ADVANCES IN EXPERIMENTAL METHOD AND ANALYSIS FOR ESTIMATION OF GEOMETRICALLY-NECESSARY DISLOCATIONS

Advances in experimental methods for determination of the geometrically- necessary dislocation (GND) tensor, based on electron backscattering diffraction, are described. Data are presented for directionally-solidified 99.999% Aluminum possessing a strong <001> columnar texture, with the primary focus being the interactions of the plastic deformation field with grain boundaries. Alternate methods of solving for the GND content are illustrated and compared. Implications of the observations for strain-gradient plasticity theory are discussed. Historically, experimental observation has shown that the constituent grains of polycrystalline materials develop complex patterns of heterogeneity over a wide range of length scales during plastic deformation. The classical theory of crystal plasticity, however, is limited to those interactions arising from the enforcement of mechanical compatibility (1). It is evident that an understanding of the behavior of grain boundaries, and how they interact with deformation mechanisms as they change their structure to accommodate deformation, has been missing. These interactions are anticipated to set the necessary length scale(s) as discussed in the context of strain and gradient plasticity (2) and as seen clearly in the Hall- Petch relationship. Recent work has shown that utilizing information obtained by electron backscattering diffraction (EBSD) and the use of Orientation Imaging Microscopy (OIM), which provides automated scanning measurements of the lattice orientation near grain boundaries may be a useful experimental technique to help provide this length scale. Computing the lattice curvature from these orientation measurements and consequently relating this curvature to estimates of the geometrically necessary dislocation (GND) density provides a promising experimental vehicle for the study of plasticity in crystalline materials (3). The primary driving force behind the work presented here is to further exploit this experimental method by investigating the most reliable GND estimation method. 2. Theoretical