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

The intermetallic compound MoSi2, which adopts the C11b 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 C11b 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 C11b 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 C11bMoSi2 and we have carefully assessed its transferability to other crystal structures and stoichiometries, notably C40, C49 and C54 MoSi2, A15 and D03 Mo3Si and D8m Mo5Si3 by comparing with ab initio structural optimizations. Comments Copyright Materials Research Society. Reprinted from Materials Research Society Proceedings, Integrative and Interdisciplinary Aspects of Intermetallics, Volume 842, 2004 . Publisher URL: http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=2702&DID=115568&action=detail This conference paper is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/95 A Bond-Order Potential Incorporating Analytic Screening Functions for the Molybdenum Silicides Marc J. Cawkwell, Matous Mrovec, Duc Nguyen-Manh, David G. Pettifor and Vaclav Vitek Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104-6272, U.S.A. Fraunhofer Institut für Werkstoffmechanik, Wöhlerstr. 11, 79108 Freiburg, GERMANY. UKAEA Fusion, Culham Science Centre, Abingdon, OX14 3DB, UNITED KINGDOM. Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UNITED KINGDOM.