A Quantum–Mechanical Study of Clean and Cr–Segregated Antiphase Boundaries in Fe3Al

Abstract

We present a quantum-mechanical study of thermodynamic, structural, elastic, and magnetic properties of selected antiphase boundaries (APBs) in Fe3Al with the D03 crystal structure with and without Cr atoms. The computed APBs are sharp (not thermal), and they have {001} crystallographic orientation. They are characterized by a mutual shift of grains by 1/2〈100〉a where a is the lattice parameter of a cube-shaped 16-atom elementary cell of Fe3Al, i.e., they affect the next nearest neighbors (APB-NNN type, also called APB-D03). Regarding clean APBs in Fe3Al, the studied ones have only a very minor impact on the structural and magnetic properties, including local magnetic moments, and the APB energy is rather low, about 80 ± 25 mJ/m2. Interestingly, they have a rather strong impact on the anisotropic (tensorial) elastic properties with the APB-induced change from a cubic symmetry to a tetragonal one, which is sensitively reflected by the directional dependence of linear compressibility. The Cr atoms have a strong impact on magnetic properties and a complex influence on the energetics of APBs. In particular, the Cr atoms in Fe3Al exhibit clustering tendencies even in the presence of APBs and cause a transition from a ferromagnetic (Cr-free Fe3Al) into a ferrimagnetic state. The Fe atoms with Cr atoms in their first coordination shell have their local atomic magnetic moments reduced. This reduction is synergically enhanced (to the point when Fe atoms are turned non-magnetic) when the influence of clustering of Cr atoms is combined with APBs, which offer specific atomic environments not existing in the APB-free bulk Fe3Al. The impact of Cr atoms on APB energies in Fe3Al is found to be ambiguous, including reduction, having a negligible influence or increasing APB energies depending on the local atomic configuration of Cr atoms, as well as their concentration.

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

“…The static lattice APB interface energy turns out to be equal to 84 mJ/m 2 . This rather low value obtained for Fe 3 Al with 3.125 at % of Cu atoms (i.e., 2 per 64 atoms) is slightly higher than the APB energy of 80 mJ/m 2 that we obtained for Fe 3 Al with the APBs characterized by the same shift in the 100 crystallographic direction and the same 64-atom supercell size in [67]. But the difference in the APB energy induced by the Cu atoms is very small and within an expected error bar of our calculations.…”

“…But the difference in the APB energy induced by the Cu atoms is very small and within an expected error bar of our calculations. This statement stems from the results of our comparative analysis of a series of supercells with different sizes and different separation of APBs in our previous study [67]. The APB energy in Fe 3 Al in our previous research exhibited values from the range of 80 ± 25 mJ/m 2 .…”

“…The first one is characterized by a shift of the interfacing grains in the 100 direction by a half of the lattice parameter defined for a 16-atom cube-shaped D0 3 cell (see red arrows in Figure 1b,c) and is associated solely with the D0 3 superlattice (D0 3 -type of APBs). We have recently computed properties of D0 3 -APBs in (i) Fe 3 Al with and without Cr additions [67] as well as (ii) in Fe-Al-Ti compounds [68]. The second type can appear both in the D0 3 superlattice and in the B2 lattice (a B2-type of APBs) when two parts of the crystal are shifted in the 111 direction by a half of the lattice parameter of the B2 cell containing two atoms (we recently examined them in Fe 70 Al 30 alloy [69]).…”

Chemical segregation and precipitation at anti-phase boundaries in thermoelectric Heusler-Fe2VAl

“…The tendency for segregation can be related to the relatively high APB energy of Fe2VAl is 0.27 ± 0.04 J/m² for a ( 2) ⟨111⟩(011) APB [26], compared to e.g. 0.08 ± 0.025 J/m² for Fe3Al [27]. The high APB energy can act as a driving force for a segregation, as segregation to APB can reduce its energy [28,29].…”

“…The high APB energy can act as a driving force for a segregation, as segregation to APB can reduce its energy [28,29]. It was shown, that Cr segregation to APBs in Fe3Al can either reduce or enhance the APB energy, depending on the position of the impurities within the matrix [27]. The locally higher composition in V, C, N can help trigger further precipitation [30].…”