• PDF / 1,837,229 Bytes
• 12 Pages / 612 x 792 pts (letter) Page_size
• 99 Downloads / 216 Views

DOWNLOAD

REPORT

---

First Principles Calculations of Defects in Unstable Crystals: Austenitic Iron G.J.Ackland, T.P.C.Klaver and D.J.Hepburn School of Physics, University of Edinburgh, Edinburgh, Scotland EH9 3JZ 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 calculations 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 structure 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 (001) 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. Another plausible reference state is the antiferromagnetic (001) single layer (AFM-1). 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 the calculated defect formation energies have fairly high dependence on the reference state chosen: in some cases this is due to instability of the reference state, a problem which does not seem to apply to AFM-d and AFM-1. Furthermore, there is a correlation between local free volume magnetism and energetics. Despite this, a general picture emerge that point defects in austenitic iron have geometries similar to those in simpler, non-magnetic, thermodynamically stable FCC metals. The defect energies are similar to those in BCC iron. 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.

INTRODUCTION First-principles calculations have proved to be a very reliable method of obtaining information about radiation-induced defects in transition metals. In early work on non-magnetic elements such as Mo and V it was shown that the pseudopotential plane wave method reproduced formation energies and barriers to within 0.1eV [1]. It also showed that the strain fields associated with interstitials were much smaller than had been predicted by interatomic potentials, such that reliable values could be calculated with supercells as small as 100 atoms. Application to steels is a more complicated task on account of their multicomponent nature and the crucial role of carbon in mechanical properties, however work on ferritic Fe and FeCr bcc alloys showed a number of unexpected outcomes. In particular, the binding energy of a Cr atom in Fe is positive, in apparent conflict with the phase diagram which shows a miscibility gap. This conundrum was resolved when it was shown