Influence of Ga on martensitic and magnetic transitions in Co-Ni-Al β-based ferromagnetic shape memory alloys

The influences of gallium concentration on martensitic transformation temperature and Curie point of Co₄₁Ni₃₂Al_27-xCa_x alloys were investigated by optical micrography, scanning electron microscopy, differential scanning calorimetry and vibrating sample magnetometer and energy dispersive X-ray detector. The results show that the martensitic transformation temperatures of Co₄₁Ni₃₂Al_27-xGa_x alloys are linearly increased with Ga content. An increase of 1% in Ga concentration can cause the martensitic transformation temperature to be increased by 25 K for alloys quenched at 1623 K or at 1573 K, respectively. The influence on Curie points of Co₄₁Ni₃₁-Al_27-xGa_x alloys is very small. However, the order degree of phase structure has great effects on Curie point. The Curie point of high ordered martensitic phase is higher than that of B2 structure phase by 32 K. Martensitic transformation temperatures increase by 43-69 K and Curie points increase by 41 K when the quenched temperatures elevate from 1573 to 1623 K. The melting points of Co₄₁Ni₃₂Al_27-xGa_x alloys decrease with the increase of Ga concentration, resulting in the partial melted phenomena of Co₄₁Ni₃₂Al₁₈Ga₇ alloy quenched at 1623 K.