Al-Fe-Ge: Phase equilibria and new ternary compounds at 400 °C

The ternary system Al-Fe-Ge was investigated for a partial isothermal section at 400 °C, evaluating the all-solid phase equilibria in the Al-rich corner of the system for the first time. Investigations with SEM/EDX were combined with extensive XRD studies using powder- as well as single-crystal techniques. Four new compounds labelled τ5 to τ8 were identified and characterized. Complex phase equilibria in the partial section with Fe< 50 at% include all ternary phases from τ1 to τ8 with the exception of τ2 which is replaced by the low-temperature phase τ5 at 400 °C. The τ5 phase (Al1−xFeGe1+x) was found to crystallize in a ternary variant of the La2Sb-type (I4/mmm) and was characterized by Rietveld refinements. The compound τ7 (Al4.5−xFeGe1+x) is related to the β-phase in the Al-Fe-Si system (Al4.5FeSi). Its crystal structure was determined by single-crystal XRD in space group Cmce. The crystal structures of τ6 and τ8 were not determined up to now, but their stability with respect to decomposition into neighbouring phases was confirmed by additional liquid-solid diffusion couple experiments at 470 °C.