Structure and mechanical properties of (Mo, Re)Si2 alloys

Hot hardness tests have been performed on arc-melted (Mo, Re)Si2 alloys at temperatures from ambient to 1300°C. Re is found to be a potent solution hardening addition to MoSi2 over the entire temperature range, much more than would be expected from their atomic size mismatch or modulus mismatch. In addition ReSi2–x has about twice the hardness of pure MoSi2 at all temperatures. This may be related to the structure of ReSi2–x which nominally has the same tetragonal C11b structure as MoSi2 but is Si-deficient (x=0.25, i.e. the composition is Re4Si7). The Si deficiency is accompanied by some unusual incommensurate periodicities in the structure which themselves give rise to orthorhombic and monoclinic distortions. The resulting microstructure probably explains the high strength of Re4Si7 and the potent hardening effect of Re in MoSi2 may be due to the accompanying Si vacancies.