Despite the supposed rarity implicit in its name, abnormal grain growth (AGG) appears to be a rather common mode of coarsening in nanocrystalline materials, regardless of composition or synthesis route. In inert gas-condensed nanocrystalline Pd90Au10, thermally induced coarsening fulfills the criteria for AGG, but with an unusual twist: here, the subpopulation of abnormally growing grains sends offshoots in many directions into the surrounding matrix. Evaluating EBSD maps of the resulting microstructure, we find a fractal-like morphology characterized by a box-counting fractal dimension of ~1.2 instead of the smooth interfaces observed in conventional samples (Braun et al., Scientific Reports 8 (2018) 1592). This is a rather unexpected finding, since the well-established phenomenon of curvature-driven grain boundary migration would be expected to smooth out boundary fluctuations. Consequently, nonstandard mechanisms and driving forces must be considered to account for grain growth in nanocrystalline Pd90Au10. The combination of fractal interface morphologies and slight fluctuations in crystal orientation — observed within the fractal grains — suggests that grain rotations could be involved in the growth process.
To gain further insight into the mechanisms and driving forces underlying fractal AGG, we address the following questions: What differences exist in the grain morphology of coarsened nanocrystalline PdAu alloys versus that of conventional samples prepared by melting and solidification? Is fractality correlated with local characteristics of the microstructure? Can the fractal dimension be influenced by variations in gold concentration, heating rate, or dwell time? Experimental evidence is accompanied by phase field simulations to identify which mechanism(s) contribute to fractal boundary migration in nanocrystalline metals.