Strain-induced variant selection in heterogeneous nucleation ofα-Ti at screw dislocations inβ-Ti

Abstract

Heterogeneous nucleation of the $\ensuremath{\alpha}$ to $\ensuremath{\beta}$ phase transition at ${\ensuremath{\langle}1\phantom{\rule{4pt}{0ex}}1\phantom{\rule{4pt}{0ex}}1\ensuremath{\rangle}}_{\ensuremath{\beta}}$-type screw dislocations in pure titanium is examined through a combination of elasticity theory and molecular dynamics simulations using a modified embedded atom method potential. These screw dislocations act as heterogeneous nucleation sites and increase the $\ensuremath{\alpha}$ phase growth rate but also restrict the orientation of the $\ensuremath{\alpha}$ nuclei to certain directions, along which the strain field of the dislocation aligns with the strain required to complete the Burgers transformation path. Simulations and elasticity theory predict the same three $\ensuremath{\alpha}$ phase variants along the same preferential directions for $\ensuremath{\alpha}$ nucleus growth in the early stages of transformation. Previous elasticity theory calculations indicate that this growth does not result in the elastically preferred habit plane for the $\ensuremath{\alpha}$ nucleus. Molecular dynamics simulations on many-layer supercells presented here show that large $\ensuremath{\alpha}$ plates will change their growth direction toward the predicted habit plane, but this rotation is resisted by the line tension of the dislocation until the $\ensuremath{\alpha}$ precipitate detaches from the dislocation.