CA2025272A1 - High-niobium titanium aluminide alloys - Google Patents

Abstract

RD-19,426 HIGH-NIOBIUM TITANIUM ALUMINIDE ALLOYS

ABSTRACT OF THE DISCLOSURE

A TiAl composition is prepared by ingot metallurgy to have higher strength and to have moderately reduced or improved ductility by altering the atomic ratio of the titanium and niobium to have what has been found to be a highly desirable effective aluminum concentration and by addition of niobium according to the approximate formula Ti48-37Al46-49Nb6-14.

Description

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RD-19.4~h ~IG~-NIQBI ~ TIT~ ~ AL ~ INIDE A~LOYS

CROSS~REFERENCE TO RELATED APPLICATIONS

The subject application relates to copending applications as follows: Serial Numbers 138,407; 138,408; ~-~
138,476; 138,481; 138,485; 138,48~; filed December 28, 1987 respectively.
The texts of these related applications are incorporated herein by reference. `~

BACKGROUND OF THE INVENTION

The present invention relates generally to alloys of titanium and aluminum. More particularly, it relates to alloys of titanium and aluminum which have been modified both with respect to stoichiometric ratio and with respect to niobium addition and which contain a higher concentration of niobium additive.
It is known that as aluminum is added to titanium metal in greater an greater proportions the crystal form of 20 the resultant titanium aluminum composition changes. Small --percentages of aluminum go into solid solution in ti~anium and the crystal fsrm remains ~hat of alpha titanium. At higher concentrations of aluminum (including about 25 to 35 atomic %) an intermetallic compound Ti3Al is formed. The Ti3Al has an ordered hexagonal crystal form called alpha-2.
At still higher concentrations of aluminum (including the range of 50 to 60 atomic % aluminum) another intermetallic compound, TiAl, is formed having an ordered tetragonal crystal form called gamma. ~ ~

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The alloy of titanium and aluminum having a gamma crystal form and a stoichiometric ratio of approximately one is an intermetallic compound having a high modulus, a low density, a high thermal conductivity~ good oxidation resistance, and good creep resistance. The rela~ionship between the modulus and temperature for gamma TiAl compounds to other alloys of titanium and in relation to nickel base superalloys is shown in Figure 1. As is evident from the figure the gamma TiA1 has the best modulus of any of the titanium alloys. Not only is the gamma TiAl modulus higher at temperature but the rate of decrease of the modulus with temperature increase is lower for gamma TiAl than for the other titanium alloys. Moreover, the gamma TiAl retains a useful modulus at temperatures above those at which the other titanium alloys become useless. Alloys which are based on the gamma TiAl intermetallic compound are attractive lightweight materials for use where high modulus is required at high temperatures and where good environmental protection is also required.
One of the characteristics of gamma TiAl which limits its actual application to such uses is a brittleness which is found to occur at room temperature. Also, the strength of the intermetallic compound at room temperature needs improvement before the gamma TiAl intermetallic compound can be exploited in structural component applications. Improvements of the gamma TiAl intermetallic compound to enhance ductility and/or strength at room temperature are very highly desirable in order to permit~use of the compositions at the higher temperatures for which they are suitable.
With potential benefits of use at light weight and at high temperatures, what is most desired in the gamma TiAl compositions which are to be used is a combination of strength and ductility at room temperature. A minimum -: ;:

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RD~13 426 ductility of the order of one percent is acceptable for some applications of the metal composition but higher ductilitles are much more desirable. A minimum strength for a composition to be useful is about 50 ksi or about 350 MPa.
However, materials having this level of strength are of marginal utility and higher strengths are often preferred for some applications.
The stoichiometric ratio of TiAl compounds can vary over a range without altering the crystal structure. The aluminum content can vary from about 50 to about 60 atom percent. The properties of TiAl compositions are subject to very significant changes as a result of relatively ~mall changes of one percent or more in the stoichiometric ratio of the titanium and aluminum ingredien~s. Also, the proper~les are similarly affected by the addition of relatively similar small amounts of ternary elements.

PRIOR ART

There is extensive li~erature cn the compositions of titanium aluminum including the Ti3Al intermetallic compound, the TiAl intermetallic compounds and the TiAl3 intermetallic compound. A patent, U S. 4,294,615, entitled "TITANIUM ALLOYS OF THE TiAl TYPE'I contains an extensive discussion of the titanium aluminide type alloys including the TiAl intermetallic compound. As is pointed out in the patent in column 1, starting at line 50, in discussing TiAl's advantages and disadvantages relative to Ti3Al:

"It should be evident that the TiAl gamma alloy system has the potential for being lighter inasmuch as it contains more aluminum. Laboratory work in the 1950's in~icated that ~itanium aluminide alloys had the potential for hi~h temperature use to about 1000C. But subsequent ;

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was that, while they had the requisite high temperature strength, they had little or no ductility at room and moderate temperatures, i.e., from 20 to 550C. Materials which are too brittle cannot be readily fabricated, nor can they withstand infrequent but inevitable minor service damage without cracking and subsequent failure. They are not useful engineering materials to replace other base allQys."

It is known that the alloy system TiAl is substantially different from Ti3Al tas well as from solid solution alloys of Ti) although both TiAl and Ti3Al are basically ordered titanium aluminum intermetallic compounds.
As the '615 patent points out at the bottom of column 1:
"Those well skilled recognize that there is a substantial difference between the two ordered phases. Alloying and - transformational behavior of Ti3AL
resemble those of titanium, as the hexagonal crystal s~ructures are very similar. However, the compound TiAl has a tetragonal arrangemen~ of atoms and thus rather different alloying characteristics. Such a distinction is often not recognized in ~he earlier literature."

The '615 patent does describe the alloying of TiAl with vanadium and carbon to achieve some property 5 improvements in the resulting alloy.
It should be pointed out, however, with re~ard to the '615 patent that there are many alloys listed in the Table 2 of this patent reference but the fact that a composition is listed should not be taken as an indlcation that any alloy which is listed is a good alloy. Most of the alloys which are listed have no indica~ion of any properties~
For example, alloy lT2A-ll9 of Table II is listed as Ti-45Al-:

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l.OHf in atomic %. This alloy corresponds to alloy 32 of applicant's Table II. The composition listed by the applicant in Table II is Ti54A145Xf1 so that it is precisely the same composition in atomic % as that lis~ed and referred in Table II o~ the '615 re~erence~ However, as is evident from the applicant's Table II, the titanium base alloy containing 45 aluminum and 1.0 hafnium is a very poor alloy ha~ing very poor ductility and, accordingly, having no valuable properties ~nd no use as a titanium base alloy. ~he alloy Ti-45Al-5.0Nb is listed in Table 2 in the same fashion, i.e., without any listing of properties or indication that the alloy has any use or any value.
A number of technical publications dealing with the titanium aluminum compounds as well as with the characteristics of these compounds are as follows:

1. E.S. Bumps, H.D. Kessler, and M. Hansen, "Titanlum-Aluminum System", _, TRA~SACTIONS AIME, Vol.
194 (June 1952).pp. 609-614.
2. H.R. Ogden, D.J. Maykuth, W.L. Finlay, and R.I.
Jaffee, "Mechanical Properties o~ Hi~h Puri~y Ti-Al Alloys", ~9~c~LJIL_Y~15, TRANSACTIONS AIME, Vol. 197.tFebruary 1953) pp. 267-272.
Three additional papers contain limited information about the mechanical behavior of TiAl base alloys modified by niobium. These three papers are as follows:

3. Joseph B. McAndrew, and H.D. Kessler, "Ti-36 Pct Al as a Base for High Temperature Alloys", ~G ~, TRANSACTIONS AIME, Vol. 206 (October 1956) pp. 1348-1353.

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RD~ 25 4. S.M.L. Sastry, and H.A. Lipsitt, "Plas~ic Deformation of Ti~l and Ti3Al", li~QuLl~m - ~Q (Published by American Society for Metals, Warrendale, Pennsylvania), Vol.
2 (1980) page 1231.

5. S.M.L. Sastry, and H.A. Lipsitt, "Fatigue Deformation of TiAl Base Alloys", ~$aLl ~ U_~L _L l- L19~3 Vol. 8A (February 1977) pages 299~308.

The first paper above contains a statement that ~'A
Ti-35 pct Al~5 pct Cb specimen had a room temperature ultimate tensile strength of 62,360 psi, and a Ti-35 pct Al-7 pct Cb specimen failed in the threads at 75,800 psi." The two above alloys referred to in the quoted passage are given in weight percent and have approximate compositions in a~omlc percentages respectively of Ti4gAlsoNb~ and Ti47AlsoNb3. It is well-known that the failure of a test specimen in the threads is a strong indication that the specimen was brittle. It is further mentioned in this paper that the niobium containing composition is good for oxidation and creep resistance.
The second paper contains a conclusion re~arding the influence of niobium additions on TiAl but offers no specific data in support o~ this conclusion.~ The conclusion is that: 'IThe major influence of niobium.additions to TiAl is a lowering o the temperature at whiCh twlnning becomes an important mode of deformation and~thus a lowering of the ~
ductile-brittle transition temperature of~TiAl." There ls no indication in this article as to whether the ductile-brittle :
transition temperature of TiAl was lowered to below room 30 temperature. The only niobium containing titanium aluminum :~
alloy mentioned without any reference to properties or oth:er - :
descriptive data is given in~weight percent and is~Ti-36Al-4Nb. This corresponds in atomic percent to Tig7 sAls1Nbl.s, a compos1tion which 1s quite dlstlnct from tho~se taught and : ~

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R~-19,~6 claimed by the Applicant herein as will become more clearly evident below.
The composition described in the fifth reference above, which contains 36.2 weight % of aluminum and 4.65 weight % of niobium in a titanium base composition, when converted to atomic composi~ion is Ti-51Al-2Nb. This composition was s~udied as is reported at the l~st sentence o~ page 301 and the first portion of page 302. As reported on the bottom of page 301 and on top of page 302, the authors concluded that:
"It has been found that the addition of Nb to the TiAl base composition improves the low temperature ductility of the base compQsition. .... The addition of Nb does not significantly alter the fatigue properties of the bàse composition as can be seen in Figure 5."

Figure 5 is quite persuasive that there is no significant alteration of the fatigue properties. There is no~indication in the article that room temperature ductility is improved by Nb additions.
BRIEF DESCRIPTION~OF THE INVENTION~

One object of the present invention is to provide a method of forming a titanium alumlnum~intermetalllc compound having improved ductility and related properties at room temperature.
Another object is to improve the properties of titanium aluminum intermetallic compounds at low and intermediate temperature~. ~
Another object is to provide an alloy of titanium and aluminum having improved properties and processability at low and intermediate temperatures. ;
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~3.~ 9,q26 Other objects will be in part apparent and in part pointed out in the description which follows.
In one of its broader aspects, the objects of the present invention are achieved by providing a nonstoichiometric TiAl base alloy, and adding a relatively higher concentration of niobium to the nonstoichiometric composition. The addition is followed by ingot processing of the niobium-containing nonstoichiometric TiAl intermetallic compound. Addition of niobium in the order of approximately 6 to 14 parts in 100 is contemplated and additions in the order of 8 to 12 parts is preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a graph illustrating the relationship between modulus and temperature for an assortment of alloys.
FIGURE 2 is a graph illustrating the relationship between load in pounds and crosshead displacement in mils for TiAl compositions of different stoichiometry tested in 4-point bending.
FIGU2E 3 is a bar graph illustrating alloyproperties on a comparative basis.
FIGURE 4 is a graph in which weight gain in mq/cm~
is plotted against dynamic exposure time in hours.
DETAILED DESCRIPTION OF THE INVENTION

It is well known, as is discussed above, that except for its brittleness and processing difficulties the intermetallic compound gamma TiAl would have many uses in industry because of its light~wei~ht, high strength at high temperatures, and relatively low cost. The composition would have many industrial ~ses ~oday if ie we e not for this basic :
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: - ~' ' : ' ~ :;~ , 9 _ ~J 'V ~ f, RD~ 426 property defect of the material which has kept it from such uses for many years.
The present inventor found that the gamma TiAl compound could be substantially ductilized by the addition of a small amount of niobium. This finding is the s~lbject of copending application Serial No. 332,088, filed April 3, 1989.
Further, the present inventor found that a chromium ductilized composition could be remarkably improved in its oxidation resistance with no loss of ductility ox strength by the addition of niobium in addition to the chromium. This later finding is the subject of copending application Serial No. 201,984, filed June 3, 1988.
The inventor has now found that substantlal further improvements in ductility can be made by additions of higher concentrations of niobium alone in the range of 8 to 13 atomic percent where this addition is coupled with ingot processing as discussed more fully below.
To better understand the improvements in the properties of TiAl, a number of examples are presented and discussed here before the examples which deal with the novel compositions and processing practices of this invention.

EXa~2LE~ 1-3:
Three individual melts were prepared to contain titanium and aluminum in various stoichiometric ratios approximating that of TiAl. The compositions, annèaling temperatures an~ test results of tests made on the compositions are set ~crth in Table I.
For each example, the alloy was first~made in~o an ingot by elec~ro arc melting. The ingot was processed into ribbon by melt spinning in a partial pressure of~argon. In both stages of the melting, a water-cooled copper hearth was used as the container for the melt in order to avoid '' ~ .

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undesirable melt-container reactions. Also, care was used to avoid exposure of the hot metal to oxygen because of the strong affinity of titanium for oxygen.
The rapidly solidified ribbon was packed into a steel can which was evacuated and then sealed. The can was then hot isostatically pressed (HIPed) at 950C (1740F) for 3 hours under a pressure of 30 ksi. The HIPing can was machined off the consolidated ribbon plug. The HIPed sarnple was a plug about one inch in diameter and three inches long.
The plug was placed axially into a center opening of a billet and sealed therein. The billet was heated to 975C (1787F) and was extruded through a die to give a reduction ratio of about 7 to l. The extruded plug was removed from the billet and was heat treated.
The extruded samples were then annealed at temperatures as indicated in Table I for two hours. The annealing was followed by aging at 1000C for two hours.
Specimens were machined to the dimension of 1.5 x 3 x ~5.4 mm (0.060 x 0.120 x 1.0 in.) for four point bending tests at room temperature. The bending tests were carried out in a 9-point bending fixture having an inner span of 10 mm (0.4 in.) and an outer span of 20 mm (0.8 in.). The load-crosshead displacement curves were recorded. Based on the curves developed, the following properties are defined:
(1) Yield s~rength is the flow stress at a cross head displacement of one thousandth of an inch. This amount of cross head displacement is taken as the first evidence of plastic deformation and the transition from elastic deformation to plastic deformation. The measurement of yield and/or fracture strength by conventional compression or tension methods tends to give results which are lower than the results obtained by four point bending as carried out in making the . ~

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~D-19 426 measurements reported herein. The higher levels of the results from four point bending measurements should be kept in mind when comparing these values to values obtained by the conventional compression or tension methods. However, the comparison of measurements' results in many of the examples herein is between four point bending tests, and for all samples measured by this technique, such comparisons are quite valid in establishing the differences in strength properties resulting from differences in composition or in processing of the compositions.

(2~ Fracture strength is the stress to fracture.

(3) Outer fiber strain is the quantity of 3.71hd, where "h" is the specimen thickness in inches, and "d"
is the cross head displacement of fracture in inches.
Metallur~ically, the value calculated represents the amount of plastic deformation experienced at the outer surface of the bending specimen at the time of fracture.

The results are listed in the following Table I.
Table I contains data~on the propertieS of samples annealed at 1300C and ~urther data on these samples in particular is given in Figure 2.

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9,426 Outer Gamma Yield Fracture Fiber Ex. Alloy Composit. Anneal Strength Strength Strain No. No. Sat.%) Temp(C)(ksi) (ksi) (%) . -- _ 1 83 Tis4Al46 1250 131 132 0.1 1300 111 120 0.1 1350 * 58 0 2 12 Tis2A148 1250 130 180 1.1 1300 98 128 0.9 1350 88 122 0.9 1400 70 85 0.2 3 85 Ti50Also 1250 83 92 0.3 1300 93 97 0.3 1350 78 88 0.4 * - No measurable value was found because the sample lacked sufficient ductility to obtain a measure-ment - It is evident from the data of this table that alloy 12 for Example 2 exhibited the best combination of properties. This confirms that the properties of Ti-Al compositions are very sensitive to the Tl/Al atomic ratios and to the heat treatment applied. Alloy 12 was selected as the base alloy for further property improvements ba~ed on further experiments which~were per~ormed as described below.
It is also evident that the anneal at~temperatures between 1250C and 1350C results in~the test specimens having desirable levels of yield strength, ~fracture strength and outer fiber strain. However, the anneal at 1400~C
results~ in a te~t specimen having a significantly lower yield strength ~about 20% lowex); lower fracture strength ~about 30% lower) and lower ductility ~about 78% lower) than a test 40 speclmen annealed at 1350C. The sharp~decline ln properties ~`
is due to a dramatlc change ln~microstructure due, iD~ turn, ~: :

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R~=19,~26 to an extensive beta transformation at temperatures appreciably above 1350C.

EX~E~ L=1~:
Ten additional individual melts were prepared to contain titanium and aluminum in designated atomic ratios as well as additlves in relatively small atomic percents.
Each of the samples was prepared as described above with reference to Examples 1-3.
The compositions, annealing temperatures, and test results of tests made on the compositions are set forth in Table II in comparison to alloy 12 as the base alloy for this comparison.
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Outer S Gamma YieldFracture Fiber Ex. Alloy Composition Rnneal Strength Strength Strain No. No. (at.%) Temp( C)(ksi~ (ksi) (%) 10 2 12 Tis2Al4s1250 130 180 1.1 1300 9~ 128 0.9 1350 88 122 0.9 4 22 TisoAl47Ni3 1200 * 131 0 S 24 Tis2A146Ag2 1200 * 114 0 1300 g2 117 0.5 6 25 TisoAl48Cu2 1250 * 83 0 13~0 80 107 0.8 . 1350 70 10~ 0.9 7 32 Tis4Al4sHfl 1250 130 136 0.1 1300 72 77 0.2 8 41 Tis2A144Pt4 1250 132 150 0.3 9 45 TislAl47C2 1300 136 149 0.1 30 10 57 TisoAl4gFe2 ~ 1250 * 8:9 0 1300 * 8I 0 1350 86 111 0.5 11 82 TisoAlq8Mo2 1250 128 140 0.2 1300 : 110 136 0.5 13S0 80 g5 0.1 12 39 TisoAl46Mo4 1200 * 143 0 ~ l~S0 135 154 0.3 1300 131 14g 0.2 :
13 20 T$4g~sAl4s.sErl +

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+ - Material fractured during machining to prepare test specimens : ~:

, - 15 ~ J ~ ~ ~?j ~ ~, , RD-19,426 For Examples 4 and 5, heat treated at 1200C, the yield strength was unmeasurable as the ductility was fou~d to be essentially nil. For the specimen of Example 5 which was annealed at 1300C, the ductility increased, but it was still undesirably low.
For Example 6, the same was true for the test specimen annealed at 1250C. For the specimens of Example 6 which were annealed at 1300 and 1350C the ductility was significant but the yield strength was low.
None of the test specimens of the other Examples were found to have any significan~ level of ductility.
It is evident from the results listed in Table II
that the sets of parameters involved in preparing compositions for testing are quite complex and interrelated.
One parameter is the atomic ratio of the ~itanium relative to that of aluminum. From the data plotted in Figure 4, it is evident that the stoichiometric ratio or nonstoichiometric ratio has a strong influence on the test properties which formed for different compositions.
Another set of parameters is the additive chosen to be included into the basic TiAl composition. A first parameter of this set concerns whether a particular additive acts as a substituent for titanium or or aluminum. A
specific metal may act in éither fashion~and there is no simple rule by which it can be determined which role an additive will play. The significance of this parameter is evident if we consider addition of some~atomic percentage of additive X. ~ -If X acts as a titanium substituent, then a composition ~i4gA14gX4 will give an effective aluminum concentration of 48 atomic percent and an~effective titanium concentration of 52 atomi~c percent.

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If, by contrast, the X additive acts as an aluminum substituent, then the resultant composition will have an effective aluminum concentration of 52 percent and an effective titanium concentration o~ 48 atomic percent.
Accordingly, the nature of ~he substitution which takes place is very important but is also highly unpredictable.
Another parameter of this set is the concentration of the additive.
Still another parameter evident from Table II is the annealing temperature. The annealing temperature which produces the best strength properties for one additive can be seen to be different for a different additive. Thls can be seen by comparing the results set forth in Example 6 with those set forth in Example 7.
In addition, there may be a combined concentration and annealing effect for the additive so that optirnum property enhancement, if any enhancement is found, can occur at a certain combination of additive concentration and annealing temperature so that higher and lower concentratlons and/or annealing temperatures are less effective in providing -a desired property improvement.
The content of Table II makes clear that the results obtainable from addition of a ternary element to a nonstoichiometric TiAl composition are highly unpredictable and that most test results are unsuccessful with respect to ductility or strength or to both.

EXAMPLE~ 14-?4:
Eleven additional samples were prepared as described above with reference to Examples 1-3 to contain titanium aluminide having compositions respectively as listed in Table III.

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RD-19,426 In addition to listing the test compositions, the Table III summarizes the bend test results on all of the alloys both standard and modified under the various heat treatment conditions deemed relevant.

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RD-19,426 TAB~
Four-Point Bend Propertie~ of Nb-Modified TiAl Alloys Outer GammaYield Fracture Fiber Ex. Alloy Composit. Anneal Strength Strength Strain No. No. (at.%) Temp(C) tksi) (ksi) (~) ~ _ , .
212 Ti52A148 1250 130 180 1.1 130~ 98 128 0.9 1350 88 122 0.9 1400 70 85 0.2 15 1478 Ti5oAl48Nb2 1250 139 143 0.1 1300 111 134 0.4 1350 57 67 0.1 15119 Ti51A145Nb4 1250 150 178 0.4 1300 *-- 69 0 1640 Ti5oAl46Nb4 1250 136 167 0.5 1300 124 176 1.0 1350 86 100 0.1 1766 Ti4gA147Nb4 1250 138 160 0.4 1300 126 167 0.8 1350 *-- 64 0 30 1855 Ti4gA148Nb4 1300 126 147 0.4 1350 10~ 135 0.6 1992 Ti46Al48Nb6 1350 *-- 8B 0 35 2052 Ti4gA144Nb8 1250 125 172 0.4 1300 *-- 131 0 1350 : *-- 125 0 2167 Ti44A148Nb8 1250 151 161 0.2 1300 140. 161 0.2 1350 119 153 0.7 2253 Ti46Al42Nbl2 1250 *-- 152 1300 *--~ 138: 0 1350 *-- :181 0 23123 Ti40Al48Nbl2 1300 *-- 67 0 ~350 107 : 138 0.8 50 24137 Ti36A148Nbl6 **~~
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* - N~ measurable value was found because the sample lacked sufficient duc~ility to obtain a measurement S **- The material was too brittle to be machined into samples for test From Table III, it is evident tha~ alloys 12, 78, 55, 92, 67, 123, and 137 contained 0, 2, 4, 6, 8 j 12, and 16 atomic percent of niobium respectively as an additive to the base composition Ti52Al48. From the data listed in Table III, it can be concluded that the rapid solidification of the listed compositions does not improve room temperature lS ductility.
If the results are compared based on the same heat treatment (1300C) being applied to each sample, then it may be concluded from the data of Table III, for the yield strength which could be measured, tha~ the progressive addition of greater concentrations of niobium xesults in a progressive increase in the yield s~rength but also resulted in a progressive decrease in the ductility. This finding is consistent with the teaching o~ McAndrew in his article 3 above, but contradicts the Sastry teaching in his above articles 4 and 5.
From Table III it is also~evident that at~the 8 and 12 atomic percent additive level ~see alloys 67 and 123) a better combination of strength and dycti~lity can be obtained ;~
if the specimens are heat treated at the 1350C Ievel but ductility is still below 1~.
For samples having lower concentrations~of niobium, : :
such as samples 78:and 55, it was:found that imparting improvements to the samples by:such heat treatment~is not feasibIe as the improvement achieved are not as:s~ignificant.
A finding results ~rom comparing the test resul~s for alloys 55, 66, 40, and 119 in Table III. This comparison , ~ ~

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RD-19,426 is made with respect to samples having a 4 atomic percent level of niobium additive but different stoichiometric ratios of titanium and aluminum. It has been discovered based on the study of these compositions ~hat the aluminum concentration can be reduced slightly to obtain significant increases in ductility without sacrificing the attractive strength. However, aluminum concentration canno~ be reduced below 46% without substantial elimination of ductility. Even where the aluminum is at 46% or above the ductility is at or below 1%.
Considering the data of Table III it is apparent that there is an optimum concentration of the niobiu~
additive of between 4 and 12 atomic percent if appropriate adjustments are made in the aluminum concentration and the annealing temperature according to ~he teaching contained in Table III.
All of the foregoing test samples were prepared by rapid solidification. Also, the testing of all of the test samples listed in the foregoing ~ables was done by four-point bending tests.

TENSILE TESTING vs. FOUR-POINT BEND TESTING:
As noted above, all of the foregoing examples were prepared by rapid solidification processing and the testing was done by four-point bending tests. All of the data lis~ed in the above tables is from this source.
The results of such preparation and testing as set forth in Examples 20 through 22 is that the material having 8 to I2 atomic percent of niobium in the titanium aluminide had very limited ductility for the most paxt with the one exception that the Ti44Al4gNbg which was processed at 1350 annealing temperature.
I have now discovered that compositions having niobium additive in the relatively larger quantities of 8-12 ~ '' .

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The principal distinguishing processing step here is that the ingo~ metallurgy technique involved a melting of the ingredients and solidification of the ingredients into an ingot. The rapid solidification method by contrast involves the formation of a ribbon by ~he melt spinning method followed by the consolidation of ~he ribbon into a fully dense coherent metal sample.
However, before getting to the ingot processing, a note of caution is warranted. The caution concerns the different measurements which are usually used in testing ingot processed samples.
The ingot processed samples are usually tested by conventional tensile tests employing tensile bars which are prepared expressly for this purpose.
In order to make a fair comparison between the properties of alloys prepared by rapld solidification and alloys prepared by conventional ingot processing a series of tests were conducted of the proper~ies of rapidly solidified alloys using conventional tensile bar testing.

TENSILE BAR TESTING OF RAPIDLY SOLIDIFIED SAMPLES
For this purpose, a series of conventional pins were prepared from the alloy samples which had been prepared by rapid solidification, most of which are listed in Table III above. In addition, however, a gamma TiAl alloy with niobium doping was prepared by the rapid solidification method described above. This alloy is identified as alloy -.

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'3 RD-19,426 132 and it contained 6 atom percent o~ the niobium dopant. A
set of pints were prepared ~rom each o~ the test alloys listed in Tahle IV below including a set of pins prepared from alloy 132.
S The different pins were separately annealed at the different temperatures listed in Table IV below. Following the individual anneals, the pins were aged at 1000C for two hours. After the anneal and aging, each pin was machined into a conventional tensile bar and conventional tensile tests were performed on the resulting bars. The results of the tensile tests are listed in Table IV immediately below.

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RD-1~,926 TA3~ ~V
Conventional Tensile Bar Testing of Room Temperature Tensile Properties of Gamma RSG Alloys Weight Loss After 48hrs Plastic Q982C in Ex. CFG Compo- Heat Treat Strength Strength tionStatic Air 10 No. No. ~ition Temp. C (ksi~ (k~ ) (mg/cm2) ,, ., . _ . _ . .. .
2 12Ti-48A1 1250 --* 88 0 1300 77 92 2.1 1350 68 81 1.1 31 14 78 Ti-48Al-2Nb 1300 90 103 1.7 1325 82 82 0.2 7 ~0 15119 Ti-45Al-4Nb 12~5 124 124 0.2 1250 120 120 0.2 1275 --* 87 0 16 40 Ti-46Al-4Nb 1275 --* 105 0 1300 101 110 0.7 4 1325 96 96 0.2 1766 Ti-47Al-4Nb 1275 109 110 0.4 1300 100 lOI 0.3 1325 95 105 0.8 1855 Ti-48Al-4Nb 1275 102 105 ~ 0.5 1325 84 ~3 1.2 1350 81 87 0.7 35 25132 Ti-46~1-6Nb 1275 --~ 120 0 1300 125 12~ 0.4 }325 --* 71 ~ 0 i992 Ti-~8Al-6Nb 1325 96 103 ~ 0.5 5 23123 Ti-48Al-12Nb 13?5 --* 106 0 1350 92 ~ 99 1.3 1375 ~4 90 0 5 I400 --* 82 0.1 - No measurable value was found because the sample lacked sufficient ductility:to obtain a measure- : ::
ment In addition~, as is evident from the~data presented in Table IV, oxidation resi~tance tests were carried out. ::

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RD-1~,426 If a comparison is made between the alloys listed in Table IV which contained different percentages of niobium dopant and the base gamma TiAl alloy which was free of the niobium ~alloy 12) it is evident that there is essentially no overall improvement in ductility. There are some alloys for which significant strength improvement is formed but in general where the strength is significantly hiqher the ductility is quite low. For example, for alloy 119, alloy strength is quite high (124 ksi and 120 ksi) but the corresponding ductility is quite low ti.e. 0.1).
There is an overall improvement in oxidation resistance from the data shown in Table IV.

E~:
INGOT METALLURGY AND TENSILE BAR TESTING
A second lot of a number of th~ alloy compositions which are listed in the tables above were prepared by conventional ingot metallurgy processing rather than by the rapid solidification processing used in the first lots ~-prepared as described in the first lots prepared as described in the earlier examples. Where the alloy composition of the ingot processed alloy is the same as an alloy of an earlier example, the same example number is repeated but the ingot processing is evidenced by adding an "A" to the example number. One additional alloy designatedi as alloy 26A was also prepared by ingot processing..
The properties of the alloys so prepared were tested and the test results are listed in Table V immediately below.

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RD-19, a26 7!A.BL~ Y
Room Temperature Tensile Properties of Cast and Forged Gamma TiP,l Alloys Weight Loss Homo- Plastic After 48hrs Gamma Atomic geni- Yield Frac~ure Elon~a- Q 982C in Ex. CFG Compo- zation Heat Treat Strength Streng~h ~ion Static Air No. No. sition TempCTemp.C tk~i)(ksi) ~) (mg/cm2) . ~
2A 12A Ti-48Al 1250 130054 73 2.6 32 1250 1325 50 71 2.3 1250 1350 57 77 2.1 16A 40A Ti-46Al-4Nb 1250 l~S0 93 96 0.8 1250 1275 89 99 1.4 1250 1300 87 100 1.6 3 18A 55A Ti-48Al-4Nb 1250 1275 70 77 1.3 1250 1350 57 78 2.3 1400 1300 65 79 2.2 : 1400 1325 62 77 2 1400 ~ 1350 63 82 2.2 26A 151A Ti-49Al-4Nb 1400 1300 53 60 1.4 1400 1325 50 63 2.1 1400 1350 52 65 2.1 1400 1375 52 66 1.6 3~5 21A 67A Ti-4~Al-8Nh 1400 1300 74 82 1.7 2 1400 1350 67 83 2.2 1400 1375 70 87 ~ 2.6 23A 123A Ti-48Al-12Nb 1400 132S 7~ 82 1.6 1400 13S0 72 8~ 2 1~00 1375 69 87 2.3 * - Example 2A corre3pond~ to Example 2 above in the composition of the alloy u~ed in the example. However, Alloy 12A of ~xamp}e 2A wa~ prepared by ingot metallurgy rather than by the rapid solidifioation method o Alloy 12 of Example 2. The tensile and elongation properties were tested by the ten3ile bar method rather than the four point S0 bending tèsting u~ed for Alloy 12 o~ Example 2. The other alloys listed in Table V were also prepared by conventional ingot metallurgy. AIl ten~ile data in Table V wa~ obtained by conventlonal to-Yil- b-F te~ing.

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~ 426 The ingot processing procedure, which is also designated cast and forge processing herein, was essentially the same for each of the alloy samples prepared and was as S follows:
In the ingot melting procedure, the ingot is prepared to a dimension of about 2" in diameter and about 1/2" thic~ in the approximate shape of a hockey puck.
Following the melting and solidification of ~he hockey puck shaped ingot, the ingot was enclosed within a steel annulus having a wall thickness of about 1/2" and having a vertical thickness which matched identically that of the hockey puck ingot Before being enclosed within the retaining ring, the hockey pucked ingot was homogenized by being heated to 15 1250C-1400C for two hours. The assembly of the hockey puck and retaining ring were heated to a temperature of about 975C. The heated sample and contàining ring were forged to a thickness of approximately half that of the original thickness.
After the foryed ingot was cooled, a number of pins were machined out of the ingot for a number of dif~erent heat treatments. The different pins were separate~ly annealed at the different temperatures lis~ed in Table V above.
Following the individual anneals, the pins were aged at 1000C for ~wo hours. After the anneal and aging, each pin was machined into a conventional tensile bar and conventional tensile tests were performed on the resulting bars. The results of the tensile tests are listed in Table V above.
As is e~ident from the table, the four samples of alloy 67A were individually annealed at the four different temperatures and specifically 1300, 1325, 1350, and 1375C.
The yield strength of these samples is significantly improved~
over the base alloy 12A. For example, the sample annealed at 1300C had a gain of about 37% in yield strength over the c~

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alloy 12A which was annealed a~ a same temperature. Other gains are of the same order of magnitude. This gain in strength was realized with a reduction in ductility but the ductility of the sample of alloy 67A annealed at 1300C is remarkably improved over a similar sample for Example 21 of Table III. The other heat-treated samples show comparable gains in strength wi~h modest reduction in ductility over the base alloy 12A and in some cases with a modest gain in ductility. The combination of improved strength with moderately reduced ductility or even moderately increased ductility when considered together make these gamma titanium aluminide compositions unique.
Returning again to consideration of the test results that are listed in Table V and by comparing it with the data, for example, listed in Table IV, it is evident thht the yield strengths determined for the rapidly solidified alloys-as reported in Table IV are somewhat higher than those which are determined for the ingot processed metal specimens as reported in Table V. Also, it is evident that the plastic elongation of the samples prepared through the ingot metallurgy route have higher ductility than those which are prepared by the rapid solidification route. The results listed, however, provide a good comparative basis in having alloy 12A which was prepared by ingot mstallurgy listed in Table V and alloy 12 which was prepared by rapid solidification listed in Table IV. However, from a general comparison of the data of Table V, with the data of Table IV, it is evident that for the higher concentration of niobium additive, the preparation of the alloy samples by the ingot metallurgy processing technique and the testing of the samples by conventional tensile bar t sting techniques demonstrates that the higher niobium alloys prepared by ingot metallurgy techniques are very desirable for those applications which require a hlgher ductillty. Generally . . : ', ' . .
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RD-19.926 speaking, it is well known that processing by ingot metallurgy is far less expensive than processing through melt spinning or rapid solidification inasmuch as there is no need for the expensive melt spinning step itself nor for the consolidation step which must follow the melt spinning when the rapid solldification processing is employed.

OXIDATION ~ESISTANCE
The alloys of this invention also display superior oxidation resistance. The oxidation tests reported in Table IV are static tests. The static tests are performed by heating the alloy sample to 98~C for 48 hours and then cooling and weighing the heated sample. The weight gain is divided by the surface area of the sample in square centimeters. The result is stated in rnilligrams of weight gain per square centimeter of surface area for each sample.
The data given in Table V is determined on the same static basis.
A number of dynamic oxidation resistance tests were performed on a number of the alloys as lis~ed in Table V.
The data from these tests are plotted in Figure 4. In Figure 4, the weight gain in mg/cm2 from oxidation of alloy samples as marked is plotted against dynamic exposure to oxidation at 850C. By dynamic or cycled exposure to an oxidizing a~mosphere at elevated temperature is meant that the test sample is cycled through a series of heatings and coolings and that the sample is weighed each time it has cooled to room temperature. The heating is to 850~C in each case and the sample is maintained at the 850C temperature during each cycle for 50 minutes. Cooling is ~ot a forced cooling but rather is a cooling in an ambient room temperature atmosphere. The cooling, weighiny, and return to the furnace for testing to the 850C temperature takes in the order of ten minutes for an average size sarnple. The heating to .

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~D-19,~2 temperature and cooling from temperature is not part of the S0-minute period during which the sample is maintained at temperature.
The data plotted in Figure 4 is a plot of the weight and of the changing weight of the four samples tested.
From the plot of Figure 4, it i~ evident that the alloys having 8 and 12 atom percent niobium dopant were by far the best compositions from the point of view o~ cyclic oxidation resistance.
Figure 3 presents similar data but on a different basis. In Figure 3, the oxidatlon resistance is displayed on the basis of the time needed for the sample to reach a weight gain level of 0.8 mg/cm2. For the Ti~4A14gNbg alloy, the time is 500 hours.
Figure 3 also presents the relevan~ strength and ductility data for the respective alloys.
Clearly, from the data plotted in Figures 3 and 4, it may be seen that the ingot processed alloy Ti48-37Al46-4gNb6-l4 is a novel and unique alloy having unusual and novel sets of properties.

.

Claims (10)

1. A niobium modified titanium aluminum alloy, said alloy consisting essentially of titanium, aluminum, and niobium in the following approximate atomic ratio;
Ti48-37Al46-49Nb6-14, said alloy having been prepared by ingot metallurgy.

2. A niobium modified titanium aluminum alloy, said alloy consisting essentially of titanium, aluminum, and niobium in the approximate atomic ratio of:
Ti46-38Al48Nb6-14, said alloy having been prepared by ingot metallurgy.

3. A niobium modified titanium aluminum alloy, said alloy consisting essentially of titanium aluminum, and niobium in the following approximate atomic ratio:
Ti46-39Al46-49Nb8-12 said alloy having been prepared by ingot metallurgy.

4. A niobium modified titanium aluminum alloy, said alloy consisting essentially of titanium, aluminum, and niobium in the approximate atomic ratio of:
Ti44-4oAl4BNb8-12 said alloy having been prepared by ingot metallurgy.

5. A niobium modified titanium aluminum alloy, said alloy consisting essentially of titanium, aluminum, and niobium in the following approximate atomic ratio:
Ti44A148Nb8 said alloy having been prepared by ingot metallurgy.

6. As an article of manufacture, a structural member, RD-19,426 said member being formed of a niobium modified titanium aluminum alloy consisting essentially of titanium, aluminum, and niobium in the following approximate atomic ratio:
Ti48-37Al46-49Nb6-l4 said alloy having been prepared by ingot metallurgy,

7. As an article of manufacture, a structural member, said member being formed of a niobium modified titanium aluminum alloy consisting essentially of titanium, aluminum, and niobium in the following approximate atomic ratio:
Ti46-38Al48Nb6-l4 said alloy having been prepared by ingot metallurgy,

8. As an article of manufacture, a structural member, said member being formed of a niobium modified titanium aluminum alloy consisting essentially of titanium, aluminum, and niobium in the following approximate atomic ratio:
Ti46-39Al46-49Nb8-12 said alloy having been prepared by ingot metallurgy,

9. As an article of manufacture, a structural member, said member being formed of a niobium modified titanium aluminum alloy consisting essentially of titanium, aluminum, and niobium in the following approximate atomic ratio:
Ti4440A148Nb8-12, said alloy having been prepared by ingot metallurgy,

10. As an article of manufacture, a structural member, RD-19,426 said member being formed of a niobium modified titanium aluminum alloy consisting essentially of titanium, aluminum, and niobium in the following approximate atomic ratio:
Ti44Al48Nb8, said alloy having been prepared by ingot metallurgy, 10. The invention as defined in any of the preceding claims including any further features of novelty disclosed.