Ordering and phase separation in b.c.c. 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°C have been investigated mainly by the diffusion couple method. The compositions of the β (B2: (Ni,Fe)(Al,Ti)) and H (L21: (Ni,Fe)2AlTi) 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 β and H phases, tie line configurations and the β–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 β+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.