Thermodynamic investigation of phase equilibria in Al-Si-V system

The Al-Si-V system is assessed to understand the phase equilibria of V-containing (3d transition metal) Al-Si alloys, which are generally of great importance for technical applications. Four annealed alloys were prepared to study the phase equilibria of Al-rich corner (V ≤ 23.3 at.%, Si ≤ 57.6 at.%) in the temperature range of 500-930 °C. The microstructure and phase constituents of the samples were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS). The existence of ternary phase τ (Al0.6Si1.4V, TiSi2-type) was confirmed with the composition ranging from 17.98 to 18.59 at.% Al and 48.89-49.20 at.% Si. However, it did not equilibrate with fcc(Al) below 868 °C, which was determined by differential scanning calorimetry (DSC). The Si3V5 appeared in Al58.5Si18.3V23.2 and Al64.7Si20.2V15.1 alloys annealed at 500 °C. It nearly disappeared after 3000 h-annealing and its thermodynamic stability was discussed. According to the measured phase relationships, the thermodynamic description of Al-Si-V system was optimized combing with the enthalpies of formation of binary and ternary compounds obtained by density functional theory (DFT). The Si-V system was modified to obtain the congruent liquidus of Si3V5 and enthalpy of formation of Si3V5 and Si2V with experimental data. By comparing the calculated phase equilibria and phase transitions with experimental data, it showed that a good agreement was reached. The description of Al-Si-V system could be used to guide the development of Al-Si alloys.