Atomistic Modeling of Iron and Steel

Abstract

Over the past few years, the continually increase in computing power together with developments in fabrication technology has enabled atomistic simulations of structures approaching the size of what can currently be fabricated and tested in the lab. In this work, a model for atomistic simulations of nanoscale pillars of α-Fe was developed. The molecular dynamics software LAMMPS was used to run indentation simulations of α-Fe pillars with a diameter of 200 Å, 400 Å and 600 Å at temperatures 5 K, 300 K and 400K. Simulations investigating the strain rate effect were run on pillars with diameter 200Åand 400 Å, with a strain rate in the range of 2 .1 107 s-1 to 5 . 108 s-1. "The smaller is stronger" α size effect referenced by literature on nanoscale structures was not observed, due to large amounts of work hardening in the larger pillars. The temperature effect observations  was as expected, with decreasing ultimate strength and slightly lower E-modulus as temperature was increased. The simulations showed a pronounced strain rate effect, as the lower strain rate simulations showed less work hardening for the larger pillars, showing a clear size effect where the smallest pillars were stronger. A rapid loss of strength was shown to correlate to the formation of twinning structures through the whole cross-section the pillar. Dislocation multiplication loops as described by Weinberger et al. were also observed, but only in pillars with diameters 400 Å or larger as the 200 Å diameter pillars were too small to contain these structures.