Coupled surface-bulk mechanisms govern grain growth under geometric confinement: Insights from time-resolved laboratory x-ray diffraction contrast tomography

Grain growth in thin samples differs markedly from bulk behavior due to surface effects, yet the governing mechanisms remain poorly understood. Here, we use time-resolved, three-dimensional (3D) measurements via laboratory x-ray diffraction contrast tomography (LabDCT) to study over 2,200 grains, 18,000 grain boundaries (GBs), and their time-evolution in a thin aluminum (Al) disc with a thickness five times the initial grain diameter. We classify grains as volumetric, surface-touching, or columnar and assess their growth behaviors against empirical topological laws, including the Aboav–Weaire and Lewis relations. In addition, we quantify the local interfacial orientations and their time-dependence in the specimen and crystallographic frames. Taken altogether, our results demonstrate, for the first time in experiment, that columnar grains persist and grow preferentially, while volumetric and surface grains are eliminated. We propose that this behavior is driven by surface energy differences that promote surface expansion, thereby introducing internal (GB) curvatures that promote faster growth rates relative to the neighboring grains. These results highlight how geometric confinement and anisotropic surface free energies combine to shape a non-self-similar grain evolution in thin specimens, providing a foundation for future mesoscale simulations and microstructural design strategies.