MD modeling of screw dislocation influence upon initiation and mechanism of BCC-HCP polymorphous transition in iron

Abstract

Abstract. The present work is devoted to classical molecular dynamics investigation into microscopic mechanisms of the bcchcp transition in iron. The interatomic potential of EAM type used in the calculations was tested for the capability to reproduce ab initio data on energy evolution along the bcc-hcp transformation path (Burgers deformation + shuffle) and then used in the large-scale MD simulations. The large-scale simulations included constant volume deformation along the Burgers path to study the origin and nature of the plasticity, hydrostatic volume compression of defect free samples above the bcc to hcp transition threshold to observe the formation of new phase embryos, and the volume compression of samples containing screw dislocations to study the effect of the dislocations on the probability of the new phase critical embryo formation. The volume compression demonstrated high level of metastability. The transition starts at pressure much higher than the equilibrium one. Dislocations strongly affect the probability of the critical embryo formation and significantly reduce the onset pressure of transition. The dislocations affect also the resulting structure of the samples upon the transition. The formation of layered structure is typical for the samples containing the dislocations. The results of the simulations were compared with the in-situ experimental data on the mechanism of the bcc-hcp transition in iron.

A method of solid–solid phase equilibrium calculation by molecular dynamics

“…In small samples (a few hundred of atoms or even larger) under uniform bulk compression or uniaxial compression, plastic deformation is absent up to rather high deformations, and, as a result, high metastability is attained. So, as reported in [18], the classical MD simulation of uniform compression of iron with the embedded atom model (EAM) shows the degree of overcompression to reach dozens GPa before the bcc-hcp transition is activated. Moreover direct MD modeling of shock loading may create metastable states arising due to extremely strong plasticity that cannot be accommodated by traditional mechanisms of plastic deformations (see for example [19,20]).…”