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

Ordering and phase separation between the B2 and L21 phases in the X2AlTi (X: Fe, Co, Ni, Cu) intermetallic compounds were investigated. The B2/L21 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/L21 ordering temperature and that of Cu for Ni brings about widening of the B2+L21 two-phase region on both the NiAl and NiTi sides. It is shown that the maximum temperatures of B2/L21 order–disorder transition (TcB2/L21max), the tricritical temperatures (TtB2/L21) and the phase boundaries of the miscibility gap between the B2 and L21 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.