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Reactive Modeling of Mo 3Si Oxidation and Resulting Silica Morphology

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Document pages: 34 pages

Abstract: Oxidation and corrosion have a significant economic footprint. Mo-based alloys are a strong candidate for structural materials with oxidation resistance at high temperatures. However, understanding of the mechanisms remains limited as experimental techniques do not reach atomic-scale resolution. To address this problem, we examined the mechanism of oxidation of Mo3Si (A15 phase) in Mo-Si-B alloys using chemically detailed reactive simulations. We used new simulation protocols for layer-by-layer oxidation and simple force fields for reactants, intermediates, and products to analyze the emergence of a superficial silica scale up to the large nanometer scale and explain available experimental data. Growth of thin superficial silica layers as a function of temperature and oxidation rate on the (001) surface involves the formation of silica clusters, rings, and chains with pore sizes of 0 to 2 nm. An increase in temperature from 800 to 1000 °C slightly decreased the pore size and lead to less accumulation of Mo oxides at the interface, consistent with observations by electron tomography and energy dispersive X-ray spectroscopy (EDS). The elimination of gaseous MoOx is essential to form open channels and much larger pores up to 100 nm size as observed by electron tomography, in-situ transmission electron microscopy (TEM) and scanning electron microscopy (SEM) as the oxide phase grows. According to the simulation, these large pores would otherwise be closed. The rate of oxidation, represented by successive oxidation of layers of variable thickness per unit time, influences the structure and cohesion of silica layers. High rates of oxidation can destabilize and break apart the silica layer, supported by a very wide pore size distribution in electron tomography. Limitations of the simulations in time scale restrict the full analysis to few-layer oxidation. Within these bounds, the proposed simulation protocols can provide unprecedented insight into the role of (hkl) surfaces, grain boundaries, and various alloys compositions in atomic-level detail up to the 100 nm scale.

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