Re effects on phase stability and mechanical properties of MoSS+Mo3Si+Mo5SiB2 alloys

In this paper, we investigate the effects of Re additions on the microstructure and mechanical properties of a ternary alloy with the composition Mo-12.5Si-8.5B (at.%). This alloy has a three-phase microstructure consisting of Mo solid-solution (MoSS), Mo3Si, and Mo5SiB2 and our results show that up to 8.4 at.% Re can be added to it without changing its microstructure or forming any brittle σ phase at 1600 °C. Three-point bend tests using chevron-notched specimens showed that Re did not improve fracture toughness of the three-phase alloy. Nanoindentation performed on the MoSS phase in the three-phase alloy showed that Re increases Young’s modulus, but does not lower hardness as in some Mo solid solution alloys. Based on our thermodynamic calculations and microstructural analyses, the lack of a Re softening effect is attributed to the increased Si levels in the Re-containing MoSS phase since Si is known to increase its hardness. This lack of softening is possibly why there is no Re-induced improvement in fracture toughness.

► Phase diagram of Mo-rich Mo–Si–B–Re system was established. ► The stable region of MoSS+Mo3Si+T2 is limited by the formation of σ phase. ► Re partitions mainly into the MoSS phase, not the intermetallic phases. ► Re addition increases the solubility of Si in MoSS phase. ► Re increases Young’s modulus, but not the fracture toughness of Mo–Si–B alloys.