FCC vs. BCC Competition in Al–Fe–Ni–X Medium-Entropy Alloys: The Role of Ni Enrichment and Electronic Structure

Abstract

This study investigates energetic competition between face-centered cubic (FCC) and body-centered cubic (BCC) configurations in Al–Fe–Ni–X (X = Cr, Co, Mn) medium-entropy alloys upon Ni enrichment, using representative 16-atom FCC/BCC supercells compared by density functional theory. Ni enrichment does not produce a universal FCC-stabilizing response: for the Cr family, the FCC–BCC energy difference increases from + 35.48 to + 73.48 meV atom − 1 (a shift of + 38.00 meV atom − 1 toward BCC), whereas the Co and Mn families decrease from + 148.36 to + 89.11 and from + 111.96 to + 84.73 meV atom − 1 , respectively (shifts of − 59.25 and − 27.23 meV atom − 1 toward FCC), although BCC remains the lowest-energy configuration in all six compositions. These energetic trends are accompanied by element- and structure-dependent changes in the d-character density of states near the Fermi level, most pronounced for Co, and by chemistry-dependent Mulliken charge redistribution between Al and the transition-metal sublattice. Structural analysis reveals a measurable BCC lattice distortion upon Ni enrichment, together with opposite volumetric responses in the two structures (BCC expansion of 5.7–10.1%, FCC contraction of 2.5–5.6%). Overall, FCC–BCC competition in Al–Fe–Ni–X alloys cannot be captured by a single descriptor such as VEC, but instead reflects a chemically specific reorganization of the electronic structure.

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