How is Electron Backscatter Diffraction (EBSD) used to detect LAB defects?

EBSD Orientation Mapping

Electron Backscatter Diffraction (EBSD) is one of the most accurate methods for detecting low-angle boundary (LAB) defects in single-crystal castings. EBSD produces detailed crystallographic orientation maps by analyzing the diffraction pattern generated when an electron beam interacts with the polished surface of the alloy. Because LABs represent slight misorientations—typically less than 15°—between adjacent dendrite arms, EBSD’s angular resolution allows engineers to visualize these subtle orientation changes with high precision.

Misorientation Angle Analysis

EBSD detects LAB defects by calculating the misorientation angle between neighboring pixels in the scan area. In a perfect single crystal, orientation changes are minimal and uniform. When LABs form, EBSD reveals a discrete increase in misorientation, often appearing as thin, linear features. These boundaries are plotted as color-coded lines or gradients in inverse pole figure (IPF) maps or misorientation distribution plots. Alloys such as CMSX-4 or Rene 41 exhibit distinct misorientation contrasts when LABs develop due to thermal instability or deformation.

Visualizing Dislocation-Based Boundaries

LABs are essentially arrays of dislocations arranged in a semi-organized structure. EBSD highlights these dislocation structures by showing subtle lattice rotations. Kernel Average Misorientation (KAM) mapping is particularly useful, as it visualizes local strain and dislocation density. Regions with high KAM values often correlate with LAB formation, making EBSD an indispensable tool for differentiating LABs from harmless lattice curvature.

Process Feedback and Quality Control

EBSD results provide critical feedback for improving casting parameters such as withdrawal speed, furnace zoning, and mold insulation in directional casting. By identifying where LABs tend to form—such as near geometric transitions or thermal anomaly regions—engineers can redesign local cooling strategies or adjust solidification conditions. Combined with destructive material testing and analysis, EBSD offers the most definitive confirmation of LAB-related crystallographic misorientation.