Phase equilibria and stability of the BCC aluminide in the Co-Cr-Al system

Phase equilibria and ordering reactions in the bcc aluminides of the Co-Cr-Al systems over a temperature range of 1000-1350°C have been investigated mainly by the diffusion couple method. The compositions of the γ (Al: fcc-Co), β (B2: CoAl), α (A2: bcc-Cr) and σ (sigma phase: D8_b) phases in equilibrium, and the critical boundaries of the A2/B2 continuous ordering transition in the bcc phase region have been determined. It is shown that both continuous and discontinuous A2 10 B2 phase ordering transitions and elliptic-ally shaped A2+B2 miscibiilly gap isiands with wide composition ranges exist in these systems in isothermal sections. The phase equilibria involving the γ, β, α and σ phases are presented and stability of the ordered bcc aluminides is discussed.