Segregation and the strength of grain boundaries

Abstract

A quantitative theory is presented which explains how solute segregation alters grain boundary strength. A simple treatment shows that, because of an incorrect assumption, previous theories in which segregation was thought to cause low temperature brittleness by lowering the grain boundary work of fracture are incorrect and that it is the brittle fracture stress that is altered. The decrease in grain boundary brittle fracture stress is shown to be proportional to the level of segregation and also to the excess size of the segregant atoms over those of the matrix. From the tem­perature dependence of the yield stress in a given alloy, it is then simple to calculate the impact transition temperature shift caused by the segrega­tion of various species. An evaluation for the temper brittle steel, AISI 3340, gives a good numerical correlation with the published transi­tion temperature shifts for the segregation of P, Sn, Sb, As, Si, Ge and Bi. The theory also explains why C, B and Be are remedial to the embrittle­ment of pure iron.