Influence of B contents on the microstructure, fracture toughness and oxidation resistance of Mo-Si-B alloys

The Mo-10Si-(0, 5, 10, 15)B (at.%) alloys located in Mo_ss (Mo solid solution) +Mo₃Si +T2 (Mo₅SiB₂) region were prepared by arc melting. The effect of B content on the microstructure, microhardness, fracture toughness and oxidation resistance of the alloys was studied. In the Mo-10Si alloy, there are primary Mo_ss and peritectic Mo₃Si phases. With the addition of B, (Mo_ss + T2) binary and (Mo_ss + T2 + Mo₃Si) ternary eutectics emerge in Mo-10Si-(5, 10)B alloys. For Mo-10Si-15B alloy, primary Mo_ss disappears and the microstructure is only composed of binary and ternary eutectics. With the increase of B content, the proportion and grain size of Mo_ss decrease. The microhardness of alloys increases with the increase of B content due to the increasing intermetallics. The fracture toughness of alloys decreases obviously in single-edge notched three-point bending testing (SENB) and the fracture mode transforms from transgranular fracture to intergranular fracture because of the refinement of Mo_ss. In the high-temperature oxidation test at 1250 °C, the oxidation weight loss of the alloys decreases with the addition of B due to the densification of the borosilicate layer.