2 Viewpoint : experimental recovery of geometrically 3 necessary dislocation density in polycrystals

8 Abstract 9 Application of electron backscattering diffraction methods to recover estimates of the geometrically necessary dis-10 location density is described. The limitations of the method arising from the opacity of crystalline materials and the 11 spatial and angular resolution limits are discussed. 15 It has long been an experimental observation 16 that the constituent grains of polycrystalline ma-17 terials develop complex patterns of heterogeneity 18 over a wide range of length scales during plastic 19 deformation. The classical theory of crystal plas-20 ticity is limited to those interactions arising from 21 the enforcement of mechanical compatibility [1]. It 22 is evident that an understanding of the behavior of 23 grain boundaries, and how they interact with de-24 formation mechanisms as they change their 25 structure to accommodate deformation, has been 26 missing. These interactions are anticipated to set 27 the necessary length scale(s) seen clearly in the 28 Hall–Petch relationship and as discussed in the 29 context of strain gradient plasticity [2]. Recent 30 work has shown that an experimental technique 31 may help provide this length scale by utilizing in-32 formation obtained by electron backscattering 33 diffraction (EBSD). Orientation imaging micro-34 scopy (OIM) provides automated scanning mea-35 surements of the lattice curvature near grain 36 boundaries. Relating this curvature to estimates of 37 the geometrically necessary dislocation densities 38 (GNDs) provides a promising experimental vehicle 39 for the study of plasticity in crystalline materials 40 [3]. The purpose of this paper is to discuss the 41 strengths and limitations of this method. The 42 principal limitations of the OIM-based methods 43 pertain to material opacity and the spatial/angular 44 resolution of EBSD. In the sections that follow 45 italics are used to emphasis strengths and weak-46 nesses of these methods.