Investigation of the influence of manganese on the dynamic strain ageing of ferritic steels – Part I: Model alloy

• In-situ TEM tensile tests are performed in a model Fe-Mn-C alloy, from 300K to 900K, to study the effect of Mn on dynamic strain ageing • Activation energies are extracted from mixed and screw dislocations velocities during deformation • Screw dislocations glide is controlled by C diffusion, whereas Mn diffusion controls mixed dislocations glide • Mn addition hinders the high-temperature diffusion-controlled Peierls glide of screw dislocations observed in binary Fe-C alloys • A mechanism for C and Mn co-segregation on mixed dislocations at about 500K is proposed Dislocation dynamics were studied in a ternary Fe-Mn-C model steel by in-situ tensile tests in a transmission electron microscope (TEM) at temperatures ranging from 300K to 900K. The influence of carbon and manganese on dynamic strain aging (DSA) is investigated, the latter being identified by a change in dislocation dynamics. In the DSA temperature range, plastic deformation proceeds by dislocation bursts or dislocation viscous glide. We measure dislocation glide velocities in the DSA regime to extract activation energies for dislocations glide. In Fe-C alloys, the activation energy for the so-called “high-temperature diffusion-controlled” (HTDC) Peierls glide of screw dislocations is close to the diffusion energy of carbon. Surprisingly, HTDC-Peierls glide of screw dislocations is not observed in Fe-Mn-C and viscous glide of mixed dislocations occurs at higher temperature with an activation energy close to that of Mn diffusion. We thus conclude that Mn addition hinders the HTDC-Peierls glide expected at about 500K, and suggest that this may result from Mn being able to co-segregate with carbon on dislocations at temperatures as low as 500K. In addition, Mn is responsible for DSA and controls dislocation viscous glide in the model Fe-Mn-C alloy. We propose a mechanism for Mn enrichment on mixed dislocations at such low temperatures, based on Mn pipe-diffuson. These findings shed new light on the elementary mechanisms controlling DSA in ferritic steels with a particular focus on the effect of C and Mn.