Papers by Author: Yoshimi Watanabe

Abstract: Shot-peening (SP) is one of the severe surface plastic deformation (SSPD) processing techniques. Due to large plastic strain by the SP, the SP for metallic materials forms crystallographic texture on the peened surface. Since the crystallographic texture formed by the SP depends on the dislocation slip, it can be expected that this texture is affected by stacking fault energy (SFE) of the materials. However, effects of the SFE on the crystallographic texture formed on the peened surface by the SP is not clear. In this study, crystallographic textures of pure Al (higher SFE) and Al-10 mass%Mg alloy (lower SFE) formed by the SP are investigated. When the pure Al is SPed, {001}+{111} double fiber texture with the <001> and <111> directions parallel to the plane normal direction of the peened surface is obtained. On the other hand, in the case of Al-10 mass%Mg alloy with the SFE close to the pure Cu, {110} fiber texture is formed as well as the pure Cu. Therefore, it is found that the crystallographic texture formed by the SP is influenced by the SFE.

Abstract: Spark plasma sintering can be carried out at a low temperature with short heating, holding and cooling times, which allows for the fabrication of non-equilibrium materials. In this article, metal-matrix-composite fabrication with thermally unstable reinforcements by spark plasma sintering is described.

Abstract: Effects of peening direction on the reverse transformation induced by the shot-peening for the Fe-33 mass%Ni alloy with large amount of martensite (α’) are investigated. When the angle between the peened surface and the peening direction (Hereafter, peening angle) is 90 ^o, the reverse transformation occurs and subsequently martensitic transformation is induced by the shot-peening. On the other hand, in case of the peening angle of 30 ^o, only reverse transformation occurs. Furthermore, the volume fraction of austenite (γ) in the specimen after the shot-peening increases as the peening angle decreases. This means that the reverse transformation induced by the shot-peening is enhanced by decreasing the peening angle. Moreover, residual compressive stress around the peened surface increases as the peening angle decreases. Since the hydrostatic compressive stress decreases phase transformation temperature, the phase transformation temperature around the peened surface would be decreased by the shot-peening. Therefore, the reverse transformation behavior depending on the peening angle can be explained by the residual compressive stress due to the shot-peening.

Abstract: In this study, joining of AlN and Al with compositional graded layer is made by centrifugal mixed-powder method (CMPM). The mixed-powder of AlN particles and Al particles is inserted into a spinning mold with bulk-shaped AlN, and then molten Al is poured into the spinning mold with the mixed-powder and bulk-shaped AlN. As a result, the molten Al penetrates into the space between the mixed-powder by the centrifugal force, and at the same time, the Al particles can be melted by heat from the molten Al. Then AlN and Al can be joined with compositional graded layer after solidification. Micromechanics-based analysis is also employed to understand the thermal stress relaxation by the compositional graded layer.

Abstract: The present study is undertaken to investigate the influence of annealing and normalizing heat treatment on the bimetallic interface microstructures of 304 stainless steel and gray cast iron. The current work is aim to control the bimetal interface microstructures by different types of heat treatment processes to improve performance of the bimetallic castings performance. For low temperature annealing, specimens are heated to 760 0C for 60 min in an electrical heating furnace. For high temperature annealing and normalizing, specimens are heated to 920 0C for 120 and 240 min. A different interface structures are obained for all heat treated samples. Annealing and normalizing induce a significant effect on the diffusion of C and Cr elements and slightly effect on the diffusion of Ni element. Thickness of interface layer 1 (austenite + carbide) increases by increasingthe annealing temperature.

Abstract: Reverse transformation behavior of thermally-induced martensite phase (α’) in martensitic stainless steel by the shot-peening is investigated. It is found that volume fraction of austenite phase (γ) on the peened surface is increased by the shot-peening under elevated temperature. This means that reverse transformation from thermally-induced α’ to γ can be induced by the shot-peening. Moreover, with decreasing the distance between blast nozzle and specimen (blast distance), the reverse transformation occurs more remarkably. This is because that larger shear strain can be induced by the shot-peening with shorter blast distance. Furthermore, thickness of the deformation-induced layer becomes larger as the blast distance decreases. It can be concluded that the reverse transformation in SUS410S with thermally-induced α’ occurs by large shear strain during the shot-peening.

Abstract: As one of processing methods of functionally graded materials (FGMs), centrifugal mixed-powder method has been proposed. The centrifugal mixed-powder method is the casting process combined of centrifugal casting and powder metallurgy. This processing method has advantage that fine ceramics-particles, whose wettability with matrix melt is low, can be compounded into metallic material. However, effects of particle size on microstructure and mechanical properties of the FGMs fabricated by the centrifugal mixed-powder method are unclear. In this study, two kinds of Al-TiO₂ FGMs rings are fabricated by the centrifugal mixed-powder method. One contains TiO₂ particles having similar diameter with Al matrix particles (hereafter, small different-size (SD) TiO₂ particles), and the other one compounds TiO₂ particles with much smaller diameter than Al matrix particles (hereafter, large different-size (LD) TiO₂ particles). In case of the Al-TiO₂ FGMs ring containing SD-TiO₂ particles, the TiO₂ particles are homogeneously dispersed in Al matrix on outer surface of the ring. On the other hand, the TiO₂ particles in the Al-TiO₂ FGMs ring with LD-TiO₂ particles are distributed along grain boundary of Al matrix. Moreover, Vickers-hardness and wear resistance around outer surface of the Al-TiO₂ FGMs ring containing the SD-TiO₂ particles is higher than that of the Al-TiO₂ FGMs ring with LD-TiO₂ particles. Since Al particles in the mixed-powder with LD-TiO₂ particles are surrounded by the TiO₂ particles, the Al particles can be hardly melted by heat of molten Al at casting process. As a result, the Al-TiO₂ FGMs ring with LD-TiO₂ particles has low hardness and wear resistance. Therefore, it is found that TiO₂ particles having similar diameter with Al matrix particles are more suitable for fabrication of the Al-TiO₂ FGMs rings.

Abstract: The effects of thermo-mechanical training on damping capacity of two types of stainless steels, Fe-18Cr-8Ni (SUS 304) and Fe-25Cr-20Ni (SUS 310S) stainless steels, are studied. The thermo-mechanical training with bending deformation is adopted, since vibration manner in internal friction measurement is bending mode. An anisotropic damping capacity as well as hardness of samples is studied. It is found that deformation induced ε-martensite is observed for trained SUS 304 sample, while deformation twins are formed in the trained SUS 310S sample. It is also found that internal friction of SUS 304 sample is larger than that of SUS 310S sample. Increase in number of training results in an increase in the internal friction and hardness. In addition, anisotropic damping capacity is observed in the samples subjected the thermo-mechanical training. To be concluded, the thermo-mechanical training is useful for enhancement of both damping capacity and strength of SUS 304 and SUS 310S stainless steels.

Abstract: Reduction of frictional coefficient at sliding position can improve wear resistance of material. In previous studies, Cu-based composites containing graphite particles have been reported. Since graphite is better lubrication material, the Cu-based composites containing graphite particles have better wear property comparing with the pure Cu. However, these composites are mainly fabricated by sintering method and its strength is relatively low. In this study, Cu-based composites containing graphite particles are fabricated by centrifugal mixed-powder casting. The centrifugal mixed-powder casting is novel centrifugal casting method combined with powder metallurgy. Using this casting method, the Cu-based composites containing graphite particles are successfully obtained. The graphite particles are distributed in the Cu matrix and no casting defects are observed. Moreover, wear resistance of these Cu-based composites are much better than pure Cu, and the frictional coefficient between these composites and bearing steel as the counter part is reduced by dispersion of the graphite particles. Furthermore, it is found that the optimum area fraction of the graphite particles to improve the wear resistance of the present Cu-based composite is from 15% to 21%.

Abstract: It is reported that alloying with Cu causes the transformation from the D0₂₂ type structure of Al₃Ti into L1₂ cubic structure. Since the lattice constant of Al_2.5Cu_0.5Ti with L1₂ structure is a = 0.3927 nm, smaller disregistry value between Al and Al_2.5Cu_0.5Ti can be achieved. In this study, novel refiner for Al cast containing Al_2.5Cu_0.5Ti intermetallic compound particles with L1₂ structure is fabricated by cold pressing. It is found that the grains of Al cast are partially refined by using the novel refiner, although microstructure of Al grain is not homogeneous. In this way, the Al_2.5Cu_0.5Ti intermetallic compound particles with L1₂ structure can become favorable heterogeneous nucleation sites for Al cast.