Effects of orientation, lattice defects and temperature on plasticity and phase transition in ramp-compressed single crystal iron

Molecular dynamics (MD) simulations have been used to investigate both deformation and bcc-hcp structural phase transition in single crystal iron under ramp compression as function of orientation, initial defects and temperature conditions using the recently developed modified version of the Ackland iron potential. At initial temperature of 50 K, significant anisotropy is observed in the deformation in the form of twinning followed by a hardening-like effect in [001] direction, elastic deformation in [111] direction, while [110] direction exhibits the most outstanding feature characterized by yielding via simultaneous twinning and massive homogeneous nucleation of hcp phase resulting in a metastable phase mixture where dislocations develop. However, after introducing microvoid defects in the initial lattice, plastic deformation is observed in all the three directions. Plasticity is mainly mediated by dislocations activities except for the [001] direction where dislocations kinetics is found to be very slow and the plasticity remains dominated by twinning. When the initial temperature is increased above 200 K, twinning is no longer observed, while dislocation density and velocity tend to increase. This dependence of elastic-plastic history on loading direction and initial conditions is found to strongly affect subsequent phase transition.