Effect of molybdenum on grain growth of W-Mo-Ni-Fe heavy alloys

The grain growth phenomena of tungsten heavy alloys with various concentrations of Mo were investigated. The early formation of a eutectic liquid phase with the alloying of Mo to W-Ni-Fe facilitated growth and spheroidization of tungsten grains in the initial stage of isothermal hold. However, when the concentration of molybdenum was so high as to leave behind non-dissolved Mo grains, coalescence of W grains around Mo grains resulted in the formation of larger W-Mo grains having Mo-rich cores. These larger grains later grew at the expense of the smaller grains and grain growth was statistically very fast. During grain growth in the later stage of isothermal hold, W had a higher potential than Mo to precipitate from the liquid phase onto the grains. This competitive behavior led to the gradual accumulation of Mo atoms in the liquid phase, which not only retarded the growth of grains, but also caused precipitation of intermetallic phases in the interfaces between the solid grains and liquid phase during cooling.