Abstract:
Plastic deformation is thought to involve highly ‘stochastic’ phenomena, caused by the generation, motion and interactions of crystal defects called dislocations and grain boundaries. Grain boundaries (or GBs) can act as the nucleation source for dislocations in polycrystalline materials, which may become the rate-limiting phenomenon in FCC metals with grain sizes less than 15 nm. Atomistic simulations were performed to study the fundamental factors of dislocation nucleation in non-equilibrium bi-crystals of aluminium. It is shown that several metastable grain boundary structures may accommodate an identical lattice misorientation, and form several non-equilibrium states not coincidental with the global-minimum energy at room temperature. Thermodynamic mechanisms for metastability are discussed, with evidence provided by a high resolution image from the literature. A recently developed post-processing technique is applied in a novel manner to analyse the non-linear, post-yield dislocation nucleation response with respect to the cumulative dislocation length. A correlation is established between the atomic free volume and the accumulation of dislocations at the final state of the simulation. A statistically valid relationship has been demonstrated, which links the initial-state (300 K, 1 atm) GB energy and free volume, with the critical resolved shear stress for dislocation nucleation under compressive loading. The mechanisms underpinning dislocation nucleation in compression are studied and linked to localised asymmetry of the GB normal stress. Overall, this work shows that by isolating key state variables, a comparative analysis of several non-equilibrium bi-crystals may provide an effective means of studying the fundamental factors of GB effects.
