novel combinatio of ~rencJ~h and ductility in certain composit:io~ls.
It Ls poin~ed ou~ in the ~rior patellt t~clt in many systems composed of two or more meta.llic elements there may appear, under certain combinations of compositions and treatment conditions, phases other than the primary solid solutions. Such other phases are commonly known as intermediate phases~ Many intermediate phases are referred to by means of the Greek symbol such as ~ or ~ '. Also they are referred to ~y formula as for example, Cu3~1, CuZn and My2Pb. The compositions of the intermediate phases which have silnple approximate stoichiometric ratios of the elements may exist over a range of temperatures as well as of compositions.
Qccasionally as in the case of Mg2Pb, which occurs in the Mg-Pb system, a true stoichiometric compound, which compound is completely ordered, is found to occur. Where each of tlle elements of the coMpound is a metallic element, the intermediate compound itself is commonly called an intermetallic compound.

. .

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~D 15,123 The intermediate phases and intermetallic compounds often e~hibit properties entirely different from those of the component metals comprising the system. They also frequently have complex crystalographic structures. The lower order of crystal symmetry and fewer planes or dense atomic population of these complex crystallographic structures may be associated with certain differences in properties, e.g. greater hardness, lower ductility, lower electrical conductivity of the intermediate phases as compared to the properties of the ~rirnary solid solutions.
Although several intermediate intermetallic coMpounds with otherwi~e desirable properties, e.g.
hardness, strenyth, stability and resistarlce to oxidation and corrosion at elevated temperatur~s "lav~
be~n identi~i~d, the~1r characte~ristic l~ck OL
ductility has posed formidable barriers to their use as structural materials. In fact soMe of these materials are so friable that they have been prepared as solids in order that they may be broken up into powdered form for use in powder metallurgical processes for fabrication of articles.
A recent article appearing in the Japanese literature disclosed that the addition of trace alaounts (0.05 to 0.1~ wt.~) of boron to Ni3Al polycrystalline material was successful in improving the ductility of the otherwise ~rittle and noll-ductile intermetallic compound. See in this reyard Journal of the Japan Institute of Metals, Vo. 43, page 35~
published in 1979 by the authors Aoki and Izumi.
Although the roorn temperature tensile strain to fracture of the Ni3Al was improved by the boron additiion to about 35~, as compared to a~out 3% for ; 35 the Ni3Al without boron, the room temperature yield strength remained at about 30 ksi. The Japanese ., RD 15,123 article did not refer at all however to rapid solidification of the boron containing composition which they studied.
By the method of the prior Unled States Patent 4,47~,791 the addition of 0.01 to 2.5 % boron demonstrated further improvements where the alloy preparation included the step of rapid solidification. In particular as it is brought out in this prior patent preferred properties are found in rapidly solidified compositions containiny between 0.5 and 2.0~ boron and an optimum combination of yield stress and strain to fracture is found in rapidly solidified compositions containing approximately 1.~%
boron or less.
BRIEF ~TATEI~ T OF ~r~E INVEI~TI~N
It is, accordingly, one o~ject of the present inv~ntion ~o provide an improve~ alloy ~or operatiorl at higher temperatures.
Another object is to provide an alloy of nickel and aluminum capable of operating at elevated ; temperatures for sustained periods of time.
Another object is to provide a nickel aluminum alloy having an L12 type crystal structure but having significant ductility and strength.
Another object is to provide an alloy of aluminum and nickel in which another element is substituted for a portion of the aluminum and which has a unique combination of physical properties.
~ther objects and advantayes of the present invention will be in part apparent and in part pointed out in the description which follows:
In one of its broader aspects, objects of the inventiorl can be achieved by providing a rapldly solidified alloy composition having an L12 crystal g RD 15,123 structure and haviny a composition as follows:

(Nio 75X0 ~5Alo.20)YBloo--y where 98 ~ y G 99. g and X is a substituent metal selected from the group consisting of vanadium and silicon.
BXI~F DESCRIPTION ~F TH~ FIGUR~S
The present invention and the description which follows will be made clearer by reference to the accornpanying fiyures in which:
FIGURE 1 is a plot of the values of the stress of the inventive alloys plotted against the strain in percent for the base ~i3Al alloy as well as alloys containing substituents for the nickel and aluminum constituents.

Surprisinyly it has IIOW been found that further property improvements are possible in the alloy system of the gamma prime N13Al intermediate phase where not only boron is present in the composition as ternary element but in addition a metal selected from a group of metals is present as a quaternary ingredien~ of such compositions as a substituent metal.
~y a substituent metal is meant a meta whicll takes the place of and in this way is substituted or another and different rnetal, where tl~e other metal is part of a combination of metals forrning an essential constituent of an alloy system.
For example, in the case of the intermediate phase system Ni3Al, the constituent metals are nickel and aluminum. The metals are present in the stoichiometric atomic ratio of 3 nickel atoms for each aluminum atom in this system. It had been discovered that a desirable crystal form and accompanying ~` ~a.2~

R~ 15,123 superior physcial properties can be achieved by forming a single crystal of Ni3Al. ~owever polycrystalline Ni3Al is quite brittle and shatters under stess such as is applied in efforts to form the material into useful objects vr to use such an article.
It was discovered that the inclusion of boron in the rapidly cooled and solidified system can impart desirable ductility to the rapidly solidified alloy as taught in United States Patent No. 4,478,791 referred to above.
Now it has been discovered that certain metals can be beneficially substltuted in part for the constituent aluminum and hence these substituted metals are designated and known herein as su~stituent lS metals i.e. as an aluminum substituent in the Ni3Al strUc~ure. ~oreoVer it has ~een discovered ttlat valllable and berle~icial proper~ie~ are iln~rted to tlle rapidly solidified compositiolls which have the stoichiometric proportions but which have a substituent metal as a quaternary ingredient of sUcn rapidly solidified alloy systems.
The alloy compositions of the present invention must contain a first or primary ingredient or component and a second ingredient or component different from the first. The compositions rnust also contain boron as a tertiary ingredient as tauyht herein and as taught in United States Patent Number 4,~78,791 referred to above, and must further contain a ~luarternary componenk or ingredient as a substituent for aluMinum as taught in the subject specification.
The first constituent or ingredient is preferably nickel.
Further, the first constituent and tile second constituent must be present in substantially stoichiometric atornic ratios. ~n example is the ~255~
R~ 1~,123 nickel aluminide in which three atoms are present as the primary component constituent for each aluminum constituent which is present.
The composition which i5 formed must have a preselected intermetallic phase havlng a crystal structure of the L12 type and must have been formed by cool ng the melt at a cooling rate of at least about 103C per second to form a solid body the principal phase of which lS the L12 type crystal lU structure in either its ordered or disordered state.
The melt composition from which the structure is formed must have the first constituent and second constituent present in the melt in an atolnic ratio of approximately 3:1.
lS As point~d out ill ~he prior United Sta~es Pat~nt Wo. ~,~7~,791 re~rr~d ~o al)ov~, colnpo~3:itl0lls having this colnbina~iotl of inyredierlts and WlliCI~ are subjected to the rapid solidification techni~ue have surprisingly high values for both the strain to fracture after yield and for the 0.2% offset yield stress. Eor boron levels ~etween 1 and 2% the values of the strain to fracture generally declines so that a preferred range for the boron tertiary additive is between 0.5 and 1.5~.
By the prior teaching it was found tllat the optimum boron addition was at about 1 atomic percent and permitted a yield strength value at room temperature of about 100 ksi to be actlieved for t~le rapidly solidified product. The fracture strain of such a product was about lU% at room temperature.
Surprisingly, it has now been found that the unusual strength properties which are obtained througll the use of the rapid solidificaton in combination with the boron additive may be increase~ to heretofore unprecedented levels with the addition of a selected quaternary component or ingxedient as a su~stituent to RD 15,123 the priinary al~minum component.
The quaternary ingredient which may ~e beneficially included in a composition for rapid solidification as a substituent to make unprecedented improvements in the properties include the elements vanadium and silicon.
Further it nas been found, observed and determined that where an e~uiaxed structure is formed with the quaternary composition by rapid solidification, the properties are substantially better on the average than in those cases where the non-e~uiaxed structure is formed. The equiaxed structure is ~elieved to result from recrystallization. It is known that recrystalli~atio lS can readily occur in a single-phase Inaterial.
The addition o~: the vanadiuln or ~i Lic~n cl~
~uaternary ingredient and as a substituent for aluminum at about a 5 atomic percent level apparently does not form borides or other phases under the influence of the rapid solidification processing.
; Regarding the improved properties achieved in the measurements made followiny the preparation of the alloys, the testing of alloys as descri~ed herein has yielded some surprising results. ~ne set of the properties and particularly the stress properties are indicated in the attached Figure 1 in whicil the stress in ksi is plotted against the strain in percent.
It is evident from Figure 1 that the alloy containing Ni3Al with 1~ boron }las the lowest stress values and that the two other samples which were tested had significantly and unexpectedly higher values. The sample with the 5 atomic percent silion had the highest values found and these were of the order of 185 ksi.
3S In the practice of this invention, an intermetallic phase having an L12 type crystal ~5~
R~ 15,123 structure is first selected. The selection criteria will depend upon the end use environment which, in turn, determines the attributes, such as strength, ductility, hardness, corrosion resistance an~ fatigue strength, required of the material selected.
An intermetallic phase typical of those of enyineeriny interest and one having particularly desirable attributes is nickel aluminide ~Ni3Al) which is found in the nickel-aluminum binary system and as the yamma prime phase in gamma/gamma prime nickel-base superalloys. ~ickel aluminide has hiyh hardness and is stable and resistant to oxidation and corrosion at elevated temperatures which makes it attractive as a potential structural material~
A:lthough si~lgle crystals of Ni3~1 exhibit good ~uctility in certain ~ry~tal:lographlc oriel~tatiorl~, the polycr~stallirle ~orm, i.e., t~e form of ~rimary significance froM an engineeriny standpoint, has low ductility and fails in a brittle manner intergranularly.
Nickel aluminide, whicil has a ordered face centered cubic (FCC) crystal structure of the CU3Al type (L12 in the Stukturbericht desiynation which is the designation used herein and in the appende~
claims) witll a lattice paramter aO = 3.589 at 75 %
Ni and melts in the range of from about 1385 to 1395C, is formed from alumirlum and nickel which have rnelting points of 660 and 1453C, respectively.
Although frequently referred to as Ni3Al, nickel 3U aluminide is an intermetallic phase and not a compound as it exists over a range of compositions as à function of temperature, e.g., about 7~.5 to 77 ~ Ni (85.1 to 87.8 wt. %) at 600C.
The selected intermetallic phase is provided as a melt w~lose composition corresponds to that of the preselected intermetallic phase. The melt composition s~
RD 15,123 _ g _ will consist essentially of the atoms of the two components of the interMetallic phase in an atomic ratio of approximately 3:1 and is modi]Eied with boron in an amount of from about 0.01 to 2.5 %.
Generally, the components wi:Ll be two different elements, but, while still maintaining the approximate atomic ratio of 3:1, one or more elements may, in some cases, be partially substituted for one or both of the two elements which form the 1~ intermetallic phase.
Although the melt should ideally consist only of the atoms of the intermetallic phase and atom~
of boron, it is recognized that occasionally and inevitably other atorns of one or more lncid~nt~l ilnpurity atolns may be preserlt in the melt.
The melt i~ nexk rapidly cooled ~t a rate Oe at ~ast a~o~lt 103C/sec ~o Eorm a solid ~ody, t~le principal phase of which is of the LL2 type crystal structure in either its ordered or diordered state.
Thus, although the rapidly solidified solid body will principally have the sa~ne crystal structure as the pre4elected interrnetallic phase, i.e., the L12 type, the present of other phases, e.g., borides, is possible. Since the cooling rates are high, it is also possible that the crysta;L structure of the rapidly solidified solid will be disordered, i.e., the atoms wiLl be located at random sites on the crystal lattice instead of at specific periodic positions on the crystal lattice as is the case with ordered solid 3~ solutions.
There are several methods by wllich the re~uisite large cooliny rates may be obtained, e.y, splat cooling. A preferred laboratory method for o~taining the re~uisite coolirly rates is the chill-block melt spinniny process.
~riefly and typically, in the chill-block ~z~
RD 15,123 melt spinning process molten metal is delivered froln a crucible through a nozzle, usually under the pressure of an inert yas, to form a free-standing stream of li~uid metal or a column of liquid metal in contact with the nozzle. r~he stream of li~uid metal is then impinyed onto or otherwise placed in conact Wit~l a rapidly moving surface of a chill-block, i.e~, a cooling substrate, made of a material such as copper.
The material to be melted can be delivered to the crucible as separate solids of the elements required and melted therein by means such as an induction coil placed around the crucible or a "master alloy~ can first be made, comminuted, and the comminuted particles placed .itl the crucible. When the lS li~uid mel~ con~cts the cold c~Lll-block, it cools rapidl~, froln a~out lU3C/sec to lO7C/sec, alld solidifies in ~he form o~ a continuous lenyth of a thin ribbon w~lose width is considerably lar~er than its thickness. A more detailed teaching oE the chill-block melt spinning process rnay be found, for example, in U.S. Patellts 2,825,108 - R. B. Pond -issued March 4, 1958; 4,221,257 M. ~. Narasi]n~lan -issued September 9, 1980 and 4,282,921 - ~. Lieberlna - issued Auyust 11, 1981.
Thé following examples are provided hy way of illustration and not by limitation to further teach the novel method of the invention and illustrate its many advantageous attributes.

A heat of a composition correspondin~ to about 3 atomic parts nickel to 1 atomic part aluminum and 1 atomic percent boron was prepared, comminuted, and about 60 grarns of the pieces were delivered into an alumina crucible of a chill-block melt spinlli}ly apparatus. The composition had the formula:

~1~ 15,123 ( Ni 75Al . 25 ) 99Bl The crucible terminated in a flat-bottomed exit section having a slot 0.25 (6~35 mm) inches by 25 mils (0.635mm) therethrough. A chill block, in the form of a wheel having faces 10 inches (25.4 cm) in diaMeter with (rim) thickness of 1.5 inches (3.8 cm), made of H-12 tool steel, was oriented vertically so that the rim surface could be used as the casting (chill) surface when the wheel was rotated about a horizontal axis passing through the centers of and perpendicular to the wheel faces. The crucible was placed in a vertically up orientation and brought to within about 1.2 to 1.6 mils (30-40~) of the castiny surface with ttle 0.25 inch length dimension of the ~310t ~rien~ed ~rp~en~icular ~o t~l~ dire~ction o~
rotation of the wheel.
r~he wheel was rotated at 1200 rpm. 'rhe rnelt was heated to between about 135U to 1450C. The melt was ejected as a rectangular stream onto the rotating chill surface under the pressure of argon at about 1.5 psi to produce a long ribbon which measured from about 40-70~ in thickness by about 0.25 inches in width.
The ribbons were tested for bend ductility and a value of 1.0 was found. This value or bend ductility designates that the ribbon can be bent fully to 130C without fracture.

rrhe procedure of Example 1 was repeated using the same equipment to prepare a master heat of the boron doped nominal ~i3Al composition but one which was modified to the following composition:

RD 15,123 (Nio ~5Alo 20Tio.05)99B:L

Ribbons were cast from the heat as descri~ed in Example 1.
The ribbons were tested for bend ductility and a value of 0.04 was found for the ribbon thus prepared. This value of bend ductility was calculated by dividing the ribbon thickness by the bend radius at which the ribbon fractures.
EXAMPLES 3 THRO~G~ 12 Ten additiorlal master heats alloys 96, 101, 111 through 117 and 125 were prepared having the compositions as set forth in Table I
below. These heats were prepared in the manller described witll reference to tlle ~irst descri~ed a~c~ve and were t~ted ~or ~end ~uctility in the ~alne rnann~r as that prepared a~ove. 'rhe values for ~end ductility which were obtained are listed in Table I.
It was also found that there is a correlation between the full bend ductility ~bend ductility = 1.0) of the samples which were prepared and the formation of an e~uiaxed configuration in the crystallographic structure which was formed. The Table indicates also those samples for which an e~uiaxed format was found and also those for which the non-e~uiaxed forïnat was found.

" ~,.,~s5~
R~ 15,123 TABLE I
Crystallo-Alloy Composition Bend graphic Example Number Formula _ Ductility Structure

2 92 ( 0.75 0.20 0.05 99 1

3 96 [~io.75 0.25)0.98 0.02 99 1 N

4 :Lll (l~io 75AlO.20Tao.o5)99 1 N

112 (NiU 75~1o.2oN~o.uo5)99 1 0.02 6 113 (L~io.75Alo.2ovo.o5)99Bl 1.0 E

~7 114 (1~iu.75~lo.2o~i~.os) 8 115 (Nio.6sFeo.l~lo.25)99Bl 9 116 (Nio 65Mno.loA10.25)99 1 117 (l~io 70Cro-osAlo 25)99 1 0.06 N

11 125 [(Nio 75Al0.2s)Reo.o3]99Bl 0.1 12 lO:L (Nio 70coo.o5Alo 25)99 1 l.U E

: N designates non-e~uiaxed; E designates e~uiaxed.

E~D 15,123 Returning to a consideration of the data plotted in Figure 1, it is evident that t~le stress in ksi of the rapidly solidified boron doped nickel aluminide base alloy containing the silicon as a partial substituent for aluminum is substantially higher than that of the similar alloy without the substituent for the aluminum.
The stress in ksi for the vana~ium modified aluminide is shown by the lower plot and this composition has a stress of 135 ksi at yield.
The stress at yield for the uppermost plot is some 37~ higher at 185 ksi and this is a significant and unexpected advance in the ability of those ~killed in this art to increase the tensile properties of tlle :L5 rapidly solidi~ied~ boron doped nickel aluminide base ~llo~.
It is furthe~r evident froln rl'abLe I that ~xample 6 which involved the incorporation of the vanadiurn in the rapidly solidified boron doped tri-nickel alulninide as a substituent for aluminuln also resulted in a composition having a bend ductility test value of 1Ø Further this compositioll was found to be equiaxed.
Based on comparison with other materials of Table I which are incorporated as substituents Eor aluminum it is evident that the silicon and vanadium provide uni~ue and advantageous improvements in the boron doped tri-nickel aluminide of the prior United States Patent Number 4,478,791.

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A rapidly solidified boron doped nickel aluminide base alloy having a crystal structure of the L12 type said alloy comprising a composition having the formula (Ni0.75X0.05Al0.20)yB100-y where 98 ? y ? 99.5 and wherein the X is selected from the group consisting of vanadium and silicon.

2. The aluminide of claim 1 in which X is vanadium.

3. The aluminide of claim 1 in which X is silicon.

4. A rapidly solidified boron doped nickel aluminide base alloy having a crystal structure of the L12 type, said alloy comprising a composition having the formula (Ni0.75X0.05Al0.20)99B1 wherein the X is selected from the group consisting of vanadium and silicon.

5. The aluminide of claim 4 wherein X is silicon.

6. The aluminide of claim 4 wherein the X
is vanadium.