Shock Synthesis of Five-component Icosahedral Quasicrystals

Five-component icosahedral quasicrystals with compositions in the range Al68–73Fe11–16Cu10–12Cr1–4Ni1–2 were recently recovered after shocking metallic CuAl5 and (Mg0.75Fe0.25)2SiO4 olivine in a stainless steel 304 chamber, intended to replicate a natural shock that affected the Khatyrka meteorite. The iron in those quasicrystals might have originated either from reduction of Fe2+ in olivine or from the stainless steel chamber. In this study, we clarify the shock synthesis mechanism of icosahedral quasicrystals through two new shock recovery experiments. When CuAl5 and Fe2+-bearing olivine were isolated in a Ta capsule, no quasicrystals were found. However, with only metallic starting materials, numerous micronsized five-component icosahedral quasicrystals, average composition Al72Cu12Fe12Cr3Ni1, were found at the interface between CuAl5 and stainless steel, demonstrating nucleation of quasicrystals under shock without any redox reaction. We present detailed characterization of recovered quasicrystals and discuss possible mechanisms for generating sufficiently high temperatures to reach melting with relatively weak shocks. We discuss the implications of our five-component quasicrystal for the stability of quasicrystals, which have previously only been considered in alloy systems with four or fewer components. Even small amounts of additional metals expand the stability range of the icosahedral phase and facilitate routine syntheses without extraordinary precision in preparation of starting mixtures.