In situ study of deformation behavior and strain partitioning in low-density medium Mn steel

• In situ tension with μ-DIC reveals the deformation mechanism of low-density medium Mn steel. • The 30-min sample, rich in block austenite, achieves the best properties (960 MPa, 35.8% ductility). • The block austenite promotes transition of deformation from low strain to high strain. • The strip austenite becomes the primary region of strain concentration at late deformation stage. In situ tensile testing, combined with EBSD analysis and μ-DIC technology, was used to reveal the deformation mechanisms of newly designed Al-containing low-density medium Mn steel. The results show that the two types of austenite (strip and block) can be obtained in the sample annealed for 30 min at 700 °C, while the other samples mainly get the strip austenite. The TRIP effect only occurs in the block austenite due to its low Mn concentration and large grain size (>1.6 μm), which results in the best tensile properties in the sample annealed for 30 min. Additionally, in situ experimental evidence indicates that strain concentration in ferrite and block austenite primarily occurs during the early and middle stages of deformation, facilitating a successful transition of deformation from low to high strain. In the later stages of deformation, some strip austenite regions exhibit the fastest rate of strain increase and become the primary location for strain concentration. Furthermore, the strain partitioning of strip austenite is influenced by both crystal orientation and geometric orientation. The strip austenite with the main slip direction and slip plane parallel to the long axis can accommodate significant strain.