Speaker
Description
Magnesium is distinguished by a highly anisotropic inelastic deformation involving a profuse activity of deformation twinning. Modeling deformation twinning poses additional challenges in comparison to plastic slip due to the critical dissimilarities in the underlying mechanisms and characteristics. A finite-strain model of coupled twinning and plastic slip is formulated by combining the phase-field method and crystal plasticity. A distinct feature of the model lies in the treatment of the kinematics of deformation twinning, which, rather than being formulated as a shear-based deformation, is expressed as a sequential operation of a volume-preserving stretch and a rigid-body rotation. The stretch-based kinematics is particularly relevant when conjugate twinning systems are crystallographically equivalent.
Instrumented micro/nano-indentation technique has been widely applied to characterize the mechanical properties of magnesium. This is usually done through the analysis of the load-indentation depth response, surface topography, and less frequently, the post-mortem in-depth microstructure. Experimental limitations, however, hinder the real-time observation of the evolving twin microstructure. Motivated by this, our model is employed to simulate the evolution of the twin microstructure and its interaction with plastic slip in a magnesium single crystal subjected to indentation. Special emphasis of the study is laid on two aspects: orientation-dependent inelastic deformation and indentation size effects. The 2D simulations reveal several interesting effects, in agreement with the existing experimental data, including an intriguing twin microstructure at large spatial scales. To further investigate size effects, we extend the model by incorporating gradient-enhanced crystal plasticity, and re-examine the notion of ‘smaller is stronger’.
