An Ab Initio Study of Vacancies in Disordered Magnetic Systems: A Case Study of Fe-Rich Fe-Al Phases.

Ivana Miháliková,Martin Friák,Mojmír Šob,David Holec,Nikola Koutná

doi:10.3390/ma12091430

Abstract

We have performed quantum-mechanical calculations to examine the impact of disorder on thermodynamic, structural and electronic (magnetic) properties of Fe-Al systems with vacancies. A series of supercells was used and their properties were computed employing density-functional theory (DFT) as implemented in the VASP package. Our case study is primarily aimed at a disordered solid solution FeAl but we have compared our results also with those obtained for the ordered FeAl intermetallic compound for which experimental data exist in literature. Both phases are found in Fe-Al-based superalloys. The Fe-18.75at.%Al solid solution was simulated using special quasirandom structures (SQS) in three different disordered states with a different distribution of Al atoms. In particular, we have considered a general disordered case (an A2-like variant), the case without the first nearest neighbor Al-Al pairs (a B2-like distribution of atoms) and also the case without both the first and second nearest neighbor Al-Al pairs (the D0-like variant, in fact, an Fe-rich FeAl phase). The vacancy formation energies as well as the volumes of (fully relaxed) supercells with vacancies showed a large scatter for the disordered systems. The vacancy formation energies decrease with increasing concentration of Al atoms in the first coordination shell around the vacancy (an anti-correlation) for all disordered cases studied. The computed volumes of vacancies were found significantly lower (by 25–60%) when compared with the equilibrium volume of the missing atoms in their elemental states. Lastly, we have analyzed interactions between the vacancies and the Fe atoms and evaluated vacancy-induced changes in local magnetic moments of Fe atoms.

Highlights

Vacancies are among the most frequently occurring defects in solids
Before we proceed with calculations of vacancies in disordered Fe-Al states, which are the main topic of our study, we first test our methodology in the case of the Fe3 Al intermetallic compound, which has been intensively studied in the past both theoretically and experimentally
Our computed vacancy formation energies for all three possible types of vacancies are summarized in Table 1 together with other computational as well as experimental data

Summary

Introduction

Vacancies are among the most frequently occurring defects in solids. Their concentration and importance grow with increasing temperature, in particular when approaching the melting point.Their higher concentrations can be, found in samples at low temperatures after quenching higher-temperature states containing higher numbers of vacancies and taking advantage of sluggish kinetics at lower temperatures.Materials 2019, 12, 1430; doi:10.3390/ma12091430 www.mdpi.com/journal/materialsThe complexity of vacancy-containing systems increases with increasing number of sublattices (where each of them represents atomic sites with a specific local crystallographic environment), the number of chemical species present in studied materials and, last but not least, other degrees of freedom involved (such as different magnetic states which we consider below). Vacancies are among the most frequently occurring defects in solids Their concentration and importance grow with increasing temperature, in particular when approaching the melting point. Their higher concentrations can be, found in samples at low temperatures after quenching higher-temperature states containing higher numbers of vacancies and taking advantage of sluggish kinetics at lower temperatures. The complexity of vacancy-containing systems increases with increasing number of sublattices (where each of them represents atomic sites with a specific local crystallographic environment), the number of chemical species present in studied materials and, last but not least, other degrees of freedom involved (such as different magnetic states which we consider below). Quantum-mechanical calculations can provide atomic-scale site-specific information about each particular type of vacancies including those in magnetic systems, see, e.g., Refs. Quantum-mechanical calculations can provide atomic-scale site-specific information about each particular type of vacancies including those in magnetic systems, see, e.g., Refs. [1,2,3,4,5,6,7,10,11,12,13,14,15,16,17,18,19,20,21]

Methods

Results

Conclusion