High Accuracy EBSD - A Review of Recent Applications, Innovations, and Remaining Challenges

Abstract

The implementation of cross-correlation analysis of EBSD patterns has allowed much improved angular sensitivity measurements to be achieved including the ability to determine the variations in elastic strains [1,2]. Cross-correlation functions between tests patterns and a reference pattern are calculated at selected sub-regions distributed across the patterns. A simple geometric analysis then allows the elastic strains and lattice rotations to be calculated from the shifts measured at these subregions. Sub-pixel resolution on the pattern shifts, typically to ±0.05 or better, leads to strain and rotation sensitivity of~±10 -4 . In many situations a convenient location on the sample can be used to generate a reference pattern for which the strain state is well known, and in most cases zero. This is the case when studying the flow fields and residual stresses near indents in single crystals. . Analysis of deformation fields near crack tips and particles are important situations where strains fall to negligible levels in the far field so that good reference patterns can be obtained from regions well away from the crack or particle [2,6,7]. In many more situations, such as a plastically deformed polycrystal, there is no obvious position on the sample from which to obtain a reference pattern corresponding to a known strain state. Fixing the absolute level of the strains in these situations remains the most significant challenge in continuing the development of the technique. One approach is to combine the local strain variations measured using the EBSD method with modeling at a longer length scale. A uniform strain state is then added to the measured EBSD strain variations to bring the average values into agreement with the model [6]. If characterizing the resulting deformation sub-structure is the main aim of the work then Nye's dislocation tensor analysis can be used to estimate the geometrically necessary dislocation (GND) density. This requires only gradients in the elastic strain and lattice rotation fields and so avoids the problem of assigning the absolute strain values. In all metal deformation studies we have made plastic flow results in rotations that are markedly larger than the elastic strains which can then be ignored when estimating the GND content. Indeed the lattice rotations can be sufficiently large so as to cause the cross-correlation analysis to give incorrect pattern shifts for parts of the pattern which change too much. We have recently implemented a robust fitting algorithm which identifies regions of the pattern for which the pattern shifts are suspect and down-weights their influence in an iterative scheme to determine the strain and rotation from the pattern shifts. This allows fuller coverage of the EBSD map as illustrated in FIG 2 (b) and (c) for a Cu sample deformed to 10% in tension. The use of simulations to generate the reference patterns is an obvious approach to solving the 'reference pattern problem' and has been explored by several groups [8,9]. The simulations must have sufficient ...

Comparison of the dislocation density obtained by HR-EBSD and X-ray profile analysis

“…HR-EBSD is a scanning electron microscope (SEM) based method, which allows determining the stress/strain in a crystalline material at the length scale of tens of nanometers. It is based on a cross-correlation method 8,[14][15][16][17][18][19] exploiting small changes in the backscattered Kikuchi diffraction patterns corresponding to a reference point and the actual point analysed. A detailed description of the technique can be found in Refs.…”

Recent Developments of Crystallographic Analysis Methods in the Scanning Electron Microscope for Applications in Metallurgy

“…[94][95][96] The resolution on the pattern shifts is in the sub-pixel range, typically §0.05, being the precision of the method »10 ¡4 in strain and 10 ¡4 rad (0.006 ) in rotation. 97,98 Even higher precision is achievable if signal to noise ratio in the patterns is improved by simple integration. 99 Additional experimental factors must be taken into account for high accuracy in the results.…”