Ordering and phase separation in the b.c.c. phase of the Fe-Al-Ti system

Ordering and phase separation in alpha-Fe alloys of the Fe-Al-Ti system have been investigated by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and electron-probe microanalysis (EPMA). The results indicate that the partial replacement of Fe with Ti in the Fe-rich Fe-Al binary alloys results in drastically stabilizing the B2 (FeAl) and D0(3) (Fe3Al) phases and increasing the A2-B2-D0(3) successive order-disorder transition temperatures. It is confirmed that there are two kinds of phase separations of the BCC phase, (A2 + D0(3)) and (B2 + D0(3)) in the composition range below 25 at.%Al. The width of the two-phase region drastically increases with decreasing Al composition. The two-phase regions close at ternary tricritical points. Calculations of BCC phase equilibria have been performed with the cluster variation method (CVM) using the irregular tetrahedron approximation. First and second nearest pair interactions as well as tetrahedron interactions are taken into account. Starting from binary and ternary phase equilibria the numerical values of these interactions have been determined and an excellent agreement between calculation and experiments was obtained. (C) 1998 Acta Metallurgica Inc.