Ordering and phase separation of BCC aluminides in (Ni, Co)–Al–Ti system

The phase equilibria and ordering reactions in the bcc aluminides of the Co–Al–Ti ternary and (Ni, Co)–Al–Ti quaternary systems were investigated mainly using the diffusion couple method. The phase boundaries between the (Ni, Co)Al (B2) and (Ni, Co)2AlTi (L21) phases, the critical compositions of the continuous ordering transition and the tricritical points were determined in the temperature range 1373–1573 K. It was found that the B2+L21 two-phase region in the composition square of NiAl–Ni2AlTi–Co2AlTi–CoAl narrows and then disappears due to increasing Co content or temperature, and that the maximum temperatures of the B2/L21 ordering reaction are lowered with the addition of Co. The phase stability of these aluminides is discussed and compared with those of the (Ni, Fe)–Al–Ti system which have been previously reported.