Structure, bonding and property changes due to grain boundary segregation

Solute segregation to grain boundaries can have profound effects on interfacial properties, such as the cohesive strength. Examples include the segregation of P, S or Sb to grain boundaries in iron causing them to fracture intergranularly and the segregation of B to Ni-rich grain boundaries in Ni3Al which results in a change in fracture mode from intergranular to transgranular. Solute segregation is expected to induce changes in both the local structure and local bonding at grain boundaries and thus influence the cohesive strength and fracture mode of boundaries. To obtain insight into the influence of solute segregation on boundary properties, studies are needed on the effects of segregation on the structure and bonding of grain boundaries. This paper reviews studies of the effects of local chemistry on the structure of boundaries in Fe and the structure and bonding of boundaries in Ni3Al. The segregation of S, Sb or Au to small and large angle [001] twist boundaries in Fe is shown to cause changes in the dislocation structure of the boundaries. The presence of different dislocation configurations and the variation in chemistry associated with them give evidence for the occurrence of a phase transformation in the interfaces induced by solute segregation. To understand the beneficial effects of B on the fracture properties of grain boundaries in Ni3Al, the chemistry and electronic structure at boundaries were studied using energy dispersive X-ray spectroscopy and electron energy loss spectroscopy in the scanning transmission electron microscope. B is shown to restore bulk Ni3Al-like bonding at grain boundaries. The changes in bonding induced by B segregation to Ni-rich grain boundaries in Ni3Al are shown to improve the cohesive strength of boundaries. The influence of Ni-enrichment on boundary properties will also be examined. The relationship between measured changes in the structure and local bonding at boundaries, and the fracture properties of grain boundaries will be discussed.