Papers by Author: Ricardo A. Lebensohn

Abstract: The understanding of the dynamics of substructures during deformation and annealing is fundamental in our ability to predict microstructural and physical properties such as rheological behaviour of crystalline materials. Here, we present an overview of new insights into substructure dynamics through a combination of in-situ heating experiments, detailed Electron Backscatter Diffraction (EBSD) analysis and numerical modelling.

Abstract: We examine the relationship between local gradients in orientation, which are quantified with the Kernel Average Misorientation, and the grain boundary network in an interstitial-free steel sheet, before and after 12% tensile strain. A portion of the unstrained microstructure is used as input to a full-field spectral viscoplastic code that simulates the same deformation. The orientation gradients are concentrated near grain boundaries in both experiments and simulation. Mapping out stress gradients in the simulation suggests that the development of orientation gradients is strongly correlated with such gradients.

Abstract: Texture evolution in plastically deformed HCP metals is strongly influenced by the nucleation and growth of deformation twins and twin variant selection. Statistically based EBSD analyses of deformed microstructures in HCP metals indicate that the nucleation of deformation twins depends on, among other factors, the local stress fields arising from neighboring grain interactions at grain boundaries [1]. Inspired by these findings a probability model for twin nucleation was developed [2,3], based on the activation of defect sources statistically occurring in grain boundaries. This nucleation model was implemented in a Visco-Plastic Self-Consistent (VPSC) code. Because the latter is based on an Effective Medium assumption and the inclusion formalism, it only provides average stress values in the grains, and the nature of local stress fields at grain boundaries had to be considered in a heuristic manner. In order to have better insight on the effect of local textures on twin nucleation, in this work we employ a viscoplastic full field Fast Fourier Transform (FFT) method as a numerical tool for conducting virtual experiments to study the role of crystal orientation and local neighbor grain interactions on stress localization close to the interfaces and, consequently, on twin nucleation in hexagonal materials, such as Zr and Mg.

Abstract: In this contribution we present how to implement the calculation of average field fluctuations inside the grains of a thermoelastic aggregate in terms of the derivatives of the stress potential given by the standard linear self-consistent (SC) model, and how this statistical information can be used to generate second-order estimates for the mechanical behavior of nonlinear viscoplastic polycrystals, by means of a rigorous non-linear homogenization procedure. To illustrate the differences between this second-order (SO) self-consistent approach and the classical first-order SC approximations, we compare them in terms of their predictions of the effective behavior of random fcc polycrystals as a function of their rate-sensitivity, and of the texture evolution in hcp ice polycrystals under uniaxial compression. In the latter case, the SO approximation is the only one able to predict a substantial accommodation of deformation by basal slip, even when the basal poles become strongly aligned with the compression direction and the basal slip systems became unfavorably oriented.