A Bond-Order Potential Incorporating Analytic Screening Functions for the Molybdenum Silicides

Abstract

The intermetallic compound MoSi₂, which adopts the C11_b crystal structure, and related alloys exhibit an excellent corrosion resistance at high temperatures but tend to be brittle at room and even relatively high temperatures. The limited ductility of MoSi₂ in ambient conditions along with the anomalous temperature dependence of the critical resolved shear stress (CRSS) of the {110)<111], {011)<100] and {010)<100] slip systems and departure from Schmid law behavior of the {013)<331] slip system can all be attributed to complex dislocation core structures. We have therefore developed a Bond-Order Potential (BOP) for MoSi₂ for use in the atomistic simulation of dislocations and other extended defects. BOPs are a real-space, O(N), two-center orthogonal tight-binding formalism that are naturally able to describe systems with mixed metallic and covalent bonding. In this development novel analytic screening functions have been adopted to properly describe the environmental dependence of bond integrals in the open, bcc-based C11_b crystal structure. A many-body repulsive term is included in the model that allows us to fit the elastic constants and negative Cauchy pressures of MoSi₂. Due to the internal degree of freedom in the position of the Si atoms in the C11_b structure which is a function of volume, it was necessary to adopt a self-consistent procedure in the fitting of the BOP. The constructed BOP is found to be an excellent description of cohesion in C11_b MoSi₂ and we have carefully assessed its transferability to other crystal structures and stoichiometries, notably C40, C49 and C54 MoSi₂, A15 and D0₃ Mo₃Si and D8_m Mo₅Si₃ by comparing with ab initio structural optimizations.