On the correlation between deformation twinning and Lüders-like deformation in an extruded Mg alloy: In situ neutron diffraction and EPSC.4 modelling

The current work focuses on the yielding and immediate post-yielding deformation of fine-grained and coarse-grained ZM20 Mg alloys obtained by extrusion. Compressive deformations along the extrusion direction, known to be governed by profuse twinning are examined in detail. It is shown that the fine-grained alloy exhibits Lüders-like plateaux suggesting heterogeneous transition from elastic to plastic deformation. This is due to the cooperative twinning of neighbouring grains which is promoted in the fine-grained alloy by the high internal stresses borne by the parent grain families in the vicinity of yielding, and the auto-catalytic nature of twin nucleation. The elasto-plastic response of tested alloys was also simulated using version 4 of the Elasto-Plastic Self-Consistent (EPSC) model. The finite initial fraction (FIF) assumption is employed to account for the stress relaxation related to the twin nucleation process. It is shown that the new EPSC.4 model is superior to its previous version as it enables realistic predictions of the development of elastic lattice strains in variously oriented grain families and the macroscopic stress-strain response of a polycrystalline aggregate undergoing profuse twinning.