Reverse Engineering the Kinetics of Grain Growth in Al-Based Polycrystals by Microstructural Mapping in 4D

Abstract

Many metallic materials used in industry are initially produced as large castings. They are further processed by deformation and annealing to obtain the final product. During the annealing step, the phenomena of recovery, recrystallization and grain growth take place. This work focuses on the final stage of annealing—grain growth—which is a key process for controlling the microstructure and achieving the desired materials properties. Although grain growth has been studied in polycrystalline materials for many years, our understanding of its underlying mechanisms remains incomplete. Even in the case of normal grain growth, computer simulations differ significantly from experimental findings. For example, the grain size distributions observed in real materials tend to be less symmetric than those predicted by simulations operating under the assumption of isotropic boundary properties, and discrepancies between the measured and simulated evolution of individual grains can be particularly pronounced. Instead of modeling grain growth in the conventional manner using boundary parameters derived from physical principles, we elected to try the opposite approach: i.e., to work backwards from experimental measurements to the kinetics of individual grain boundaries. A reverse engineering type of analysis was established, allowing the boundary reduced mobility (the product of boundary mobility and energy) to be extracted from 3D microstructural evolution. Employing synchrotron-based 3D x-ray diffraction (3DXRD) microscopy, we investigated thermally induced coarsening in two different Al-based alloys. We collected and reconstructed a series of microstructural snapshots between isothermal annealing steps. In these time-resolved 3D datasets, we followed the morphology, misorientation, inclination and migration of thousands of boundaries in a single sample, forming the basis for a robust statistical analysis of local growth kinetics. In an Al-1 wt% Mg specimen, the microstructural evolution and the associated grain statistics demonstrated that abnormal grain growth took place. In comparison, the coarsening of an Al-5 wt%Cu specimen resembled normal grain growth. On the grain boundary level, many low-angle grain boundaries were found in AlMg. Through the reverse engineering procedure, the reduced mobility of grain boundaries was extracted and related to the boundary misorientation and inclination. The results show that the reduced mobility manifests a clear trend with respect to misorientation. Beyond this general trend, the data spreads reveal that grain boundaries of similar misorientation may vary significantly in their reduced mobility. Such spreads were found to be larger in the AlMg sample, for which the extreme values in reduced mobility were often associated with the boundaries of abnormal grains. From an analysis of the extracted reduced mobility, we determined that factors beyond the usual capillary force must have altered the migration of grain boundaries in AlMg, facilitating the abnormal grain growth. Moreover, in both samples the mobility of individual grain boundaries was found to vary over time (rather than maintaining a constant value), suggesting that the reduced mobility depends on atomic-scale details of the growth mechanism that are overlooked by mesoscale grain growth simulations.