Patent Publication Number: US-4842817-A

Title: Tantalum-modified titanium aluminum alloys and method of preparation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The subject application relates to copending application as follows: Ser. Nos. 138,407; 138,408; 138,476; 138,481; and 138,486; filed Dec. 28, 1987 respectively. 
     The tests 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 tantalum 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 Ti 3  Al is formed. The Ti 3  Al 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. 
     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 FIG. 1. 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 lighweight 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 temperture. 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 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 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 Ti 3  Al intermetallic compound, the TiAl intermetallic compounds and the Ti Al 3  intermetallic compound. U.S. Pat. No. 4,294,615, entitled &#34;Titanium Alloys of the TiAl Type&#34; 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&#39;s advantages and disadvantages relative to Ti 3  Al: 
     &#34;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&#39;s indicated that titanium aluminide alloys had the potential for high temperature use to about 1000° C. But subsequent engineering experience 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 550° C. 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.&#34; 
     It is known that the alloy system TiAl is substantially different from TiAl 3  (as well as from solid solution alloys of Ti) although both TiAl and Ti 3  Al are basically ordered titanium aluminum intermetallic compounds. As the U.S. Pat. No. 4,294,615 points out at the bottom of column 1: 
     &#34;Those well skllled recognize that there is a substantial difference between the two ordered phases. Alloying and transformational behavior of Ti 3  Al resemble those of titanium as the hexagonal crystal structures are very similar. However, the compound TiAl has a tetragonal arrangement of atoms and thus rather different alloying characteristics. Such a distinction is often not recognized in the earlier literature.&#34; 
     The U.S. Pat. No. 4,294,615 patent does described the alloying of TiAl 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, &#34;Titanium-Aluminum System&#34;, Journal of Metals, June, 1952, pp. 609-614, TRANSACTIONS AIME, Vol. 194. 
     2. H. R. Ogden, D. J. Maykuth, W. L. Finlay, and R. I. Jaffee, &#34;Mechanical Properties of High Purity Ti-Al Alloys&#34;, Journal of Metals, February, 1953, pp. 267-272, TRANSACTIONS AIME, Vol. 197. 
     3. Joseph B. McAndrew, and H. D. Kessler, &#34;Ti-36 Pct Al as a Base for High Temperature Alloys&#34;, Journal of Metals, October, 1956, pp. 1348-1353, TRANSACTIONS AIME, Vol. 206. 
     This latter paper discloses on page 1353 a composition of titanium-35 weight percent aluminum and 7 weight percent tantalum. On an atomic percent scale this is equivalent to Ti 47 .5 Al 51  Ta 1 .5. This composition is stated to have an ultimate tensile strength of 76,060 psi and a ductility of about 1.5% and is dicussed further below. 
     BRIEF DESCRIPTION OF THE INVENTION 
     One object of the present invention is to provide a method of forming a titanium aluminum intermetallic 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 temperatures. 
     Another object is to provide 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 an nonstoichiometric TiAl base alloy, and adding a relatively low concentration of tantalum to the nonstoichiometric composition. The addition may be followed by rapidly solidifying the tantalum-containing nonstoichiometric TiAl intermetallic compound. Addition of tantalum in the order of approximately 3 to 5 parts in 100 is contemplated. 
     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 
     FIG. 1 is a graph illustrating the relationship between modulus and temperature for an assortment of alloys. 
     FIG. 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. 
     FIG. 3 is a graph similar to that of FIG. 2 but illustrating the relationship with respect to tantalum modified titanium aluminide. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     EXAMPLES 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 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 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 rapidily solidifed ribbon was packed into a steel and which was evacuated and then sealed. The can was then hot isostatically pressed (HIPped) at 950° C. (1740° F.) 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 975° C. (1787° F.) 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 1000° C. for two hours. Specimens were machined to the dimension of 1.5×3×25.4 mm (0.060×0.120×1.0 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. Based 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 of cross head displacment 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 points bending as carried out in making the measurements reported herein. The higher levels of the results from four points 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 measurement results in the examples herein is between four point bending tests for all samples measured and 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 1300° C. and further data on these samples in particular is given in FIG. 2. 
     
                       TABLE I                                                     
______________________________________                                    
                                           Outer                          
     Gamma    Com-     Anneal                                             
                             Yield  Fracture                              
                                           Fiber                          
Ex.  Alloy    posit.   Temp  Strength                                     
                                    Strength                              
                                           Strain                         
No.  No.      (wt. %)  (°C.)                                       
                             (ksi)  (ksi)  (%)                            
______________________________________                                    
1    83       Ti.sub.54 Al.sub.46                                         
                       1250  131    132    0.1                            
                       1300  111    120    0.1                            
                       1350  *      58     0                              
2    12       Ti.sub.52 Al.sub.48                                         
                       1250  130    180    1.1                            
                       1300  98     128    0.9                            
                       1350  88     122    0.9                            
                       1400  70     85     0.2                            
3    85       Ti.sub.50 Al.sub.50                                         
                       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 measurement.                                       
 
    
     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 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 1250° C. and 1350° C. 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 test specimen having a significantly lower yield strength (about 20% lower); lower fracture strength (about 30% lower) and lower ductibility (about 78% lower) than a test specimen annealed at 1350° C. The sharp decline in properties is due to a dramatic change in microstructure due in turn to an extensive beta transformation at temperatures appreciably above 1350° C. 
     EXAMPLES 4-13 
     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, 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. 
     
                                           TABLE II                                
__________________________________________________________________________
                          Yield                                           
                               Fracture                                   
     Gamma Composit.                                                      
                   Anneal Strength                                        
                               Strength                                   
                                    Outer Fiber                           
Ex. No.                                                                   
     Alloy No.                                                            
           (at. %) Temp. (°C.)                                     
                          (ksi)                                           
                               (ksi)                                      
                                    Strain (%)                            
__________________________________________________________________________
2    12    Ti.sub.52 Al.sub.48                                            
                   1250   130  180  1.1                                   
                   1300    98  128  0.9                                   
                   1350    88  122  0.9                                   
4    22    Ti.sub.50 Al.sub.47 Ni.sub.3                                   
                   1200   --*  131  0                                     
5    24    Ti.sub.52 Al.sub.46 Ag.sub.2                                   
                   1200   --*  114  0                                     
                   1300    92  117  0.5                                   
6    25    Ti.sub.50 Al.sub.48 Cu.sub.2                                   
                   1250   --*   83  0                                     
                   1300    80  107  0.8                                   
                   1350    70  102  0.9                                   
7    32    Ti.sub.54 Al.sub.45 Hf.sub.1                                   
                   1250   130  136  0.1                                   
                   1300    72   77  0.1                                   
8    41    Ti.sub.52 Al.sub.44 Pt.sub.4                                   
                   1250   132  150  0.3                                   
9    45    Ti.sub.51 Al.sub.47 C.sub.2                                    
                   1300   136  149  0.1                                   
10   57    Ti.sub.50 Al.sub.48 Fe.sub.2                                   
                   1250   --*   89  0                                     
                   1300   --*   81  0                                     
                   1350    86  111  0.5                                   
11   82    Ti.sub.50 Al.sub.48 Mo.sub.2                                   
                   1250   128  140  0.2                                   
                   1300   110  136  0.5                                   
                   1350    80   95  0.1                                   
12   39    Ti.sub.50 Al.sub.46 Mo.sub.4                                   
                   1200   --*  143  0                                     
                   1250   135  154  0.3                                   
                   1300   131  149  0.2                                   
13   20    Ti.sub.49.5 Al.sub.49.5 Er.sub.1                               
                   +      +    +    +                                     
__________________________________________________________________________
 *See asterisk note to Table I.                                           
 + Material fractured during machining to prepare test specimens.         
 
    
     For Examples 4 and 5 heat treated at 1200° C., the yield strength was unmeasurable as the ductibility was found to be essentially nil. For the specimen of Example 5 which was annealed at 1300° C., the ductility increased, but it was still undesirably low. 
     For Example 6 the same was true for the test specimen annealed at 1250° C. For the specimens of Example 6 which were annealed at 1300° and 1350° C. the ductility was significant but the yield strength 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 FIG. 2 it is evident that the stoichiometric ratio or non-stoichiometric 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 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 Ti 48  Al 48  X 4  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 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 enhancement 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 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 non-stoichiometric TiAl composition are highly unpredictable and that most test results are unsuccessful with respect to ductility or strength or to both. 
     EXAMPLES 14-18 
     Five 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. 
     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. 
     
                                           TABLE III                               
__________________________________________________________________________
FOUR-POINT BEND PROPERTIES OF Ta-MODIFIED TiAl ALLOYS                     
                  Annealing                                               
                         Yield                                            
                              Fracture                                    
   Gamma Alloy                                                            
           Composition                                                    
                  Temperature                                             
                         Strength                                         
                              Strength                                    
                                   Outer Fiber                            
Ex.                                                                       
   Number  (at. %)                                                        
                  (°C.)                                            
                         (ksi)                                            
                              (ksi)                                       
                                   Strain (%)                             
__________________________________________________________________________
 2 12      Ti.sub.52 Al.sub.48                                            
                  1250   130  180  1.1                                    
                  1300    98  128  0.9                                    
                  1350    88  122  0.9                                    
                  1400    70   85  0.2                                    
14 42      Ti.sub.52 Al.sub.46 Ta.sub.2                                   
                  1250   131  163  0.6                                    
                  1300   112  146  0.4                                    
                  1350    83   90  0.1                                    
15 68      Ti.sub.50 Al.sub.48 Ta.sub.2                                   
                  1250   125  147  0.7                                    
                  1300   106  139  0.8                                    
                  1350    97  131  1.0                                    
16 43      Ti.sub.50 Al.sub.46 Ta.sub.4                                   
                  1250   123  138  0.1                                    
                  1300   --    86  0                                      
17 60      Ti.sub.48 Al.sub.48 Ta.sub.4                                   
                  1250   120  147  1.1                                    
                  1300   106  141  1.3                                    
                  1350    97  137  1.5                                    
                  1400    72    92 0.2                                    
18 108     Ti.sub.46 Al.sub.48 Ta.sub.6                                   
                  1250   136  158  0.4                                    
                  1300   117  134  0.3                                    
__________________________________________________________________________
 
    
     On the same heat treatment basis, the tantalummodified alloys all maintained their strengths at least similar to the base alloy or higher. 
     However, the outer fiber strain was reduced for alloys 42 and 43, while both properties were increased for alloy 60, particularly when annealed at higher temperatures. 
     For alloy 68 to yield strength is generally improved relative to base alloy 12 but the outer fiber strain remains about the same. 
     For alloy 108 the yield strength is also generally improved relative to base alloy 12 but the outer fiber strain is substantially reduced to less than half that of base alloy 12. 
     Alloy 60 when heat treated at 1300+ to 1350° C. thus has the optimum combination of room temperature properties. 
     This remarkable increase in ductility of alloy 60 was an unexpected result. 
     The increased ductility appears to be a result of the reduced Al/Ti ratio, the high tantalum modification, and the use of rapid solidification processing. 
     Like the base alloy, alloy 60 also undergoes a beta transition above 1350° C. The properties are precipitously dropped above that temperature. 
     Regarding now the Ogden reference listed above and entitled &#34;Mechanical Properties of High Purity Ti-Al Alloys&#34;, this reference teaches a titanium alloy having 35 weight percent aluminum and 7 weight percent tantalum. As noted above this is equivalent to a composition having the formula in atomic percentages of Ti 47 .5 Al 51  Ta 1 .5. 
     As also noted above the author reported an ultimate tensile strength of 76,060 psi and a ductility of about 1.5%. No yield strength of that alloy was reported in that paper. 
     As is evident from the data set forth in Table III above this ductility of 1.5% reported by Ogden is about equivalent to that of the alloy 60 which was annealed at 1350° C. However, the ultimate tensile strength of alloy 60 annealed at this temperature is about 137 ksi. In other words the fracture strength of alloy 60 is about 80% higher than the highest values reported by Ogden. Neither the unexpected benefits of the higher tantalum concentrations, nor the criticality of achieving a specific aluminum to titanium ratio were recognized by Ogden. 
     As has been pointed out above it is the combination of strength and ductility which is most critical in judging the comparative advantages of an alloy. A gain of 80% in strength with no loss in ductility is a remarkable advance in the technology of TiAl alloys.