Tailoring Grain-Boundary Segregation to Control Mechanical Properties

Abstract

ABSTRACTRecent advances in our understanding of the role of the chemistry of grain boundaries in controlling the mechanical properties of materials (in particular intergranular brittle fracture) are reviewed. It is now possible in a modem field-emission gun (FEG) analytical transmission electron microscope (AEM) to measure the chemistry of sub-nanometer films of GB segregants while at the same time observing the effect (if any) on the bonding of the atoms within a nanometer of the boundary plane. This has been accomplished by the development of X-ray mapping (XRM) a powerful new tool for the study of segregation. For the first time, in the same instrument, on the same grain boundary, any changes in the boundary chemistry can be correlated with the occurrence or absence of brittle failure, which is often associated with boundary segregation. There is strong evidence that boundary segregation is extremely nonuniform, even in some strongly embrittling systems (e.g. Cu-Bi) and in these same systems, embrittling segregants introduce subtle but consistent changes in the bonding. Non-embrittling segregants (e.g. Ag in Cu) do not introduce detectable bonding changes.