Revealing the effect of inverse dislocation pileups on the mechanical properties of multi-principal element alloys

In this work, we utilize atomistic simulations and dislocation mechanics to explore the formation of inverse pileups in CrCoNi model alloys and elucidate their unique impact on the strength and ductility of multi-principal element alloys (MPEAs). The present atomistic simulations on single crystals reveal that during the deformation of CrCoNi, stress gradients lead to the formation of novel inverse dislocation pileup. We find that this unique dislocation pattern in a confined volume is due to the elevated lattice friction and significant stress gradient present in the material. Furthermore, this phenomenon can be notably promoted by lowering the temperature, increasing the loading rate, and introducing chemical short-range ordering. Additional simulations on bicrystals show that these inverse pileups play a critical role in suppressing dislocation transmission, reflection, and grain boundary (GB) migration. As a result, they effectively mitigate stress concentration and reduce damage accumulation at GBs, lowering the risk of catastrophic failure due to GB damages. In our theoretical analysis, we utilize dislocation mechanics to predict the formation of the inverse pileup and its subsequent strengthening effect, considering scenarios with and without obstacles. Our investigations encompass various lattice frictions and stress gradients. Remarkably, our results shed light on the prevailing impact of dislocation hardening in the plastic deformation of CrCoNi even under the presence of a linear stress gradient, while the contribution of GB strengthening is found to be comparatively limited. These findings provide valuable insights into the deformation mechanisms of MPEAs in general and significantly aid their applications as promising structural materials.