Entropy versus enthalpy in hexagonal-close-packed high-entropy alloys

The addition of hexagonal-close-packed (hcp) non-rare-earth elements Zr, Ti and Co, to the 10-component hcp rare-earth-based high-entropy alloys (HEAs) with a composition of ScYLaNdGdTbDyHoErLuX (X = Zr, Co and Ti) was investigated. The enthalpy of mixing between elements was found to have a significant effect on the formation of phases. The addition of Co combines with elements that had a strong chemical affinity to form intermetallic compounds by the effect of enthalpy. Ti was added with all elements with poor chemical affinity and exhibited rejection to form a phase alone. These were the two terminal manifestations of the role of enthalpy over entropy. Part of Zr was soluble in the matrix under the action of entropy, while the other part had a greater affinity for Sc than the other elements to form a precipitate under the action of enthalpy. This was the result of the local balance between the effect of enthalpy and entropy. The solid solution of the elements had different degrees of strengthening effect, among which Zr had the most excellent strengthening effect from 185 to 355 MPa, so the solid solution strengthening model and precipitation strengthening model were proposed to predict the strength of the alloy with the addition of Zr effectively. Graphical abstract