α2, oxygen and the micromechanics of α-titanium

The micromechanical behaviour of titanium alloys underpins their use in critical rotating parts of aero engines. Time-dependent and spatially heterogeneous slip are key contributors to dwell fatigue crack initiation, and the aim of this work has been to contribute mechanistic understanding of factors controlling slip character and dwell sensitivity. This thesis addresses the phase metallurgy and mesoscale plasticity of Ti–Al–X and Ti–Sn alloys, which model the hcp α phase of many structural alloys. The influence of O, V and Mo on α2 Ti3Al formation in Ti–7Al (wt.%) alloys was characterised using transmission electron microscopy (TEM), atom probe tomography (APT) and small-angle X-ray scattering (SAXS). Isothermal ageing was conducted at 550 ° C for times up to 120 d. Precipitates reached sizes of 10–20 nm and 6–10% volume fraction. O was found to increase volume fraction, while V and Mo had little effect. All solutes, particularly Mo, promoted higher nucleation density and smaller precipitates. The effect of solutionising temperature on nucleation density was also investigated. Results supported the proposal that α2 nucleation is mediated by vacancy concentration prior to ageing and subsequent coarsening. Ti–7Al–0.05O and Ti–7Al–0.25O alloys in both quenched and aged conditions were investigated using tensile, creep and stress relaxation testing to explore the effects of O and α2 on micromechanics. TEM analysis of dislocation configurations after stress relaxation showed that both O and α2 restricted dislocation motion to slip bands, but that O encouraged appreciable cross-slip while α2 prevented it. In situ far-field high energy X-ray diffractometry (ff-HEDM) during creep testing revealed that interstitial oxygen promotes more frequent, smaller slip events, while α2 encouraged larger slip avalanches. These results indicate that interstitial oxygen helps smooth the stochastic nature of slip at the grain scale, while α2 worsens it. The relative merits of Al and Sn as α-stabilisers and solid solution strengtheners were assessed. For three compositions in each system around the α/α+α2 boundary, solutionised and aged microstructures were compared using TEM, SAXS and tensile testing. It was determined that, at the solubility limit for each solute, Al provides greater specific strengthening while avoiding the risk of deleterious crystallographic ordering. These results also allowed assessment of the position of phase boundaries. Drawing on the mechanistic insights for α2 formation and slip heterogeneity in α-Ti, implications for dwell fatigue crack initiation and proposals for alloy development strategies are outlined.Open Acces