In situ three-dimensional investigation of Ostwald ripening

Owing to their destructive nature, conventional techniques for investigating Ostwald ripening — the thermally induced coarsening of two-phase microstructures — do not allow repeated observation of the same volumetric region in a sample. In this work, we employed laboratory microcomputed tomography (microCT) to carry out nondestructive, three-dimensional characterization of an Al-5 wt.% Cu alloy undergoing Ostwald ripening. The sample was repeatedly annealed at 630∘C, inducing a semisolid state with a volume fraction VV = 60% of the coarsening (solid) phase. Each annealing step was followed by a microCT scan performed at room temperature. When semisolid, the system consists of Al-rich grains surrounded by a liquid phase of higher Cu content. Tomographic contrast between the two phases arises from their differing attenuation factors for x-ray radiation. During cooling, the liquid phase tends to retreat into the triple junctions, resulting in incomplete coverage of the boundaries separating the coarsening grains. Correction for the latter effect was accomplished by a watershed-transform-based image processing routine, enabling reliable grain segmentation and the evaluation of local and global microstructural parameters in 3D. These can, in turn, be compared to theoretical and simulation results.