Numerical study of the size-dependent deformation morphology in micropillar compressions by a dislocation-based crystal plasticity model

In recent years, size-dependent deformation morphology has been observed in uniaxial micropillar compression experiments. After deformation, large pillar shows a barrel shape while small pillar shows a clear shear band. Meanwhile, one or more slip systems are activated at different sizes. In order to reveal the underlying mechanism of this size-dependent phenomenon, a dislocation-based crystal plasticity model is developed in this paper to simulate the uniaxial compression tests for different sizes of pillars with material imperfection. The simulation results show that: (1) The transition from barrel shape to severe shear with decreasing pillar size is well captured by the newly developed model. Two slip systems operate equally in large pillar and only one slip system is activated in small pillar. (2) The back stress induced by dislocation mutual interactions plays an important role in size-dependent deformation morphology. High back stress in small pillar impedes the second slip system to operate. (3) The critical size of transition from double slip (or multiple slip) to single slip is obtained and it is quantitatively comparable with experiments in a specific case. (4) Material softening is necessary to trigger slip band. It can be concluded that the competition between the short range back stress and the external resolved shear stress results in the transition from barrel shape to shear band during the micropillar compression tests.