Establishing effective criteria to link atomic and macro-scale simulations of dislocation nucleation in FCC metals

Processes at fundamental length scales contribute collectively, in a statistical manner, to the macro-scale effects observed at length scales several orders of magnitude higher. To derive useful quantities pertaining to real material properties from atomic scale simulations, it is critical to evaluate the cumulative effect of multiple atomic- scale defects at the 'meso'- and 'micro'- scales. This study aims to develop a phenomenological model for atomic scale effects, which is a critical step towards the development of a comprehensive meso-scale simulation framework. In moderate loading conditions, dislocations in FCC metals are dictated by thermally activated processes that become energetically favourable as the stress approaches a threshold value. The nudged elastic band technique is ideal for evaluating the energetic activation parameters from atomic simulations, in order to evaluate the stress, temperature and rate dependence of a process. On this basis, a constitutive mathematical model is developed for simulations at the meso-scale with respect to the atomic activation parameters, to evaluate the critical (local) shear stress threshold. Once models are established for multiple effects, such as dislocation junction formation, cross-slip, and nucleation, the threshold temperature and stress for a transition between different effects can be evaluated. For example, the threshold temperature can be evaluated during heating, beyond which an immobilised dislocation in a junction will be activated for cross-slip and will shift into an adjacent mobile slip system. This is useful to predict the rate-limiting dislocation process at each simulation timestep, by evaluating the simulation condition-dependent criteria.