Probing the Lattice-Distortion Effect on Yield Strengths in High Entropy Alloys

High entropy alloys (HEAs) have attracted great attention due to their impressive properties induced by the severe lattice distortion in comparison to the conventional alloys. However, the effect of severe lattice distortion on the mechanical properties in face-centered-cubic (FCC) and body-centered-cubic (BCC) structured HEAs is still not fully understood, which are critically important to the fundamental studies as well as the industrial applications. Herein, a theoretical model for predicting the lattice-friction resistance and the yield stress in the FCC and BCC HEAs accounting for the lattice distortion is presented. Both the calculated lattice-friction resistance and the yield strength are compared to the experimental results, to verify the rationality of the built theoretical model. Moreover, the effect of the grain-size distribution on the yield strength is theoretically considered, which reveal the origin of multistage grain structure strengthening. The numerical predictions considering the severe lattice-distortion effect agree well with the experimental results for both FCC and BCC HEAs, in terms of the yield strength and the lattice-friction resistance. The grain-boundary strengthening dominates the yield strength in the FCC Al0.3CrCoFeNi HEA. The yield strength is governed by the lattice-friction resistance in the BCC TaNbHfZrTi HEA, agreeing with the previous work. In AlxCrCoFeNi HEAs, the Al concentration dominates the lattice-friction resistance, and the atomic-radius mismatch and shear-modulus mismatch induced by other four-principal-elements govern the lattice-friction lattice. The atomic-size mismatch dominates the lattice distortion in HEAs, and this viewpoint differs from the traditional knowledge that the increasing incorporated principal element controls the lattice distortion. These results provide the insight into the effect of the severe lattice distortion on the yield strengths in HEAs from the theoretical perspective, for discovering advanced high-strength HEAs.