Structural models of quasicrystals: significance of positron annihilation studies

Abstract

We discuss implications of our results of positron annihilation studies of variotts icosahedral quasicrystalline alloys on the structural models of this new phase. We have studied AI-Mn, A1-Mn-Si, AI-Cu-Li and AI-Cu-Fe and a two component positron annihilation lifetime spectrum seems characteristics of all these icosahedral quasicrystalline alloy phases. Analysis of the lifetime spectra yields vacancy sizes varying from monovacancies to hexavacancies with concentration of I to 8 ppm in variotts quasicrystalline alloys. These vacancy concentrations are about three orders of magnitude less than that observed in metallic glasses. We also notice that in the case of metastable quasicrystalline alloys such as A1-Mn and AI-Mn-Si, these vacancy clusters disappear during the crystallization process resulting in single lifetime spectra. On the other hand, for stable quasicrystalline alloys, two component life time spectra, which indicate the presence of vacancy clusters, continue to exist even after prolonged heat treatment. Our results seem incompatible with the space filling Penrose tiling or random tiling models and favour cluster-based models.