Martensitic transformation and microstructure of polycrystalline Ni–Mn–Ga and C-doped Ni–Mn–Ga Heusler alloys

The martensitic transformations of Ni–21·7Mn–23·8Ga (at.-%) (NiMnGa) and Ni–19·4Mn–22·7Ga–1·6C (at.-%) (NiMnGaC) alloys were investigated by the measurement of resistivity. Two kinds of martensitic transformations occur in NiMnGa alloy. The first martensitic transformation is thermoelastic, which exhibits a steep increasing in resistivity. The second transformation exhibits a larger thermal hysteresis compared with the first transformation. NiMnGaC alloy only shows a single martensitic transformation and the C addition increases the first martensitic transformation temperatures. The first martensitic phase of NiMnGa alloy is of five layered structure while the martensitic phase of NiMnGaC alloy is of non-modulated structure. Combined with the observation of optical microscopy and TEM, NiMnGa alloy exhibits much wider martensite twins than NiMnGaC alloy does.