Fractal-Based Analysis of Layer-Specific Grain Boundary Network Evolution in Surface-Deformed LPBF AlSi10Mg Alloy

The thermal stability of the shot-peened gradient microstructure in LPBF AlSi10Mg during annealing at 500 °C for up to 8 h was investigated. EBSD boundary maps were analyzed using the box-counting method to determine fractal dimension, D, as a quantitative descriptor of grain-boundary geometrical complexity. It was found that D decreased with depth from the dynamically recrystallized surface layer (0–10 µm; D = 1.73; ECD = 0.8 ± 0.2 µm) through the transition layer (10–40 µm; D = 1.56; ECD = 2.4 ± 0.7 µm) to the matrix (>40 µm; D = 1.16; ECD = 3.0 ± 0.7 µm). After 5 min, the surface network simplified (D = 1.63; ECD = 2.4 µm), whereas the transition layer exhibited increased complexity (D = 1.72; ECD = 3.7 µm), suggesting a strong contribution of near-surface particle pinning and extensive recovery/polygonization within the subsurface. The matrix showed a transient increase in D (1.16 → 1.71), associated with fragmentation of the cellular Si network. Continued annealing reduced D in the surface and transition layers to 1.50 and 1.64 after 1 h due to progressive boundary smoothing and consumption of deformation substructure. Prolonged exposure triggered sparse discontinuous recrystallization exclusively within the transition layer, producing abnormally large grains that migrated bi-directionally into both the pinned surface layer and the bulk matrix. After 8 h, the gradient microstructure collapsed and the boundary trace became disconnected, yielding an apparent exponent D ≈ 0.97 at the map scale.