Crystal nucleation and growth in high-entropy alloys revealed by atomic electron tomography

High-entropy alloys combine multiple principal elements and can exhibit exceptional mechanical properties and catalytic activity. However, how they crystallize remains poorly understood because early nuclei are small, transient and chemically complex. Here we advance atomic electron tomography to determine the three-dimensional atomic structures and local chemical order of 8,160 high- and medium-entropy alloy nuclei. We find that nucleation proceeds through gradient ordering, in which structural order is highest at the core, decreases smoothly towards the boundary and is coupled to local chemical order. Most nuclei coalesce with nearly aligned crystal lattices, whereas a minority form twin boundaries. We develop the gradient nucleation pathways model, which generalizes classical nucleation theory by incorporating spatially varying structural order within each nucleus. The model captures diffuse, partially ordered nuclei, recovers classical nucleation theory in the sharp-interface limit and reveals multiple intermediate states. These results provide an atomistic framework for understanding crystal nucleation and growth across a broad range of materials.