First-principles study of the effect of iron on the crystal structure, stability and chemical bonding in the β-based AlCu ordered η _2-phase and the pretransition state of a solid solution

摘要

First-principles calculations showed that the thermodynamic stability of β-based ordered η _2-AlCu phase doped with Fe is due to iron substitution in the copper sublattice (Fe _(Cu)), which corresponds to the maximum number of Fe-Al bonds in the first cubic coordination polyhedron. This iron localisation leads to stable ω-like atomic displacements and pentagonal Al-nets in the (010) plane of η _2-AlCu(Fe). This phase with iron substituting copper (e/a=1.925) is an energetically preferred -based non-canonical approximant of the icosahedral phase (e/a=1.86). The energy gain for the Fe _(Cu) position is determined by strong covalent Fe3d-Al3p bonding, while there is a weak Fe3d-Cu4s3d hybridisation for the Fe _(Al) substitution. Using a composite cluster model, we demonstrate that short-range order in the pretransition state of the β-Al-Cu-Fe solid solution observed prior to the precipitation of η-phase is stabilised due to formation of Fe-Al bonds in the first cubic coordination polyhedron of the composite cluster.