Transmission Electron Microscopy Analysis of Yielding in Ultrafine-Grained Medium Mn Transformation-Induced Plasticity Steel

The microstructural changes occurring during yielding of ultrafine-grained medium Mn transformation-induced plasticity steel intercritically annealed at 873 K and 923 K (600 °C and 650 °C) were analyzed by transmission electron microscopy. The tensile properties and the work hardening behavior of the 7 wt pct Mn steel were found to have a pronounced dependence on the intercritical annealing temperature. A low intercritical annealing temperature of 873 K (600 °C) resulted in a martensite-free ultrafine-grained two-phase microstructure consisting of ferrite and austenite. After annealing at 923 K (650 °C), the ultrafine microstructure contained ferrite, low stability retained austenite, and athermal martensite. Yielding of the steel annealed at 873 K (600 °C) was associated with the initiation and propagation of Lüders deformation bands, whereas yielding of the steel annealed at 923 K (650 °C) was initiated at a lower tensile stress by stress-induced austenite transformation. The progression of the martensite transformation during yielding strongly suggests that the yielding and strain hardening of ultrafine-grained medium Mn transformation-induced steel are controlled by the stability of the ultrafine-grained austenite phase. Suppression of plastic flow localization is achieved when the UFG steel has an initially high rate of strain hardening resulting from transformation-initiated yielding.