Modelling of dislocations, twins and crack-tips in HCP and BCC Ti

Abstract

<p>Ti exhibits complex plastic deformation controlled by active dislocation and twinning systems. Understandings on dislocation cores and twin interfaces are currently not complete or quantita-tive, despite extensive experimental and simulation studies. Here, we determine the dislocation core, twin, and crack properties in both HCP and BCC Ti using a Deep Potential (DP), DFT and linear elastic fracture mechanics. We compute the core structures, critical resolved shear stresses and mobilities of the (a), (c+a), (c) dislocations in HCP and the (111)/2 dislocations in BCC Ti. The (a) slip consists of slow core migration on pyramidal-I planes and fast migration on prism-planes, and is kinetically limited by cross-slips among them. This behaviour is consistent with "locking-unlocking"phenomena in TEM and is likely an intrinsic property. Large-scale DFT calculations provide a peek at the screw (c + a) core and glide behaviour, which is further quantified using DP-Ti. The screw (c + a) is unstable on pyramidal-II planes. The mixed (c + a) is nearly sessile on pyramidal-I planes, consistent with observations of long dislocations in this orientation. The edge and mixed (c+a) are unstable against a pyramidal-to-basal (PB) transition and become sessile at high temperatures, corroborate the difficulties in (c)-axis compression of Ti. Cracks on basal planes are predicted and shown to be intrinsically brittle, consistent with cleavage facets primarily on this plane in experiments. Twin boundary energies vary considerably but all twin boundary structures possess a mirror reflection symmetry. Finally, in BCC Ti, the (111)/2 screw has a degenerate core with average glide on {112} planes; the (111)/2 edge and mixed dislocations have non-dissociated cores on {110} planes. This work paints a self-consistent, complete picture on all dislocations in Ti, rationalises previous experimental observations on Ti plasticity and fracture, and points to future HRTEM examinations of unusual dislocations such as the mixed and PB transformed (c + a) cores.<br></p>