Influence of point defects and grain boundaries on plasticity and phase transition in uniaxially-compressed iron

Abstract Using Molecular Dynamics (MD) computations we have reported recently a shear stiffening (hardening-like) effect in [001]-oriented defect-free iron single crystal. This effect makes shift the structural phase transition to a particularly high pressure level under ramp compression, but not under shock compression, so that the question arises about the essential role of the compression path and wave evolution in the solid state response including phase transition. Here, plasticity and phase transition are studied for iron samples under non-equilibrium uniaxial compression. Our focus is put on the effects of grain size and point defects such as atoms vacancy. Consistently with our previous results, it is found that, after yielding via twinning, both defect-free and defective iron single crystals exhibit a hardening-like effect while the twins progressively recede upon further compression and no dislocations activity is detected. This leads to mounting of the deviatoric stress. Yet, the transition onset pressure shift and temperature behavior are found to be defect-density dependent. Considering polycrystalline lattice, plasticity consists in complex interplay of intra-grain twinning and dislocation activities from grain boundaries. The elastic stiffening is attenuated now as compared to single crystal so that the phase transition is found to occur at much lower pressure in polycrystalline iron.