Combined experimental and CALPHAD investigation of equimolar AlCoCrFeNiX (X=Mo,Ta,W) High-Entropy Alloys

Aiming to evaluate the effect of refractory metal additions to a quinary AlCoCrFeNi High-Entropy Alloy (HEA), three novel equimolar AlCoCrFeNi-X (X = Mo, Ta, W) HEAs were designed, arc-melted, annealed, and characterized by SEM-EDS, XRD and microhardness measurements. CALPHAD thermodynamic calculations were exploited to design compositions and thermal treatments of the selected HEAs as well as to predict constitution and interpret microstructure of the samples. On the other hand, the experimental results contributed to the validation of the in-house built GHEA thermodynamic database (including the Al, Co, Cr, Fe, Ni, Mo, Ta, W elements) used for the calculations. No TCP intermetallic was found to form in the quinary AlCoCrFeNi alloy. However, the formation of sigma, Laves-C14 and mu phases was observed in the samples containing Mo, Ta, and W, respectively, in agreement with the most accepted VEC-based phases stabilization criteria. The addition of the refractory metals led to a microhardness increase for all the investigated alloys. Overall, good agreement was observed between experiments and calculations, especially for compositional trends and phase amounts, allowing the database validation and supporting its applicability to phase equilibria simulation in the sixcomponent HEAs belonging to the Al-Co-Cr-Fe-Ni-X (X = Mo, Ta, W) systems.