Growth morphology of icosahedral Al–Mn–Pd single quasicrystals

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Abstract
Stoichiometric as-cast ingots of composition Al70Mn9Pd21, Al70Mn15Pd15 and Al70Mn17Pd13 have been heat treated in several different ways and have subsequently been investigated by scanning electron microscopy (SEM) and electron channelling studies in order to characterize the growth morphology as well as the degree of structural order of the stable icosahedral quasicrystal phase. Icosahedral single-quasicrystal units, typically 2 mm in size, are formed for the first of these three compositions, while in the two other specimens up to 0.5 mm units have been found. Icosahedral Al-Mn-Pd single quasicrystals have a strongly developed cleavage normal to the fivefold axes and to a lesser degree also normal to the twofold and threefold axes. Curved surfaces of crushed single quasicrystals are microscopically rough, i.e. they are composed of steps. The ideal growth shape of stable icosahedral Al70Mn9Pd21 quasicrystals is found to be the Archimedian {4,6,10} polyhedron. This shape has been found on the basis of SEM observations on faceted microholes in slowly cooled icosahedral single quasicrystals, and it is suggested to be the equilibrium shape of the icosahedral Al-Mn-Pd quasicrystal. Faceted single quasicrystals of an Al70Mn17Pd13 alloy showed on their surfaces isolated nucleation terraces with a step height of 1·5 μm. The facets of similar single quasicrystals in an Al70Mn15Pd15 alloy were completely covered by fractal-like dendrites while in an Al70Mn9Pd21 alloy these units had smooth surfaces. These characteristics as well as the estimated quasicrystal growth velocities for these three compositions appear to be in qualitative agreement with the predictions of a simple model of kinetic roughening by J. Toner (1991, Phys. Rev. B, 43, 915).