The study of icosahedral quasicrystals (i-phases) is of considerable interest due to their unique physical properties and potential applications. However, the synthesis of i-phases, especially those containing a large number of components, is often a complex task, requiring long heat treatments and strict control over the composition. Shock synthesis, which is the effect of a short-term high-pressure pulse on the material, is a promising method for the rapid production of complex phases, including quasicrystals.
This paper presents a method for shock synthesis of five-component i-phases based on the Al-Cu-Fe-Cr-Si system. The initial powders of the elements were mixed in a stoichiometric ratio and exposed to a shock wave generated by an explosion. The parameters of shock compression (pressure, temperature, exposure time) were varied to optimize the synthesis conditions. The obtained samples were studied by X-ray diffraction, electron microscopy, and energy-dispersive spectroscopy.
In the course of experiments on modeling impact processes similar to the impact on the Khatyrka meteorite, icosahedral quasicrystals consisting of five elements (Al68–73Fe11–16Cu10–12Cr1–4Ni1–2) were obtained. The synthesis was carried out by impact action on a mixture of CuAl5 and olivine (Mg0.75Fe0.25)2SiO4 in a chamber made of grade 304 steel. It was assumed that the source of iron in the quasicrystals could be the reduction of Fe2+ from olivine or the chamber material.
To clarify the mechanism of quasicrystal formation during impact, additional experiments were conducted. In particular, a mixture of CuAl5 and olivine containing Fe2+ was placed in a tantalum capsule, in which the formation of quasicrystals was not observed. However, when using metallic precursors, numerous micron-sized icosahedral quasicrystals consisting of five elements with an average composition of Al72Cu12Fe12Cr3Ni1 were found at the boundary of CuAl5 and stainless steel. This indicates that quasicrystals are formed during impact without oxidation-reduction reactions.
Detailed characteristics of the obtained quasicrystals are presented and possible mechanisms for generating the high temperatures required for melting under relatively weak impact effects are discussed. The effect of the inclusion of five components on the stability of quasicrystals previously studied in systems with four or fewer elements is also considered. Even small additions of metals expand the stability region of the icosahedral phase and simplify the synthesis without strict accuracy in the preparation of the starting materials.
The results showed that shock synthesis allows obtaining the i-phase in the Al-Cu-Fe-Cr-Si system in a significantly shorter time than with traditional methods. The phase composition and microstructure of the obtained materials depend on the shock compression parameters. Optimization of these parameters allows obtaining samples with a high proportion of the i-phase and a minimum content of impurity phases. The shock synthesis method opens up new possibilities for the rapid production and study of complex quasi-crystalline materials.
Author: Julius Oppenheim, Chi Ma, Jinping Hu, Luca Bindi, Paul J. Steinhardt & Paul D. Asimou
Institute: Department of Geological and Planetary Sciences, California Institute of Technology, 1200 California Blvd., M/C170-25, Pasadena, California, 91125, USA