An atomistically informed energy-based theory of environmentally assisted failure

Abstract

Abstract The critical energy release rate, defined as G c =2γ+γ p (where γ is the cohesive energy and γ p is the plastic work), is widely used as a macroscopic fracture criterion. During embrittlement of aluminum by gallium, impurity segregation is localized to a small region along the grain boundary and the bulk plastic properties are unaffected. Yet, the critical energy release rate (which is predominantly described by the plastic work [γ p] since γ p>>γ) significantly decreases. In this work, we recognize that as the cohesive energy (γ) decreases during corrosion due to an increase in impurity concentration, the stress needed at the notch tip to form the crack decreases, and this, in turn, decreases the plastic work by reducing dislocation emission at the notch tip. We study two different models proposed in the past that can capture this dependence of γ p on γ during liquid metal (Ga) embrittlement of aluminum alloy (Al 7075). The parameters in these models are computed from first principles atomistic calculations and recent experiments. We compare and contrast these models on their ability to describe various aspects of embrittlement such as fracture toughness, K IC, and subcritical value of stress intensity, K Iscc. Extension of the approach to predict threshold fatigue crack initiation in Al7075 is suggested.