Plastic deformation behavior of two-phase intermetallic compounds based on $L1_2 (Al,Cr)_3Ti$ was investigated using compression test. $L1_2$ single phase alloys and two-phase alloys consisting of mainly $L1_2$ phase and a few or 20% second phases were selected from Al-Ti-Cr phase diagram. The two-phase alloys in this study showed higher yield strength and lower strain to failure than the $L1_2$ single phase alloy, Al-25Ti-10Cr. However, the Al-21Ti-23Cr alloy containing 20% $Cr_2Al$ as a second phase showed the best ductility among the two-phase alloys. Homogenization of arc melted ingots substantially reduced the amount of second phases but introduced extensive pore. When Cr content increased in $L1_2$ single phase alloys after the homogenization, the volume fraction of pore in the alloys decreased, and no residual pore was observed in two-phase alloys consisting of a few % $Cr_2Al$ phase. Environmental effect on the ductility of the alloys was investigated using compression test in two different atmosphere, air and vacuum. Environmental embrittlement was least significant in Al-25Ti-10Cr alloy consisting of $L1_2$ single phase among the alloys tested in this study. However, Al-21Ti-23Cr alloy consisting of 20% $Cr_2Al$ showed more desirable ingot cast structure than that of Al-25Ti-10Cr alloy. In addition, the yield strength of Al-21Ti-23Cr is about two times higher than that of Al-25Ti-10Cr over the entire temperature range tested. Based on the combined estimation of the pore formation, environmental embrittlement and ingot cast structure, Al-21Ti-23Cr alloy consisting of 20% $Cr_2Al$ as a second phase is expected to show the best tensile elongation behavior.
The effect of second phase morphology on deformation behavior of the two-phase alloy was investigated using compression tests for the directionally solidified Al-21Ti-23Cr alloys with three different orientations. The yield strength and the strain to failure of the directionally solidified Al-21Ti-23Cr alloy show the second phase orientation dependence. In addition, Al-25Ti-10Cr and Al-21Ti-23Cr alloys were prepared using three different methods, homogenization, HIP and directional solidification, and then tensile deformation behavior of the alloys was investigated using tensile test at R. T. and 1073K. Both of the alloys failed prior to yielding at R. T. but were deformed plastically to about 3% at 1073K. Al-21Ti-23Cr alloy, however, showed higher yield strength and better tensile ductility than Al-25Ti-10Cr alloy at 1073K. This is attributed to the vanishment of the pore due to the precipitation of $Cr_2Al$ phase. Based on the results obtained, it is suggested that the deformation behavior of two-phase alloys could be changed by the second phase alignment, which shows the possibility of improving the strength and ductility of $L1_2(Al,Cr)_3Ti$-based two-phase intermetallic compounds through controlling the second phase morphology.
Effects of V and Zr additions on the microstructures and the mechanical properties of two-phase intermetallic compounds consisting of $L1_2$ matrix and 20% $Cr_2Al$ were investigated. Among the V-added two-phase intermetallic compounds, Al-21Ti-20Cr-3V showed the best ductility as well as relatively high yield strength. V was found to substitute for Cr and improve the ductility of $Cr_2Al$ phase. In the case of Zr-added two-phase alloys, the particle size of $Cr_2Al$ was refined, and Zr was found to substitute for Ti of $L1_2$ matrix. As Zr content increased in the Zr-added two-phase alloys, the yield strength increased, but the ductility decreased. It was found in Zr-added two-phase alloys that the loss of ductility in $L1_2$ matrix due to Zr additions was more significant than the ductilizing effect due to second phase refinement. Zr-added two-phase alloys, however, showed higher yield strength than V-added two-phase alloys over the entire temperature range tested. Based on results obtained, plastic deformation behavior of two-phase intermetallic compounds alloyed with V and Zr were examined. Al-19Ti-21Cr-2V-2Zr showed the best mechanical properties among the alloys tested, which is suggested to be the alloy composition to optimize the effects of V and Zr additions simultaneously.
Ti-48Al specimens were coated with Al-21Ti-23Cr film by RF magnetron sputtering. Ti-48Al specimen coated at 200W, 0.8Pa and 573K showed the best oxidation-resistant property in the isothermal oxidation test. Al-21Ti-23Cr film was amorphous after deposition, but crystallized and formed a protective Al_2O_3 layer on the surface during oxidation. The oxidation behavior of the coated specimens has been investigated through isothermal oxidation tests at 1073K, 1173K and 1273K for 100h and cyclic oxidation test at 1273K for 100 cycles. The isothermal oxidation curves showed that the Al-21Ti-23Cr film was very effective in decreasing the oxidation rate of Ti-48Al, and the excellent oxidation resistance is attributable to the formation of a protective $Al_2O_3$ layer on the surface of the Al-21Ti-23Cr film. The protective $Al_2O_3$ scale formed on the surface of the Al-21Ti-23Cr film has been shown to be relatively stable during a cyclic oxidation test at 1273K for 100 cycles although thermal stress is induced by temperature changes. After the oxidation test at 1273K, the matrix of the Al-21Ti-23Cr film had transformed into the TiAlCr phase due to the depletion of Al during oxidation and the diffusion of Ti from the substrate, and the extent of mass gain of the specimen increased compared with that of specimens tested at lower temperatures. The tensile deformation properties of the coated specimens were also investigated before and after oxidation. It was found that the protective $Al_2O_3$ layer on the surface of the Al-21Ti-23Cr film enabled the Ti-48Al to maintain its tensile properties in an oxidizing environment.
