Investigating intergranular fracture in nickel via atomistic simulations

Abstract

Alloys based on face-centered cubic (FCC) elements such as nickel (Ni) are among the most resistant to fracture. However, when embrittled by impurities, they lose their toughness and crack along grain boundaries. Though long known, this phenomenon remains poorly understood. In this thesis, we use large-scale molecular dynamics (MD) simulations to study the effects of grain boundaries (GBs) on various aspects of fracture properties in Ni, including intergranular fracture mechanisms, fracture toughness as well as crack healing. By performing statistical analysis on crack tip processes for fracture along different GBs, we revealed three distinct crack propagation mechanisms. For fracture along coherent twin boundary with the crack front along the [112] direction, no bond breaking is observed and crack advance is solely attributed to the slip of atoms at its tip due to the emission of dislocations. The dislocation process leads to the blunting of the crack tip. For fracture along [Sigma]265(100) symmetrical tilt GB, we discovered a new crack propagation mechanism, decohesion restrained by emission of dislocations (DRED). In it, bursts of brittle fracture initiate emission of dislocations, which pre- vent cracks from propagating more than a few nanometers in a single burst. For fracture along coherent twin boundary with the crack front along the [110] direction, crack propagates by brittle decohesion, which initiates dislocation emission in a similar way as DRED. However, the dislocation process does not arrest the crack due to the local hardening mechanism, which constraints the motion of dislocations. Using the method developed to calculate the critical energy release rate Gc from atomistic simulations, we also compared the toughness of fractures by these three mechanisms. In the course of investigating intergranular fracture, we discovered a new mechanism for crack healing in a 2D model. This mechanism relies on the generation of disclination dipoles due to GB migration, which can interact with the crack, causing it to advance or heal. We also demonstrate the healing of nanocracks in realistic 3D microstructures.