Phase transformation and microstructures in Ni-Mn-Ga ferromagnetic shape memory alloys

Composition dependence of martensitic transformation and magnetic transformation has been studied on Ni-Mn-Ga ferromagnetic shape memory alloys. A good correlation was found between the transformation temperatures and valence electron density (e/a), Martensitic transformation temperature increases rapidly with e/a, while the Curie temperature exhibits only a mild dependence. Crystal structure of martensite also changes with e/a. At a lower e/a (near the stoichiometric Heusler composition) the martensite was bct with the modulation of five layers period of {110}(L21) planes; at a higher e/a, it was fct without the modulation, Transformations from the parent phase to an intermediate phase and then to martensite phase were observed in situ using a cold stage for Ni-23.0Mn-25.6Ga. The sample exhibits so-called "tweed" structures in the parent phase. The tweed contrast becomes more pronounced as the temperature approaches M,. This is accompanied by increasing intensity of diffuse streaks along <110> directions. At 230 K the satellite spots appeared at 1/6<220> positions, corresponding to the intermediate phase. On further cooling martensite plate formed and the structure changed continuously to the modulated bct.