Temperature and strain rate effect of the deformation-induced phase transformation in pure titanium nanopillars oriented along [0 0 0 1]

The tensile deformation behavior is studied in pure titanium (Ti) nanopillars subjected to loading along the [0 0 0 1] orientation based on molecular dynamics (MD) simulations. The double yielding phenomenon is displayed in stress-strain curves when the deformation temperature is less than 380 K. One new type of deformation-induced phase transformation from the hexagonal close-packed (hcp) to face-centered cubic (fcc) phase has been predicted. The effects of temperature and strain rate on this type of phase transformation are systematically investigated. It is revealed that {101¯2}〈101¯1〉 twinning plays an essential role in inducing the phase transformation, which is produced through dislocation glide of multiple Shockley partial dislocations inside the {101¯2}〈101¯1〉 twin. A group of high-density stacking faults is accumulated though the continuous glide of multiple Shockley partial dislocations inside the twinning region, eventually leading to the allotropic phase transformation from the hcp to fcc phase. After twinning, two thermally activated dislocation slip processes compete with one another: Shockley partial dislocations and full dislocation slip. The deformation mechanism changes from phase transformation to dislocation slip when the temperature is higher than 380 K or the strain rate is lower than 10⁸ s⁻¹. The dislocation slip on the {101¯1} pyramidal plane is clearly observed under tensile loading at higher temperatures. Furthermore, our simulations indicate that the nucleation rate has a strong effect on the deformation mechanism on the nanoscale.