An experimental study of phase transformations and a comparison with calculated phase equilibria in Ti-Al-Mn alloys

The solid-state phase transformations and phase equilibria in Ti-Al-Mn alloys containing 5 and 10 at.% Mn (with a TI:Al atomic ratio of 1.14) have been investigated both experimentally and through CALPHAD methods. The alloys, prepared by plasma are melting, were studied by differential thermal analysis (DTA) and microstructural characterization of high temperature annealed and water quenched (WQ) samples. On heating, DTA endotherms were observed to occur at temperatures consistent with the microstructural changes identified in annealed and WQ specimens. The 5% Mn alloy is found to transform from (beta + alpha + gamma) to (beta + alpha) on heating from 1145 to 1330 degrees C. The 10% Mn alloy undergoes the following sequence of phase changes on heating from 1090 to 1330 degrees C: (beta + gamma + (MnAl)(2)Ti) --> (beta + gamma) --> (beta + alpha + gamma) --> (beta + alpha) --> beta. Both the alpha and beta phases decompose to a variety of products on WQ. A comparison between calculated and predicted equilibrium phase compositions shows good agreement for the (alpha + beta + gamma) tie triangle over a range of temperatures, but the calculated two-phase (alpha + beta) region is somewhat displaced from the high temperature experimental phase compositions. (C) 1998 Acta Metallurgica Inc.