Bimodal α-Mo grain structure inducing excellent oxidation resistance in Mo-12Si-8.5B alloy at 1100 °C

Abstract

Bimodal Mo-12Si-8.5B-1.0wt%ZrB2 alloys were fabricated via hot pressing sintering of a mixture of mechanical alloying Mo-Si-B-ZrB2 powders and unmechanical alloying Mo-Si-B-ZrB2 powders in selected proportion. The bimodal Mo-12Si-8.5B-1.0wt%ZrB2 alloys consisted of fine and coarse α-Mo grains, showing mainly bimodal structure with a part of submicron-scaled α-Mo grains and partial micron-scaled α-Mo grains, where the intermetallics showed weak bimodal distribution but with fine grain structure. Oxidation behavior of the alloys was studied at 1100 °C in air. When average grain sizes of the fine and coarse α-Mo grains in the bimodal alloy were 0.45 μm and 2.15 μm, respectively, and their volume fraction ratio was approximately 1.7:1, a superior oxidation resistant property was achieved. Its mass loss in the transient stage was decreased, showing a 62% decrease compared to that of the ultrafine-grained alloy, and the oxidation recession rate of the alloy substrate was decreased significantly. Additionally, a thinner borosilicate glass scale was formed after oxidation for 30 h which exhibited excellent protective capability. The improved oxidation resistance at 1100 °C was mainly because of the combination of lower oxidation rate of the coarser α-Mo grains and the higher lateral flow rate of the borosilicate glass due to the presence of partial fine α-Mo grains and intermetallics.