Papers by Author: Takaaki Sakuma

Abstract: The Zn-22Al alloy with fine-equi-axed has been well known as a typical superplastic metallic material [1]. In the present study, The Zn-22Al alloy ingot of 20mm thickness was homogenized, either air-cooled or water-quenched, and then hot-rolled to a thickness of 2mm. Microstractural observation, showed that in the air-cooled specimens lammellar microstructure was formed after homogenization, and become fragmented to fine-grained microstructure as the hot rolling process proceeded. In the water-quenched specimens, equi-axed fine-grained microstructure with grain size under 2.1μm was attained and maintained throughout the hot rolling process.

Abstract: The Zn-Al eutectoid alloy has been well known as a typical superplastic metallic material with small grain sizes. In the present study, controlling conditions in hot rolling such as temperature, total number of passes, reduction in a pass, etc. have been investigated in a ZnAl eutectoid alloy to obtain finegrained microstructures. Homogenized ingot specimens were hotrolled by a total rolling reduction of 90% with different pass schedule. Microstructural observation on the final sheet showed that nearly equiaxed finegrained microstructure with grain size of about 2µm was formed when the ingot was waterquenched after homogenization.

Abstract: During superplastic deformation (SPD) of tetragonal zirconia polycrystals containing 3 mol% yttria (3Y-TZP) at high strain-rates, a number of crack-like flat cavities having very narrow gaps lying along grain boundaries mostly normal to the tensile axis are produced in addition to conventional cavities. The formation and growth of these flat cavities are responsible for the strain softening that appears on the true stress versus true strain curves. The growth and coalescence of the flat cavities were a main cause of the degradation of elongation to fracture. We have found that a simple treatment, in which the superplastic deformation is temporally stopped, i.e., the cross-head movement is reversed and accordingly the applied load is removed, and then the specimen is kept at the test temperature for several minutes, is surprisingly effective to reduce the flat cavities. Carrying out this simple treatment repeatedly, after 30% nominal stain during the SPD, led to an increace in total elongation by about three times larger than that of a specimen not subjected to such a treatment. This treatment can also recover the strength and accordingly mechanical properties of the superplastically deformed 3Y-TZP to that of the undeformed state. This finding is believed to be quite significant for practical applications of superplasticity in 3Y-TZP, because the flat cavities can be closed very simply by keeping a product at the forming temperature after or during the superplastic forming process.

Abstract: The 4mol%Sc2O3 doped ZrO2 (4ScSZ) showed a superplastic-like large elongation in a range of strain-rate at 1773K. The large elongation was caused by both of high strain-rate sensitivity and high strain hardening during the deformation. Ion conductivity of the 4ScSZ, deformed superplastically at a relatively high strain-rate region, was higher than that of non-deformed one, suggesting that superplastic deformation can improve its conductivity. These results are discussed with probable microstructural evolution.

Abstract: The effects of Zr ion irradiation on the mechanical properties of a typical superplastic ceramic, 3mol% yttria stabilized tetragonal zirconia polycrystal (3Y-TZP), were examined and discussed. The specimens were irradiated by Zr11+ ions with 130MeV at fluence level of 3.5×1012 and 2.1×1013 ions/cm2 in the TANDEM accelerator at Tokai Reasearch Establishment of JAEA. Microstructures after annealing and bending deformations to fracture of Zr ion irradiated 3Y-TZP were examined. It was found that the ratio of intergranular fracture to intragranular fracture was increased in the region that was affected by Zr ion irradiation. It seemed that grain boundary cohesion became relatively weak in the irradiated surface region. The influence of Zr ion irradiation on the mechanical properties almost disappeared when the irradiated 3Y-TZP was subsequently heated to 1173K.