Tetragonality mapping of martensite in a high-carbon steel by EBSD

The locally varying tetragonality in martensite grains of a high-carbon steel (1.2 mass percent C) was resolved by electron backscatter diffraction (EBSD) with a spatial resolution in the order of 100 nm. Compared to spatially integrating X-ray diffraction, which yielded an average tetragonality of c/a = 1.05, the EBSD measurements in the scanning electron microscope allowed to image a local variation of the lattice parameter ratio c/a in the range of 1.02 < c/a < 1.07. The local variation of tetragonality is confirmed by two different EBSD data analysis approaches based on the fitting of simulated to experimental EBSD patterns. The resulting EBSD-based tetragonality maps are pointing to a complex interaction of carbon concentration and local lattice distortions during the formation process of martensitic structures.