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 to B2 phase ordering transitions and elliptically shaped A2+B2 miscibility gap islands 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.