Craig Daniels, Brigham Young University
Engineering
Martensite is a steel phase that has a body-centered tetragonal crystal lattice. It significantly affects the material properties of steel, particularly hardness and strength. Electron Backscatter Diffraction (EBSD) is a microscopy technique that is used to characterize the crystal and grain structure of metals by analyzing diffraction patterns. Martensite is traditionally difficult to identify using EBSD techniques because its diffraction patterns are too similar to the ferrite phase, which is body-centered cubic (BCC). The martensite crystal is modeled as BCC, but with an imposed tetragonal strain. “High resolution” EBSD can reveal the absolute strain of the crystal lattice, and is sensitive enough to measure the strain imposed in the model. This technique uses cross-correlation to compare experimental diffraction patterns to kinematically simulated patterns, and returns the absolute strain tensor. The tensor is rotated into the crystal frame, and the principle strains are used to create a tetragonality index. This tetragonality index can then be used to identify martensite. Further, the lattice parameters of martensite are related to the carbon content. A theoretical tetragonality index can be created using only lattice parameters. If the experimental index can be fitted to the theoretical index, this technique could be used to characterize the carbon content of steel at a sub-grain level.