Abstract
Abstract : The primary objective of the present project was to gain a basic understanding of the effects of microstructure on the creep behavior of a new class of Mo-Si-B alloys that are being considered for very high temperature structural applications. A second objective was to obtain insight into the oxidation behavior of these materials. During this one-year project, thermal effects on microstructure evolution in a Mo-7.44Si-8.51B (at.%) alloy were studied. The results indicate that it is possible to exert some control over microstructure and properties by very high temperature heat treatments. Significant changes in volume fraction of alpha-Mo, Mo3Si and T2 phases occur at temperatures >- 1700 deg C. The cyclic oxidation behavior in air at temperatures between 800 - 1100 deg C were also studied in a three-phase Mo-12Si-12B (at.%) and near-single T2 phase Mo-12.5Si-25B alloy: The results indicate that catastrophic oxidation occurs in both alloys at/below 800 - 900 deg C; performance and oxidation protection is better at >- 1000 deg C. A porous, non-protective borosilicate/B-SiO2 layer forms at low temperatures, which permits easy oxygen diffusion and increased weight loss through volatilization of Mo as Mo3 gas. A stable, dense silica scale forms at/above 1000 deg C, which provides protection from oxidation and reduced weight loss.
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