Correlations between two EBSD-based metrics Kernel Average Misorientation and Image Quality on indicating dislocations of near-failure low alloy steels induced by tensile and cyclic deformations

Abstract

Two metrics Kernel Average Misorientation (KAM) and Image Quality (IQ) widely used in Electron Back-Scatter Diffraction (EBSD) mapping are compared with each other on indicating the dislocations of type 40Cr low alloy steel induced by tensile and cyclic deformations approaching failure limits respectively. The tensile deformation up to 12 % just before necking increased the KAM and decreased the IQ obviously, thus a negative correlation was established between KAM and normalized IQ in this case. In contrast, the cyclic deformation with small strain amplitudes (±0.4 % and ±0.7 %) until fatigue failure only decreased the IQ obviously but almost produced no effect on KAM, thus the above negative correlation no longer existed in this case. The different correlations between KAM and normalized IQ in tensile and cyclic cases were attributed to the different ratios between Geometrically Necessary Dislocations (GNDs) and Statistically Storage Dislocations (SSDs). GNDs contribute to not only the lattice distortion that can be indicated by IQ mapping, but also the lattice curvature that can be indicated by KAM mapping, while SSDs only contribute to the lattice distortion. In tensile case, GNDs occupy the major part of total dislocations, thus KAM and IQ indicate the similar dislocations distribution. In cyclic case with the applied strain amplitude lower than ±0.7 %, SSDs play a leading role, thus KAM and IQ disagree with each other on indicating the dislocations distribution.