Phase equilibria and stability of B2 and L2 1 ordered phases in the vicinity of half-metallic composition of Co–Cr–Ga Heusler alloy system

Abstract

Recently, half-metallic properties have been extensively investigated from theoretical and practical viewpoints. In the present study, the phase equilibria, A2/B2 and B2/L2 1 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 (D8 b) phases in equilibrium, the critical compositions corresponding to the A2/B2 continuous order–disorder transition, and the critical temperatures of the B2/L2 1 order–disorder transition and the paramagnetic/ferromagnetic transition in the L2 1 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/L2 1 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.