X-rays structural analysis and thermal stability studies of the ternary compound α-AlFeSi

From literature data presently available, the decomposition temperature and the nature of the decomposition reaction of the ternary compound α-AlFeSi (also designated as αH or τ5) are not clearly identified. Moreover, some uncertainties remain concerning its crystal structure. The crystallographic structure and thermochemical behaviour of the ternary compound α-AlFeSi were meticulously studied. The crystal structure of α-AlFeSi was examined at room temperature from X-ray single crystal intensity data. It presents hexagonal symmetry, space group P63/mmc with unit cell parameters (293 K) a=12.345(2) Å and c=26.210(3) Å (V=3459 Å3). The average chemical formula obtained from refinement is Al7.1Fe2Si. From isothermal reaction-diffusion experiments and Differential Thermal Analysis, the title compound decomposes peritectically upon heating into θ-Fe4Al13(Si), γ-Al3FeSi and a ternary Al-rich liquid. Under atmospheric pressure, the temperature of this reversible transformation has been determined to be 772±12 °C.