First-principles study of the effect of iron on the crystal structure, stability and chemical bonding in the β-based AlCu ordered η₂-phase and the pretransition state of a solid solution

Abstract

First-principles calculations showed that the thermodynamic stability of β-based ordered η₂-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 η₂-AlCu(Fe). This phase with iron substituting copper (e/a = 1.925) is an energetically preferred η-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.

Keywords:

• aluminium alloys
• ordered intermetallics
• phase stability
• electronic structure
• solid solution
• approximants

Acknowledgements

This work was supported by an RFBR grant (010-02-00602).