Investigation of Strain Heterogeneities Between Grains in Ferritic and Ferritic-Martensitic Steels

This work uses microlithography, digital image correlation and tensile test in order to investigate the reasons behind the heterogeneous strain distribution at the grain scale. Scanning Electron Microscope images are taken to examine the relationship between microstructure features and strain heterogeneity. The study is carried out on single phase ferritic steel and two dual phase steels with ferrite and different hard particle martensite contents. Useful image correlation is obtained in grains with diameters of 2–3 μm for the martensite and ranging from 10 to 20 μm for the ferrite. To prevent a decrease of image correlation success, some technical aspects as the microgrid step and bar width are extensively tackled with for intermediate deformations (>10 %). The different levels of longitudinal intragranular strains observed inside the ferrite grains are not correlated with their orientation, shape, size or the presence (and content) of hard phase in the material.