Phase stability of the X2AlTi (X: Fe, Co, Ni and Cu) Heusler and B2-type intermetallic compounds

Ordering and phase separation between the B2 and L2 1 phases in the X 2 AlTi (X: Fe, Co, Ni, Cu) intermetallic compounds were investigated. The B2/L2 1 continuous ordering was determined using the diffusion couple technique in the temperature range of 1273-1573 K. It was found that the substitution of Co for Fe results in raising the B2/L2 1 ordering temperature and that of Cu for Ni brings about widening of the B2+L2 1 two-phase region on both the NiAl and NiTi sides. It is shown that the maximum temperatures of B2/L2 1 order-disorder transition (Tc B2/L2 1 max ), the tricritical temperatures (Tt B2/L2 ) and the phase boundaries of the miscibility gap between the B2 and L2 1 phases in the XAl-XTi pseudobinary systems can be well described by the 3d+4s electron concentration of the elements occupying the X site. On the basis of this finding, the phase stability and the interchange energies between Al and Ti in the next nearest neighbors of X-Al-Ti (X: Ti, V, Cr, Mn, Fe, Co, Ni and Cu) system are discussed.