Influence of Boundary Migration Induced Softening on the Steady State of Discontinuous Dynamic Recrystallization

Abstract

During discontinuous dynamic recrystallization (DDRX), new dislocation-free grains progressively replace the initially strain-hardened grains. Furthermore, the grain boundary migration associated with dislocation elimination partially opposes strain hardening, thus adding up to dynamic recovery. This effect, referred to as boundary migration induced softening (BMIS) is generally not accounted for by DDRX models, in particular by “mean-field” approaches. In this paper, BMIS is first defined and then analyzed in detail. The basic equations of a grain scale DDRX model, involving the classical Yoshie–Laasraoui–Jonas equation for strain hardening and dynamic recovery and including BMIS are described. A steady state condition equation is then used to derive the average dislocation density and the average grain size. It is then possible to assess the respective influences of BMIS and dynamic recovery on the strain rate sensitivity, the apparent activation energy, and the relationship between flow stress and average grain size (“Derby exponent”) of the material during steady state DDRX. Finally, the possible influence of BMIS on the estimation of grain boundary mobility and nucleation rate from experimental data is addressed.

Why Do Grains Remain Roughly Equiaxed during Steady State Discontinuous Dynamic Recrystallization?

“…where h and r are the strain hardening and dynamic recovery parameters, and V the volume of the grain. The last term on the right reflects the effect of BMIS [3]. Expressing…”

“…In order to determine ρ , a ″closure equation″ is used, which means that upon steady state each grain gives birth to one unique new grain in its lifetime. This condition can be written in the form [1,3]: denotes the surface of the grain, which means that DDRX nucleation occurs mainly at grain boundaries, and the factor 1/ 2 accounts for the fact that each boundary is shared by two grains. N k is a nucleation parameter which characterizes the nucleation rate.…”

“…A simple grain scale (or mean field) model has been formerly developed by the authors [1] to analyze various aspects of DDRX steady state, such as the influence of the specific strain hardening and dynamic recovery behaviour of the material [2] or the effect of grain boundary migration induced softening (BMIS) [3]. On the other hand, the combined effects of interphase-boundary advection and migration on the shape change of a second phase particle during growth or dissolution has been recently investigated [4].…”

Dynamic Recrystallization Behaviours in Metals and Alloys

“…Finally, a theoretical paper by Montheillet [13] points out the importance of the softening induced by grain boundary migration (BMIS) during DDRX, in particular for the estimation of grain boundary mobility from experimental data. Two contributions are devoted to near-β titanium alloys deformed in uniaxial compression in both the α and α + β domains: Zhou et al [10] investigate a low-cost iron-containing alloy, while Buzolin et al [11] develop mesoscale models to predict the flow stress and microstructure evolutions of the Ti-5553 and Ti-17 grades.…”

“…Finally, a theoretical paper by Montheillet [13] points out the importance of the softening induced by grain boundary migration (BMIS) during DDRX, in particular for the estimation of grain boundary mobility from experimental data.…”