Electron holography studies on the pattem formation preceding to displacive phase transformations

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The aim of this research project is to reveal the mechanism of the macroscopic pattern formation that occurs before the onset of martensitic phase transformations in some class of ferromagnetic shape memory alloys. This macroscopic pattern formation was discovered by the head investigator recently, while the mechanism remained unsolved.Detailed electron holography studied were carried out in order to reveal the temperature dependence of magnetic flux distribution in the parent phase of Ni-Fe-Ca, Fe-Pd, and Ni-Co-Mn-In alloys. In the Ni-Fe-Ca and Fe-Pd alloys, the magnetic flux pattern in the parent phase changes dramatically with an approach to Ms (martensitic transformation start temperature). Interestingly, the magnetic flux pattern observed at a temperature slightly higher than Ms was inherited to the magnetic domain structure in the martensitic phase. On the contrary, this type of pattern formation was not observed in a Ni-Co-Mn-In alloy. Diffuse scattering analysis by energy-filte … More red electron diffraction has revealed that the transverse lattice displacement on {110} planes is pronounced upon cooling in the parent phase of Ni-Fe-Ca and Ni-Co-Mn-In alloys, while the signal of the lattice displacement is quite weak in the Ni-Co-Mn-In alloy. This result implies that the change in the magnetic flux pattern in the Ni-Fe-Ca and Ni-Co-Mn-In alloys. In the Ni-Fe-Ga and Fe-Pd alloys, the magnetic flux pattern in the parent phase changes dramatically with an approach to Ms (martensitic transformation start temperature). Interestingly, the magnetic flux pattern observed at a temperature slightly higher than Ms was inherited to the magnetic domain structure in the martensitic phase. On the contrary, this type of pattern formation was not observed in a Ni-Co-Mn-In-alloy. Diffuse scattering analysis by energy-filtered electron diffraction has revealed that the transverse lattice displacement on {110}planes in pronounced upon cooling in the parent phase of Ni-Fe-Ga and Ni-Co-Mn-In alloys, while the signal of the lattice displacement is quite weak in the Ni-Co-Mn-In alloy. This result implies that the change in the magnetic flux pattern in the Ni-Fe-Ga and Ni-Co-Mn-In alloys is triggered by the premonitory lattice distortion in the parent phase. TEM observations of the martensite variant structures, which were obtained in an applied magnetic field (1.9T) and zero-field, respectively, supported the above interpretation.The structure of martensite variants is a very important factor that is responsible for the shape memory properties. The result of the present work sheds light on the understanding the formation mechanism of martensite variants in ferromagnetic shape memory alloys. Less