Papers by Author: Akihiko Chiba

Abstract: Porous Ti compacts reinforced by ultra-high molecular weight polyethylene (UHMWPE) were fabricated and their mechanical properties were evaluated. Ti powder atomized by plasma rotating electrode process (PREP) was sintered at temperatures ranging from 1473 K to 1673 K for 7.2 ks in a vacuum. The porous Ti compacts contain the porosity of about 40%, irrespective of the sintering temperature. Porous Ti/UHMWPE composites were successfully fabricated by compressing UHMWPE powder into the porous Ti compacts. The compacts exhibit open pore structure and enables the penetration of UHMWPE into pores in the compacts. Young’s modulus of the composites is higher than that of the porous Ti compacts. The increment in Young’s modulus is not simply explained by the rule of mixture because Young’s modulus of the UHMWPE is approximately 1.3 GPa. Three-point bending strength of the composites is improved, presumably due to the local stress relief by UHMWPE in the vicinity of neck in the composites.

Abstract: Nitrogen-doped and -undoped Ti powders were obtained by Plasma Rotating Electrode Process (PREP) in an Ar atmosphere using Ar-2 vol.%N2 and pure Ar, respectively, for plasma flame. Auger electron spectroscopy (AES) revealed that nitrogen is enriched at the surface of nitrogen-doped Ti powder and decreases along with the depth direction. Microstructure of a compact prepared by hot pressing the nitrogen-doped Ti powder shows a shell structure. The outer area is considered to form by the diffusion of nitrogen. This microstructure disappears after annealing at 1373 K for 3.6 ks. It is found that the compressive yield strength of porous Ti compacts can be improved by introducing nitrogen in Ti powder and is superior to that of human cortical bone.

Abstract: A recrystallized Co-Ni-Cr-Mo based superalloy was produced by cold working of 72% and subsequent recrystallization heat treatment. Microstructural observation revealed that a full recrystallization of the cold-worked alloy occured when heat treatment was performed at and above 1273K for 1h. So that, recrystallization heat treatment was carried out in a temperature range from 1273K to 1473K for 1h~24h, by which the average grain size was controlled to 28µm～238µm. Tensile tests were carried out from room temperature (RT) to 1073K in order to understand the effect of grain size on the mechanical properties of the Co-Ni-Cr-Mo based superalloy. At RT and 943K, yield strength, tensile strength and elongation of the recrystallized alloy were improved with decreasing grain size. The alloy having a grain size less than 42µm exhibited a steady-state flow behavior in the true stress-true strain curve at 943K. However, the alloy having a grain size of 28µm showed lower yield strength than that of 42µm at 1073K. It was found that the steady state flow is closely related to the occurrence of {111}<112> deformation twinning in the Co-Ni-Cr-Mo based superalloy.

Abstract: Mechanical property of carbon-free Co-Ni-Cr-Mo based superalloy was investigated at room temperature and at 943 K. Cold working and subsequent recrystallization heat treatment was carried out to produce an equiaxed grain structure. The average grain size was controlled to 28 and 238 µm in order to understand the effect of grain size on mechanical property and flow behavior. At room temperature and 943 K, 0.2% offset yield strength increased with decreasing grain size to exhibit grain size dependence. At 943 K, a specimen with the average grain size of 28 µm showed higher yield strength than that obtained with a specimen having the average grain size of 238 µm. A steady-state like behavior leading to an extensive ductility was observed in the fine-grained specimen. {111}<112> deformation twinning was found to correlate with the steady-state like behavior in the true stress-true strain curve. It was suggested that grain refinement is favor to enhance the fracture strength and allow the deformation twinning to occur.