In Situ Investigation of Temperature and Strain-Rate-Dependent Serrated Flow and Crack Evolution in Mn13 High–Manganese Steel

Temperature- and strain-rate-dependent serrated flow, localized plastic instability, and qualitative microcrack evolution in Mn13 Hadfield steel were investigated using stress–strain analysis and in situ tensile observation. Solution treatment at 1050 °C for 1–1.5 h followed by water quenching yielded homogeneous single-phase austenite. Type A and Type B serrations were dominant at room temperature at 1 × 10−3 s−1 and at 100 °C at 1 × 10−2 s−1, whereas the room-temperature specimen tested at 1 × 10−2 s−1 showed delayed serration onset and pronounced Type B stress drops only at high strains. Type C serrations occurred mainly near fracture at 100 °C at 1 × 10−3 s−1, 200 °C at 1 × 10−2 s−1 and 200 °C at 5 × 10−2 s−1. In situ observations further reveal that different serration types correspond to distinct localized deformation modes. During Type A serrations, the tracked feature exhibited unidirectional stepwise migration involving forward motion, arrest, and renewed advance. Type B serrations were associated with discontinuous pulse-like migration characterized by rapid forward motion, brief arrest, local backward motion, and renewed advance. No regular trajectory was observed during Type C stress drops; only local brightness changes and slight positional shifts occurred. These observations link macroscopic serrations to localized deformation in Mn13 steel and provide qualitative in situ evidence for grain-boundary microcrack initiation and evolution during deformation.