Evidence of shear stress component in Lüders front based on an in-situ synchrotron high-energy X-ray diffraction experiment

The formation and propagation of Lüders bands are important phenomena in the plastic deformation of some critical structural materials. The propagating Lüders front, which is structurally unstable, plays a key role in this process. Yet, the microstructural and stress states of the Lüders front are challenging to characterize and remain insufficiently understood. The present study utilized in-situ synchrotron-based high-energy X-ray diffraction on a fine-grained medium-Mn transformation-induced plasticity steel exhibiting typical Lüders banding behavior. Detailed analysis of evolving diffraction patterns was performed regarding peak intensity, full width at half maximum, and measured lattice strain. An abnormal measured lattice strain asymmetry was observed from the Debye rings that were collected when the Lüders front overlapped with the irradiated volume. This allows for a discussion of the local microstructural and stress states of the Lüders front, evidencing the possibility of a local shear stress component with a tilted principal stress axis. The work offers new insights into the micro-mechanisms of Lüders banding. It provides a practical and efficient analytical method for studying the dynamics of localized deformations, particularly when deformation is inhomogeneous within the characterized volume or inconsistent with macroscopic deformation.