Transitional grain boundary structures and the influence on thermal, mechanical and energy properties from molecular dynamics simulations

The thermo-kinetic characteristics that dictate the activation of atomistic crystal defects significantly influence the mechanical properties of crystalline materials. Grain boundaries (GBs) primarily influence the plastic deformation of FCC metals through their interaction with mobile dislocation defects. The activation thresholds and atomic mechanisms that dictate the thermo-kinetic properties of grain boundaries have been difficult to study due to complex and highly variable GB structure. This paper presents a new approach for modelling GBs which is based on a systematic structural analysis of metastable and stable GBs. GB structural transformation accommodates defect interactions at the interface. The activation energy for such structural transformations was evaluated with nudged elastic band analysis of bi-crystals with several metastable 0 K grain boundary structures in pure FCC Aluminium (Al). The resultant activation energy was used to evaluate the thermal stability of the metastable grain boundary structures, with predictions of transition time based on transition state theory. The predictions are in very good agreement with the minimum time for irreversible structure transformation at 300 K obtained with molecular dynamics simulations. Analytical methods were used to evaluate the activation volume, which in turn was used to predict and explain the influence of stress and strain rate on the thermal and mechanical properties. Results of molecular dynamics simulations show that the GB structure is more closely related to the elastic strength at 0 K than the GB energy. Furthermore, the thermal instability of the GB structure directly influences the relationship between bi-crystal strength, temperature and strain rate. Hence, theoretically consistent models are established on the basis of activation criteria, and used to make predictions of temperature-dependent yield stress at a low strain rate, in agreement with experimental results. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.