J Computer-Aided Mater Des (2007) 14:203–209 DOI 10.1007/s10820-007-9069-6 Monte Carlo simulations of Fe-Cr solid solution Mikhail Yu. Lavrentiev · Duc Nguyen-Manh · Ralf Drautz · Peter Klaver · Sergei L. Dudarev Received: 10 May 2007 / Accepted: 8 June 2007 / Published online: 6 March 2008 © Springer Science+Business Media B.V. 2008 Abstract Iron-chromium alloys are characterised by a complex phase diagram, the small negative heat of formation at low Cr concentrations in bcc α-structure of Fe and by the inversion of short-range order parameter. We present Monte Carlo simulations of Fe-Cr alloy based on cluster expansion (CE) approximation for the enthalpy of the system. The set of cluster expansion coefficients is validated versus the DFT results on small clusters in bcc structure. The enthalpy of mixing is negative at small Cr concentrations up to high temper- atures. Also, at small concentrations chromium atoms are well separated from each other. Clustering of Cr atoms begins at concentrations of about 10% at 800 K and 20% at 1400 K. Short-range order parameters were calculated and it was confirmed that negative values of the first and second parameters at low Cr concentrations change sign at about 10.5% Cr, in agreement with experiment. We demonstrate that complex ordering reactions in Fe-Cr and its properties may be described by 12 concentration-independent expansion coefficients. Keywords Fe-Cr alloys · Monte Carlo · Cluster expansion · Simulation 1 Introduction The research and development of improved materials for future fusion power plant is widely recognized as the key element in the development of new energy sources [1]. One of the most serious challenges for the current work on prospective fusion reactors is the development of materials able to perform reliably for long time under specific conditions, which include high irradiation levels (total dose of 50 to 200 dpa) and high temperature (up to 1000 ◦ C). M. Yu. Lavrentiev (B ) · D. Nguyen-Manh · S. L. Dudarev EURATOM/UKAEA Association, Culham Science Centre, Abingdon OX14 3DB, UK e-mail: Mikhail.Lavrentiev@ukaea.org.uk R. Drautz Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK P. Klaver School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, Northern Ireland, UK 123