The Impact of Vibrational Entropy on the Segregation of Cu to Antiphase Boundaries in Fe3Al

Martin Friák,Miroslav Černý,Mojmír Šob

doi:10.3390/magnetochemistry7080108

Abstract

We performed a quantum mechanical study of segregation of Cu atoms toward antiphase boundaries (APBs) in Fe3Al. The computed concentration of Cu atoms was 3.125 at %. The APBs have been characterized by a shift of the lattice along the ⟨001⟩ crystallographic direction. The APB energy turns out to be lower for Cu atoms located directly at the APB interfaces and we found that it is equal to 84 mJ/m2. Both Cu atoms (as point defects) and APBs (as extended defects) have their specific impact on local magnetic moments of Fe atoms (mostly reduction of the magnitude). Their combined impact was found to be not just a simple sum of the effects of each of the defect types. The Cu atoms are predicted to segregate toward the studied APBs, but the related energy gain is very small and amounts to only 4 meV per Cu atom. We have also performed phonon calculations and found all studied states with different atomic configurations mechanically stable without any soft phonon modes. The band gap in phonon frequencies of Fe3Al is barely affected by Cu substituents but reduced by APBs. The phonon contributions to segregation-related energy changes are significant, ranging from a decrease by 16% at T = 0 K to an increase by 17% at T = 400 K (changes with respect to the segregation-related energy difference between static lattices). Importantly, we have also examined the differences in the phonon entropy and phonon energy induced by the Cu segregation and showed their strongly nonlinear trends.

Highlights

The Fe3 Al intermetallic compound belongs to a promising class of Fe-Al-based materials possessing relatively low density, unusual resistance to oxidation, and low cost of raw materials [1,2,3]; some of them suffer from room-temperature brittleness [4,5,6,7,8]
Focusing on the main topic of our work, that is, the segregation of Cu atoms toward the antiphase boundaries in Fe3 Al, we evaluated the segregation-related energy difference ∆U static determined from the static lattice calculations
When simulating antiphase boundaries (APBs), one Cu atoms was approaching each of the two identical APBs inside the supercells

Summary

Introduction

The Fe3 Al intermetallic compound belongs to a promising class of Fe-Al-based materials possessing relatively low density, unusual resistance to oxidation, and low cost of raw materials [1,2,3]; some of them suffer from room-temperature brittleness [4,5,6,7,8]. Earlier experimental studies of these materials [9,10,11,12,13,14,15,16,17,18,19] were recently complemented by research focused on their use in high-temperature coatings [20,21,22,23,24,25,26] or composites [27,28,29,30,31], novel preparation techniques [32,33,34,35], and their materials properties [36,37]. Our current research is focused on extended defects called antiphase boundaries (APBs). They separate two parts of the crystal mutually shifted such that the sublattices that would normally not interface do so. As the D03 structure of Fe3 Al has three sublattices (one with the Al atoms and two with the Fe atoms), APBs are among defects previously simulated in Fe3 Al [61,62,63,64]

Methods

Results

Conclusion