From refined crystal structure to structural evolution of the Al-Fe-Si system: Single-crystal X-ray analysis revealing more than expected

Abstract

The Al–Fe–Si system contains a variety of complex intermetallic compounds, among which the cubic α_c-AlFeSi has been extensively studied due to its intricate crystal structure as a quasicrystal approximant phase. In the present work, a high-pressure-synthesized (HPS) Al₇₄Fe_18.5Si_7.5 alloy and its differential scanning calorimetry (DSC)-treated counterpart were characterized mainly by single-crystal X-ray diffraction (SXRD), along with scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Firstly, the SXRD can confirm the coexistence of the HPS cubic α_c-AlFeSi phase and hexagonal τ₁₁-Al₄Fe_1.7Si phase as implied by powder X-ray diffraction (PXRD). Secondly, the transition from the cubic α_c-AlFeSi phase to the α_h-AlFeSi phase can be detected solely by SXRD instead of the dedicated electron backscatter diffraction (EBSD) technique. Thirdly, SXRD can discriminate the twins of θ-Al₁₃Fe₄, coexisting with γ-AlFeSi and pure Al, all of which are the byproducts of DSC-treated HPS sample, which could otherwise only be characterized by cumbersome EBSD and/or high-resolution transmission electron microscopy (HRTEM). Fourthly, the refined crystal structures of the aforementioned α_c-AlFeSi, τ₁₁-Al₄Fe_1.7Si, α_h-AlFeSi, and γ-AlFeSi can be provided by the SXRD, contributing to the topological analysis of the building units of these complex intermetallics. Last but not least, it also reveals that SXRD can discriminate the subtle differences in site occupancies of Al atoms in the α_c-AlFeSi and α_c-AlMnSi crystal structure models with the Im$\overline{3}$ space group. The fundamental differences between the Pm$\overline{3}$ and Im$\overline{3}$ space groups were fully understood by comparative analysis of both refined crystal structure models obtained by SXRD in the Al-Fe-Si and Al-Mn-Si systems.