Multi-method identification and characterization of the intermetallic surface layers of hot-dip Al-coated steel: FeAl3 or Fe4Al13 and Fe2Al5 or Fe2Al5+x

Until now, several different chemical formulae are used to refer to the intermetallic θ- and η-layers formed at the surface of hot-dip aluminized steel. To clear up the subsequent confusion, both layers were identified and characterized using several experimental techniques combined with DFT calculations.11EDX: energy dispersive X-ray spectroscopy; APT: atom probe tomography; EBSD: electron backscatter diffraction; XRD: X-ray diffraction; ILEEMS: Integral Low-Energy Electron Mössbauer spectroscopy; DFT: density functional theory. EDX- and EBSD-mappings were performed on the cross section of a hot-dip aluminized steel and showed the presence of the two single-phased intermetallic layers, θ and η, just beneath the aluminum top-layer. The XRD-pattern of a sample of which the top aluminum layer was removed, confirmed the foregoing observations. The compositions of both layers were determined by APT and yield Fe4Al13 and Fe2Al5.6. DFT calculations showed the stability of the Fe4Al13 phase and predicted Fe4Al13 to be the only stable composition in that area of the Fe-Al phase diagram. Based on the DFT results, the ILEEMS spectrum of the θ-layer could be successfully analyzed and, for the first time, fully interpreted in accordance with the crystallographic structure of Fe4Al13. Fe4Al13 is suggested for referring to the composition of the θ-layer, instead of other formulae, e.g. FeAl3. The ILEEMS η-layer spectrum was best reproduced by a model-independent quadrupole splitting distribution, which supports the dis-ordered structure model for the η-layer. An analysis based on the results of the DFT calculations, confirmed this finding. Because of the partially dis-ordered structure of the η-layer and the width of the related region in Fe-Al phase diagrams, Fe2Al5+x is suggested for referring to the composition of the η-layer.