The Effect of Impurity Elements on Chemical Bonding at Grain Boundaries

Impurity elements, which have very low bulk concentrations in metals, often segregate to grain boundaries where their concentrations can be greatly enriched. As a results of this segregation, the grain boundaries are often weakened so that they become preferred paths for brittle fracture. This paper presents the results of fully quantum mechanical cluster calculations which have been applied to this problem. The particular system which we have considered is the embrittlement of nickel by sulfur. Our results will show that embrittling elements are electronegative with respect to their host metal, and draw charge off of the host metal onto themselves. As a result of this charge transfer, there is less charge available to participate in metal-metal bonds and they are weakened. It is the weakening of these bonds that leads to embrittlement. The results further show that, although the qualitative model for embrittlement is independent of the cluster geometry, the degree of charge transfer and hence the amount of weakening of the metal-metal bonds does depend on geometry. Therefore, grain boundary embrittlement is not strictly a function of concentration of the impurity but of the structure as well.