Strain-induced structural transformation of single-phase Al–Cu–Fe icosahedral quasicrystal during mechanical milling

Abstract

A single-phase stable icosahedral quasicrystalline sample of high quality with the composition Al_62.5Cu₂₅Fe_12.5 was produced by the spray forming technique. The material was further investigated by mechanical milling under an argon atmosphere to avoid oxidation during milling. At the initial stages of milling (within 5 h) a significant broadening of the diffraction peaks was observed, indicating a reduction of crystallite size and the introduction of lattice strain, which can be linked to phason strain of the quasilattice. Line broadening was noticed to increase with increasing milling time and in the material milled for longer time only a few broad diffraction peaks, which can be identified as a nanoscale bcc phase (i.e. disordered B2 phase, a ∼ 2.9 Å), were visible. At this stage the diffraction signals belonging to the quasicrystals were no longer observable, indicating a complete transformation of the quasicrystals into the bcc phase. Finally, the bcc phase formed during milling transformed back to the quasicrystalline phase during subsequent annealing treatment. The microhardness measured on the milled powders was found to decrease with increasing milling time, most likely as a consequence of the increased volume fraction of the ductile bcc phase. Attempts are made to rationalize the structural transformation.

Keywords:

• quasicrystalline alloy
• ball milling
• nanomaterial
• phase transformation
• intermetallic
• indentation hardness

Acknowledgements

The authors would like to thank Prof. G.V.S. Sastry and Prof. R.K. Mandal for many stimulating discussions. FA acknowledges the PIEAS for financial support. KBS is grateful for the financial support provided by the DAAD. The author (NKM) would like to thank Alexander von Humboldt Foundation for Research Fellowship during which a part of the work was completed. The partial financial support from Department of Science and Technology (DST), New Delhi, India, is also thankfully acknowledged.