Many issues in forming are influenced to some degree by the internal structure of the material which is commonly referred to by the materials science community as microstructure. Although the term microstructure is commonly only thought of in the context of grain size, it more properly encompasses all relevant aspects of internal material structure. For the purposes of forming, the most relevant features are the crystallographic orientations of the grains ( texture ) and the locations of the grain boundaries, or, equivalently, the size, topology and shape of the grains. In order to perform realistic simulations one needs to specify the initial state of the material, e.g. on a finite element mesh, with sufficient detail that all these features are reproduced. Measuring microstructure at the scale of individual grains is possible in the synchrotron but scarcely practicable for an analyst. Cross-sections or surfaces are easily evaluated through automated diffraction in the scanning electron microscope (SEM), however. Therefore this paper describes a set of methods for generating statistically representative 3D microstructures based on microscopy input for both single-phase and two-phase materials. Examples are given of application of the technique for generating input structures for recrystallization simulation, dynamic deformation and finite element modeling.

This article outlines the importance of anisotropic interfacial properties for microstructure evolution. The anisotropic properties of interfaces profoundly affect the development of microstructure during thin film deposition, sintering, grain growth and recrystallization, to name but a few processes. The properties of interfaces vary from mildly anisotropic, as for the energy of the solid-liquid interface, to strongly anisotropic as in the case of diffusion rates along grain boundaries. As a companion to this set of articles on interfacial anisotropy, this article demonstrates the connection between grain boundary anisotropy, primarily in mobility, and texture development during grain growth. A Monte Carlo model is used to investigate the evolution of the so-called cube texture component during grain growth of a polycrystal in which the texture is the result of prior deformation.

The grain boundary character distribution in commercially pure Al has been measured as a function of lattice misorientation and boundary plane orientation. The results demonstrate a tendency to terminate grain boundaries on low index planes with relatively low surface energies and large interplanar spacings. The most frequently observed grain boundary plane orientation is (1 1 1). However, there are also instances where boundaries terminated by higher index planes have significant populations. For example, certain twist configurations on 1 1 w planes, which correspond to symmetric [1 1 0] tilt boundaries, also have relatively high populations. The population of symirietric [1 1 0] tilt boundaries exhibits an inverse relationship with previously measured energies.

Techniques are described that have been used to create a statistically representative three-dimensional model microstructure for input into computer simulations using the geometric and crystallographic observations from two orthogonal sections through an aluminum polycrystal. Orientation maps collected on the observation planes are used to characterize the sizes, shapes, and orientations of grains. Using a voxel-based tessellation technique, a microstructure is generated with grains whose size and shape are constructed to conform to those measured experimentally. Orientations are then overlaid on the grain structure such that distribution of grain orientations and the nearest-neighbor relationships, specified by the distribution of relative misorientations across grain boundaries, match the experimentally measured distributions. The techniques are applicable to polycrystalline materials with sufficiently compact grain shapes and can also be used to controllably generate a wide variety of hypothetical microstructures for initial states in computer simulations.

Aluminum alloys exhibit recrystallization kinetics that vary strongly with composition. The conventional understanding is that certain alloying elements, e.g. chromium, retard grain boundary motion due to the formation of fine dispersions of second phase particles, giving rise to particle drag of boundaries. There is countervailing evidence, however, that suggests that solute drag provides a stronger influence on grain boundary mobility. This paper presents new evidence for a pronounced effect of solute based on experiments in which individual boundaries migrate under the driving pressure of stored energy from prior plastic strain. As supported by the literature, boundaries exhibit a maximum mobility for a 38-39 degree <111> misorientation in initial annealing experiments. Specifically, this mobility maximum is asymmetric with a sharp cutoff below 38-39 degrees but a more gradual decrease at misorientations beyond 40 degrees. The occurrence of other, smaller mobility peaks is discussed within the context of the sharpening of evolving maxima with discussed within the context of the sharpening of evolving maxima with increased recrystallization. The presence of a minimum at 38-39 degrees is found at both higher temperatures and higher solute concentrations. This transition from a local mobility maximum to a minimum is discussed within the context of recent theories solute drag activity.

Abnormal subgrain growth has been proposed as the nucleation mechanism for recrystallization. To test this hypothesis, Monte Carlo Potts model simulations of subgrain growth were performed on single-phase, strain-free subgrain structures with experimentally validated microstructure, texture, boundary character, and boundary properties. Results indicate that abnormal growth events emerge spontaneously during evolution in such systems, and abnormal subgrains behave as predicted by mean field theory. An analysis predicts the frequency of abnormal growth events as a function of local neighborhood and the boundary misorientation distribution. A recrystallization model is derived based on the abnormal subgrain growth analysis. Using data for aluminum subgrain structures, the model predicts reasonable recrystallized grain sizes as a function of von Mises strain. The extension of these results to abnormal grain growth is discussed. Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.

An Expert System is proposed in this work. to conduct computational exploration of the deformation and restoration behavior of a medium C-Mn steel under high strain rate conditions, at elevated temperatures and complex strain paths that occur in rod rolling process. The expert system computes appropriate thermomechanical parameters necessary for describing rod rolling process in detail and then utilizes these parameters in mathematical models to determine microstructure evolution during a typical industrial-scale rod rolling process. Microstructure simulation in rod rolling is a challenging problem due to the fact that several softening mechanisms may operate sequentially or concurrently during each deformation step. Different softening mechanisms have very different impact on microstructure development and therefore it is important to investigate the particular combinations of processing conditions under which transition of operating softening mechanisms occurs. In the present work, the transition from dynamic to metadynamic recrystallization is studied in detail based on the criteria of critical strain, austenite grain size and Zener-Hollomon parameter when the interpass (interdeformation) time is very short of the order of few milliseconds during the later stages of rod rolling. Computational results are subsequently validated by comparing the program output to in-plant measured microstructure data. The proposed expert system is designed as an off-line simulation toot to examine and assess the various options for thermomechanical process optimization.