Highly oriented microtube arrays of decagonal quasicrystal approximants in Al20Si20Mn20Fe20Ga20 high-entropy alloy

The unique structures of micro/nanotubular materials have received extensive attention, while the micro/nanotubular structures of quasicrystals or quasicrystal-related phases, especially oriented arrays, are rarely found. Here, oriented microtube arrays of high-entropy decagonal quasicrystal approximants are prepared in Al20Si20Mn20Fe20Ga20 alloy by simple melt spinning. Transmission electron microscopy revealed microtubes are mostly composed of alternating arrangements of (1/0, 2/1) quasicrystal approximant domains (lattice constants: a = 0.73 nm, b = 1.23 nm, and c = 2.24 nm) and defective (1/0, 2/1) domains. However, heat treatment leads to the change of the structure and morphology of microtubes. At 873 K, a primitive cubic phase existed, and at 973 K, microtube morphology disintegrated and the (1/0, 2/1)-related domain transformed into complex domains, showing instability. By nanoindentation testing, microtubes show the hardness of 10.9 GPa and the elastic modulus of 189.3 GPa. However, after heat treatment at 873 K, these values decreased to 5.3 GPa and 128.4 GPa, and to 3.2 GPa and 107.9 GPa at 973 K. We demonstrate that the formation of microtubes is mainly related to the temperature gradient, twinning growth of approximant domains, and also the insufficient supply of raw materials in the later stages of solidification.