Sequence of martensitic transformations in Ni-Mn-Ga alloys

The martensitic transformation sequence in two single crystalline Ni-Mn-Ga alloys with a martensite start temperature (M_s) of about 400 K has been investigated using dynamic mechanical analysis, calorimetry, dilatometry, three-point bending tests, and transmission electron microscopy. A two-step thermally induced martensitic transformation during cooling and one-step reverse transformation during heating have been found in both alloys, although involving different modulated martensitic phases in each of the alloys. In one of them, the first martensitic transformation leads to a martensitic phase having a commensurate modulated lattice characterized by a periodic stacking sequence of ten {110}_P planes along the 〈110〉_P direction. On further cooling, a second transformation to a modulated martensite with a periodicity of seven planes has been observed. In the other alloy, the transformation sequence observed on cooling is from the parent phase to an eight-layered modulated martensite followed by a second transformation to a martensite having a tetragonal lattice (c/a>1) without modulation.