Effect of Si/B ratio on the microstructure and mechanical properties of lanthanum oxide-doped Mo-Si-B alloys

An investigation of the effect of the Si/B ratio on the microstructure and mechanical properties of high temperature Mo-Si-B alloys, doped with 0.9 wt.% lanthanum oxide (La₂O₃), is presented. Mechanical properties are measured at room temperature for three α-Mo-Mo ₃Si-Mo₅SiB₂ containing alloys produced by powder metallurgy processing. It is found that the alloys with Si/B ratios of 2.4 and 1.4 possess a continuous α-Mo matrix with homogeneously distributed intermetallic particles while the alloy with the Si/B ratio of 0.7 is a homogeneous mix of α-Mo, Mo₃Si and Mo₅SiB ₂ phases without continuous ones obsevered. Measurements on mechanical properties show that the flexural and compression strength, and fracture toughness of Mo-Si-B alloys were increased with the Si/B ratio raised. The continuous α-Mo phase with high volume fraction is beneficial for ameliorating the room temperature strength and fracture toughness.