Oxidation behavior of multiphase molybdenum-molybdenum silicide-molybdenum silicide boride intermetallics

Voramon Supatarawanich Dheeradhada, Purdue University


Multiphase Mo-Mo3Si-Mo5SiB2(T2) alloys of various compositions and microstructures were investigated for oxidation resistance at 800 and 1300°C. The alloys varied in Si/B ratio from 1:1 to 1:2 and their microstructures were processed to have the Mo phase distributed as a continuous phase or as discrete particles. At 800°C, the alloys exhibited poor oxidation resistance regardless of Si/B ratio or their microstructures. Oxide scale consisted of MoO 3 and boron-containing glass. Alloys with lower Si/B ratio yielded silicate glass with lower viscosity that resulted in lower weight loss compared to the high Si/B ratio alloys. Intermediate layers of Mo and Mo-Si-O were observed adjacent to Mo5SiB2 and Mo3Si phases, respectively. At 1300°C, the multiphase alloys developed a protective glass scale with an intermediate layer of (Mo+glass) between the base alloy and outer glass scale. Boron-lean silicate glass was essential for the protection from further oxidation. For oxidation resistance at 1300°C, alloys require a high Si/B ratio. In terms of microstructures, the multiphase alloys consisting of small Mo particles uniformly dispersed in fine spacing of silicide phases provide better oxidation resistance than the alloys with continuous Mo dendrites. Glass preferentially formed at Mo3Si rather than at T2 phase. Vapor-solid diffusion experiments were carried out with single phase alloys containing either Mo3Si or T2 as the major phase, and the multiphase Mo-10Si-10B(at%) alloy. The alloys were exposed to vacuum to induce silicon loss resulting in the formation of a Mo layer. In addition, microprobe analysis was carried out for the determination of composition profiles across the (Mo+glass) layer and the external glass scale formed on the Mo-13.2Si-13.2B (at%) alloy oxidized at 1300°C. The microprobe data were analyzed for interdiffusion fluxes and interdiffusion coefficients with the aid of a computer program called "MultiDiFlux ©". The effective interdiffusion coefficients for Mo, Si, B, and O in various phases were also determined. The motion of the (metal/Mo+glass) and (Mo+glass/glass) interfaces was examined. An average effective interdiffusion coefficient of silicon in molybdenum at 1300°C was estimated as 10 -17 m2/s. Moreover, interdiffusion coefficients for each component in the Mo+glass layer were evaluated on the basis of Mishin's analysis.




Dayananda, Purdue University.

Subject Area

Materials science

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