Unintuitive alloy strengthening by addition of weaker elements

A positive correlation between strength and elastic modulus is generally observed in metallic alloys, where the addition of a stronger element such as Mo, W, or Cr increases both the strength and elastic modulus. Our density functional theory (DFT) calculations explain an opposite experimentally measured trend, i.e., the addition of a weaker element such as Ti, Hf, or Zr enhances the yield strength in specific high entropy alloys (HEAs). We show that the underlying mechanism is the lower bond stiffness of the weaker element, which causes larger local lattice distortion (LLD). Higher lattice distortion pins the movement of dislocations, causing solid solution strengthening, thereby raising the strength in body-centered cubic (BCC) refractory HEAs. We show this unintuitive behavior in Ti-based HEAs, i.e., TixMoNbTaW, and compare it with the conventional behavior in MoxNbTiV0.3Zr.