Sonntag, N.; Cabeza, S.; Kuntner, M.; Mishurova, T.; Klaus, M.; Kling e Silva, L.; Skrotzki, B.; Genzel, C.; Bruno, G.: Visualisation of deformation gradients in structural steel by macroscopic magnetic domain distribution imaging (Bitter technique). Strain 54 (2018), p. e12296/1-15
10.1111/str.12296

Abstract:
While classically used to visualise the magnetic microstructure of functional materials (e.g., for magnetic applications), in this study, the Bitter technique was applied for the first time to visualise macroscopic deformation gradients in a polycrystalline low‐carbon steel. Spherical indentation was chosen to produce a multiaxial elastic–plastic deformation state. After removing the residual imprint, the Bitter technique was applied, and macroscopic contrast differences were captured in optical microscopy. To verify this novel characterisation technique, characteristic “hemispherical” deformation zones evolving during indentation were identified using an analytical model from the field of contact mechanics. In addition, near‐surface residual stresses were determined experimentally using synchrotron radiation diffraction. It is established that the magnetic domain distribution contrast provides deformation‐related information: regions of different domain wall densities correspond to different “hemispherical” deformation zones (i.e., to hydrostatic core, plastic zone and elastic zone, respectively). Moreover, the transitions between these three zones correlate with characteristic features of the residual stress profiles (sign changes in the radial and local extrema in the hoop stress). These results indicate the potential of magnetic domain distribution imaging: visualising macroscopic deformation gradients in fine‐grained ferromagnetic material with a significantly improved spatial resolution as compared to integral, mean value‐based measurement methods.