Carbon-induced negative strain-rate sensitivity in a quenching and partitioning steel

The present work reports an abnormal negative strain-rate sensitivity (SRS) in a room-temperature quenching and partitioning (RT-Q&P) steel. The mechanisms responsible for such negative SRS were systematically investigated. Continuous and interrupted tensile tests at quasi-static (10− 3 s − 1) and high strain rate (600 s − 1) were performed. It is found that the flow stress after yielding exhibits abnormal negative SRS. Interestingly, similar martensitic transformation occurs at both 10− 3 s − 1 and 600 s − 1, implying that transformation induced plasticity (TRIP) effect is not responsible for the negative SRS. The designed interrupted rate-change tests and interrupted high-strain-rate load-unload-load tests were performed to reveal the significant effect of interstitial carbon atoms on the negative SRS. Moreover, the Cottrell atmospheres with carbon atoms segregated at dislocations were further confirmed by atom probe tomography (APT). The Cottrell atmosphere can be continuously rebuilt during deformation at quasi-static strain rate, but cannot be rebuilt at high strain rate since there is no sufficient time for carbon atoms to diffuse to the high-velocity dislocations. The lack of Cottrell atmospheres at high strain rate results in (1) absence of extra carbon dragging force and (2) a lower dislocation density, both of which contribute to a lower flow stress at high strain rate and therefore the negative SRS.