Papers by Author: Yutaka Hiraoka

Abstract: Fractography was used in order to investigate the effects of Ta addition on the diffusion behavior of carbon in molybdenum. Mo-1.5mass%Ta alloy was carburized by the solid-state carburization process at a temperature between 1073 and 1473 K. Fracture surface of the specimen was observed before and after the carburization process and carbon diffusion distance was estimated from the change in fracture mode as a function of distance from the specimen surface. Results are as follows. (1) Ta addition slightly decreased the carbon diffusion distance. (2) Ta addition had almost no effect on the activation energy for diffusion. (3) These results suggest preferential combination between carbon atoms and Ta atoms is only minor.

Abstract: A series of industry-scale and laboratory-scale W-Cu composites containing copper of 19-51 vol% were prepared by infiltrating a tungsten skeleton with liquid copper. These composites were subjected to a three-point bend test at temperatures between 77 and 363 K, and yield and maximum strengths were evaluated as a function of both the temperature and the composition. Fracture surface was observed using SEM and its fracture mode was examined. Results are summarized as follows. (1) Critical Cu content for the transition from a brittle to a ductile deformation behavior at room temperature was about 20 vol%. (2) Critical Cu content for the transition from a bcc-type to a fcc-type deformation behavior was 38-45 vol%.

Abstract: Bend tests were performed at temperatures between 273 and 363 K for W-19vol%Cu, W-22vol%Ag and W-19vol%(BAg-8) composites. Yield and/or maximum strengths and ductility of the composite were discussed in terms of microstructure and fractography. Results are summarized as follows. (1) Almost no difference was recognized in yield strength between the composites. In contrast, a large difference was recognized in maximum strength and ductility between the composites. Maximum strength and ductility of W-Ag and W-(BAg-8) composites were generally much inferior to those of W-Cu composite. (2) Inferior mechanical properties of W-Ag composite to W-Cu composite are attributed to heterogeneous distribution of Ag-phases, whilst inferior mechanical properties of W-(BAg-8) composite to W-Cu composite are attributed to large pores at grain boundaries.

Abstract: Room-temperature deformation behavior in the localized regions of W-80vol%Cu composite was investigated in this study. Plastic strain of about 10 % was added to the specimen at room temperature using a three-point bend testing machine. Then deformation behavior in a localized region was investigated using a high-magnification SEM photograph before and after deformation. Results are summarized as follow. (1) In a localized region near a large pore, heterogeneous deformation had occurred as expected. (2) In a region apart from a pore, mostly homogeneous deformation occurred but occasionally heterogeneous deformation occurred.