The Almo0.5nbta0.5tizr Refractory High Entropy Superalloy : Experimental Findings and Comparison with Calculations Using the Calphad Method

A detailed microstructural characterization of the AlMo0.5NbTa0.5TiZr refractory alloy in the as-cast state is reported for first time, showing a dendritic structure, with a mixture of A2/B2 phases in both the dendritic region and in the transition towards interdendritic regions. A phase rich in Al and Zr is found inside the interdendritic region, which shows amorphous and crystalline portions. Annealed samples at 1400 °C for 24 h were also characterized, showing a combination of A2/B2 phases with the Al-Zr-rich phase completely crystallized in the grain boundaries, as it was reported in previous contributions. Thermodynamic stabilities and solidification paths were calculated using Scheil-Gulliver model solidification based on two different databases of ThermoCalc software and compared with the experimental data. The experimental results from the as-cast alloy do not show a good agreement with the Scheil calculations whereas the equilibrium thermodynamic calculations show a better agreement with the experimental results from the annealed alloy. A mechanism of spinodal decomposition resulting in the patterned A2/B2 microstructure during the annealing is confirmed via the CALPHAD calculations. The current alloy has an apparition of the liquid phase at temperature higher than the one for Ni-based alloys, making it attractive for high temperature applications