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The core aim of this research project was to improve understanding of the effects of microstructure and crystallographic texture on the high strain rate plastic deformation behaviour of the industrially important Titanium alloy, Ti-6Al-4V. To facilitate this study, four rolled plates of Ti-6Al-4V, with varying thermo-mechanical processing histories, were provided by TIMET Corp., the world’s largest supplier of Titanium product. To determine the nature of each plate’s microstructure and the crystallographic texture of the dominant α phase, the four Ti-6Al-4V plates were microstructurally characterised using techniques such as optical microscopy and electron backscatter diffraction (EBSD). To determine the effects of the measured microstructures and textures on the strain rate dependent plastic deformation behaviour of the four Ti-6Al-4V plates, uniaxial compression and tension tests were carried out in the three orthogonal material orientations at quasi-static (10^-3 s^-1) and high strain rates (10^3 s^-1) using a standard electro-mechanical test device and split-Hopkinson pressure bars (SHPB), respectively. To provide further understanding of the effects of microstructure and texture on the plastic deformation behaviour of Ti-6Al-4V, this time under complex impact loading conditions, the classic Taylor impact experiment was adapted to include an optical measurement and geometry reconstruction technique. A novel experimental setup was designed that consists of an ultra-high speed camera and mirror arrangement, allowing the Taylor impact specimen to be viewed from multiple angles during the experiment. Using the previously mentioned optical measurement and geometry reconstruction technique, it was then possible to gain valuable, previously unobtainable, data on the deformation history of Taylor impact specimens in-situ, such as the major/minor axes of the anisotropically deforming elliptical specimen cross-sections as a function of time and axial position, true strain as a function of time and axial position, and the true strain rate as a function of axial position. The technique was verified by testing a specimen cut from the in-plane material orientation of a clock-rolled high purity Zirconium plate. The output measurements from a post-deformation image frame were compared with measurements of the recovered specimen made using a coordinate measurement machine (CMM), with analysis showing excellent agreement between the two techniques. The experiment was then carried out on specimens cut from the two orthogonal in-plane material orientations of one of the four Ti-6Al-4V plates. Analysis of the data from these experiments gave significant insight into the plastic deformation behaviour of macroscopically textured Ti-6Al-4V under complex impact loading. Recovered Ti-6Al-4V specimens from the outlined Taylor impact experiments were then sectioned along specific planes and microstructurally characterised using EBSD, with comparisons made between the pre and post-deformation microstructures. From this analysis, and the previously discussed geometry reconstruction technique, insight was gained into the effects of micro-texture on the general anisotropic plastic deformation behaviour of Ti-6Al- 4V plate materials and in particular the role of micro-texture on the formation of deformation twins. Finally, the understanding gained from these experiments, and a detailed review of the literature, was used to inform a novel, physically based material modelling framework, capable of capturing the effects of microstructure and texture on the strain rate and temperature dependent plastic deformation behaviour of Ti-6Al-4V. The model was implemented in the computational software package, MATLAB, and verified by comparison with the mechanical characterisation results from one of the Ti-6Al-4V plates. A number of frameworks are discussed for implementing the new Ti-6Al-4V model within finite element (FE) analysis software packages, such as ABAQUS, LS-DYNA and DEFORM. It is hoped that the new Ti-6Al-4V model can be used to optimise the design of Ti-6Al-4V components and structures for impact loading scenarios.

The optimisation of compressor stage aerofoil and fan blade design remains an important area of titanium alloy research and development for aerospace gas turbines. Such research has important implications for critical and sensitive component integrity and efficiency. In particular, a better understanding of how deformation twinning interacts with microstructural features in titanium alloys is required, because such twinning facilitates plastic deformation at a higher strain rate than dislocations. To investigate this behaviour, commercial purity titanium and the titanium alloy Ti–6Al–4V were subjected to ballistic impact testing at room temperature with a high strain rate of 10³s⁻¹. In addition, a detailed analysis was conducted of three manufacturing processes of Ti–6Al–4V (wt. %) that are likely to cause deformation twinning: metallic shot peening, laser shock peening and deep cold rolling. The results presented in this thesis have furthered the understanding of the role of deformation twinning in the plastic deformation of α-phase titanium. Key findings of the research include the characterisation of deformation twinning types and the conditions that favour certain deformation twinning types. From the analysis of the ballistic testing of commercial purity titanium, the first definitive evidence for the existence of {112‾4} twinning as a rare deformation twinning mode at room temperature in coarse-grained commercial purity titanium is presented. In addition, the ballistic testing results of the Ti–6Al–4V alloy highlighted very different deformation twinning characteristics. Commercial purity titanium deformed plastically by a combination of {101‾2} and {112‾1} tensilve twinning and {112‾4} and {112‾2} compression twinning modes. By contrast, the deformation twinning of Ti–6Al–4V was limited to only the {101‾2} and {112‾1} tensile twinning modes. The two tensile deformation twinning types have very different morphologies in equiaxed fine grained Ti–6Al–4V. {112‾1} deformation twins span multiple grain boundaries and {101‾2} deformation twins reorient entire grains to a twinned orientation. This observation provides evidence for whole grain twinning of equiaxed fine grained Ti–6Al–4V by {101‾2} twinning. Grain boundary interactions between various deformation twinning types and alpha phase grain boundaries in commercial purity titanium and Ti–6Al–4V are reported and analysed. In commercial purity titanium {101‾2} as well as other deformation twinning types were observed interacting across alpha phase boundaries and higher angle alpha phase grain boundaries. The analyses of the manufacturing processes of Ti–6Al–4V highlight the very different dislocation and deformation twinning structures in surfaces processed by these techniques. A notable feature of material processed by laser shock peening is the almost complete absence of deformation twinning, contrasting with the frequent observation of extensive deformation twinning observed in the material processed by metallic shot peening and deep cold rolling. Therefore, the findings suggest that there is a strain rate limit above which deformation twinning is suppressed. The implications of this research are that a better understanding of the conditions that that favour certain deformation twinning types or propagation behaviours will enable more accurate plasticity modelling and better alloy design. This is important for the design and the manufacturing of titanium components and the high strain rate deformation to which titanium components in aerospace gas turbines can be subjected because of bird strike, foreign object debris ingestion or fan blade failures.