Phase stability of L12-based alloys in Al-Ti-Cr systems

Compressive tests on Al-21Ti-23Cr (L12+Cr2Al), Al-21Ti-15Cr (L12+Al17Cr9) and Al-30Ti-15Cr (L12+TiAl+TiAlCr) alloys were performed to investigate the effects of alloy compositions on the phase stability of L12-based alloys at temperatures below 1000 °C. In Al-30Ti-15Cr alloy, yield strength increased significantly after exposure at 800 °C as compared to the other two alloys. Microstructure observation and phase identification confirmed that the Al-30Ti-15Cr alloy with Ti-rich composition exhibited complete decomposition of the L12 phase in contrast to partial decomposition of the L12 phase in Al-21Ti-23Cr and Al-21Ti-15Cr alloys under the same conditions. This results from the fact that the L12 phase field is shifted in the direction of the Al-rich corner with decreasing temperature in the Al-Ti-Cr phase diagram. The fracture toughness of Al-30Ti-15Cr alloy from three-point bend tests was more degraded after exposure at 800 °C than the other two alloys because the Al-30Ti-15Cr alloy exhibited poor phase stability due to its higher Ti content. Therefore, L12-based alloys with higher Al-rich composition are more appropriate as coating materials when considering phase stability coupled with fracture toughness.