Effect of thermal treatments on the martensitic transformation in Co-containing Ni–Mn–Ga alloys

The effect of the addition of Co to a Ni–Mn–Ga polycrystalline alloy has been analyzed through a comparative calorimetric study of the effect of thermal treatments on both Co-free and Co-containing alloys. The martensitic transformation (MT) and Curie temperatures of the ternary alloy show an increase as a function of the heat treatment temperature, while in the cobalt-containing alloy, a decrease and a subsequent increase of the MT temperature take place. The behavior of the ternary alloy agrees with the occurrence of an ordering process, while in the quaternary alloy, a process of elimination of defects seems to occur at the same time, slightly affecting the evolution of the transformation temperatures. This indicates that the addition of cobalt, besides increasing the martensitic and magnetic transformation temperatures, increases the quench-in defects influence on these transformations.