Papers by Author: Mamoru Mabuchi

Abstract: The GB embrittlement mechanism of Fe enhanced by P segregation has been investigated by first-principles tensile tests because a P atom is a famous GB embrittler in Fe. The first-principles tensile tests have been performed on Fe with two P-segregated GBs, where P atoms are located at the different sites, and with a nonsegregated GB. The tensile strength and the strain to failure in the P-segregated GBs were lower than those in the nonsegegated GB. The first bond breaking occurred at the Fe-P bond owing to the covalent-like characteristics, although the charge densities were high at the Fe-P bonds even just before the bond breaking. This premature bond breaking of Fe-P was independent of the location of the P atom.

Abstract: Cross-roll rolling with different rolling routes (unidirectional rolling and reverse rolling) was carried out on a commercial Mg alloy sheets. In the cross-roll rolling, the roll axis was tilted by 7.5 degrees against the transverse direction. As a result of the Erichsen tests at 433 – 493 K, the stretch formability of the reverse cross-rolled specimen was higher than that of the unidirectional cross-rolled specimen. The reverse cross-rolled specimen showed lower average Lankford value compared with the unidirectional cross-rolled specimen. This is likely to be responsible for the enhanced stretch formability of the reverse cross-rolled specimen. The higher stretch formability was attributed to reduction of (0002) texture intensity, which was originated from random grain distribution.

Abstract: Porous copper specimens with relative densities of 0.22–0.96 were produced by spacer method and their compressive properties were investigated. In the low relative density range (relative density < 0.5–0.6), porous copper showed a density exponent n of 2.3, where n represents the relative density dependence of yield strength. In this range, the bending and buckling of cell walls and the formation of macroscopic deformation bands were observed. On the other hand, porous copper with a higher relative density (0.5–0.6 < relative density < 0.9–1) had an n value of approximately 1, where the dominant deformation mode of cell walls was yielding and no clear deformation band was observed. Also, in the highest relative density range (relative density is very close to 1), the compressive properties degraded markedly with decreasing density, indicating that stress concentration around the minimal pores occurred in this density range.

Abstract: Nanoporous gold was fabricated by dealloying and their pore characteristics were further modified by thermal or acid treatment. The fabricated nanoporous gold had a ligament size of approximately 5 nm. Thermal treatment on the nanoporous gold increased the ligament size to approximately 500 nm. During the thermal treatment, ligaments are bonded across the cracks which had been generated during the dealloying. Acid treatment also increased the ligament size to approximately 500 nm; however, the acid treatment had a different effect on the pore characteristics from the thermal treatment. As a result, nanoporous gold prism microassembly with anisotropic structure was spontaneously fabricated by the acid treatment. The mechanical properties of nanoporous gold were also examined. It is estimated that the yield strength of nanosized ligaments in nanoporous gold is very high and close to the ideal strength of gold.

Abstract: Spacer method is excellent technique of processing porous metals with well-controlled pore characteristics such as porosity (up to 90%) and pore size (as small as several hundred micrometers). Compressive properties of porous aluminum fabricated by the spacer method are investigated. They were subjected to monotonic compression tests at room temperature, and showed less fluctuated flow stress during their compressive deformation than conventional porous aluminum alloy, reflecting their homogeneous pore characteristics. Also, shortening behavior of the porous aluminum fabricated by the spacer method during cyclic compression was significantly differed from that of conventional porous aluminum alloy. Therefore, it can be concluded that the homogeneity of pore characteristics is responsible for compressive properties of porous metals. Monotonic compression tests on porous copper specimens with various porosities, which were made by the spacer method, were also conducted. The yield stress of the porous copper with high porosity (or low relative density) depended on the relative density more strongly than that of the porous copper with low porosity (or high relative density). It is presumed that porous metals with high porosity and ones with low porosities have different deformation mechanisms.

Abstract: Superior mechanical properties of the recycled specimen by solid-state recycling were introduced at first. AZ31 Mg machined chips were recycled by extrusion at 673 K with the different extrusion ratios. The oxide contaminants were dispersed more uniformly in the recycled specimen with the high extrusion ratio (1600:1). There was a remarkable increase in tensile strength and 0.2% yield stress for the recycled specimen with the high extrusion ratio compared with an extrusion reference subjected to the same deformation history. Next, superior corrosion resistance of the recycled specimen was introduced. The recycled specimen with low extrusion ratio (45:1) possessed superior corrosion resistance compared with the extrusion reference. The enhancement of corrosion resistance for the recycled specimens was attributed to the presence of oxide contaminants which were distributed parallel to the extrusion direction.

Abstract: In this study, two different cross rolling processes, which are effective rolling processes for a reduction of (0002) plane texture, are introduced. In the first cross rolling process, a sheet specimen is rotated around the rolling plane normal after each pass. In the second cross rolling process, the roll axis is tilted against the transverse direction (TD) in the rolling direction (RD) - TD plane. The two cross-rolling processed were carried out on a AZ31 alloy, and the press formability of cross-rolled sheets was compared with that of unidirectionally rolled sheets determined by Erichsen tests at 433 – 493 K. Both the cross-rolled specimens exhibited a high press formability, compared to an unidirectionally rolled alloy. The high press formability of the specimen by the first cross rolling was due to a reduction in (0002) texture intensity. The high press formability of the specimen by the second cross rolling was due to not only a reduction in (0002) texture intensity but also grain refinement.

Abstract: Porous Al specimens with a pore size range from 212-300 to 610-700 μm, a porosity from 85 to 95% and a specimen thickness from 2 to 20 mm were produced by the spacer method, and their sound absorption capacity was investigated. For these specimens, sound absorption coefficient increased with increasing porosity. On the other hand, sound absorption coefficient varied inconsistently with the variation of pore sizes. The latter may be attributed to variation of aperture sizes of each specimen because the porous Al specimens with differerent pore sizes produced by the spacer method should have different aperture sizes. Sound absorption coefficient increased at the frequency below 2000 Hz with increasing specimen thickness.

Abstract: Porous metals, or metallic foams, are emerging ecomaterials that can be applied to structural use, shock absorber, filter, heat exchanger, etc. Their very low densities and peculiar deformation behaviors will facilitate the application. The control of pore characteristics such as porosity and pore size distribution can be successfully achieved by spacer method. In this paper, fabrication of porous aluminum via the spacer method is introduced and their excellent properties due to homogeneous pore characteristics are exhibited

Abstract: Superplastic forming characteristics of AZ31 Mg alloy recycled by solid-state process were investigated. Cylindrical scraps and machined chips were recycled by hot extrusion and hot rolling in air. As a result of tensile test, elongation to failure of the specimens recycled from small and large cylindrical scraps was almost the same as that of the rolled specimen from a virgin ingot. On the other hand, the specimen from machined chips had much lower elongation at elevated temperatures. The oxygen concentration of the specimen recycled from machined chips showed about 60 times higher value compared with that of the other specimens. In superplastic forming tests, the specimen from small and large cylindrical scraps exhibited excellent formability similar to the rolled specimen from a virgin ingot. However, the specimen from machined chips showed poor formability. Thus, oxide contamination adversely affected the formability of recycled Mg alloy.