The study of microstructural evolution in polycrystalline materials has been active for many decades so it is interesting to illustrate the progress that has been made and to point out some remaining challenges. Grain boundaries are important because their long-range motion controls evolution in many cases. We have some understanding of the essential features of grain boundary properties over the five macroscopic degrees of freedom. Excess free energy, for example, is dominated by the two surfaces that comprise the boundary although the twist component also has a non-negligible influence. Mobility is less well defined although there are some clear trends for certain classes of materials such as fcc metals. Computer simulation has made a critical contribution by showing, for example, that mobility exhibits an intrinsic crystallographic anisotropy even in the absence of impurities. At the mesoscopic level, we now have rigorous relationships between geometry and growth rates for individual grains in three dimensions. We are in the process of validating computer models of grain growth against 3D non-destructive measurements. Quantitative modeling of recrystallization that includes texture development has been accomplished in several groups. Other properties such as corrosion resistance are being related quantitatively to microstructure. There remain, however, numerous challenges. Despite decades of study, we still do not have complete cause-and-effect descriptions of most cases of abnormal grain growth. The response of nanostructured materials to annealing can lead to either unexpected resistance to coarsening, or, coarsening at unexpectedly low temperatures. General process models for recrystallization that can be applied to industrial alloys remain elusive although significant progress has been made for the specific case of aluminum alloy processing. Thin films often exhibit stagnation of grain growth that we do not fully understand, as well as abnormal grain growth. Grain boundaries respond to driving forces in more complicated ways than we understood. Clearly many exciting challenges remain in grain growth and recrystallization.

A 2D cellular automaton model developed for the simulation of grain growth in hexagonal metals is presented here. It allows the direct use of experimental measurement as input data. Texture evolution of a titanium alloy and a zirconium alloy are simulated on the basis of simple hypothesis and compared with experimental evolution as well as the results from a 3D Monte Carlo model. Results from both models are discussed with regards to their characteristics.

Grain boundary character in samples of Zr701 annealed at two different temperatures has been investigated in terms of lattice misorientation. The main difference between the two samples was the extent of grain growth post-recrystallization. The textures were typical for the material. Differences between the texture-based misorientation distribution function (T-MDF) and the microstructure-based MDF (M-MDF) revealed significant preferences for certain grain boundary types, notably those with < 11-20 > rotation axes.

Monte Carlo (MC) simulations were used to model microstructural evolution in Pt thin films with epitaxial seed grains buried in a polycrystalline matrix. The key to achieving purely epitaxial films via templated grain growth in such materials is the lateral coalescence of the seeds into a single epitaxial grain. The primary factors in determining whether this event takes place, for a given set of interfacial mobility/energy functions, are the relative initial sizes of the seed grains and polycrystalline matrix grains, and the initial degree of surface coverage of the epitaxial seeds. These characteristics are evaluated by varying the films initial microstructural parameters, including seed grain size, seed number density, seed surface coverage and polycrystalline matrix grain size. Additional simulations were carried out to investigate the effect of varying the energy and the mobility of seed-matrix interfaces. The critical values of seed-matrix grain size depend on the energy/mobility used, though seed coalescence remains the key criterion for epitaxial grain growth. (c) 2007

The subgrain structure of hot rolled aluminum alloy AA 5005 has been characterized on as-received samples using Electron Backscatter Diffraction (EBSD). Based on the OIM scans of RD-ND and TD-ND, 3 dimensional microstructures of subgrains are built up using the 3D Microstructure Builder, which is a method for developing statistically representative digital representations of microstructures. Following the generation of microstructure, different textures were fit into these constructed 3D microstructures, based on individual components such as Brass and S textures. For this study, the Brass texture was chosen as an exemplary case. Monte Carlo simulation was used to model subgrain coarsening and visualization was a key to detecting abnormal grain growth in such structures. The main objective is to understand the circumstances under which we can expect abnormal (sub-)grain growth leading to nucleation of recrystallization.

A three-dimensional, Potts model of liquid phase sintering in a system with full solid wetting is introduced to investigate the coarsening kinetics and microstructures associated with this process. Kinetic Monte Carlo simulation is used to probe coarsening dynamics and to obtain the properties of solid particles, including the volume of critical nuclei and the distribution of particle size as a function of time. It is found that the average particle volume increases linearly with time and that the particle size distributions are consistent with those obtained experimentally, as in the W-Ni-Fe and Sn-Pb systems. In obtaining these results careful consideration is given to the role of initial microstructural features in the subsequent evolution of the system.