Grain boundary sliding and critical grain size for dislocation storage

Aluminum Σ3 grain boundary sliding enhanced by vacancy diffusion

N Du et al

Grain boundary sliding is an important deformation mechanism for elevated temperature forming processes. Molecular dynamics simulations are used to investigate the effect of vacancies in the grain boundary vicinity on the sliding of Al bi-crystals at 750 K. The threshold stress for grain boundary sliding was computed for a variety of grain boundaries with different structures and energies. These structures included one symmetrical tilt grain boundary and five asymmetrical tilt grain boundaries. Without vacancies, low energy Σ3 grain boundaries exhibited significantly less sliding than other high energy grain boundaries. The addition of vacancies to Σ3 grain boundaries decreased the threshold stress for grain boundary sliding by increasing the grain boundary diffusivity. A higher concentration of vacancies enhanced this effect. The influence of vacancies on grain boundary diffusivity and grain boundary sliding was negligible for high energy grain boundaries, due to the already high atom mobility in these boundaries.

Critical grain size for dislocation storage and consequences for strain-hardening of nanocrystalline materials

O Bouaziz et al

We consider strain-hardening of nanostructured materials and propose a physically based interpretation of their low strain-hardening capability in terms of a reduced storage rate of dislocations. The model suggested provides a modification of the Kocks-Mecking-Estrin (KME) evolution law for dislocation storage for nanostructured materials and predicts a critical grain size below which the strain-hardening rate drops off.