Development of Light Weight-High Temperature Structural L1_2 Compounds in Al_3Ti-Base Alloys.

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The intermetallic compound Al_3Ti is very attractive as a potential high-temperature structural material. Some of the positive characteristics of this compound are its good oxidation resistance, low density of 3.3 g cm^<-3>, and relatively high melting point of about 1673 K.However, it has the low-symmetry tetragonal D0_<22> structure and is brittle at room temperature. Successful development of this aluminide depends on whether we can overcome its brittleness at ambient temperature. One possible approach to ductilize ordered intermetallics with low-symmetry crystal structures is to change their structures to those of higher symmetry. Indeed, it is known that replacing a certain amount of aluminum atoms in Al_3Ti with Zn, Ni, Cu, or Fe changes the structure of Al_3Ti(D0_<22>) to the L1_2 structure. The authors found the formation of new ternary L1_2 compounds in Al_3Ti-base alloys containing Mn, Cr, Ag, Rh, Pt, and Au. Many studies on the Al-Ti-X (X=Ni, Fe, Cu, Mn, Cr, Ag and Pd etc.) … More alloys have focused on the mechanical properties of the ternary L1_2 trialuminide compounds. However, the ternary L1_2 compounds are still brittle in tension and/or bending, although they exhibit appreciable compressive ductility at ambient temperature.Recently, much work has been made of the microstructure, especially second phases and porosity generally induced by a homogenization treatment, and microstructure-mechanical property relationships of the ternary L1_2 trialuminide compounds. The present authors more recently reported also the effects of manganese on the microstructure and mechanical properties of ternary L1_2 intermetallic compounds in the Al_3Ti-based alloys, and that the titanium trialuminide containing Mn does posses some bend ductility at room temperature when porosity is reduced.In the present study, sintering of elemental powders was carried out to form ternary L1_2 intermetallic compounds in Al-Ti-Cr alloys. The L1_2 phase field and equilibrium phases surrounding the L1_2 phase at 1423K have been established. Besides the L1_2 phase, the pertinent second phase fields have been also estimated ; TiAl, TiAl_2, Ti_2Al_5, Al_3Ti, Al_<17>Cr_9, AlCr_2, and TiAlCr phase fields. The vertical section of the Al-25mol%Ti-Cr alloys has been experimentally determined, and the presence of the L1_2+ Liquid phase field in the present composition and temperature ranges was confirmed. Button ingots of some ternary L1_2 compounds in the Al-Ti-Cr alloys were prepared by arc-melting to investigate their microstructure (especially second phases and porosity), microhardness, and bend ductility. In general, homogenization of as arc-melted alloys substantially reduced the amount of second phases but introduced extensive porosity. Therefore, the formation of porosity is closely associated with second phase solutionized during homogenization treatment. However, no residual porosity was observed in higher Cr content alloys. The porosity after homogenization was observed when Al_<17>Cr_9 phase is formed and not when TiAlCr or AlCr_2 phase is formed. The ternary L1_2 compounds in the Al-Ti-Cr alloys had some bend ductility at ambient temperature, which can be increased to about 0.9% (plastic bend strain) by increasing the Cr content when porosity is reduced. Less