Grain Boundary Structure and Chemistry

Abstract

The importance of understanding the atomic and chemical structure of grain boundaries in intermetallic compounds for a fundamental comprehension of their fracture behavior is the principal theme of this contribution. Since intermetallics are the prime examples of quasi-brittle materials, we first discuss general features of brittle fracture in ductile materials. Ll2 intermetallic compounds, in particular Ni3Al, have been studied most extensively; therefore, we review in detail the present state of our understanding of the atomic and chemical structure of grain boundaries in these alloys and discuss possible reasons for intrinsic brittleness of their grain boundaries. Results of recent atomistic studies of grain boundaries in Ll2 alloys are then described. First, we concentrate on comparison of the boundary structures in Ni3Al and Cu3Au, the two alloys with the same crystal structure but rather different propensities to ordering. Second, we discuss the effects of temperature and bulk nonstoichiometry on the structure and chemistry of grain boundaries in Ni3Al. At this point, the most important finding is that significantly different structures are invoked by segregation of nickel and aluminum, respectively, which may be related to the fact that only nickel-rich alloys may be ductilized by boron alloying. Finally, we discuss briefly the structure and properties of grain boundaries in NiAl and assess the possibilities and limits of further research on atomic level behavior of interfaces in intermetallic compounds.