TEM investigation of intermediate phase transformation and micromodulation in Ni–Mn–Ga ferromagnetic shape memory alloys

Abstract

Intermediate phase transformation, martensitic transformations and modulated structures in Ni–Mn–Ga ferromagnetic shape memory alloys were investigated by TEM. The transformation from high temperature L21 phase to intermediate phase was characterised by the appearance of diffuse satellite spots at 0˙167q220 positions in electron diffraction patterns and intense fine striations in bright field images. These results are consistent with the reported results of neutron diffraction measurements related to the phonon anomaly. Also, high resolution TEM observations and fast Fourier transformation analysis on the near intermediate phase state in Ni–Mn–Ga–Gd were conducted. It was revealed that the coarse tweed contrast was associated with a long wavelength strain field while the fine striations are associated with the satellite spots and domains of micromodulation aligned parallel to {110}.