Intermetallic TiAl-Ti3Al composite materials

Abstract: A TiAl-Ti₃Al composite material having improved mechanical strength and ductility in which Al is contained in the range of 33 to 35% by weight percentage, the balance is substantially Ti, and TiAl-intermetallic compound contains Ti₃Al-intermetallic compound in the range of 10 to 35% by volume percentage.

Description: This invention Relates to a TiAl -Ti₃Al composite material for use as a raw material of parts required to have excellent heat resistance and toughness, for example, parts to move rotationally such as a gas turbine, a turbine blade of a jet engine and a turbocharger rotor, and parts to move reciprocally such as an engine valve, a piston and a valve rocker arm. Heretofore, in various parts to move rotationally or reciprocally mentioned above, Ni based heat-resisting alloys have been used for the turbine blade, the rotor and so on, heat-resisting steels have been used for the engine valve, casting aluminum alloys have been used for the piston, and cast irons have been often used for the valve rocker arm. Meanwhile, titanium and titanium alloys (for example, Ti -6Al -4V) are used as lightweight materials superior in strength in the field of aircraft industry Furthermore, TiAl -intermetallic compound based materials have attracted attention as lightweight materials having improved heat resistance. Said TiAl -intermetallic compound based materials are light and excellent in heat resistance, however they do not get to the stage of practical application even now because of the poor toughness (ductility) at room temperature. US-A-3,203,794 discloses binary Ti-(34-46%) Al alloys which exhibit a single phase, face-centred, tetragonal microstructure. Accordingly, the addition of the third element and the dispersion of Ti₃Al -intermetallic compound into TiAl -intermetallic compound has been attempted in order to improve the ductility of the TiAl -intermetallic compound based materials at room temperature. For example, in Japanese Patent Disclosure No.56-41344, Ti-Al alloy having duplex phase structure is disclosed and it is described that the main phase is γ -phase (TiAl) and globular α ₂-phase (Ti₃Al) is contained a little in said γ -phase. And in Japanese Patent Disclosure No. 61-41740, equivalent to US-A-4,661,316, an intermetallic compound TiAl-based heat resisting alloy is disclosed, which is added with Mn into TiAl -intermetallic compound based alloy. However, concerning the duplex Ti-Al alloy dispersed with Ti₃Al - intermetallic compound in TiAl -intermetallic compound in said Japanese Patent Disclosure No.56-41344, there is only description that the main phase is γ -phase (TiAl ) and the γ -phase contains a little and globular α ₂-phase (Ti₃Al), but the optimum amount of Ti₃Al - intermetallic compound in TiAl -intermetallic compound does not become clear. Therefore, there is a problem in that sufficient clarification is not yet given about the improvement of the ductility at room temperature by dispersing Ti₃Al - intermetallic compound. The present invention is made in view of the above mentioned problem of the prior art, it is an aim to provide TiAl -Ti₃Al composite material having improved ductility at room temperature which has the excellent light weight and heat-resisting characteristics of TiAl -intermetallic compound, furthermore by using Ti₃Al intermetallic compound. The inventors have found out that the ductility is improved remarkably (the elongation of alloy at room temperature gets to 1.5% or more) in an alloy containing Ti₃Al -intermetallic compound in a specified range in TiAl - intermetallic compound. This was determined by investigating the amount of Ti₃Al -intermetallic compound and the strength in detail by X-ray diffraction and the tensile testing of ingots melted so as to contain aluminum in the range of 33.2 to 34.9% wt% in titanium. That is,the TiAl -Ti₃Al composite material according to this invention has the construction that Al is contained in the range of 33.2 to 34.9% by weight percentage, the balance being Ti and unavoidable impurities, and wherein the TiAl-intermetallic compound contains 11.7 to 31% of fine lamellar Ti₃Al-intermetallic compound by volume percentage. And the TiAl - Ti₃Al composite material according to one of the preferred aspects of this invention has the construction that Al is contained from 33.2% to less than 34% by weight percentage, the balance being Ti and unavoidable impurities and wherein the TiAl-intermetallic compound contains from 15 to 31% of fine lamellar Ti₃Al-intermetallic compound by volume percentage. It is characterized by adopting such a construction of TiAl -Ti₃Al composite material as a means for solving the aforementioned problem. In the TiAl -Ti₃Al composite material according to this invention, Al is contained in the range of 33.2 to 34.9% by weight percentage, the balance is Ti and unavoidable impurities, and the TiAl-intermetallic compound contains 11.7 to 31% of fine lamellar Ti₃Al -intermetallic compound by volume percentage. Tensile ductility of TiAl -intermetallic compound is at its highest when Ti₃Al -intermetallic compound is contained in the range of 11.7 to 31%, preferably 15 to 31%, by volume percentage in TiAl -intermetallic compound at a state in which Ti₃ Al-intermetallic compound is distributed finely in lamellar form. And it has been found that it is nesessary to contain Al in the range of 33.2 to 34.9%, preferably from 33.2% to less than 34% by weight percentage in order to contain Ti₃Al - intermetallic compound in the range of 11.7 to 31%, preferably 15 to 31%, by volume percentage and to precipitate it finely into lamellar form. Furthermore, it has also become clear that the most suitable condition is obtained in case of containing 33.5% of Al . And, when Al content is less than 33.2% by weight percentage, the amount of Ti₃Al-intermetallic compound increases and tensile ductility deteriorates, and when Al content exceeds 34.9% by weight percentage , it is undesirable because the amount of Ti₃Al-intermetallic compound decreases conversely. Therefore, Al content is defined in the range of 33.2 to 34.9% by weight percentage so that Ti₃Al-intermetallic compound in the range of 11.7 to 31% by volume percentage in order to obtain excellent tensile ductility. Said Ti₃Al-intermetallic compound in TiAl -inter metallic compound is variable by not only the composition but also by heat treatment at the time of manufacturing. Namely, if TiAl -intermetallic compound solidified in a non-equilibrium state at the time of manufacturing is maintained at high temperature (more than 1100°C) for a long time, Ti₃Al -intermatallic compound dissolves in TiAl -intermetallic compound, the amount of Ti₃Al-intermetallic compound decreases, and Ti₃Al-intermetallic compound coheres and becomes gross, so that the deterioration of ductility is induced. Accordingly, it is undesirable to perform the heat treatment for a long time at high tempetature in excess of 1100°C. In the case of a heat treatment at 1150°C, for example, it has been found that it is desirable not to exceed 8 hours or so. It is possible to obtain the fine lamellar phase of Ti₃Al -intermetallic compound in the range of 11.7 to 31% by volume percentage in TiAl -intermetallic compound by casting. It is also possible to obtain it by powder metallurgy, which is a method of processing the sintered mixture of TiAl -intermetallic compound powder and Ti₃Al -intermetallic compound powder. The TiAl -Ti₃Al composite material according to this invention contains 33.2 to 34.9% of Al by weight percentage and the balance is Ti and unavoidable impurities, wherein the TiAl-intermetallic compound contains 11.7 to 31% of Ti₃Al-intermetallic compound by volume percentage, so Ti₃Al-intermetallic compound is dispersed in proper quantity in TiAl -intermetallic compound. Consequently, said Ti₃Al -intermetallic compound dispersed in proper quantity in TiAl -intermetallic compound produces a remarkable improvement in toughness (especially ductility) of TiAl-intermetallic compound which is lightweight and excellent in heat resistance. The present invention will now be described in more detail by way of the following non-limiting Examples. EXAMPLES Using sponge titanium and high purity granulated aluminum as raw materials, Ti-Al alloys having chemical composition shown in Table 1 as Nos.1 to 10 were melted in an atmosphere of Ar using a plasma skull crucible furnace, and then cast into ingots of approximately 5 kg in weight. Next tensile test specimens were cut from respective ingots, and 0,2% proof stress, tensile strength, elongation and reduction of area were measured by tensile test without heat treatment (as cast) except alloy No.9. And the amount of Ti₃Al-intermetallic compound in TiAl-intermetallic compound was investigated by X-ray diffraction. The specimen of alloy No.9 having the same chemical composition as that of alloy No.3 was subjected to heat treatment at 1150°C for 24 hours and then air cooled with the intention of reducing the amount of Ti₃Al - intermetallic compound. These results are also shown in Table 1. As is apparent from the results shown in Table 1, in example alloys Nos.1 to 5 according to the TiAl -Ti₃Al composite material of this invention in which Al is contained in the range of 33.2 to 34.9% by weight percentage and Ti₃Al intermetallic compound is contained in TiAl-intermetallic compound in the range of 11.7 to 31% by volume percentage, it is evident that the mechanical strength is high and the elongation is more than 1.5%, and so, it has been recognized that they are excellent in toughness. And since the mechanical strength has a tendency to drop in the alloy Nos. 4 and 5 containing Al more than 34% by weight percentage, in this regard it is more preferable to define Al content as less than 34% by weight percentage. And the mechanical strength and the the elongation have a tendency to drop in the alloy No.5 containing Ti₃Al - intermetallic compound less than 15% by volume percentage in TiAl -intermetallic compound, in this regard it has been found that it is more preferable to define the lower limit of the amount of Ti₃Al -intermetallic compound in TiAl -intermetallic compound as 15% by volume percentage. On the contrary, comparative alloy No.6 short of Al content, which is less than 33.2% by weight percentage, is remarkably inferior in the ductility due to Ti₃Al -intermetallic compound being in excess. Comparative alloy No. 7 having an excess of Al, that is more than 34.9% by weight percentage, is considerably inferior in ductility similar to example No. 6 due to the lack of Ti₃Al -intermetallic compound. And it has been found that comparative alloy No.8 containing more Al than that of alloy No.7 has inferior mechanical strength and ductility because Ti₃Al -intermetallic compound content is lower. In comparative alloy No.9 containing Al in proper quantity and subjected to heat treatment at high temperature over a long time, it is impossible to obtain an elongation of more than 1,5% because Ti₃Al-intermetallic compound content decreases by the heat treatment at high temperature over a long time. Furthermore, in comparative alloy No.10 containing V as the third element, it is impossible to obtain an elongation of more than 1.5% due to Al content exceeding 34.9% by weight percentage, Nevertheless, the mechanical strength is comparatively high. As described above, the TiAl -Ti₃Al composite material according to this invention contains 33.2 to 34.9% of Al by weight percentage and the balance is Ti and unavoidable impurities, wherein the TiAl-intermetallic compound contains 11.7 to 31% of Ti₃Al - intermetallic compound by volume percentage. Therefore, it is lightweight and excellent in heat resistance, more over excellent in mechanical strength and elongation. And it is a composite material suitable for a raw material of rotational parts required to have heat resistance and toughness such as a turbine blade and a turbocharger rotor. An excellent effect is obtained in that it results in a weight reduction. Furthermore, it is a composite material suitable for a raw material of reciprocating parts such as an engine valve, a position, and a valve rocker arm. Another excellent effect obtained is that it contributes to an increase in engine speed by the reduction of inertial mass.

Claims: A TiAl-Ti₃Al composite material comprising from 33.2 to 34.9 wt% of Al and a balance of Ti and unavoidable impurities, wherein Ti₃Al-intermetallic compound is contained in the range of from 11.7 to 31 vol.% in the form of fine lamellar phase in TiAl-intermetallic compound. A TiAl-Ti₃Al composite material according to claim 1, wherein said composite material contains from 33.2 to less than 34 wt% of Al. A TiAl-Ti₃Al composite material according to claim 1, wherein said composite material contains 33.5 wt% of Al. A TiAl-Ti₃Al composite material according to any foregoing claim wherein said Ti₃Al-intermetallic compound is contained in the range of from 15 to 31 vol.% in the form of fine lamellar phase in TiAl-intermetallic compound.

Inventors: IIKUBO TOMOHITO NO S- OF ESPOI, ISOBE SUSUMU NO OF LIONS-MANSI, NODA TOSHIHARU, IIKUBO, TOMOHITO NO. S-302 OF ESPOIR ATSUTA, ISOBE, SUSUMU NO. 508 OF LIONS-MANSION, NODA, TOSHIHARU
Applicants: DAIDO STEEL CO LTD, DAIDO TOKUSHUKO KABUSHIKI KAISHA
Application year: 1989
Main classification: C22C14
IPC codes: C22C14