Atomic-Scale Stress Delocalization Enabled by Local Chemical Heterogeneity Modulates Brittle-to-Ductile Transition in BCC Multi-Principal Element Alloys

Body-centered cubic (BCC) metals often exhibit limited ductility at finite temperatures, and elemental synergy in multi-principal element alloys (MPEAs) offers a promising route to overcome this limitation. Here, we employ atomistic simulations with a high fidelity machine-learning potential—which outperforms traditional empirical potentials at capturing complex chemical interactions—to investigate mode-I crack propagation in the engineering-relevant NbMoTaW MPEA versus pure Mo. While Mo fails by brittle cleavage, the NbMoTaW undergoes ductile fracture mediated by dislocation activity. We demonstrate that the average-based Rice criterion demands local corrections to sample diverse chemical environments along the advancing crack. Specifically, the MPEA's intrinsic chemo-mechanical heterogeneity produces atomic-scale stress delocalization at the crack tip, generating a chemically-driven shielding effect that mitigates the singular stress concentration responsible for cleavage. In particular, Ta-related local chemical heterogeneities act as precursors that nucleate localized shear and dissipate plastic energy. By embedding such heterogeneities into a brittle Mo matrix, we find that a sufficient level of interconnectivity is required to amplify stress delocalization and promote dislocation nucleation, enabling a brittle-to-ductile transition of the fracture mode. These findings reveal a stress delocalization mechanism by which local chemical heterogeneities control intrinsic ductility in BCC alloys and provide a theoretical guide for composition design.