Twin nucleation and variant selection in Mg alloys: An integrated crystal plasticity modelling and experimental approach

Extension twin nucleation and variant selection in magnesium alloy WE43 is investigated in experimentally characterised and deformed microstructures replicated in crystal plasticity models. Total stored (dislocation) energy density is found to identify the experimentally observed locations of twins which are not otherwise explained by global Schmid factors or local resolved shear stress criteria. A critical total stored energy of the order 0.015 Jm-2 is determined below which twin nucleation does not occur. The total stored energy density explains the locations of the observed twins and the absence of twins in parent grains anticipated to be favourable for twin nucleation. Twin variant selection has been shown to be driven by minimising locally stored shear energy density, while the geometric compatibility and strain compatibility factors only aid in partial prediction. All experimentally observed variants were correctly determined

Micromechanics of twin nucleation and growth in magnesium alloys

This thesis presents an integrated experimental and computational study that investigates the mechanistic drivers of twin nucleation, variant selection and twin growth in magnesium alloys. The analyses investigate microstructure sensitivity of twin nucleation in both 2D free surfaces and full 3D microstructures, while twin growth is extensively studied in pseudo-3D microstructures. It is shown that the total stored (dislocation) energy density identifies the experimentally observed locations of both classical (favourably oriented parent grains) and non-classical (unfavourable parent grains) twins. In both the cases, a critical total stored energy density of the order 0.015 Jm-2 is determined below which twin nucleation does not occur. In the case of classical twins, the local twin resolved shear stresses drive the variant selection, while it is the local shear stored energy density (that stored within the twin embryo) in the case of non-classical twins. Once these twins propagate, it is shown that the local deformation characteristics that govern their subsequent growth are influenced mainly by their crystallographic orientations. The experimental observations indicate that the intra-twin geometrically necessary dislocations (GND) density and twin-resolved shear stress (TRSS) differences within twin vary with twin crystallographic orientation, which is characterised in terms of inclination angle (that between the loading direction and twin c-axis). Further, model predictions, where the twin transformation is considered as a sequential process of reorientation followed by shear, and experimental measurements show that intra-twin average GND density increases with inclination angle, and that a reversal in the sign of intra-twin TRSS occurs as the inclination angle increases. This implies that the TRSS (backstress) within a twin is not always negative, which further suggests that the rate of twin growth is influenced by its own crystallographic orientation instead of global Schmid factor (which is based on parent grain orientation).Open Acces

Statistical analyses of the relationship between inclination angle and twin growth in uniaxial compression of Mg alloys

Inclination angle (IA), which is that between the c-axis of a twin and the global loading direction, satisfactorily captures trends observed in area fractions of both Schmid and non-Schmid twins. The detailed analyses also show that the non-Schmid twins form preferentially in smaller grains compared to that of Schmid ones.</p