Interplay of plasticity and phase transformation in shock wave propagation in nanocrystalline iron

Nina Gunkelmann,Eduardo M Bringa,Herbert M Urbassek,Diego R Tramontina

doi:10.1088/1367-2630/16/9/093032

Abstract

Strong shock waves create not only plasticity in Fe, but also phase transform the material from its bcc phase to the high-pressure hcp phase. We perform molecular-dynamics simulations of large, 8-million atom nanocrystalline Fe samples to study the interplay between these two mechanisms. We compare results for a potential that describes dislocation generation realistically but excludes phase change with another which in addition faithfully features the bcc → hcp transformation. With increasing shock strength, we find a transition from a two-wave structure (elastic and plastic wave) to a three-wave structure (an additional phase-transformation wave), in agreement with experiment. Our results demonstrate that the phase transformation is preceded by dislocation generation at the grain boundaries (GBs). Plasticity is mostly given by the formation of dislocation loops, which cross the grains and leave behind screw dislocations. We find that the phase transition occurs for a particle velocity between 0.6 and 0.7 km s−1. The phase transition takes only about 10 ps, and the transition time decreases with increasing shock pressure.

Highlights

Fe is a prototypical material showing phase transformation both induced by temperature (from the low-temperature bcc to the high-temperature fcc phase) and by pressure
Fe is a prototypical material showing phase transformation both induced by temperature and by pressure
In this paper we present large-scale shock simulations in polycrystalline Fe with a new interatomic potential that describes both the phase transition and dislocation-based plasticity in the bcc phase, as it has been recently shown for homogeneous compression of Fe samples [17]

Summary

Introduction

Fe is a prototypical material showing phase transformation both induced by temperature (from the low-temperature bcc to the high-temperature fcc phase) and by pressure. Up until now the typical three-wave structure could not be found in the simulation [15]; more precisely, recent molecular-dynamics (MD) simulations [14] show a phase transition lacking previous dislocation activity in the Fe samples. We use MD simulations to study the propagation of shock waves through a nanocrystalline Fe sample with the aim of identifying the interplay of plasticity and phase transformation.

Results

Conclusion