Shear response of grain boundaries with metastable structures by molecular dynamics simulations

Grain boundaries (GBs) can play a role as the favored locations to annihilate point

defects, such as interstitial atoms and vacancies. It is thus highly probable that

different boundary structures can be simultaneously present in equilibrium with each

other in the same GB, and thus the GB achieves a metastable state. However, the

structural transition and deformation mechanism of such GBs are currently not well

understood. In this work, molecular dynamics simulations were carried out to study

the multiple structures of a Σ5(310)/[001] GB in bicrystal Al and to investigate the

effect of structural multiplicity on the mechanical and kinetic properties of such a

GB. Different GB structures were obtained by changing the starting atomic configuration

of the bicrystal model, and the GB structures had significantly different

atomic density. For the Σ5(310) GB with metastable structures, GB sliding was the

dominant mechanism at a low temperature (T=10K) under shear stress. The

sliding mechanism resulted from the uncoordinated transformation of the inhomogeneous

structural units. The nucleation of voids was observed during GB

sliding at the low temperature, and the voids subsequently evolved to a nanocrack at

the boundary plane. Increasing the temperature can induce the structural transition of

local GB structures and can change their overall kinetic properties. GB migration

with occasional GB sliding dominated the deformation mechanism at elevated

temperatures (T=300 and 600 K), and the migration process of the metastable GB

structures is closely related to the thermally assisted diffusion mechanism.