3DXRD Characterization and Modeling of Solid-State Transformation Processes
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Characterization and Modeling of Solid-State Transformation Processes
D. Juul Jensen, S.E. Offerman, and J. Sietsma Abstract Three-dimensional x-ray diffraction (3DXRD) allows nondestructive characterization of grains, orientations, and stresses in bulk microstructures and, therefore, enables in situ studies of the structural dynamics during processing. The method is described briefly, and its potential for providing new data valuable for validation of various models of microstructural evolution is discussed. Examples of 3DXRD measurements related to recrystallization and to solid-state phase transformations in metals are described. 3DXRD measurements have led to new modeling activity predicting the evolution of metallic microstructures with much more detail than hitherto possible. Among these modeling activities are three-dimensional (3D) geometric modeling, 3D molecular dynamics modeling, 3D phase-field modeling, two-dimensional (2D) cellular automata, and 2D Monte Carlo simulations.
Introduction Processes involving grain nucleation and growth are ubiquitous in materials science. Nevertheless, at the present time, studying the underlying mechanisms involved in these processes is extremely challenging experimentally. Herein, we describe these difficulties and introduce three-dimensional x-ray diffraction (3DXRD) as an approach to overcome some of the problems. We focus on two important metallurgical processes, namely, solid-state phase transformations and recrystallization. For solid-state phase transformations, the experimental difficulty stems from several facts. The critical nuclei are very small (in the range of nanometers) and exist as nuclei only for a short time before they grow into larger grains. These nuclei often form at defects or interfaces in the bulk of the material. From classical nucle-
ation theory, it is known that the energies of the grain boundaries between the parent grains as well as between the parent and nucleating grains are very important. To determine the energies of these interfaces, the local atomic arrangement and chemistry must be known, which requires a local probe with atomic resolution. At the same time, a large volume of the material needs to be probed in order to obtain sufficient statistics about the different types of potential nucleation sites available in the heterogeneous microstructure of the alloy. Moreover, for accurate measurements, the grain-boundary energy between parent grains should be obtained before the nucleus forms, and the energy of the interface between the nucleus and the matrix should be determined at the moment of formation of a critical nucleus.
MRS BULLETIN • VOLUME 33 • JUNE 2008 • www.mrs.org/bulletin
Notwithstanding the great improvements and sophistication in experimental techniques, no technique is currently available that simultaneously fulfills all of the requirements just described. As a result, studies of nucleation during solid-state phase transformations have been limited to either high-spatial-resolution measurements with instruments suc
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