A multiscale investigation on the preferential deformation mechanism of coarse grains in the mixed-grain structure of 316LN steel

Mixed-grain structure with millimeter-grade coarse grains (MCGs) is an intolerable defect often found in heavy forgings. Eliminating this microstructural defect through thermomechanical operations requires a thorough understanding on the microscopic deformation heterogeneity associated with the uneven grain size. Both in-situ and interrupted micro-tensile tests combined with a grain-level digital image correlation, microstructure tracking, surface slip tracing, and fractography examination were conducted on the intentionally prepared samples of mixed-grain structure with MCGs. Mesoscale full-field crystal plasticity (CP) modeling, incorporating a grain-level Hall-Petch law and a grain-size enhanced hardening rule, was proposed to describe the uneven grain size effect of mixed-grain structure. The agreements of simulation results with the experiments demonstrated that the proposed CP model is competent to capture the mesoscale deformation characteristics of the structures with MCGs. It was found that, during the deformation, the large deformation band covering the MCG and its surrounding fine grains (FGs) was preferentially generated because MCGs take the priority of plastic deformation, even if MCGs were hard-oriented. At large deformation, some FGs in the vicinity of MCGs inside the deformation band sustained more strain over the MCGs to coordinate the enforced deformation in the band, resulting in the intense strain and stress localizations. It was further found that the preferential and continuous deformation characteristic of the MCG is related to its ‘lake’ shape distribution of the number of activated slip systems (Ns), meaning that Ns is smaller in the interior than at the boundary of the MCGs. Both the TEM characterizations and CP simulations demonstrate that the MCG interior deformed in single-slip at early deformation and in primary-slip at large deformation. Compared with the FG structure, the MCGs have weaker initial slip resistance and hardening rate due to the less GB-related multiplication and storage of dislocations, and dislocation interaction in the multiple slip scenario.