Icosahedral quasicrystalline and hexagonal approximant phases in the Al-Mn-Be alloy system

Abstract

We report the formation of a primitive icosahedral quasicrystal with increased stability in Al-Mn-Be alloys close to the compound Al15Mn3Be2 by melt spinning and injection casting. The crystal structure of this compound was unknown. We show that in as-cast as well as heat treated condition the intermetallic phase HI has a hexagonal structure with lattice parameters a = 1.2295 nm and c = 2.4634nm. The space group is P63/mmc. In the injection-cast samples, the quasicrystal coexists with another closely related hexagonal phase H2 with a = 1.2295nm and c = 1.2317nm with a possible space group of P63/mmc. This phase exhibits specific orientation relationships with the icosahedral quasicrystal given by [0001]hex//2f QC and [0110]hex//5f QC, where 2f QC and 5f QC represent twofold and fivefold axes respectively. Electron diffraction patterns from both phases exhibit a close resemblance to the quasicrystalline phase. It is shown that the H1 phase is closely related to μ-Al4Mn with the same c parameter while the a parameter is reduced by τ. Following Kreiner and Franzen, it is postulated that both structures (H1 and H2) can be understood by a simple hexagonal packing of 113 clusters.