Materials Science Forum Vols. 715-716

Abstract: The derivation of an equivalent 3-D Von Neumann equation and the corresponding kinetics equation in terms of geometrical characteristics of a grain is shown and the formulation is provided in the framework of the statistical theory of grain growth. The topological relationships between number of grain faces, grain size, number of corners and edges and how these can be calculated in a real microstructure with a statistical approach are discussed. A quadratic law for the linkage between number of faces and grain size is found and compared with available experimental results. Inside the above description a basic formulation of the statistical theory will be derived based on simple geometrical and statistical principles without any independent assumption

Abstract: The annealing behavior of cold rolled Type 430 ferritic stainless steel is the subject of this paper. The steel was cold rolled 79%, then heated at 6 °C/s to the soaking temperature of 841 °C, which is just below the Ae₁ temperature. During heating, specimens were quenched from selected temperatures between 650 and 841 °C and after various times at 841 °C. These quenched samples underwent metallographic examination and micro-hardness determination. The results indicated that under the prevailing experimental conditions, the hardness appeared to correlate strongly with the extent of recrystallization. The kinetics of recrystallization appeared to originate in the cold worked state, where three kinds of grain were found: (i) smooth elongated, featureless of α-fiber orientation {001}<100>; (ii) irregular fishbone grains of the γ-fiber orientations {111}<112> plus {111}<110>; and (iii) twisted grains of the η-fiber orientation {001}<100>. It was found that the twisted grains of the η-fiber were the first to recrystallize, with the fishbone grains of the γ-fiber second, and the smooth elongated, featureless grains of the α-fiber last. It was found that the grains of the α-fiber orientation {001}<100> and the η-fiber orientation {001}<100> were replaced with grains of the γ-fiber orientations as recrystallization progressed. These results are discussed in terms of recrystallization and texture development.

Abstract: We have used high energy x-ray diffraction microscopy (HEDM) to study annealing behavior in high purity aluminum. In-situ measurements were carried out at Sector 1 of the Advanced Photon Source. The microstructure in a small sub-volume of a 1 mm diameter wire was mapped in the as-received state and after two differential anneals. Forward modeling analysis reveals three dimensional grain structures and internal orientation distributions inside grains. The analysis demonstrates increased ordering with annealing as well as persistent low angle internal boundaries. Grains that grow from disordered regions are resolution limited single crystals. Together with this recovery behavior, we observe subtle motions of some grain boundaries due to annealing.

Abstract: In the present study, a modified cellular automaton CORe (Cellular Operator for Recrystallization) was used to predict the recrystallization (RX) texture and microstructure of 70% cold rolled commercial AA8079L alloy at 300°C. The nucleation of the Cube orientation is of considerable scientific interest since the Cube texture component influences significantly the anisotropy of material properties. Experimental data collected during this investigation were used for subsequent modeling. By studying the annealed material by EBSD, an average nucleation rate at Cube bands was established and used in the model. The simulated microstructure reveals, in general, good agreement with experiment. The texture prediction shows the right tendency, but the modeled intensity of the Cube texture is about 2 times smaller than in experiment.

Abstract: n-situ 3D X-ray diffraction (3DXRD) annealing experiments were conducted at the ID-11 beamline at the European Synchrotron Radiation Facility in Grenoble. This allowed us to non-destructively document and subsequently analyse the development of substructures during heating, without the influence of surface effects. A sample of deformed single crystal halite was heated to between 260-400 °C. Before and after heating a volume of 500 by 500 by 300 μm was mapped using a planar beam, which was translated over the sample volume at intervals of 5-10 µm in the vertical dimension. In the following we present partially reconstructed orientation maps over one layer before and after heating for 240min at 260 °C. Additional small syn-heating maps over a constrained sample rotation of 12-30º. The purpose of this was to illuminate a few reflections from 1 or 2 subgrains and follow their evolution during heating. Preliminary results show that significant changes occurred within the sample volume, for which, surface effects can be excluded. Results show a number of processes, including: i) change in subgrain boundary misorientation angle and ii) subgrain subdivision into areas of similar lattice orientation with new subgrain boundary formation. These results demonstrate that 3DXRD coupled with in-situ heating is a successful non-destructive technique for examining real-time post-deformational annealing in strongly deformed crystalline materials with complicated microstructures.

Abstract: The results of investigations of magnetically driven grain boundary migration in high purity (99.995%) zinc bicrystals are presented. In-situ measurements were conducted by means of a specially designed and fabricated polarization microscopy probe. The migration of planar tilt grain boundaries with various misorientation angles in the range between 60° and 90° was studied. The absolute grain boundary mobility and its temperature dependence was measured in the regime between 330°C and 415°C and the corresponding migration activation parameters were determined. The results revealed that there is a pronounced misorientation dependence of grain boundary mobility in the investigated angular range. The migration activation enthalpy was found to vary between 1.18 eV and 2.15 eV. The obtained activation parameters comply with the compensation law, i.e. the migration activation enthalpy changes linearly with the logarithm of the pre-exponential factor.

Abstract: We revisit grain growth and the puzzle of its stagnation in thin metallic films. We bring together a large body of experimental data that includes the size of more than 30,000 grains obtained from 23 thin film samples of Al and Cu with thicknesses in the range of 25 to 158 nm. In addition to grain size, a broad range of other metrics such as the number of sides and the average side class of nearest neighbors is used to compare the experimental results with the results of two dimensional simulations of grain growth with isotropic boundary energy. In order to identify the underlying cause of the differences between these simulations and experiments, five factors are examined. These are (i) surface energy and elastic strain energy reduction, (ii) anisotropy of grain boundary energy, and retarding and pinning forces such as (iii) solute drag, (iv) grain boundary grooving and (v) triple junction drag. No single factor provides an explanation for the observed experimental behavior.

Abstract: In this research, recrystallization of AA7020 aluminum alloy after hot compression testing was predicted using a framework being a combination of physical modeling and Monte Carlo simulation. Stored energy was calculated as a function of subgrain size related to the Zener Hollomon parameter. The as-deformed grain structure was mapped into the Monte Carlo simulation from experimental results. Calculated stored energy was assigned to the mapped structure, considering the length scale of the simulation. Results were validated by comparing the microstructures obtained from the model predictions with those from experimental results and a reasonable agreement was reached. The predicted grain size was found to be 15 % smaller than the experimental values. Predicted fractions recrystallized showed a similar trend to the experimental results. However, a discrepancy between the model predictions and experimental results in terms of recrystallization kinetics was found, which was attributed to neglecting the effect of subgrain growth and resulting reduction of the stored energy during recovery on the recrystallization kinetics in the present simulation.

Abstract: A heating stage as been developed to perform in-situ annealing in a SEM equipped with an EBSD system in order to study recrystallization mechanisms. High temperature treatments could then be performed inside the SEM, up to 1180°C and with high heating-and cooling-rates (~100°C.s^-1). Samples were cooled down to room temperature to perform EBSD orientation mapping in between successive short-duration heat-treatments. Microstructure evolution snapshots obtained this way are presented in this paper to show recrystallization in Zircaloy4 and in pure tantalum.

Abstract: A physically-based model for nucleation during discontinuous dynamic recrystallization (DDRX) has been developed and is coupled with polyphase plasticity and grain growth models to predict the macroscopic stress and grain size evolution during straining. The nucleation model is based on a recent description for static recrystallization and considers the dynamically evolving substructure size. Model predictions are compared with literature results on DDRX in pure Cu as a function of initial grain size, deformation temperature and strain-rate. The characteristic DRX features such as single to multiple peak stress transitions and convergence towards a steady-state stress and grain size are quantitatively reproduced by the model.