Deformation behavior of ultrafine-grained TRIP steel observed by neutron diffraction

抄録

Ultrafine-grained (UFG) metastable austenitic steels, produced through plastic deformation and annealing, retain good ductility due to deformation-induced martensitic transformation (DIMT), unlike most UFG metals. However, the effects of grain size on DIMT and deformation behavior are not fully understood. This study addresses this gap by conducting in situ tensile tests with neutron diffraction on coarse-grained (CG: 35 μm) and UFG (0.5 μm) Fe-24Ni-0.3C steels to explore how grain refinement affects deformation and DIMT. Results showed that grain size significantly affects dislocation density and lattice strain partitioning among austenite grains during deformation. The UFG specimen showed a rapid increase in dislocation density within austenite grains and complete grain DIMT, but in a smaller fraction of grains compared to the CG specimen. The dislocation arrangement, represented by the M* parameter, evolved differently between the two specimens, suggesting distinct nucleation mechanisms. In the CG specimen, DIMT occurred in many austenite grains, resulting in significant stress relaxation. In contrast, the UFG specimen experienced DIMT in fewer austenite grains, resulting in reduced stress relaxation. These findings underscore the impact of grain refinement on martensite formation and the mechanical properties of metastable austenitic steels.