Retained-austenite transformation precedes grain fragmentation in carbon-partitioned QP1180 steel

Understanding the mechanistic interplay between phase transformation and grain fragmentation is critical for microstructural control in advanced structural steels subjected to severe shear. Here, we investigate the activation sequence of retained-austenite transformation and grain fragmentation along the radial strain gradient of a single QP1180 steel disk processed by high-pressure torsion. Synchrotron-based high-energy X-ray diffraction and microscopy reveal a pronounced austenite (γ) → martensite (α′/α) transformation that saturates at a critical equivalent von Mises strain εT ~ 8.5. Concomitantly, γ grain size decreases sharply up to εT , while γ peak broadening and microstructural analysis suggest limited grain fragmentation of austenite during transformation. These findings demonstrate that γ-phase reduction is primarily driven by phase transformation prior to the onset of defect-induced fragmentation. This mechanistic activation order and the critical strain εT provide key inputs for calibrating physics-based constitutive models and defining robust process windows for industrial forming operations and component design.