Embedded-atom study of grain boundary segregation and grain boundary free energy in nanosized iron-chromium tricrystals

We present a theoretical study of equilibrium grain boundary (GB) segregation in a tricrystal setup using a thermodynamically accurate iron-chromium embedded-atom potential. Through continuous variation of the chemical potentials, the full concentration range is explored in the temperature range of 600 K - 1100 K, evaluating segregation below and above the critical temperature of the miscibility gap. Key findings are: i) the GB excess entropy is rather small and shows only weak variation with temperature; ii) due to the small lattice mismatch in the iron-chromium system, segregation can reasonably be predicted by comparing the T = 0 energies of iron and chromium; iii) every atomic site in the defects undergoes the occupation probability transition at a different chemical potential, resulting in stepwise segregation isotherms; iv) the extraordinary thermodynamic properties of the iron-chromium system lead to negative GB free energies on the chromium-rich side; v) the width of the GB segregation zone decreases with temperature and depends significantly on the bulk concentration; vi) the additional triple junction (TJ) excess segregation is small relative to the GBs and for the most part a geometric effect of the finite width of the GB segregation zones. Available experimental data of GB segregation on the iron-rich side are in good quantitative agreement with the presented results. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.