Enhancing strengthening effect of topologically close-packed superlattices in medium-entropy alloys via enabling imperfect atomic packing

Medium- or high-entropy alloys (MEAs or HEAs) with a face-centered cubic (FCC) lattice exhibit low strength, despite their considerable ductility. Introducing topologically close-packed (TCP) phases offers a promising avenue to significantly enhance their strength. However, precipitation of these crystallographically ordered phases is usually limited to their strict composition, giving rise to a high energy barrier for nucleation. Essentially, the challenge lies in stimulating the nucleation of these strengthening media and optimizing the microstructure and mechanical properties of the alloy. In this work, we chose the CrCoNi MEA as the prototype and substituted Cr with refractory elements like W and Mo to adjust the compositions and atomic configurations of the TCP phase (i.e., the μ phase). Our investigation reveals that the addition of W leads to the formation of μ phase particles with a non-stoichiometric composition, inducing imperfect crystallographic packing (i.e., planar defects) within the particles. This trend lowers the nucleation energy barrier, resulting in an increased volume fraction of the μ phase and then effectively restraining the growth of FCC matrix grains. Consequently, both precipitation strengthening and grain boundary strengthening effects are enhanced, leading to a dramatic increase in yield strength from 427 MPa in CrCoNi to 1356 MPa in W0.3Cr0.7CoNi. Conversely, the addition of Mo fails to alter the composition and atomic configuration of the μ phase particles, resulting in a weaker strengthening effect. Our findings unveil that manipulating atomic-scale configurations can effectively enhance strengthening effect of the TCP phase, which holds promise for the development of advanced metallic materials.