Determination of elastic strain fields and geometricallynecessary dislocation distributions near nanoindents using electronback scatter diffraction

摘要

The deformation around a 500-nm deep Berkovich indent in a large grainedFe sample has been studied using high resolution electron back scatterdiffraction (EBSD). EBSD patterns were obtained in a two-dimensionalmap around the indent on the free surface. A cross-correlation-basedanalysis of small shifts in many sub-regions of the EBSD patterns was usedto determine the variation of elastic strain and lattice rotations across themap at a sensitivity of —±10-4. Elastic strains were smaller than latticerotations, with radial strains found to be compressive and hoop strainstensile as expected. Several analyses based on Nyes dislocation tensor wereused to estimate the distribution of geometrically necessary dislocations(GNDs) around the indent. The results obtained using different assumeddislocation geometries, optimisation routines and different contributionsfrom the measured lattice rotation and strain fields are compared. Ourfavoured approach is to seek a combination of GND types which supportthe six measurable (of a possible nine) gradients of the lattice rotations aftercorrection for the 10 measurable elastic strain gradients, and minimise thetotal GND line energy using an L1 optimisation method. A lower boundestimate for the noise on the GND density determination is —±1012 m-2for a 200-nm step size, and near the indent densities as high as 1015 m-2were measured. For comparison, a Hough-based analysis of the EBSDpatterns has a much higher noise level of f1014m2 for the GND density.