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 10$3$s$\mathrm{<mrow\; data-mjx-texclass="ORD">–</mrow>}1$. 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 {11$\overline 2$4\}\; twinning\; as\; a\; rare\; deformation\; twinning\; mode\; at\; room\; temperature\; in\; coarse-grained\; commercial\; purity\; titanium\; is\; presented.\; In\; addition,\; the\; ballistic\; testing\; results\; of\; the\; Ti–6Al–4V\; alloy\; highlighted\; very\; different\; deformation\; twinning\; characteristics.\; The\; deformation\; twinning\; of\; Ti–6Al–4V\; was\; limited\; to\; only\; the\; \{10$\overline 1$2\}\; and\; \{11$\overline 2$1\}\; tensile\; twinning\; modes.\; The\; two\; tensile\; deformation\; twinning\; types\; have\; very\; different\; morphologies\; in\; equiaxed\; fine\; grained\; Ti–6Al–4V.\; \{11$\overline 2$1\}\; deformation\; twins\; span\; multiple\; grain\; boundaries\; and\; \{10$\overline 1$2\}\; deformation\; twins\; reorient\; entire\; grains\; to\; a\; twinned\; orientation.\; This\; observation\; provides\; evidence\; for\; whole\; grain\; twinning\; of\; equiaxed\; fine\; grained\; Ti–6Al–4V\; by\; \{10$\overline 1$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 {10$\overline 1$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.