Stability of the ternary ₁₁-Al₄Fe_1.7Si intermetallic phase from experiment and Ab initio calculations

The Al-Fe-Si ternary system provides an opportunity for the design of new lightweight low cost alloys that have high melting temperature, superior strength, and excellent high-temperature corrosion and oxidation resistance. This system is exceptionally complex and exhibits up to 21 reported binary and ternary compounds. The intermetallic compound tau(11)-Al4Fe1.7Si is of particular interest due to its low theoretical density, promising mechanical properties, and low cost constituent elements. Knowledge of the exact composition range and thermal stability of this phase is crucial for further development. In this study, Al-Fe-Si alloys were fabricated and equilibrated at 800 and 950 degrees C. The composition range of tau(11)-Al4Fe1.7Si was analyzed using scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction to confirm the presence of the phase. Density functional theory calculations show that entropic contributions drive and stabilize the tau(11)-Al4Fe1.7Si phase. Based on experimental measurements, the composition range of tau(11)-Al4Fe1.7Si phase was determined to be Al-(24.3-25.5)Fe-(8.2-10.8)Si at% and Al-(24.6-25.2)Fe-(9.6-11.0)Si at% at 800 degrees C and 950 degrees C, respectively.