Atomic electron tomography mapped 8,160 crystal nuclei in high- and medium-entropy alloys in three dimensions. Structural order was strongest at each core, faded smoothly toward the boundary and varied with local chemical order. The result adds a gradient pathway to the classical account of nucleation, which treats the interface as sharp.
The UCLA team resolved atomic positions and local chemical order for 8,160 nuclei in 25 rapidly cooled high- and medium-entropy alloy nanoparticles. The nuclei did not show one sharp surface separating an ordered interior from disordered surroundings. Structural order peaked at the centre and declined gradually toward the boundary, and that variation was coupled to chemical order inside each nucleus.[1], [2]
Most nuclei merged with nearly aligned crystal lattices, while a minority formed twin boundaries. Those two paths show that growth did not proceed only by one nucleus expanding outward; the orientation in which neighbouring nuclei joined also helped determine the resulting structure.[2]
The researchers' gradient nucleation pathways model allows structural order to vary across a nucleus and returns to classical theory when the interface becomes sharp. It therefore extends the older equations to cover the observed diffuse boundaries instead of discarding them. The direct evidence remains limited to the two alloy classes and the nanoparticle scale examined here. Whether the same gradient appears in other compositions and larger structures requires new atomic-scale measurements.[2]
