Grain refinement in a metastable beta Ti alloy deformed to large strains at high strain rates

Shear punching to large plastic strains up to 40 was conducted on a metastable beta Ti alloy at various speeds, creating severe plastic deformation at high shear rates (<(gamma)over dot>) up to 200 s(-1). Grain refinement in the resulting shear bands (SBs), which experienced even faster deformation due to strain localisation, was studied using extensive TEM. Significant grain refinement occurred at <(gamma)over dot> < 40 s(-1), producing beta grains of similar to 5 -10 nm, considerably smaller than those obtained by high pressure torsion (HPT) to a much higher strain of 240 at a slow shear rate of 0.5 s(-1). The applied shear strain rate of 2 s(-1) was found to be an optimum for attaining a pure nano-beta grain structure thanks to maximum dislocation activity and complete alpha '' to beta reverse transformation. However, some stress induced alpha '' remained as <(gamma)over dot> deviated from this value and much coarser 13 grains formed at <(gamma)over dot> > 40 s(-1). The effects of strain rates on grain refinement and the minimum achievable grain sizes at high strain rates are discussed based on classical dislocation dynamics. (C) 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.