Martensitic and magnetic transition of Ni₅₄A1₂₅Fe_21-xCo_x alloy

Ni-Al-Fe alloy with a β phase crystal structure is a conventional shape memory alloy. The alloy exhibits ferromagnetic characteristics with the increasing of Fe element concentration and thus it is a promising ferromagnetic shape memory alloy. The influence of cobalt concentration x on martensitic transformation temperature and Curie point of Ni₅₄A1₂₅ Fe_21-xCo_x alloys were investigated by optical micrography, SEM, DSC and VSM and EDX analysis. It is found that the martensitic transformation temperatures of Ni₅₄A1₂₅ Fe_21-xCo_x alloys are linearly increased with the increasing of Co concentration x.1 % Co can increase the martensitic transition temperature by 30 K for the alloys quenched from 1623 K and by 38 K for the alloys quenched from 1373 K. Martensitic transformation temperature and Curie point decrease quickly for the alloys quenched at lower temperature. With the increasing of Co concentration x, the influence of quenching temperature on martensitic transformation temperature and Curie point is weakened. No significant influence of Co concentration on Curie point is observed. A T_c peak exists on T_c-x curves. For example, the maximum value of T_c is 256 K for Ni₅₄A1₂₅ Fe₁₉Co₂ alloy quenched from 1623 K. Martensitic transformation temperatures of the alloys are discussed by average s+d electron concentration (e/a), meanwhile, their Curie points are explained by the combined effects of Al content and crystal structure ordered degree of β phase at first time.