Papers by Author: Kenji Higashi

Abstract: Nanocrystalline materials with high strength have been reported in large numbers. In particular, there has been considerable research on electrodeposited nanocrystalline Ni (nc-Ni) and nc-Ni alloys. However, reported data vary widely especially in ductility. Therefore, it is necessary to obtain the true characteristic value of nc-Ni and nc-Ni alloys. In the present study, nc-Ni and nc-Ni-W was electrodeposited under different conditions in order to obtained bulk nc-Ni and nc-Ni-W with high tensile strength and good ductility. At first, bulk nc-Ni-W was fabricated using a sulfamate bath. Although the resulting bulk nc-Ni-W had inhomogeneous grain size and W-concentration, this sample exhibited plastic deformation behavior. Then, nc-Ni was fabiricated by four types of sulfamate baths. As a result, the nc-Ni obtained from a sulfamate bath containg added saccharine and 2-butyne-1,4-diol exhibited brittle behavior. In contrast, bulk nc-Ni obtained from sulfamate bath with a grain size of about 60 nm exhibited a tensile strength of about 1000 MPa and ductility of 8.8 %.

Abstract: We investigate the energy of segregation of solute Ca at symmetric tilt grain boundary in aluminum from the first-principles calculations. As energy of segregation of Ca is negative, Ca atoms tend to segregate at the grain boundary. Furthermore, on basis of the Rice-Wang model, we study the effect of the segregation of Ca on the grain boundary embrittlement of aluminum. Our first-principles calculations of energies of segregation at grain boundary and free surface show that Ca behaves as embrittler.

Abstract: We have developed a new surface treatment, Mgbright, which produces a luster on AZ31 magnesium alloy surfaces. The treatment consists of a chemical process and a coating process, and the primary characteristics are that of developing a luster on AZ31 magnesium alloy and imparting a high resistance to corrosion. When Mgbright was applied, the value of surface roughness, Ra, required for developing luster was 1.5 µm. As the target Ra value of press-formed articles was set to less than 1.5 µm, mass-production press-forming was tested by means of the 100,000 times square-cup drawing method, and the target was achieved. We applied the Mgbright treatment techniques and the square-cup drawing test, and produced a laptop computer chassis.

Abstract: High-temperature plastic flow and tensile ductility in pre-annealed Zr65Al10Ni10Cu15 bulk metallic glass are examined in a supercooled liquid region. The specimen annealed at 673K for 1.8ks before the tensile test exhibits uniform deformation, and large elongation of 1100% is obtained in the tensile test at 673K and in a constant true strain rate of 1×10-2s-1. In the specimen, initial stress decreases in comparison with that of the specimen without annealing, but apparent strain hardening arise during deformation. SEM analysis after the deformation reveals that the cavitation is very limited even at the fracture tip. XRD and TEM analyses reveal that icosahedral phase is observed only in gage section but not in grip section after the deformation. The limited cavitation due to the accommodation of stress concentration through the grain boundary amorphous phase possibly makes a positive contribution to the large elongation in the annealed specimen, as well as the neck stability due to the strain hardening by the interaction between the icosahedral phase particles increasingly crystallized by the local strain.

Abstract: High-strength Mg-Y-Zn alloy plate was obtained by friction stir processing (FSP) after casting. In this study, the effect of processing order on the strength of processed specimens was discussed. It was revealed that the microstructures and strength of doubly stirred zone depended on the direction of overlapping pass. In the area of the doubly stirred zone that was affected by the thermal history of Retreating-side, the hardness was 121Hv. It was twice as hard as Base Metal, and harder than the doubly stirred zone that was affected by thermal history of Advancing-side.

Abstract: Strength of Al5083-O FSW joints was evaluated, in relation to FSW conditions. Static strength of the joints was found to be enough because base metal fracture occurred in the tensile tests of joints for some FSW conditions. However, the fatigue strength of those joints with fine static strength varied significantly. For example, in tensile strength of joints with the FSW condition with tool rotation: 800 rpm- welding speed: 100 mm/min (800-100), 800 rpm-200 mm/min (800-200) and 500 rpm- 100 mm/min (500-100), each condition of FSW joint efficiency were all hundred percent. In contrast, joint efficiency for fatigue strength varied from 75% for 500-100 to 31% for 800-200. Crack path in fatigue test was always initiated at the center of back surface of FSW weld zone, and propagated through stir zone. The variance of fatigue strength is believed to be attributed to the stirred condition at back surface.

Abstract: Friction stir processing (FSP) is the effective method of the grain refinement for light metals. The aim of this study is to acquire the fine grained bulk Mg-Y-Zn alloy by ingot metallurgy route much lower in cost. Such bulk alloy can be formed by the superplastic forging. The microstructure of as-cast Mg-Y-Zn alloy was dendrite. The dendrite arm spacing was 72.5 [(m], and there are the lamellar structures in it. FSP was conducted on allover the plate of Mg-Y-Zn alloy for both surfaces by the rotational tool with FSW machine. The stirring passes were shifted half of the probe diameter every execution. The dendrite structures disappeared after FSP, but the lamellar structure could be observed by TEM. The matrix became recrystallized fine grain, and interdendritic second phase particles were dispersed in the grain boundaries. By using FSP, cast Mg-Y-Zn alloy could have fine-grained. This result compared to this material produced by equal channel angular extrusion (ECAE) or rapid-solidified powder metallurgy (RS P/M). As the result, as-FSPed material has the higher hardness than materials produced by the other processes at the similar grain size.

Abstract: The effect of second phase particles on tetragonal to monoclinic (t-m) phase transformation of zirconia is examined, which causes the degradation of zirconia in vivo. Silica is selected as the second phase particles and dispersed 0.1 to 10mol% into 3mol% yttria stabilized tetragonal zirconia polycrystal (3Y-TZP). As the result, phase transformation is promoted in all the samples examined in comparison with 3Y-TZP. Non-dispersed specimen exhibits better phase stability than that in silica doped one. Since the thermal expansion coefficient is smaller in order of silica and zirconia, the residual stresses induced during cooling from the sintering temperature must be the dominant factor to determine the phase stability in these materials.

Abstract: Phase stability of cation co-doped zirconia ceramics is examined. As the result, in contrast to the result in small amount of single cation doped zirconia, phase stability of co-doped zirconia ceramics can not be simply explained from ionic radius and valency of dopant or from the change in axis ratio. We focus on oxygen vacancy concentration and binding energy between oxygen vacancy and doped cation. By estimating phase stability from these factors, it is found that concentration of oxygen vacancy and the binding energy between the dopant and the oxygen vacancy are important factors for understanding the phase stability of zirconia ceramics.

Abstract: Many studies have been conducted on mechanical properties in nanocrystalline Ni-W alloys. However, since these results are obtained in the specimens whose thickness is less than 100 μm and whose homogeneity is not strictly controlled, an inherent potential of the nanocrystalline Ni-W alloy may be hidden. Therefore, it is necessary to fabricate the bulk Ni-W alloy with sufficient thickness and homogeneity. In the present study, we develop novel electroforming process and fabricate the homogeneous nanocrystalline Ni-W alloys. The homogeneities of W-concentration in micrometer scale are confirmed by the W-concentration profiles obtained by the linear analyses of the energy dispersed spectroscopy (EDS). The single-phase nanocrystalline bulk Ni-W alloy with the thickness above 2 mm and minimized W-concentration gradient and fluctuation is featured for the first time.