Role of slip and twinning on strain hardening, and correlation with geometric hardening, latent hardening, and grain boundary strengthening in Titanium

Abstract

This work investigates the role of slip & twinning on the strain hardening in Titanium (Ti) that develops profound {101¯2}<101¯1¯> extension twins (ET) during compression. The microstructure and texture evolution studies were carried out by ex-situ electron backscattered diffraction (EBSD) at intermittent strains. The strain hardening could be catagorized into multiple plastic deformation stages governed by twin-induced geometrical hardening & softening, intrinsic latent hardening, and grain boundary strengthening. Crystal plasticity was applied to simulate the flow curve and texture evolution, thereby obtaining slip and twin activities. The initial axisymmetric {101¯0} - {112¯0} fiber texture was favourable for prismatic <a> slip and ET evolution on compression. ET nucleate till ε=0.10 and broadens to consume almost the entire microstructure up to ε=0.22. The ET reorient the microstructure to a geometrically hard orientation with c-axis ∼∥ to compression direction (CD) escalating the Taylor factor (M). The ET leads to sigmoidal flow behavior from ε=0.06 to 0.17, increasing SHR in stage II. The ET domains majorly deform by pyramidal <c+a> and basal slips, though having high shear strength, and produce {112¯7}−{101¯7} fiber texture from ε=0.1 to 0.69. Beyond ε=0.11, {112¯2}<112¯3¯> contraction twins (CT) also evolve inside ET-domains, consuming ∼0.29 of its fraction. The CT reorient the ET domains to a softer {112¯1}−{112¯2} fiber texture with a low M. The geometrically soft CT domains, latent hardening and grain fragmentation contribute toward flattening the strain hardening curve and decreasing SHR in stage III after ε=0.17.