Effect of atomic ordering on the phase transformations in Ni-Mn-Ga shape memory alloys

The influence of the L21 order degree in single crystalline Ni-Mn-Ga alloys is analysed in the present work. The alloys studied display a sequence of martensitic (MT) and intermartensitic (IMT) transformations on cooling, the reverse transformation to austenite, on heating, taking place in a single step. Quenching from different temperatures produces distinct effects on the MT and IMT: while the MT temperatures are not practically affected by the performed heat treatments, the IMT shifts towards lower temperatures after quenching from increasing temperatures. Such evolution can be related to changes in the L21 order degree, in the sense that ordering favours the occurrence of the IMT while it hardly affects the MT temperatures. The closeness of the free energies of the different martensitic structures and the differences between the MT and IMT entropy changes allow this behaviour to be explained. In turn, the Curie temperature increases with the L21 ordering for an alloy undergoing magnetic transition in martensite phase, no changes being detected if the Curie temperature is above the martensitic transformation