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The effect of alloying elements Cr and Mn on the cohesion of the gamma -iron Sigma 11[1(1) over bar 1]/(11(3) over bar) grain boundary (GB) is investigated based on the thermodynamic model of Rice-Wang by using the first-principles density functional theory. The electronic properties are studied fur Cr/Fe and Mn/Fe systems. In these systems, the chemical effect of Cr and Mn is in favor of enhancing the cohesion of the grain boundary due to the anisotropic bonding which weakens the bonds in the grain boundary plane, but strengthens those in planes perpendicular to the grain boundary. However, the structural relaxation effect is detrimental to the cohesion of the grain boundary. After synthesizing these two effects, Cr can act as a cohesion enhancer and Mn is an embrittler. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.