Precipitation mechanisms of intermetallic compounds in W–Mo–Ni–Fe alloys

Abstract

W–Mo–18at.%Ni–8at.%Fe alloys with different ratios of W to Mo (Mo: 15–59 at.%) were liquid phase sintered, in order to investigate the effect of Mo on the microstructural evolution of W–Mo–Ni–Fe alloys, phase composition, and precipitation mechanism of the intermetallic compounds. Results indicated that the increased concentration of Mo in the matrix, as a result of increased Mo in the alloy, not only reduced the solidification temperature of the matrix phase from 1415 °C to 1336 °C, but also promoted the formation of a MoNi type intermetallic compound, (W 1− a Mo a ) 0.5− X -(Ni 1− b Fe b ) 0.5+ X , ( X = 0–0.04). The molar ratio of Mo in this intermetallic compound was found to increase with increase in the Mo concentration in the alloy, but the precipitation temperature of this intermetallic compound was very consistent, ranging from 1349 °C to 1355 °C. Owing to the varying solidification temperature of the matrix phase with the variation of alloy composition, the phase transformation mechanism of the intermetallic compound was found to be determined by the molar ratio of Mo to W and Mo (M Mo/M W + M Mo) in the original alloy composition. It is a monoeutectic reaction when this ratio is higher than 0.66, or eutectic reaction when this ratio ranges between 0.5 and 0.66. When this ratio is lower than 0.5, the precipitation of the intermetallic compound takes place by either eutectoid or peritectoid reaction. However, no intermetallic compound was found for a ratio lower than 0.2, because of the low diffusion rates of atoms associated with a large temperature difference between the solidification temperature of the matrix phase and the precipitation temperature of the intermetallic compound.