放電焼結の基礎と応用 放電プラズマ焼結法によって作製した遷移金属炭化物分散Ｍｏ焼結合金の低温破断挙動

Abstract

In order to improve both fracture strength and ductility at low temperature, fine-grained sintered molybdenum alloys dispersed with 0.8mol% TaC or 1.6mol% TiC were prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The mixtures of fine molybdenum powder with grains measuring 20nm in diameter and carbide particles which were prepared by MA, were sintered at 1870K for 300s under 49MPa by SPS. The sintered alloys had relative densities of over 99% and grain sizes of approximately 2-5μm. The low-temperature fracture strength and ductility were evaluated by a three-point bending test at 77K to 360K. The results were compared with those for sintered alloys dispersed with 0.4-1.6mol% TaC or TiC which were prepared by MA and hot isostatic pressing (HIP). The following results were obtained.(1) Both low-temperature fracture strength and ductility increased by dispersion of carbides. This result was the same as those obtained from sintered alloys prepared by MA and HIP.(2) The alloy dispersed with TaC had a low dutile-to-brittle transiton temperature of 157K and a high fracture strength of 1860MPa at the temperature.(3) High fracture strength of the present alloy was attributed to both carbide dispersion and grain refining, though the former effect was much effective.(4) The alloy dispersed with TaC exhibited mostly intergranular fracture mode. The intergranular fracture area comprised 70% of the total fracture area. In contrast, the alloy dispersed with TiC exhibited a mixed mode of intergranular and transgranular fracture. The intergranular fracture area comprised 50%.