Papers by Author: Shunichiro Tanaka

Abstract: Since the matrix phase is transformed to martensitic phase in shape memory alloys (SMAs) during plastic deformation, complicated residual stresses may arise during deformation, and they may affect the shape recovery ability of the alloys. Thus, it is important to be able to characterize the residual stresses formed in SMAs during plastic deformation and annealing. In this study, X-ray diffraction was used to characterize the residual stress formed in a Fe-Mn-Si-Cr SMA, which was deformed in the tensile direction and subsequently annealed. The results showed that the compressive stress persisted in the tensile direction of the face-centered cubic (fcc) matrix upon tensile deformation and unloading. Compressive stress is believed to result from the hexagonal close-packed (hcp) phase formed during stress-induced martensitic transformation. After the deformed samples were annealed to recover their shapes, the residual stress was considerably reduced. This is believed to be due to the decrease in the formation of the hcp phase or to the recovery of their shapes during annealing. Our results indicated that residual stress in the fcc matrix phase is associated with the shape recovery characteristics of the alloys after martensitic and reverse martensitic transformations.

Abstract: In-situ HRTEM technique was applied to various substrates and the reaction processes between substrates and molten alloy were compared. Substrates used were SiC, Si3N4, Si wafers, an amorphous carbon film, and a carbon nanotube. Ti-containing Ag-Cu eutectic alloy foil was placed on the substrate and the combined specimen mounted on the heating holder of an HRTEM and heated in the microscope to melt the alloy foil. In the case of SiC, Si3N4, carbon materials, the molten alloy spreading on the substrates were observed after melting of the alloy. SiC polar plane nano-steps appeared with the SiC dissociation by the molten alloy. On the surface of the carbon nanotube, thin film precursor spreading was observed. In contrast, Si reaction with the molten alloy produced big holes at the contacted area and molten alloy spreading was not observed.