Grain size effect on the strain hardening behavior of Fe-24Ni-0.3C metastable austenitic steel studied by in-situ neutron diffraction

Mao, W.; Gao, S.*; Gong, W.   ; Park, M. H.*; Bai, Y.*; Shibata, Akinobu*; Tsuji, Nobuhiro*

Deformation induced martensitic transformation (DIMT) during plastic deformation of metastable austenitic steels plays an important role in enhancing their strain hardening, leading to an outstanding combination of strength and tensile ductility. In this study, Fe-24Ni-0.3C metastable austenitic steel specimens having mean grain sizes ranging from 1.3 micrometer to 35 micrometers were fabricated by cold rolling and subsequent annealing processes. The effect of the grain size on the strain hardening behavior and DIMT in the material was investigated by tensile test at room temperature with in-situ neutron diffraction. Results obtained by the in-situ neutron diffraction showed that the enhanced strain-hardening rate was caused not only by the increase of the volume fraction of martensite but also by the rapid increase of the internal stress within martensite. When the grain size changed within the coarse grained region (35 micrometers to 4 micrometers) the influence of the grain size on the stress partitioning between austenite and martensite was relatively small, thus the work-hardening behavior was mainly determined by the increasing rate of the volume fraction of martensite. However, when the grain size decreased down to ultrafine grain regime (smaller than 2 micrometers), the internal stress in martensite significantly increased, which resulted in the increasing work-hardening rate. The increasing stress in martensite in the ultrafine grained specimens is explained by the enhanced elastic stress associated with the incompatibility between martensite and austenite phases.