Mo-Silicide Alloys for High-Temperature Structural Applications

The challenges of a high-temperature environment (T > 1,400°C) impose severe material performance constraints in terms of melting point, oxidation resistance, and structural functionality. A number of ceramic materials, intermetallic compounds, and refractory metals with high melting temperatures are available as material choices. However, in a single-component single-phase form, these materials do not satisfy all the aforementioned requirements. One clear message from the evolutionary development of high-temperature alloys is the importance of developing multicomponent alloys with multiphase microstructures and the capability to control phase fractions and morphologies to satisfy a number of mechanical property requirements. Besides the essential structural requirements, elevated temperatures often also involve aggressive environments that require a material to display an inherent oxidation protection that can be further enhanced by coating. Among the leading candidates to advance beyond the capability of the current nickel (Ni)-base superalloys, the multiphase microstructures that can be developed in the molybdenum-silicon-boron (Mo-Si-B) system involving a high melting temperature (>2,100°C) ternary-based intermetallic Mo5SiB2 (T2) offer an attractive performance. Most of the attention has been on three-phase alloys comprised of Mo(ss), T2, and Mo3Si that offer high-temperature stability and robust microstructures, but new alloy designs are in development. In this review the recent advances in the development of Mo-silicide alloys are discussed in terms of alloy design, microstructure control, structural performance, environmental resistance, and component analysis.