Micromechanics of twin nucleation and growth in magnesium alloys
File(s)
Author(s)
Paramatmuni, Chaitanya
Type
Thesis
Abstract
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).
Version
Open Access
Date Issued
2020-07
Date Awarded
2020-12
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Dunne, Fionn
Sponsor
Imperial College President's Scholarship
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)