Concerning sigma-phase in molybdenum-rhenium alloys

Molybdenum-rhenium alloys containing 40 to 48% Re by weight are commonly used for many high-technology applications. Higher rhenium contents bring the alloys into a two-phase region with the presence of brittle σ-phase, an intermetallic compound with the approximate composition of Mo2Re3. Alloys with lower rhenium contents do not possess sufficient low-temperature ductility and high-temperature strength. Available phase diagrams for the molybdenum-rhenium system contain conflicting information about the single-phase/two-phase borderlines. Further, the formation of σ-phase and its influence on mechanical properties depends on the forming process and heat treatment, diffusion rate, precipitation mechanism, and morphology. Very few data on these factors are available in the literature. Three alloys were selected for study of the formation of σ-phase: Mo-41Re, Mo-44.5Re, and Mo-47.5Re. Applying different production parameters, especially heat treatments, the properties of the alloys were evaluated using microstructure examinations, electron probe microanalysis, and hardness testing. It was found that any harmful influence of σ-phase can be largely eliminated by properly adjusting and controlling the processing parameters. Modification of the chemical composition allows the presence of σ-phase to be avoided with no deterioration of low-temperature properties