An Atomistically-informed Energy Based Theory of Environmentally Assisted Failure

The critical energy release rate defined as Gc = 2γ + γp (where, γ is the cohesive energy and γp 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 is unaffected. Yet, the critical energy release rate (which is predominantly described by the plastic work (γp) since γp >>γp ) significantly decreases. In this work, we recognize that as the cohesive energy (γ) decreases during corrosion due to 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 γp 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, KIC and subcritical value of stress intensity, KIscc. Extension of the approach to predict threshold fatigue crack initiation in Al7075 is suggested.