Grain boundary mechanics: universal description and coarse graining
To gain a better understanding of deformation of polycrystals, we investigate the mechanisms of grain boundary and bulk deformation under a combination of shear and tensile strain by means of ab-initio calculations. The derived stress-strain relationships can be used as traction-separation laws for continuum models of deformation and fracture.
The γ-surface is a theoretical concept to calculate plasticity, that is applied to gain insight into the atomistic mechanisms of bulk and grain-boundary sliding. Density functional theory (DFT) calculations are performed to quantify energy-displacement curves and generalized-stacking fault energy (γ) surfaces of bulk aluminum and of different tilt and twist grain boundaries. Furthermore we investigate the bulk aluminum (111) plane shear process with different shear methods (affine, alias (pure and simple)) to obtain the theoretical shear strength and critical shear strain. From these results we can determine the possible sliding paths, slip vectors, atomistic structures, works of separation, energy barriers, theoretical shear strengths and critical shear strain for grain boundary sliding in particular directions. Further investigations show the dependency of the characteristic parameters for sliding on superimposed tensile strain.
Atomic structures of Σ3(111)60° twist grain boundary with different shifts. (a) and (c) represent bulk and intrinsic stacking fault configurations; (b) and (d) structures represent the unstable stacking fault and run-on configurations.
Gamma-surface of Σ3(111)60° twist grain boundary calculated by VASP. The energy landscape shows the rotational symmetry of the interface.
Gamma-surface of Σ11(1-13)129.5° tilt grain boundary calculated by VASP. The energy landscape shows a pronounced easy-sliding path // tilt axis.