Study On High Temperature Oxidation Resistance Of Mo-Si-B Ternary Silicides

Mo-Si-B alloys are attractive materials in the field of high temperature structural applications for their high melting temperature (above 2000°C), excellent strength, good creep resistance and oxidation resistance at high temperatures. In this paper, Mo-Mo₅SiB₂-Mo₃Si and Mo₅SiB₂-based alloys were chose to study. Mo- Mo₅SiB₂-Mo₃Si samples with the same composition were prepared by arc-melting, directional solidification, vacuum annealing, and laser-remelting. Effects of microstructure on oxidation behavior of Mo-Mo₅SiB₂-Mo₃Si alloys were investigated, and a useful process for improving the oxidation resistance of this alloy was suggested. Mo₅SiB₂-based alloys were prepared by arc-melting then vacuum annealing. Effects of small variations of alloy compositions on microstructure and elevated temperature oxidation resistance of Mo₅SiB₂-based alloys were investigated.The microstructures of directionally solidified Mo-10Si-14B alloys are composed of significant coarse Mo, Mo₅SiB₂ and Mo₃Si phases. With growth rate increasing, microstructures of directionally solidified alloys refine. The average sizes of Mo phase of 10 mm/h and 30 mm/h samples are 18.88μm and 11.85μm, respectively. Rapid oxidation has been observed for the directionally solidified Mo-Mo₅SiB₂- Mo₃Si samples. The cyclic oxidation kinetics data exhibit initial fast mass loss followed by slow mass loss at all test temperatures. Compared with the sample of growth rate of 30 mm/h, the sample of growth rate of 10 mm/h oxidizes faster at all tested temperatures. In the initial oxidation, Mo phase is subject to oxidation preferentially in Mo-Mo₅SiB₂-Mo₃Si alloys. Quite irregular oxide scales are formed on the surface of samples with the growth rate of 10 mm/h and 30 mm/h, and regular oxide scales are not formed even after 100 h oxidation. The reason of poor oxidation resistance for directionally solidified samples is due to very coarse Mo phase. On exposure to air, since large size Mo particles oxidize into MoO3, after volatilization of MoO3, large pores are left in the scale. The pores are too large to be filled by B-SiO₂ flow in short time, so continuous, compact and protective oxide scales can't easily form on the surface of directionally solidified samples. Laser-remelting significantly refines the microstructure of annealed Mo- Mo₅SiB₂-Mo₃Si alloy. The average sizes of Mo phase of annealed and laser-remelted samples are 4.62μm and 1.00μm, respectively. Compared with annealed and directionally solidified samples, laser-remelted samples have the best oxidation resistance at all tested temperatures. During oxidation, the diffusion of Si along the grain boundaries is accelerated in the fine grain alloy, and the formation of protective scales (B-SiO₂) on the surface of laser-remelted Mo-Mo₅SiB₂-Mo₃Si alloy is promoted in the early stage of oxidation. For 10 min exposure, laser-remelted sample forms uniform and continuous scale; however, annealed sample forms relatively sound and continuous scale after 100 h oxidation. Therefore, laser-remelting treatment is a promising process for improving the oxidation resistance of this alloy.The arc-melted microstructure of Mo-12.5Si-25B alloy consists of a 3-phase Mo-Mo₅SiB₂-Mo₃Si eutectic embedded in Mo₅SiB₂ matrix. The cyclic oxidation kinetics data exhibits initial rapid mass loss followed by slow mass loss. In the initial oxidation, a discontinuous, irregular SiO₂ scale is formed. Then a continuous, porous SiO₂ scale is formed, meanwhile, the SiO₂ transforms from crystalline and amorphous to amorphous. In the last stage, a compact, continuous scale is formed. The regions containing the 3-phase eutectic are the preferential oxidation sites, the preferentially oxidized products are SiO₂ and Mo-Si-O oxide. After the formation of the steady oxide scale, Mo₃Si is preferentially oxidized in the Mo-Mo₅SiB₂-Mo₃Si eutectic.The microstructure of annealed Mo-12.5Si-25B alloy consists of a 3-phase Mo- Mo₅SiB₂-Mo₃Si eutectic embedded in Mo₅SiB₂ matrix with discrete Mo particles of the order of 1μm distributed in the matrix. The microstructure of annealed Mo-14Si-28B alloy is composed of Mo₅SiB₂ as matrix with Mo5Si3 and MoB as dispersions. Because of the existence of Mo5Si3 and MoB phases and higher Si and B content, a sound, continuous B-SiO₂ layer is easily formed on Mo-14Si-28B alloy in the early stage of oxidation. This is consistent with the result that the oxidation resistance of Mo-14Si-28B alloy is much better than that of Mo-12.5Si-25B alloy at 1200°C and 1300°C according to cyclic oxidation kinetics data.