First Principles Calculations of Defects in Unstable Crystals: Austenitic Iron

TL;DR

This study uses first principles calculations to analyze defect energies in austenitic iron, addressing challenges posed by its mechanical instability at zero Kelvin and examining impurity effects relevant to radiation damage.

Contribution

It introduces a stable reference state for austenitic iron in first principles calculations and explores impurity effects on defect energetics under irradiation conditions.

Findings

The AFM-d state is a suitable reference for austenitic iron calculations.

Defect formation energies strongly depend on the chosen reference state.

Ni and Cr impurities have minimal impact on defect properties.

Abstract

First principles calculations have given a new insight into the energies of point defects in many different materials, information which cannot be readily obtained from experiment. Most such calculation are done at zero Kelvin, with the assumption that finite temperature effects on defect energies and barriers are small. In some materials, however, the stable crystal structre of interest is mechanically unstable at 0K. In such cases, alternate approaches are needed. Here we present results of first principles calculations of austenitic iron using the VASP code. We determine an appropriate reference state for collinear magnetism to be the antiferromagnetic double-layer (AFM-d) which is both stable and lower in energy than other possible models for the low temperature limit of paramagnetic fcc iron. We then consider the energetics of dissolving typical alloying impurities (Ni, Cr) in the materials, and their interaction with point defects typical of the irradiated environment. We show that using standard methods there is a very strong dependence of calculated defect formation energies on the reference state chosen. Furthermore, there is a correlation between local free volume magnetism and energetics. The effect of substitutional Ni and Cr on defect properties is weak, rarely more than tenths of eV, so it is unlikely that small amounts of Ni and Cr will have a significant effect on the radiation damage in austenitic iron at high temperatures.