Papers by Author: Minoru Nishida

Abstract: Ni₄Ti₃ precipitates with a heterogeneous distribution growing in a polycrystalline Ni_50.8Ti_49.2 alloy have been investigated in a Dual-Beam FIB/SEM system. The volume ratio, mean volume, central plane diameter, thickness, aspect ratio and sphericity of the precipitates in the grain interior as well as near to the grain boundary were measured or calculated. The morphology of the precipitates was classified according to the Zingg scheme. The multistage martensitic transformation occurring in these kinds of samples is interpreted in view of the data of this heterogeneous microstructure of matrix and precipitates.

Abstract: Metallic glass or amorphous alloys have been gaining popularity due to their low density, high strength, high fracture toughness, good corrosion and good wear resistance. Especially, bulk metallic glasses (BMGs), which are made in bulk form with a thickness of ~10mm at slow cooling rates of the order of 1~100K/s, have been noted as an industrial application. The welding to other materials becomes very important. Explosive welding of most popular Zr41.2Ti13.8Cu10Ni12.5Be22.5 bulk metallic glass to crystalline pure Cu plate is investigated in this paper and the BMG was found to retain the amorphous structure. The sound bonding with other materials is expected to push forward the application of BMGs for industrial usage.

Abstract: Ti-Sn binary alloys (Ti-5 to 20 mol% Sn) were diffusion-bonded to high carbon steel between 1073 and 1273 K for 3.6 ks in a vacuum to investigate the influence of the alloy composition on the interfacial microstructures. Ti-5 and 10 mol% Sn alloys were attached firmly to the steel at a bonding temperature of 1273 K. A continuous TiC layer was formed along the interface, while voids were observed between the TiC layer and the steel. Although the joints with Ti-15 and 20 mol% Sn alloys were also prepared at 1273 K, these joints separated near the interface after the bonding treatment. The TiC layer was formed in the separated surface of Ti-Sn alloy, and Fe in the steel diffused into the Ti-Sn alloy. This indicates that the Ti-15 and 20 mol% Sn alloys established contact with the steel at elevated temperatures until just before the separation. The specimens bonded at 1173 K also denoted the same tendency. However, the Ti-15 mol% Sn/steel joint bonded at 1073 K showed a shear strength of more than 50 MPa. The mechanism and the application of the interface separation are discussed on the basis of the microstructural observations.

Abstract: The relationship between the microstructure and mechanical properties on the ternary ZrCoNi alloys has been investigated. Zr50Co50-xNix alloys undergo martensitic transformation from the B2 to B33 structure by the substitution of Ni for Co. The tensile ductility at room temperature increases remarkably without the loss of yield strength by substituting Ni. Zr50Co39Ni11 alloy has the extremely high plastic elongation of 21%. There are many {021}B33 deformation twins in the B2 parent phase just near the tensile fractured area. These twins are the strain induced martensitic phase due to the tensile deformation. Consequently, the remarkable improvement of the ductility of ZrCoNi alloys is due to the transformation induced plasticity.