Development of molybdenum alloy with high toughness at low temperatures

The irradiation embrittlement and recrystallization embrittlement of molybdenum and its alloys are major problems for their application to plasma facing components in the future fusion reactors. In order to overcome the problems, the basic idea of alloy design and microstructure control was proposed, and a TiC dispersed molybdenum alloy was developed by applying mechanical alloying and hot isostatic pressing treatments. The alloy, designated as MTC-02, contained an additive of 0.2 mass% titanium carbide. TEM observations revealed that the characteristic of microstructures of the alloy was extremely small grain size, a high density of dislocations and a fairly large number of very fine particles of Ti-oxy-carbide which exist along grain boundaries and in grain interiors. The low-temperature toughness of MTC-02 in four different states, i.e., the as-rolled, stress-relieved, recrystallized and neutron irradiated states, was examined by impact three-point bending tests using miniaturized hat specimens (1 mm x 1 mm x 20 mm). For comparison, the same examination was conducted for TZM alloy which is known to have the highest low-temperature toughness among the commercially available molybdenum alloys. It is shown that in each of the four states MTC-02 exhibits a much lower ductile-brittle transition temperature and a considerably higher strength than TZM, and the fracture of MTC-02 is completely transgranular, indicating that the grain boundaries are strengthened. It is argued that MTC-02 is unsusceptible to the detrimental effect of oxygen impurity.