Micromechanical Characterization of Nano-bainite Steel

Microtensile tests combined with crystal plasticity finite element simulations were performed on a single-packet structure of a nano-bainite steel. The steel microstructure consisted of bainitic ferrite and retained austenite, and its constituents were formed according to the Nishiyama–Wassermann crystallographic orientation relationship. The single-packet specimens exhibited plastic anisotropy in their yielding behavior, similar to those of lath martensite and upper bainite. The deformation-induced transformed martensite variants demonstrated high kernel average misorientation values owing to the introduction of a large number of dislocations, and these crystallographic orientations were similar to those of the adjacent bainite variants, in which the slip system with the highest Schmid factor was nearly parallel to the transformation system. A numerical analysis incorporating a crystal plasticity constitutive model that accounted for the martensitic transformation closely represented experimental stress–strain responses, demonstrating the effectiveness of the proposed method. When applied to polycrystalline structures, multiaxial stress distributions promoted martensitic transformations, leading to significant strain hardening. Assuming a high austenite strength in the analysis model, the strength increased owing to the bainitic matrix, whereas strain hardening was limited because of the suppressed martensitic transformation. In contrast, assuming low-strength austenite in the model, the yield stress was slightly reduced while the martensitic transformation was enhanced, which resulted in pronounced strain hardening and a high tensile strength. These findings suggest that the incorporation of low-strength austenite within a high-strength matrix can optimize the balance between strength and ductility.