Title of Invention

"A PROCESS FOR THE PREPARATION OF TITANIUM BASED EITERMETALLIC ALLOY"

Abstract

This invention relates to a process for the preparation of Titanium based intermetallic alloy characterised by adding in atomic ratios Al 16 to 26; Nb 18 to 28; Mo 0 to 2; Si 0 to 0.8, Ta 0 to 2; Zr 0 to 2 and balance Ti provided that Mo+Si+Zr+Ta > 0.4%, melting to form an ingot of homogeneous composition; deformation at high speed bringing about a reduction of the grain size; isothermal forging at a temperature between T ß -25°C and T ß -125°C at deformation rates in the range of 5 x 10 4/s 1, Heat treatment by solutionising at a temperature ranging of T ß -35°C and T ß +15°C for a duration of 2 hours, ageing at a temperature range of 750°C and 950°C for a duration of more than 16 hours permitting development of orthorhombic hardening phase 0, treatment is carried out at the temperature of 100°C around the operating temperature determined for the material, the cooling rates between the heat treatment stages being determinded in accordance with the requirement of the alloy for a particular application by considering their effect on the lath size of the orthorhombic hardening phase 0.

Full Text

The present invention relates to a process for the preparation of titanium baised intermetallic alloys which combine a set of specific mechanical properties comprising o-f a high yield srtrength, high creep strength and an adequate ductility at room temperature. The intermetallic alloys o-f type Ti3A1 have shown significant and specific mechanical properties Some ternary alloys, particulaerly with additions of Mb . have beers tested and their mechamical praperties coupled with a density much 1 over than that of mickel based alloys as comprised between 4 and 5,5 according to the percentage of Mb. are highly advantageous for aeronatical applications. Besides, these alloys have a resistance to titanium fire, more significant than the titanium based allays previously used in the manufacture of turbomachines. The envusage applications relates to massive structural parts such as casings, massive rotary parts like impellers or composite material materix for monoblock varied rings, the studied ranges, of operating tempers tares; go upto 650 or 700°C in the case of c omposite mater i al with long fibres Thus (U.S. 4.. 292-077 . and U. S.. 4.71.6,, 020 describe the results obtained in titanium based intefmetallic alloys comprising ot 24 to 27 Al and ii to 16 Mb in atomic ratios. U.S. 5.(332.357 has shown improved resuits due to an increase of the Mb percentage- Generally, the inter metallic alloys obtained in this.. case shown a microstructure consisting of3 two phases., a B2. phase rich in niobion constituting the matrix of the material and which ensures a, ductility at roam temperature. a phase called 0 of definite camposition Ti 2-AiMo,. with artbarhambic structure and farming laths in the B2 matrix. Phase 0 is present upto aoout 1000°C and canfers the material with its properties of heat resistance during creep and during tension . The formerly known alloys however show some disadvantages, particularly an insufficient ductility at room temperature anf a significant p1astic deformation during primary creep which actually limits their use. Consequently, the present invention relates to a process for the preparation of intermetallic titanium based alloys. overcoming the disadvantages; of the. previously cited known alloys and which are distinguished by preparing a composition in atomic percent,, belonging to the following ranges Al 16 to 26s Mb 18 to 28; Mo 6' to 2; Si 0 to 0 8: 'a i.3 to 2 :2r 0 to 2 and balance 'fi provided that Mo si zr+Zr + Ta 0.4%, Some suitable thermomechanical treatments besides a method of use, are defined for these patented intermetallic alloys, allowing improvement of their mechanical properties especially for increasing the ductility at room temperature and limiting the plastic deformation during primary creep. STATEMENT OF INVENTION This invention relates to a process for the preparation of Titanium based intermetallic alloy characterised by adding in atomic ratios Al 16 to 26; Nb 18 to 28; Mo 0 to 2; Si 0 to 0.8, Ta 0 to 2; Zr 0 to 2 and balance Ti provided that Mo+Si+Zr+Ta > 0.4% wherein the intermetallic material comprises of, a) melting of alloys to form an ingot of homogeneous composition; b) deformation at high speed bringing about a reduction of the grain size; c) isothermal forging at a temperature between Tß -25°C and T ß -125°C at deformation rates in the range of 5 x l0-2s1, d) Heat treatment by, dl) solutionising at a temperature ranging of Tß -35°C and Tß +15°C for a duration of 2 hours, d2) ageing at a temperature range of 750°C and 950°C for a duration of more than 16 hours permitting development of orthorhombic hardening phase 0, d3) treatment is carried out at the temperature of 100°C around the operating temperature determined for the material, the cooling rates between the heat treatment stages being determinded in accordance with the requirement of the alloy for a particular application by considering their effect on the lath size of the orthorhombic hardening phase 0. The justification for selection of composition range retained, as well as the description of tests carried out, resulting in the definition of the method of preparation and formation are given below indicating the results obtained in measurements of mechanical properties and compared to the properties of previously known alloys with reference to the enclosed drawings in which Figure 1 represents the results of creep tests at 550°C/500 MPa and the results of tensile tests represented by the time in hours in 1% strain and the yield strength in MPa respectively on the Y-axis for various alloy compositions, Figure 2 represents the creep test results at 550°C/500 MPa represented by yield strength in MPa on the Y-axis and time in hours for 0.5% strain on the X-axis for various alloy compositions, Figure 3 shows a microstructure obtained as a result of production of intermetallic alloy conforming to the invention, Figure 4 diagramatically represents by zones the results of mechanical tests carried out on four different types of alloys, represented by the percentage of elongation on X-axis and specific yield strength on the Y-axis, at ambient temperature, Figure 5 and 6 represent by Larson-Miller diagrams the results of creep resistance, respectively for 1% strain and at fracture, represented by the Larson Miller parameter on the X-axis and the specific stress in MPa on the Y-axis, for different alloys, Figures 7, 8 and 9 represent the mechanical test results obtained for an alloy conforming to the invention and show UTS and YS in MPa at 20°C and 650°C, uniform elongation in percentage at 20°C and 650°C and finally the time in hours for 1% strain during the creep at 550°C/500 MPa, depending on four different ranges of heat treatment applied to the alloy, Figure 10 represents the creep test results during compression for a previously known alloy and two alloys conforming to the invention. Experimental results have shown that the contents retained for the three major elements of the composition, titanium, aluminium and niobium are most suitable, namely : Al 16 to 26; Nb 18 to 28 and Ti basic element Variation of the contents within the indicated limits, permits a combination of properties according to the type of application and the corresponding range of working temperature. Characteristics of Al, Si; a-forming elements : These two elements are the elements which stabilise the O phase and hence they increase the heat resistance of the alloys. However, they have the tendency to decrease the ductility particularly at ambient temperature. The plastic strain during primary creep reduces from 0.5% to 0.25% with the addition of these elements (0.5% of Si or an increase of 22% to 24% of Al). On the other hand, the YS and ductility are greatly reduced (from 1.5% to 0.5%). An increase of the percentage of aluminium from 22 to 24 greatly reduces the YS which drops from 600 MPa to 500 MPa at 650°C. The beneficial effect of addition of 0.5% Si on the creep strength is illustrated in Figure 2. Characteristics of Nb, Mo and Ta; p-forming elements : These elements stabilise the B2 phase which is ductile at ambient temperature, they promote the stability of B2 phase at operating temperatures. A reduction of niobium percentage (from 25% to 20%) mainly affects the creep strength, the tensile properties being only slightly modified, as shown by the results represented in Figure 1. It will be shown that the addition of molybdenum permits a significant increase of the yield strength by 100 MPa at room temperature and 200 MPa at 650°C and this, without reduction of ductility at ambient temperature. Molybdenum also permits a better resistance to creep, it reduces very clearly the plastic deformation during primary creep (from 0.5% to 0.25%) and decreases the rate of plastic deformation during the secondary stage. These gains are accentuated when the alloys also contained silicon. These results obtained during creep at 550°C/500 MPa, are illustrated in Figure 2 for alloys comprising of additions of Mo, Si or of the two elements. Tantalum is a p-forming element very similar to niobium with which it is often mixed in minerals. In titanium alloys, it increases their mechanical resistance and confers them with a better resistance to corrosion and to oxidation. Characteristics of Zr : ß-neutral element : Zirconium is a neutral element and the remelting and foundry practices may introduce it voluntarily or not, depending on the origin of alloying elements used and on recycling practices. It can also be introduced intentionally. The atomic percentage retained for patented intermetallic alloys for Zr, as for Ta, is situated between 0 and 2%. These specifications and experimental tests carried out for the composition of intermetallic alloys, have resulted in retaining the additional elements in the following atomic ratios besides the three major elements noted above. Mo 0 to2; Si 0 to 0.8; Ta 0 to 2 and Zr 0 to 2 with the additional condition of presence of at least one of the additional elements: Mo + Si + Zr + Ta > 0.4% Preparation and forming procedures In this preparation, the first stage consists of a homogenization of the composition of the material, for example, by using the VAR process (Vacuum Arc Remelting). This stage is important as it determines the homogeneity of the material. The material is then deformed at high speed to reduce the grain size either by drop forging in the \\ range, or by an extrusion at high speed always in the \) range. These bars are then cut into slabs to undergo the final stage of the thermomechanical treatment; the isothermal forging. This isothermal forging is done in a range of temperatures starting from Tß-125°C to Tß-25°C and with deformation speeds of 5 X 10"V1 to 5 X 10"2s"1. T|5 is the transformation temperature at which single phase ß undergoes transformation to two phase α2+B2, α2 phase known as Ti 3AI can transform to 'O' phase at 900°C and below. As an example, Tß is 1065°C for an alloy with composition Ti-22AI-25Nb. For special applications, the bars obtained by forging or extrusion can, alternatively, be subjected to a rolling operation where the deformation speeds are in the order of 10-1s-1. Precision forging can also be carried out in the two phase αa2+B2 domain. This will yield an equiaxed grain structure with globular α2/0 phase. In this case, forging takes place in a temperature domain extending form Tp-180°C to Tp-30°C. The final step of processing is accomplished by a heat treatment consisting of three stages. The first stage of heat treatment is a stage of solutionising at a temperature in the range of Tp-35°C to Tp+15°C for less than 2 hours. The second stage permits the development of the hardening phase, O, and this ageing is carried out between 750°C and 950°C for at least 16 hours. The third treatment is done in a temperature range of 100°C around the operating temperature of the material. The selection of the cooling rate between various stages is important as it determines the size of the laths of hardening phase O. Determination of a particular heat treatment schedule is done based on the characteristics required in the alloy for a specific application. Figure 3 illustrates an example of the microstructure obtained as a result of this preparation of an intermetallic alloy in confirmity with the invention. In the case where equiaxed grained structure by precision forging in the domain of α2+B2 is desired, the solution treatment temperature during the first stage of heat treatment should be in the vicinity of forging temperature. The choice of this temperature is critical because it influences not only the size and relative proportion of the remaining globular primary hardening phase but also secondary hardening phase which is formed in the subsequent stages of heat treatment. In actual practice, it is shown that thermomechanical treatments have a large influence on the mechanical properties : Effect of the forging temperature : Forging at a high temperature ensured a better resistance to creep at 550°C, the time to rupture increases tenfold and the ductility at creep rupture increases from 0.8 to 1.3%, this with an increase of 50°C of the forging temperature. Effect of the forging speed : For a speed 20 times higher, time to rupture reduces tenfolds during creep at 550°C/500 MPa. Heat treatment around the Tp transition temperature causes the recrystallization of B2 grains and allows a significant increase of the creep strength at 650°C. However, this treatment reduces the yield strength, but increases the ductility around 350°C. A heat treatment at a temperature much distant (-25°C) from that of the transition Tp, increases the yield strength and the creep strength at 550°C. Moreover, this treatment allows the attainment of a ductility level around 10% from 200°C upto 600°C. These observations result from following tests : Example 1 -Role of the forging temperature : The effect of two forging temperatures on the creep strength was examined. Forging is followed by the same heat treatment at high temperature. Thus, it is shown that the forging temperature is important for the creep strength as it determines the structure of the phases present in the material, as shown by the results hereunder of the creep strength of a Ti-22AI-25Nb alloy at 550°C / 450 MPa. (Table Removed ) Finally, the creep strength of the alloy Ti-22AI-25Nb at 650°C/300 MPa relative to the isothermal forging temperature gives the following results : (Table Removed ) Example 2 - Effect of the heat treatment: Here, the effect of the solutionising temperature on the mechanical properties and creep strength, for the blank forged at high temperature was studied. It was observed that solutionising at high temperature leads to a recrystallization and a reduction of tensile properties. On the other hand, these two treatments allow the choice of the temperature at which the material is resistant to creep either at 550°C or at 650°C. A low solutionising temperature permits a high creep strength at 550°C while a much higher temperature permits a better creep strength at 650°C for all the properties; rupture life, primary strain, secondary strain rate. The following results have been obtained for yield strength measured in MPa, at different test temperatures for two temperatures of solutionising. (Table Removed ) Moreover, the following results have been obtained for creep strength at 550°C/500 MPa for different temperatures of solutionising. (Table Removed ) Example 3 - Ductility optimisation at ambient temperature : The ductility obtained at ambient temperature will be influenced by the temperature of the final heat treatment. The duration of this treatment is can be between 16 and 48 hours. It can be noted that higher the temperature of final treatment, higher is the ductility. These results have been obtained on a quaternary alloy containing molybdenum. Hence, it is possible to obtain a suitable ductility adapted for a particular use, as indicated below : (Table Removed ) Some intermetallic alloy samples whose composition belongs to the patented range, have been tested and have shown the improvements of the results obtained in comparison with the previously known alloy of standard composition Ti-22AI-25Nb. Example 4 - Effect of Molybdenum : The table below shows the yield strength for different temperatures and the effect of addition of 1% Mo on the yield strength is noted. (Table Removed ) In the second table, the advantage of the presence of molybdenum on the creep strength is shown. The materials have been subjected to the same thermomechanical treatment. This thermomechanical treatment is characterized by a forging at low temperature Tp-100 and solution treating at Tp-25°C before holding 24 hours at 900°C and an ageing at 550°C for at least 2 days. (Table Removed ) Example 5 - Effect of Silicon : Addition of silicon on the creep strength is shown, as in other cases, based on materials prepared by applying the thermomechanical treatment described in example 4. Thus, the reduction of plastic strain in primary creep and significant reduction of the secondary creep rate are shown . (Table Removed ) Example 6 - Effect of Tantalum : The castings of a standard alloy Ti-24AI-20Nb and of a modified alloy of composition Ti-24AI-20Nb-1Ta, the values being given in atomic percent, have been prepared in conformity with the invention. The cylindrical samples have been machined and heat treatments applied have been : 1160°C/30 minutes, cooling in furnace upto 750°C, then stabilized for 24 hours. Mechanical compression tests carried out have given the following results : (Table Removed ) Example 7 - Effect of Zirconium : The same operations as in example 6 for an alloy Ti-24AI-20Nb-1Zr have yielded the following results : (Table Removed ) The creep tests in compression in these two examples have also shown the advantage of elements Ta and Zr in increasing the creep strength by decreasing the amplitude of primary creep and reducing the secondary creep rate. The results are given in Fig. 10 for creep tests in compression at 650°C/310 MPa, on curve 5 for the alloy Ti-24AI-20Nb, on curve 6 for the alloy Ti-24AI-20Nb-1Ta, and curve 7 for the alloy Ti-24AI-20Nb-1Zr. The experimental results obtained show the advantages noted previously for alloys conforming to the invention. Besides, Fig.4 shows a comparison of characteristic mechanical properties of tensile strength at ambient temperature of these alloys with those of alloys currently used in aeronautics, of the type with nickel or titanium base or in the course of development such as the intermetallic alloys g-Ti'AI and these results confirm the advantage of the patented alloys. Likewise, the results compared from the creep strength of known nickel based alloys such as Inco 718 and a nickel based superalloy A conforming to EP-A-0 237/378, titanium based alloy such as IMI 834 or intermetallic alloy g-Ti-AI and an alloy conforming to the invention, are illustrated in Figs. 5 and 6 using Larson-Miller diagrams. Finally, the results obtained in mechanical tests on an alloy conforming to the invention, of composition of atomic ratios 22AI, 25Nb, 1Mo and balance Ti have been referred to in diagrams of Figs.7, 8 and 9 where the levels 1a g correspond to a heat treatment: Dissolution at 1030°C/1 hour; Ageing at 900°C/24 hours; Tempering at 550°C/48 hours; The levels 2a....g correspond to the heat treatment: Dissolution at 1030°C/1 hour, Ageing at 900"C/24 hours; The levels 3a g correspond to the heat treatment: Dissolution at 1060°C/1 hour; Ageing at 900°C/24 hours; Tempering at 550°C/48 hours; The levels 4a....g correspond to the heat treatment : Dissolution at 1030°C/1 hour, Ageing at 800°C/24 hours; Tempering at 600°C/48 hours. I CLAIM: 1. A process for the preparation of Titanium based intermetallic alloy characterised by adding in atomic ratios Al 16 to 26; Nb 18 to 28; Mo 0 to 2; Si 0 to 0.8, Ta 0 to 2; Zr 0 to 2 and balance Ti provided that Mo+Si+Zr+Ta > 0.4% wherein the intermetallic material comprises of, a) melting of alloys to form an ingot of homogeneous composition; b) deformation at high speed bringing about a reduction of the grain size; c) isothermal forging at a temperature between T ß -25°C and T ß -125°C at deformation rates in the range of 5 x l0-4/s1, d) Heat treatment by: dl) solutionising at a temperature ranging of Tß -35°C and Tß +15°C for a duration of 2 hours, d2) ageing at a temperature range of 750°C and 950°C for a duration of more than 16 hours permitting development of orthorhombic hardening phase 0, d3) treatment is carried out at the temperature of 100°C around the operating temperature determined for the material, the cooling rates between the heat treatment stages being determinded in accordance with the requirement of the alloy for a particular application by considering their effect on the lath size of the orthorhombic hardening phase 0. 2. A process as claimed in claim 1, wherein said melting is carried by vaccum arc melting process. 3. A process for the preparation of titanium based intermetallic alloys substantially as herein described and exemplified in the examples.

Documents:

2491-del-1998-abstract.pdf

2491-del-1998-claims.pdf

2491-del-1998-correspondence-others.pdf

2491-del-1998-correspondence-po.pdf

2491-del-1998-description (complete).pdf

2491-del-1998-drawings.pdf

2491-del-1998-form-1.pdf

2491-del-1998-form-19.pdf

2491-del-1998-form-2.pdf

2491-del-1998-form-3.pdf

2491-del-1998-gpa.pdf

2491-del-1998-petition-137.pdf

2491-del-1998-petition-138.pdf