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 Al 15 Mn 3 Be 2 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 H1 has a hexagonal structure with lattice parameters aˆ1:2295 nm and cˆ2:4634 nm. The space group is P6 3 =mmc. In the injection-cast samples, the quasicrystal coexists with another closely related hexagonal phase H2 with aˆ1:2295 nm and cˆ1:2317 nm with a possible space group of P6=mmm. This phase exhibits speci®c orientation relationships with the icosahedral quasicrystal given by ‰0001Š hex ==2f QC and ‰011 10Š hex ==5f QC , where 2f QC and 5f QC represent twofold and ®vefold axes respectively. Electron diOE raction patterns from both phases exhibit a close resemblance to the quasicrystalline phase. It is shown that the H1 phase is closely related to m-Al 4 Mn with the same c parameter while the a parameter is reduced by ½. Following Kreiner and Franzen, it is postulated that both structures (H1 and H2) can be understood by a simple hexagonal packing of I13 clusters.

Hexagonal phases consisting of icosahedral chains/networks in (0 0 1) layers in Al–Cr–Si alloys

“…7. The simulated [0 0 1] EDP based on this model agrees with that observed by Kim et al [12]. In this context, it should be mentioned that a chain of four interpenetrated icosahedra occurs also often in orthorhombic approximants of quasicrystals with a lattice parameter of 1.24 nm [31,32].…”

“…Owing to the large Y atoms, this interpenetrated icosahedral chain is enlarged to a length of 1.4579 nm. Recently, Kim et al [12] reported another hexagonal phase in Al 15 Mn 3 Be 2 with a = 1.2295 nm and they pointed out that this is τ times smaller than that of the -Al 4 Mn. Its structure is possibly related to the four interpenetrated icosahedra part of the 4+2 chain of -Al 4 Mn.…”

“…For comparison purpose, the lattice parameters of the hexagonal H phase found recently in Al 15 Mn 3 Be 2 [12] are also included in this table. The crystal structure of the hexagonal ␤ is well known in the binary Al-Mn system as the -Al 10 Mn 3 type [11] and in the ternary Al-Mn-Si system as the ␤-Al 9 Mn 3 Si type [10].…”

“…On the other hand, the crystalline and quasicrystalline phases in Al-Cr-Si alloys are less studied. Though the cubic Al 13 Cr 4 Si 4 [6,7] and Table 1 Hexagonal Al-Cr-Si phases (for comparison purpose, the hexagonal H-(AlMnBe) phases [12] are also included) consisting of interpenetrated icosahedral chains along 1 0 0 directions (2) Deviated ∼+23 • from 1 0 0 (Fig. 11), non-periodic a Proposed in the present paper.…”

Quasicrystalline phase in melt-spun Al–Mn–Be ribbons

“…However, it was discovered that beryllium strongly increased the quasicrystalline forming ability in well-known Al-Mn quasicrystalline alloys [9][10][11]. The metastable quasicrystalline phase was present in the microstructure even when conventional casting methods were used; with relatively slow cooling rates in the range of 100 K/s.…”