Phase equilibria and structural investigations in the system Al-Fe-Si.

The Al-Fe-Si system was studied for an isothermal section at 800 °C in the Al-rich part and at 900 °C in the Fe-rich part, and for half a dozen vertical sections at 27, 35, 40, 50 and 60 at.% Fe and 5 at.% Al. Optical microscopy and powder X-ray diffraction (XRD) was used for initial sample characterization, and Electron Probe Microanalysis (EPMA) and Scanning Electron Microscopy (SEM) of the annealed samples was used to determine the exact phase compositions. Thermal reactions were studied by Differential Thermal Analysis (DTA). Our experimental results are generally in good agreement with the most recent phase diagram versions of the system Al-Fe-Si. A new ternary high-temperature phase τ ₁₂ ( cF 96, NiTi ₂ -type) with the composition Al ₄₈ Fe ₃₆ Si ₁₆ was discovered and was structurally characterized by means of single-crystal and powder XRD. The variation of the lattice parameters of the triclinic phase τ ₁ with the composition Al _{2+ x} Fe ₃ Si _{3- x} (-0.3 <  x  < 1.3) was studied in detail. For the binary phase FeSi ₂ only small solubility of Al was found in the low-temperature modification LT-FeSi ₂ ( ζ _β ) but significant solubility in the high-temperature modification HT-FeSi ₂ ( ζ _α ) (8.5 at.% Al). It was found that the high-temperature modification of FeSi ₂ is stabilized down to much lower temperature in the ternary, confirming earlier literature suggestions on this issue. DTA results in four selected vertical sections were compared with calculated sections based on a recent CALPHAD assessment. The deviations of liquidus values are significant suggesting the need for improvement of the thermodynamic models.