PHASE-EQUILIBRIA AND STABILITY OF THE BCC ALUMINIDE IN THE CO-CR-AL SYSTEM

Phase equilibria and ordering reactions in the bce aluminides of the C o-Cr-Al systems over a temperature range of 1000-1350 degrees C have b een investigated mainly by the diffusion couple method. The compositio ns of the gamma (Al: fee-Co), beta (B2: CoAl), alpha (A2: bcc-Cr) and sigma (sigma phase: D8(b)) phases in equilibrium, and the critical bou ndaries of the A2/B2 continuous ordering transition in the bce phase r egion have been determined. It is shown that both continuous and disco ntinuous A2 to B? phase ordering transitions and elliptically shaped A 2+B2 miscibility gap islands with wide composition ranges exist in the se systems in isothermal sections. The phase equilibria involving the gamma, beta, alpha and sigma phases are presented and stability of the ordered bce aluminides is discussed.