Phase equilibria and stability of B2 and L21 ordered phases in the vicinity of half-metallic composition of Co-Cr-Ga Heusler alloy system

Recently, half-metallic properties have been extensively investigated from theoretical and practical viewpoints. In the present study, the phase equilibria, A2/B2 and B2/L21 order-disorder transition, and ferromagnetic/paramagnetic transition in the Co-rich portion, that is, in the vicinity of half-metallic composition, for Co-Cr-Ga system were examined by electron probe microanalysis (EPMA), differential scanning calorimetric (DSC) measurement, and transmission electron microscope (TEM). The compositions of the γ (A1; fcc-Co), α (A2; bcc-Cr), β (B2; CoGa), ɛ (A3; hcp-Co), and σ (D8b) phases in equilibrium, the critical compositions corresponding to the A2/B2 continuous order-disorder transition, and the critical temperatures of the B2/L21 order-disorder transition and the paramagnetic/ferromagnetic transition in the L21 phase region have been determined. It has been confirmed that the β single-phase region at 1000 °C exists in a wide composition range, and the B2/L21 order-disorder transition temperature is higher than that of the Co-Cr-Al system. Furthermore, it has been disclosed that the phase stability of the CoGa B2 phase, estimated from the Bragg-Williams-Gorsky (BWG) calculation as a linear extrapolation of the continuous order-disorder transition temperatures, is lower than that of CoAl.