Papers by Author: Minoru Nishida

Abstract: In this work, WC-17Co powder was thermally sprayed onto mild steel using HVOF spray technique. The coated specimen was heat treated at 1100°C in a vacuum chamber and was then studied by using transmission electron microscopy (TEM). Post heat treatment resulted in recrystallization of the amorphous phase, formed during thermal spraying, into low carbon eta phase like Co6W6C. TEM results of the heat treated specimens showed that these new nucleated eta phases had very clear crystallographic structure without any crystalline defects. Heat treatment could also transform high carbon carbides like WC and W2C in the as sprayed samples to high carbon eta phases like Co3W3C. High density of dislocations and staking faults noticed in TEM of these phases might be an indication of possible shear mechanism in formation of these carbides.

Abstract: Recently, a number of amorphous alloys that possess high glass-forming ability and a wide supercooled liquid region before crystallization have been discovered. Especially, bulk metallic glasses, which are made in bulk form with a thickness of ~10 mm at slow cooling rates of the order of 1~100 K/s, have been noted as an industrial application. Hence the welding of bulk metallic glasses to other materials is very important. Explosive welding of most popular Zr41.2Ti13.8Cu10Ni12.5Be22.5 bulk metallic glass to crystalline pure Ti and SUS304 plates is investigated in this paper. The BMGs was found to retain the amorphous structure and the original mechanical properties. The sound bonding with other materials is expected to push forward the application of bulk metallic glass for industrial usage.

Abstract: Diffusion bonding of Ti and Ti-Al (Ti-10, 20 and 40 mol%Al) alloys to high carbon steel was carried out at 1073 and 1273 K for 3.6 ks in a vacuum. It has been found that the joint with Ti-20 mol%Al alloy is separated in the vicinity of the interface promptly after the bonding treatment at 1273 K. Such a phenomenon could not be observed in other Ti-Al alloy/steel joints, and the Ti-20 mol%Al alloy/steel joint bonded at 1073 K showed a high strength of about 170 MPa. Therefore, this phenomenon depends on the bonding temperature and the composition of the Ti-Al alloys. From the observation results of the interface, it is thought that the diffusion of constituent elements across the interface is part of the reason for the separation phenomenon. The separated surface of the Ti-20 mol%Al alloy side showed diffraction peaks of TiC phase by XRD. Its Vickers hardness was about 1200 and approximately 4 times higher than that before the bonding treatment. It is expected that the separation phenomenon at the Ti-20 mol%Al alloy/steel interface serves surface modification of titanium materials, which show poor wear resistance.