Segregation effects in the fracture of brittle materials: CaAl 2O 3

Abstract

The fracture properties of the model ceramic system CaAl 2O 3 have been studied by strength measurements at controlled flaw sizes and direct observations of crack propagation, with a view to determining the effect of grain-boundary Ca segregation. In contrast to the assumptions of previous works the fracture properties are not controlled by the grain-boundary properties alone, but by an increasing toughness with crack extension, a T-curve, observed in all the microstructural variations examined (changing grain-size and grain-boundary Ca concentration). The origin of the T-curve is identified as the formation and subsequent rupture of ligamenting bridges of material, acting as restraining elements behind the crack tip. Models of the bridging process are developed to quantify the T-curves underlying the observed strength behavior, highlighting the equal roles played by the intrinsic interfacial (grain-boundary) properties, and the toughening mechanisms, in determining the overall fracture response of these and similar polycrystalline materials.