Ordering and phase separation in bcc aluminides of the Ni-Fe-Al-Ti system

The phase equilibria and ordering reactions in the b.c.c. aluminides of the Ni-Al-Ti-Fe system at 800-1300 degrees C have been investigated mainly by the diffusion couple method. The compositions of the beta (B2: (Ni,Fe)(Al,Ti)) and H (L2,: (Ni,Fe)(2)AlTi) phases in equilibrium, the critical boundaries of the continuous ordering transition and the tricritical points have been determined. It is shown that the miscibility gap boundary between the beta and H phases, tie line configurations and the beta-H second order reaction can all be depicted inside a composition square whose corners are made up of NiAl-NiTi-FeAl-FeTi, respectively, i.e. (Ni,Fe)-50 at.% (Al,TI). The stability region of the Heusler phase and the beta + H immiscibility gap in the NiAl-NiTi pseudo-binary system are narrowed down by increasing the Fe content. The phase stability of these ordered b.c.c. aluminides is presented and discussed. (C) 1997 Elsevier Science S.A.