Crystal structure of the τ11 Al4Fe1.7Si phase from neutron diffraction and ab initio calculations

Abstract

The intermetallic τ11 Al4Fe1.7Si phase is of interest for high-temperature structural application due to its combination of low density and high strength. We determine the crystal structure of the τ11 phase through a combination of powder neutron diffraction and density functional theory calculations. Using Pawley and Rietveld refinements of the neutron diffraction data provides an initial crystal structure model. Since Al and Si have nearly identical neutron scattering lengths, we use density-functional calculations to determine their preferred site occupations. The τ11 phase exhibits a hexagonal crystal structure with space group P63/mmc and lattice parameters of a = 7.478 Å and c = 7.472 Å. The structure comprises five Wyckoff positions; Al occupies the 6h and 12k sites, Fe the 2a and 6h sites, and Si the 2a sites. We observe site disorder and partial occupancies on all sites with a large fraction of 80% Fe vacancies on the 2d sites, indicating an entropic stabilization of the τ11 phase at high temperature.