Origin of the phase separation into B2 and L21 ordered phases in X–Al–Ti (X: Fe, Co, and Ni) alloys based on the first-principles cluster variation method

Abstract

The phase separation behaviors from a single B2 ordered phase into two separate B2 and L21 ordered phases in X–Al–Ti (X: Fe, Co, and Ni) alloys are analyzed utilizing the cluster variation method (CVM), based on interaction energies derived from electronic band structure calculations. A cubic approximation of the CVM is adopted for X2Al2−xTix () alloys limiting the interchange between Al and Ti atoms on the - and -sublattices of an L21 ordered structure with X atoms fixed on the -sublattice. The phase stabilities of the B2 and L21 structures are examined, and phase diagrams at the pseudo-binary section, XAl–XTi, are determined. The two-phase regions of the B2 and L21 phases (i.e. phase separation behavior) are successfully produced in Co– and Ni–Al–Ti alloy systems, and no phase separation is observed in the Fe–Al–Ti alloy. The origins of phase separation in the Co– and Ni–Al–Ti alloys are mechanical instability and a combination of mechanical instability and chemical repulsions of unlike pairs, respectively.