Thermal and microstructural evolution under ageing of several high-temperature Ni-Mn-Ga alloys

Alloys with a high content of Ni exhibit a relatively poor thermal stability, caused either by chemical decomposition or by fast precipitation of Ni-rich phases, which can promote loosing most of the transformation even after few cycles in the DSC. Two types of precipitates have been observed: lath-like precipitates with compositions not very far from the ones of the matrix and ellipsoidal L12 (γ′) precipitates with chemical composition rather different to the matrix. On the other hand, Mn-rich alloys with Ni close to 50 at% do not show precipitates even after 7 × 107 s (27 months) at 770 K then keeping the martensitic transformation similar to non-aged samples. Therefore, the thermal stability of some compositions of the Ni-Mn-Ga system is comparable to the most stable high-temperature shape memory alloys. Some features of the thermal and microstructural evolution under ageing as well as strategies to optimize these alloys are also discussed in terms of the Ni-Mn-Ga equilibrium phase diagram.