Ordering effect on the mechanical, electronic and magnetic properties of the β-based non-canonical approximant phases: β-Al₅₀Cu₃₃Fe₁₇, η-Al₅₀Cu₄₄Fe₆ and φ-Al_47.5Cu_49.5Fe₃

Abstract

Using transmission electron microscopy and X-ray diffraction, we established that the ordered η1-Al₅₀Cu₄₄Fe₆ and φ-Al_47.5Cu_49.5Fe₃ (Fmm2) alloys with nano-sized domain structure are formed by slowly cooling, whereas β-solid solutions with a short-range order were found in quenched states. The φ′-modification which exhibits the additional long-period superstructure was also observed in Al_47.5Cu_49.5Fe₃. The studies of low temperature magnetic susceptibility and heat capacity did not reveal any another phase transitions in these alloys. The indentation test showed that hardness and Young’s modulus consistently grow as β-Al₅₀Cu₃₃Fe₁₇ → η1-Al₅₀Cu₄₄Fe₆ → (φ+φ′)-Al_47.5Cu_49.5Fe₃ and approach to those in icosahedral phase. The same trend in the Young’s modulus was obtained for alloys containing β-solid solution with a short-range order. Ab initio calculations, however, predicted the opposite tendency in cubic β-Al₅₀Cu_50−xFe_x with a decrease in x, which was explained by the weakening of the covalent Fe 3d – Al sp bonding. This discrepancy between the results for β- and ordered phases, we related to a crucial effect of ordering which is accompanied by a progressive distortion of cubic local structure in the series β-Al₅₀Cu₃₃Fe₁₇ → η1-Al₅₀Cu₄₄Fe₆ → φ-Al_47.5Cu_49.5Fe_3. As we demonstrated for η-Al(Cu, Fe), these distortions lead to the strengthening of the both covalent Fe–Al and Cu–Al bonds and the higher modules.

Keywords:

• Aluminium alloys
• approximants
• phase transformations
• transmission electron microscopy
• indentation
• mechanical properties
• ab initio
• electronic structure

Acknowledgements

The authors thank Dr A.N. Yksysnikov for technical assistance in preparing samples for TEM study. The electron microscopy investigations were carried out at the Centre for Collective Use ‘Electron Microscopy’ (Ural Branch of the Russian Academy of Sciences, Ekaterinburg, Russian Federation).