Variation of phase composition of Mo-Si-B alloys induced by boron and their mechanical properties and oxidation resistance

Mo-12Si-xB (x = 5, 8.5, 17 at%) alloys, doped with 0.9 wt% nanometer La2O3, were fabricated via a powder metallurgical process. The distribution of the alpha-Mo, Mo3Si and Mo5SiB2 phases existed in these alloys was dependent on the volume fractions of the individual phases. The Mo-12Si-5B (-52 vol% of alpha-Mo) and Mo-12Si-8.5B (similar to 46 vol% of alpha-Mo) alloys possessed a continuous alpha-Mo matrix where Mo3Si/Mo5Si132 particles distributed dispersedly and the former exhibited the highest fracture toughness. Our theoretical analysis demonstrated that the crucial contribution of crack trapping mechanism to the toughness and the alpha-Mo volume fraction increasing effectively promoted this mechanism. Besides, the alpha-Mo/intermetallic interface debonding and the formation of intragranular microcracks induced by La2O3 provided extra contributions to toughness. The Mo-12Si-17B alloy (similar to 65 vol% of intermetallic) formed an intermetallic matrix with embedded semi-continuous alpha-Mo colonies and other isolated alpha-Mo islands, having the highest compression strengths at 800-1300 degrees C. Especially at 1200-1300 degrees C, the intermetallic matrix with low plastic deformation played a significant role in reinforcing alloy. Besides, the intragranular La2O3 particles strengthened alloy by pinning down and accumulating dislocations in the grain interior. The Mo-12Si-17B was also found to have the excellent oxidation resistance at 1000 degrees C owing to exhibiting the shortest duration and minimal mass loss in the transient oxidation stage. Moreover, higher B content decreased the viscosity of the borosilicate and thereby formed a dense and protective glassy borosilicate scale quickly, which substantially reduced the growth of inner oxide layers in the steady oxidation stage. In addition, the La2O3 as a blocking phase could impede the permeation of MoO3 bubbles/clusters through the flowing SiO2 glass scale.