Relating strain localisation to failure mechanisms in titanium alloys

Abstract

This thesis aims to provide an improved understanding of the deformation mechanisms in polycrystalline alpha titanium. For this purpose, the present study employed high resolution digital image correlation (HRDIC), in conjunction with Electron Backscattered Diffraction (EBSD) and transmission electron microscopy (TEM), to investigate the strain localisation behaviour induced by dislocation-based slip and deformation twins during uniaxial tensile loading at ambient temperature. To start with, the effect of aluminium on strain localisation was studied in binary Ti-Al alloys at the initial stage of plasticity. The comparison of HRDIC strain maps identified a significant transition of slip character between 2 wt.% and 4 wt.% aluminium additions, which is consistent with the change of dislocation arrangements from diffuse to planar as observed in TEM. Slip trace analysis demonstrated that the dominant slip system switched from prismatic to basal at low strain levels once Al concentration reached 6 wt.%. These observations are closely related to the presence of ordering and the change of critical resolved shear stress of individual slip systems. The automated in-situ HRDIC study in CP-Ti revealed that the preference of localised strain fields varied between tension twinning and slip trace formation with different loading directions and strain levels. Additionally, several typical twinning situations were characterized with local strain distribution associated with lattice misorientations. Furthermore, the development of in-situ HRDIC mapping combined with grain orientations information enabled the monitoring of twinning evolution with unprecedented detail. The study found that twinning generation accompanied by lateral growth was favoured in grains with c-axes oriented towards the loading direction and persisted almost throughout the entire plastic process until material fracture. The three-stage strain hardening was found to be associated with the competition between tension twins and slip bands for accommodating the localised strains. In addition, the examination of geometrical compatibility emphasized the significance of slip activity in stimulating different twin variants in the neighbouring grains.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Preuss (Supervisor) & Joao Quinta Da Fonseca (Supervisor)