CA2010681A1 - Silicon-modified titanium aluminum alloys and method of preparation - Google Patents

Abstract

RD-19,570 C-I-P of RD-17,813 SILICON-MODIFIED TITANIUM ALUMINUM ALLOYS
AND METHOD OF PREPARATION

ABSTRACT OF THE DISCLOSURE
A TiAl composition is prepared to have high strength and to have improved ductility by altering the atomic ratio of the titanium and aluminum to have what has been found to be a highly desirable effective aluminum concentration by addition of silicon according to the approximate formula Ti59-51Al39-44Si2-5.

Description

R~-19,570 - C-I-P of RD-17,813 SI~ICON-~ODIFlED TI~ANI~ ~LUMINU~ ALhO~S
AND ~T~OD OF PREP~RATION 20~06S~
.. .. .. . .. _ . . .. .

CROSS~REFEREN OE TO RELATED APPLICATIONS

The subject application relates to copending applications as follows:
Serial Nos. 138,408; 138,476; 138,481; 138,485; and 138,486; filed December 28, 1987 respectively.
This application is a continuation-in-part of application Serial No. 138,407, filed December 28, 1987, and now allowed.
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 whicll have been modified both with respect to stoichiometric ratio and with respect to silicon addition.
It is known that as aluminum is added to titanium metal in greater and greater proportions the crystal form of the resultant titanium aluminum composition changes. Small percentages of aluminum go into solid solution in titanium and the crystal form remains that 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 ., ~. , .
; ~

RD~
C-I-P of RD-17,813 compound, TiAl, is formed having an ordered tetragonal crystal form called gamma. 20~6~
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 relationship between the modulus and temperature for TiAl compounds to other alloys of titanium and in relation to nickel base superalloys is shown in Figure l. As is evident from the figure, the TiAl has the best modulus of any of the titanium alloys. Not only is the TiAl modulus higher at temperature but the rate of decrease of the modulus with temperature increase is lower for TiAl than for the other titanium alloys. Moreover, the TiAl retains a useful modulus at temperatures above those at which the other titanium alloys become useless. Alloys which are based on the 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 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 TiAl intermetallic compound can be exploited in structural component applications. Improvements of the 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 TiAl compositions which are to be used is a combination of strength and ductility at room temperature. A minimum : .

~ .

~ ~C70 2 0 ~ 0 6 ~ P of RD-17,813 ductility of the order of one percent is acceptable for some applications of the metal composition but higher ductilities are much more desirable. A minimum room 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 small changes of one percent or more in the stoichiometric ratio of the titanium and aluminum ingredients. Also, the properties are similarly affected by the addition of relatively similar small amounts of ternary elements.

PRIOR ART

There is extensive literature on the compositions of titanium aluminum including the Ti3Al intermetallic compound, the gamma TiAl intermetallic compounds and the Ti3Al intermetallic compound. A patent, U.S. 4,294,615, entitled "Titanium Alloys of the TiAl Type" 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 1~50's indicated that titanium aluminide alloys had the potential for high temperature use to about 1000C. But subsequent RD-l3~570 n ~ ~C-I-P of RD-17,813 ~ ~ ~ ~ ~ ~- 5/30/89 engineering experlence with such alloys 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 alloys."

It is known that the alloy system TiAl is substantially different from Ti3Al (as 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 structures are very similar. However, the compound TiAl has a tetragonal arrangement oE atoms and thus rather different alloying characteristics. Such a dLstinction is often not recogr. zed in the earlier literature."

The '615 patent does describe the alloying of TlAl with vanadium and carbon to achieve some property improvements in the resulting alloy.
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, "Titanium-Aluminum System", Jour~L of Metal~, June 1952, pp.
609-614, TRANSACTIONS AIME, Vol. 194.

RD-19 .~Q
C-I-P of RD-17,813 Z0~68~. 5/30/89 2. H.R. Ogden, D.J. Maykuth, W.L. Finlay, and R.I.
Jaffee, "Mechanical Properties of High Purity Ti-Al Alloys", J~urna1 of Metal.~, February 1953, pp. 267-272, TRANSACTIONS
AIME, Vol. 197.

3. Joseph B. McAndrew, and H.D. Ke~sler, "Ti-36 Pct Al as a ~ase for High Temperature Alloys ", Jo~xnal Qf L~, October ~956, pp. 1348-1353, TRANSACTIONS AIME, Vol.
206.

BRIEF DESCRIPTION OF THE INVENTION

One object of the present invention is to provide a method of forming a titanium aluminum intermetallic compound ha~ing improved ductility and related properties at room temperature.
Another object is to improve the properties of titanium aluminum intermetalflic compounds at low and intermediate temperatures.
Another object is to provi.de an alloy of titanium and aluminum having improved properties and processability at low and intermediate temperatures.
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 low concentration of silicon to the nonstoichiometric composition. The addition may be followed by rapidly solidifying the silicon-containing nonstoichiometric TiAl intermetallic compound. Addition of silicon in the order of approximately 2 to 5 parts in 100 is contemplated.

RD-19,57Q
8 ~ I-P of RD-17,813 The rapidly solidified composition may be consolidated as by isostatic pressing and extrusion to form a solid composition of the present invention.

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..
FIGURE 3 is a graph illustrating the properties of a silicon modified TiAl in relation to those of Figure 2.
FIGURE 4 is a bar graph illustrating th,e results of a bending test for silicon modified TiAl in relation to Tis2Al4 a -DETAILED DESCRIPTION OF THE INVE~TION

EXAMe~ 1-3:
Three individual melts were prepared to contain titanium and aluminum in various stoichiometric ratios approximating that of TiAl. The compositions, annealing temperatures and test results of tests made on the compositions are set forth in Table I.
For each example, the alloy was first made into an ingot by electro arc melting. The ingot was processed in~o 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 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.

-.

R3-13 5,0 C-I-P of RD-17,813 Z~S8~. 5/30/83 The rapidly solidified ribbon was packed into a steel can which was evacuated and then sealed. The can was then hot isostatically pressed (HIPped) at 950C (1740F) for 3 hours under a pressure of 30 ksi. The HIPping can was machined off the consolidated ribbon plug. The HIPped sample 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 is extruded through a die to give a reduction ratio of about 7 to 1. 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 l.S x 3 x 25.4 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 4-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. E3ased on the curves developed, the following properties are defined:

(1) Yield strength is the flow stress at a cross head displacement of one thousandth of an inch. This amount o~ 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 obtalned by four point bending as carried out in making the 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 R~-19 ~7Q
2~ 5t30/89 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 9.71hd, where "h" is the specimen thickness in inches, and "d"
is the cross head displacement of fracture in inches.
Metallurgically, 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 further da~a on these samples in particular is given in Figure 2.

~L~5~Q
C-I-P o~ RD-17,813 ~ . 5/30/89 TAsLE I
Outer GammaYield Fracture Fiber Ex. Alloy Composit. Anneal Strength Strength Strain No. No. (at.%~ Temp(C)(ksi) (ksi) (%) . _ _ 1 83 Tis4Al46 1250 131 132 0.1 1300 111 120 0.1 1350 * 58 0 2 12 Tis2Al48 1250 130 180 1.1 1300 98 128 Q.9 1350 88 122 0.9 1400 70 85 0.2 3 85 TisoAlso 1250 83 92 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. ~his confirms that the properties of Ti-Al compositions are very sensitive to the Ti/Al atomic ratios and to the heat treatment applied. Alloy 12 was selected as the base alloy for further property improvements based on further experiments which were performed 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 strenath and outer fiber strain. However, the anneal at 1400C
results in a test specimen having a significantly lower yield strength (about 20% lower); lower fracture strength (about 30% lower) and lower ductility (about 78% lower) than a test specimen annealed at 1350C. The sharp decline in properties ~2-l9,570 C-I-P of RD-17,813 ~ ~ ~ ~ ~c~ 5/30/89 is due to a dramatic change in microstructure due, in turn, to an extensive beta transformation at temperatures appreciably above 1350C.

EXAM~ 4-1~:
Ten additional individual melts were prepared to contain titanium and aluminum in designated atomic ratios as well as additives in relatively small atomic percents.
Each of the samples was prepared as described above with reference to Examples 1-3.
The compositions, annealiny 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.

RD-I 9 A_~7 ~2010!681.5 / 30 / 89 TABLE I I
Outer Gamma Yield Fracture Fiber Ex.Alloy Composition Anneal Strength Strength Strain No. No. (at.%) Temp(C) (ksi) (ksi) (~) 2 12 Ti52Al481250 130 180 1.1 1300 98 128 0.9 1350 88 122 0.9 4 22 Ti50A147Ni3 1200 * 131 0 24 Ti52A146Ag2 1200 1~ 114 0 1300 92 117 0.5 6 25 Ti5oAl48cu2 1250 ~ 83 0 1300 80 107 0.8 1350 70 102 0.9 7 32 Tis4Al45Hfl 1250 130 136 0.1 1300 72 77 0.2 8 41 Ti52A144Pt4 1250 132 150 0.3 9 45 Ti51A147C2 1300 136 149 0.1 57 TisoAl48Fe2 1250 * 89 0 1300 * 81 0 1350 86 111 0.5 11 82 Ti50Al48Mo2 1250 128 140 0.2 1300 110 136 0.5 1350 80 95 0.1 12 39 Ti5oAl46Mo4 1200 * 143 0 1250 135 154 0.3 1300 131 149 0.2 13 20 Ti49.5A149.5Crl + + +
*-See asterisk note to Table I
+-Material fractured during machining to prepare test specimens .

R~ 70 6~3~ C-I-P of RD-17, 813 For Examples 4 and 5, heat treated at 1200C, the yield strength was unmeasurable as the ductility was found to be essentially nil. For the specimen of Example S 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 stren~th was low.
None of the test specimens of the other Examples were found to have any significant 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 titanium relative to that of aluminum. From the data plotted in Figure 2, it is evident that the stoichiometric ratio or nonstoichiometric ratio has a strong influence on the test properties which are found from testing of from testing oE different compositions.
Another set of parameters is the additive chosen to be included into the basic TlAl composition. ~ first parameter of this set concerns whether a particular additive acts as a substituen~ for titanium or for aluminum. A
specific metal may act in either 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 Ti4gA14gX4 will give an effective aluminum concentration of 48 atomic percent and an effective titanium concentration of 52 atomic percent.
If, by contrast, the X additive acts as an aluminum substituent, then the resultant composition will have an 2~ P of RD-17,813 effective aluminum concentration of 52 percent and an effective titanium concentration of 48 atomic percent.
Accordingly, the nature of the 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. This 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 optimum property enhancement, if any enhance~ment is found, can occur at a certain combination of additive~ concentration and annealing temperature so that higher and lower concentrations and/or annealing temperatures are le:ss effective in providing a deqired property improvement.
The content of Table II makes clear that the results obtainable from addition o~ 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.

~E~Q:
Five additional examples were prepared in the manner described above with reference to Examples 1-3 to contain silicon modified compositions respectively as listed in Table III.
Table III summarizes the bend test results on all of the alloys both standard and modified under the various heat treatment conditions deemed relevant.

O ~ C-I-P of RD-17,813 5~30/89 TABLE III
Four-Point~..R~n~ Properti~.~ of Si-~Q ~
Outer Gamma Yield Fracture Fiber Ex. Alloy Composition Anneal Strength Strength Strain No. No. (at.%) Temp(C)(ksi) (ksi)t%) .. . .
2 12 Ti52A14a 1250 130 180 1.1 1300 98 128 0.9 1350 88 122 0.9 14 19 Ti52Al46si2 1250 * 154 0 1300 142 145 0.1 15 35 Ti52Al44Si4 1300 160 164 0.1 16 121 Ti54Al42si4 1250 * 183 0 1300 167 175 0.1 1350 120 161 0.6 17 59 Ti56Al40Si4 1250 184 205 0.2 1300 160 214 0.6 1350 155 206 0.5 18 71 Ti56Al43sil 1300 135 146 0.1 * - No measurable value was found because the sample lacked sufficient ductility to obtain a measurement If the test results are compared for the examples ~, 14 and 15, it is evident that as the silicon concentration is increased from 0 to 2 atomic percent and then to 4 atomic percent, and assuming that the silicon substitutes for aluminum, ~hen the strength of the alloys formed increases above that of the base alloy but the ductility is reduced.
If the test results are compared for Examples 15, 16 and 17, it becomes evident that as the concentration of aluminum is lowered from 44 atomic percent to 42 atomic percent and then to 40 atomic percent respectively, there is ~ fi81 P of RD-17,813 improvement in the ductllity from essentially brittle for alloys 35 and 121 to about 0.6 for alloy 59.
Considering next, Example 18, the conclusion is reached that as the aluminum and silicon concentrations are reduced the strength and ductility are also reduced.
As is evident from the table, alloy 35 exhibited strengths which are more than 60% greater than those of the base alloy, while the outer fiber strain for thls alloy was significantly reduced.
Alloy 59 exhibited similar or greater strength improvements. More interestingly, the outer fiber strain for the alloy 59 was maintained at the 0.6% level under two heat treatment conditions. Alloy 59 was, accordingly, found to have the best combination of properties at room temperature.
The combination of high strength and ductllity observed for alloy 59 was an unexpected result.
Figure 3 shows the crosshead displacement of alloy 59 in relation to the three stolchiornetric compositions of TiAl of Figure 2.
Figure 4 is a bar graph illustrating graphically the ~racture strength, yield strength and outer fiber strain of alloy 59 in relation to that of Ti52A148-~AM~L~ 2~ 20 and 21:
Three additional alloys were prepared in the mannerdescribed with re~erence to Examples 1 through 3 above.
These three alloys were identified as alloys 110, 111 and 98.
Tests were conducted of the physical properties of these alloys. These tests were no~ four-point bending tests as are described above. Rather, the tes~s were tensile tests which were performed using conventional tensile bars. Conventional tensile testing was also carried out on alloys 59, 121 and 2.
The results of the conventional tensile testing are listed in Table IV.

RD-19,~70 20~0~81 of RD-17,813 TABLE IV
~oom Temperature Tensile Properties of ~1-Modifi~d TiAL A11QYS Determln~d from Tensile Bar~
Plastic Gamma Yield Fracture Elon-Ex. Alloy Composition Anneal Strength Strength gation No. No. (at.%) Temp~C)(ksi) (ksi) (%) ."." . _ . _ 2a 12 Ti52Al48 1300 77 92 2.1 l9 110 Ti54Al44Si2 1300 111 ll9 0 9 1325 110 120 1.3 1350 * 92 0.1 21 98 Ti5gAl38si4 1300 * 139 0 1325 * 143 0 1350 * 114 0 17a59 Ti55Al4osi4 1300 * 123 0.1 1325 127 136 0.5 1350 122 127 0.4 16a 121 Tis4Al42si4 1325 * 137 0 1350 108 118 1.0 1375 95 120 1.3 111 Ti56Al38si6 1325 125 128 0.3 1350 125 127 0.2 1375 131 136 0.3 * - see footnote of Table III

It is evident from the test results listed in Table IV that the range of concentrations of siliconr when used as an additive in a TiAl composition, which provide valuable combinations of strength and ductility extends from about 2 to about 5 parts.
Further, the range of titanium and aluminum concentrations extend from about 51 to about 59 for titanium and from about 39 to about 44 for aluminum.

RD-l9~570 20~06~1. C-I-P of RD-17,813 Accordingly, the overall composition of the silicon doped TiAl of this invention may be written as follows:

Ti59-51A139-44Si2-5 Preferred ranges of the ingredients of the compositions have smaller ranges as follows:

Ti57-53A140-42Si3-5 A still narrower preferred range is one with about 4 parts of silicon as follows:

Ti56-54Al42-49si4 The advantageous properties of these several alloy compositions are set forth in Tables III and IV above.

.: ~

Claims (12)

1. A silicon modified titanium aluminum alloy consisting essentially of titanium, aluminum and silicon in the following approximate atomic ratio:
Ti59-51Al39-44Si2-5 .

2. A silicon modified titanium aluminum alloy consisting essentially of titanium, aluminum and silicon in the approximate atomic ratio of:
Ti57-53Al40-42Si3-5 .

3. A silicon modified titanium aluminum alloy consisting essentially of titanium, aluminum and silicon in the following approximate atomic ratio:
Ti56-54Al40-42Si4 .

4. A silicon modified titanium aluminum alloy consisting essentially of titanium, aluminum and silicon in the approximate atomic ratio of:
Ti56-54Al42-44Si2 .

5. The alloy of claim 1, said alloy being rapidly solidified from a melt and consolidated.

6. The alloy of claim 2, said alloy being rapidly solidified from a melt and consolidated.

7. The alloy of claim 3, said alloy being rapidly solidified from a melt and consolidated.

8. The alloy of claim 4, said alloy being rapidly solidified from a melt and consolidated.

RD-19,570 C-I-P of RD-17,813

9. The alloy of claim 1, said alloy having been rapidly solidified from a melt and then consolidated and given a heat treatment at a temperature between 1300 and 1400°C.

10. The alloy of claim 2, said alloy having been rapidly solidified from a melt and then consolidated and given a heat treatment at a temperature between 1300 and 1350°C.

11. The alloy of claim 3, said alloy having been rapidly solidified from a melt and then consolidated and given a heat treatment at a temperature between 1300 and 1350°C.

12. The alloy of claim 4, said alloy having been rapidly solidified from a melt and then consolidated and given a heat treatment at a temperature between 1300 and 1350°C.