(TiZrNbTa)-Mo high-entropy alloys: Dependence of microstructure and mechanical properties on Mo concentration and modeling of solid solution strengthening

In this work, the evolution of microstructure and fundamental mechanical properties with Mo concentration in the arc-melted (TiZrNbTa)(100-x)Mo-x (0 <= x <= 20) high-entropy alloys (HEAs) are investigated. The arc-melted (TiZrNbTa)(100-x)Mo-x alloys structurally consist of two bcc phases. The change in volume fractions of two phases in the microstructure is insignificant with Mo concentration, at levels of similar to 75% for bcc1 and similar to 25% for bcc2. The increases in microhardness and Young's modulus of the alloys linearly scale with Mo concentration. To compromise the strength and ductility, the (TiZrNbTa)(90)Mo-10 exhibits an optimal combination of stiffness (E = 137 GPa), compressive yield strength (sigma(y) = 1370 MPa) and deformability (epsilon(p) approximate to 25%). In addition, it is indicated that dislocation widths in bcc lattice of refractory HEAs are insensitive to the alloying complexity, reflecting that the Peierls barrier is excluded as a predominant factor of strengthening. Furthermore, a simple model is proposed to reveal the solid solution hardening (SSH) in bcc refractory HEAs, in which the solid solutions are treated as an imposition of distortion-free and distorted lattices. By applying this model to archived refractory HEAs, the predicted yield strength agrees well with experimentally measured values. It provides a simple empirical tool used for predicting the strength of bcc refractory HEAs and to assist new alloy design.