Phason-strain field and grain growth: a scanning focused ion beam and transmission electron microscopy study in Al–Li–Cu icosahedral phase

We succeeded to favour grain growth in single-phased Al–Li–Cu quasicrystal samples by annealing treatment under confined atmosphere. Using scanning focused ion beam (FIB) microscopy we can image the monograins with a good spatial and angular resolution. FIB observations carried out on Al–Li–Cu quasicrystal samples show that grain size, which is below 0.1 mm in the starting samples, reaches 0.5 mm after 10 days annealing. It reveals regular grain boundaries both before and after annealing. According to electron diffraction performed on these samples, the grain growth is accompanied by a reduction of the deviations to icosahedral symmetry, i.e., a phason-strain field reduction. Since, for symmetry reasons, the phason-strain field in Al–Li–Cu quasicrystal is not coupled to the phonon strain field, its reduction cannot be a consequence of phonon strain elimination. Our observations suggest that phason-strain field elimination occurs during grain growth. We propose an interpretation for such effect based on the grain boundary displacement. In this frame it is shown that grain boundary displacement is associated with phason-strain relaxation.