Place: MSE 2016: Materials Science and Engineering Congress, Darmstadt, Germany
In many interface-dominated nanostructured materials the role of interfaces during deformation is not yet completely clarified. Very fine spacing of interfaces leads to a competition between dislocation controlled and grain boundary sliding based plasticity. To improve our understanding of this competition we have to investigate the atomistic origin of deformation in the interface region.
We present the results of molecular dynamics simulations of sliding at γ/γ interfaces in lamellar TiAl alloys, which can explain existing, seemingly contradictory, experimental results on the role of interfacial sliding during creep. We suggest that the origin of the controversy lies in the pronounced in-plane anisotropy of the shear strength of the individual interfaces, which is observed in the simulations. Experimentally, the orientation of in-plane directions with respect to the loading axis has not been monitored so far.
A multi-scale concept is introduced to capture effects of both the electronic and the atomistic level. On the one hand we carried out quasi-static calculations of multi-planar generalized stacking fault energy (MGSFE) surfaces of the interface plane as well as the adjacent layers. On the other hand molecular dynamics simulations guided by ab initio GSFE-surface calculations were carried out for different bicrystal cells under different shear loading conditions. The critical stresses for shearing some of these interfaces, which were derived from these bicrystal shear simulations, are of the same order of magnitude or even lower as those for dislocation motion in a γ-single crystal, showing that these mechanisms are competitive. In total four shear mechanisms, twin nucleation and migration/absorption, interfacial partial dislocation nucleation, rigid grain boundary sliding and grain boundary migration were observed. The comparison with the MGSFE surfaces allows to create a link between (quasistatic) physical properties such as stacking fault energies, and the (dynamic) deformation mechanisms, and hence between the results of ab-initio calculations and molecular dynamics simulations. Constitutive relationships for grain boundaries from atomistic simulations for multiscale mechanical modelling of metallic microstructures