Formation, stability and crystal structure of the σ phase in Mo–Re–Si alloys

Abstract

The formation, stability and crystal structure of the σ phase in Mo–Re–Si alloys were investigated. Guided by thermodynamic calculations, six critically selected alloys were arc melted and annealed at 1600 °C for 150 h. Their as-cast and annealed microstructures, including phase fractions and distributions, the compositions of the constituent phases and the crystal structure of the σ phase were analyzed by thermodynamic modeling coupled with experimental characterization by scanning electron microscopy, electron probe microanalysis, X-ray diffraction and transmission electron microscopy. Two key findings resulted from this work. One is the large homogeneity range of the σ phase region, extending from binary Mo–Re to ternary Mo–Re–Si. The other is the formation of a σ phase in Mo-rich alloys either through the peritectic reaction of liquid + Mo ss → σ or primary solidification. These findings are important in understanding the effects of Re on the microstructure and providing guidance on the design of Mo–Re–Si alloys.