Introducing local chemical ordering to trigger a planar-slip-initiated strain-hardening mechanism in high entropy alloys

This study demonstrates that we can break the strength-ductility paradox in face-centered cubic (FCC) high-entropy alloys (HEAs) by introducing local chemical ordering. Employing high-resolution transmission electron microscopy, atom probe tomography, and electron-chanelling contrast imaging in CoFeNi and CoCrFeNi HEAs with (Al/Ti) additions, we report a planar slip-induced strain-hardening mechanism that operates at significantly higher stresses compared to the well-known transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) mechanisms. Our results provide clear evidence that the introduction of chemical ordering in HEAs promotes localized slip along multiple {111} planes. Importantly, the interactions between planar slip activities on non-parallel {111} planes triggers a dynamic Hall-Petch like effect that continually refines the slip length. This is in contrast to hexagonal closed packed (HCP) alloys, where such glide plane softening typically causes catastrophic failure. Consequently, this newly identified mechanism increases the yield-strength while maintaining a better combination of strength and ductility in our FCC-based HEAs. These findings, together with our previous results [33,34], establish a compelling alloy design paradigm for the discovery of FCC-based HEAs that can circumvent the strength-ductility paradox.