Mamoru Nakamura

Microstructural evolution and superplasticity of rolled Mg-9Al-1Zn

Abstract Microstructural evolution and superplasticity of a Mg-9Al-1Zn alloy rolled at 673 K were investigated at 573 K and 1.5×10−3 s−1. The grain size of the as-rolled Mg alloy was 39.5 μm. However, the grain size of the specimen deformed to a true strain of 0.6 was 9.1 μm. The grain refinement was attributed to dynamically continuous recrystallization during an initial stage of tensile test. Stabilization of subgrain boundaries by fine particles and stimulation of continuous recrystallization by prior warm-deformation were not needed to attain dynamically continuous recrystallization in the Mg alloy. As a result of the grain refinement, the rolled Mg alloy exhibited superplastic behavior.

Microstructure and mechanical properties of a Mg-4Y-3RE alloy processed by thermo-mechanical treatment

Abstract Thermo-mechanical treatments consisting of hot extrusion at 573 or 673 K and artificial aging at 473 K were carried out on a Mg-4Y-3RE alloy. The material extruded at 573 K, followed by aging for 2 h, showed fine needle-like precipitates and the high density of a dislocation cell. This material exhibited a high strength of 370 MPa at room temperature. The material extruded at 673 K exhibited a very small grain size of 1.5 μm and fine spherical precipitates. This material showed not only a high strength of about 300 MPa from room temperature to 473 K, but also superplastic behavior at a high strain-rate of 4×10−1 s−1 and at 673 K. Furthermore, a good combination of high strength and high ductility at room temperature was attained for the material extruded at 673 K. These qualities can likely be attributed to the very small grain size.

Surface treatment of AZ91D magnesium alloy by aluminum diffusion coating

Magnesium and magnesium alloys are being increasingly used in varied areas of industry due to their low densities from 1.75 to 1.85 g/cm 3 and high specific strength. These very light materials are particularly suitable for weight-saving constructions in aircraft industry, vehicle production, and portable electric devices. However, for many applications, magnesium and its alloys are inferior in corrosion and wear resistance due to their high reactivity and low hardness (HV60–80). In general, the hardness improvement by bulk alloying with other elements requires high quantities of additives. It may cause an increase in cost and density of alloys. Thus, surface treatments are frequently applied to practical applications as circumstances demand. As the surface hardening process, dry processes such as anodizing, plating, and PVD (physical vapor deposition) are frequently adopted on the mechanical parts and structural magnesium alloys [1]. In general, however, thin films produced by these processes are insufficient in machine parts used under harsh conditions. For example, vapor coatings by PVD are too thin to endure being under large stress. Plating has a problem that adherence strength of coating is so weak that there is possibility of peeling off. The stiffening layers of hundreds of microns are necessary so that wear and abrasion resistance are improved sufficiently. On the other hand, diffusion coating process is one of the promising methods of surface treatment since its adherence strength is high and it does not need any expensive equipment like PVD and CVD (chemical vapor deposition) [2–9]. In this research, in order to obtain a surface coating layer over hundreds-micron, diffusion coating was carried out on the AZ91D magnesium alloy. As a result, Al-Mg intermetallic compound layer of 750 μm was formed by a heat treatment o f 1 h in aluminum powder. The aluminum powder whose purity was 99.5% and whose particle diameter was 100 μm was used for the source of diffusion element. Furthermore, zirconia powder was mixed with aluminum powder so that sintering of aluminum powder was avoided. Magnesium specimens (AZ91D alloy), in form of 15 mm × 10 mm× 5 mm plate, were cut out from a cast ingot that had been performed a solid solution treatment at 420◦C for 20 h. Both the mixed powder of aluminum and AZ91D specimens were put into an alumina crucible together. The crucible was settled in a ceramic tube, and heating elements outside the tube heated the tube and the crucible. To avoid oxidation of specimens and powder in the crucible, pure argon gas (purity 99.995%) was flowed through the tube during the heat treatment. The heat treatment for diffusion coating was carried out at 450 ◦C for 1 h. The specimens were cooled down until 100 ◦C in the furnace, and were taken out to the air. After the processing, the surface of the specimen was analyzed by X-ray diffractometer to identify formed intermetallic compounds. The cross sections were observed with SEM (scanning electron microscope) and a composition analysis by EPMA (electron probe X-ray micro analyzer) was carried out. Furthermore, hardness distributions of depth

Experimental study of a structural magnesium alloy with high absorption energy under dynamic loading

It has been demonstrated that pure Mg exhibits low ductility under dynamic loading at room temperature owing to its HCP structure. Very limited data are currently available for magnesium alloys under dynamic loading. In order to be used for structural components, it is necessary to improve the mechanical properties of magnesium alloys. Lahaise et al. reported the yield strength of the AZ91 magnesium alloy increased with refining its microstructure. Mohri et al. has already been reported the ductility enhancement of a Mg-Y-RE(Rare Earth) alloy by hot extrusion. They mentioned the enhancement of ductility is due to the refining microstructure of magnesium. Thus refining microstructure enables to raise the possibility for the development of a structural magnesium alloy with high ductility at dynamic strain rate. In this paper, the possibility of a fine-grained WE43 magnesium alloy is investigated to raise the high speed impact performance against the foreign object damage by the enhancement of ductility and absorption energy under dynamic loading.

Processing of Cellular Magnesium Materials

Especially cellular aluminum materials have been extensively developed and investigated in the recent years. But also magnesium is a suitable metal for cellular metals due to its low density. By a special casting method open-cellular magnesium with a very loin density of 0.05 g/cm 3 is fabricated that shows high potential for usage as energy absorbers.

Effects of heat treatment on compressive properties of AZ91 Mg and SG91A Al foams with open-cell structure

Compressive properties were investigated for the as-cast and T6 heat-treated AZ91 Mg and SG91A Al foams with open-cell structure. The foams showed an elastic region at an initial stage, then a plateau region with a nearly constant flow stress to a large strain of about 60%, and finally a densification region where the stress increased rapidly. The relative stress of the as-cast foams was higher than that of the T6 foams, taking into consideration the influence of the relative density.

Compressive properties of open-cellular SG91A Al and AZ91 Mg

Abstract Mechanical properties of open-cellular SG91A Al and AZ91 Mg have been investigated by compressive tests. The strain to densification of the cellular AZ91 Mg was almost the same as that of the cellular SG91A Al, though the AZ91 Mg solid with the relative density of 100% showed much lower ductility than the SG91A Al solid. By compensation with the yield stress of the solid and the relative density, the stress–strain relation of the cellular AZ91 Mg was in agreement with that of the cellular SG91A Al.

Compressive deformation behavior of Al2O3 foam

Deformation behavior of an Al2O3 foam with an open-cellular structure was investigated by compressive tests. The variation in flow stress with strain was significantly large and there was no densification region. Breakage of the columns is responsible for the large variation in flow stress with strain and no densification region. The relative stress of the Al2O3 foam was lower than the value predicted by Gibson and Ashby. This is probably because of the high degree of cracking in the columns and the presence of partial closed-faces.

Surface damage in ZrB2-based composite ceramics induced by electro-discharge machining

Electro-conductive ZrB2-based composite ceramics, containing SiC and B4C, were machined with an electro-discharge machining (EDM) process. The EDMed surfaces were covered with resolidified ZrB2 layers. Many open pores and surface cracks were observed on the surfaces. The strength degradation of the ceramics caused by machining was evaluated by three point bending tests of the partially EDMed bending specimens. The effects of pulsed current, pulse duration and duty factor on the strength and the roughness of EDMed surfaces are discussed. The strength of EDMed specimens was increased with decreases in pulse current, pulse duration and duty factor. The roughness of EDMed surfaces was decreased with decreases in pulse current, pulse duration and duty factor. The reliability of the ceramics EDMed with the appropriate conditions was as high as that of the ceramics ground with a 400 grit diamond wheel. It would be possible to use the carefully EDMed ZrB2-based composite ceramics as structural components without any additional finishing processes.

Mechanical properties of a powder metallurgically processed Mg–5Y–6Re alloy

Abstract A rapidly solidified powder of Mg–5 wt.%Y–6 wt.%Re alloy (chemical composition: Mg–4.74 wt.%Y–3.36 wt.%Nd–1.76 wt.%Pr–0.71 wt.%Ce) was extruded at 573 K with a reduction ratio of 20:1 in a vacuum. Mechanical properties of the P/M Mg alloy were investigated by velocity-constant tensile tests at room temperature to 773 K. The powder metallurgy Mg alloy had a mean grain size of 0.5 μm. The alloy exhibited a high strength of 536 MPa at room temperature and superplastic behavior at a high strain rate of 1.7×10 −1 s −1 at 773 K. These characteristic mechanical properties might be attributed to the sub-micron grained structure.