Back-stress-induced strengthening and strain hardening in dual-phase steel

Strain hardening for ductility remains challenging especially at high yield strength when dislocation plasticity is usually invalid. The hetero-deformation provides an effective route to induce extra back stress hardening specifically in hetero-structures inherently with large mismatch of mechanical responses, e.g. flow stress and strain hardening etc., upon applied loading. In this paper, both strengthening and strain hardening were investigated in a dual- phase steel, consisting of ductile γ-austenite and almost non-deformable B2 intermetallic phase as the second phase of volume fraction of 23%. The chemical composition was 0.86C, 16Mn, 10Al, 5Ni, balance Fe (wt.%). Of special note is two distinct hetero-deformation responses during both tensile and interrupted unload-reload testing. One is the yield-drop, while the other is hysteresis loop. Both unceasingly appear even from the elasto-plastic yield stage up to whole uniform deformation. The measured back stress and resultant back stress hardening account for a large proportion of global flow stress and strain hardening. Further, both Schmid factor and Kernel average misorientation (KAM) values were measured after tensile deformation. Un-expected, only γ-grains bordering on B2-phase show a significant decrease in the average Schmid factor, relative to almost unchanged in left γ-grains still next to γ-ones as well as B2-phase. This indicates that γ-grains adjacent to B2-phase bear the vast majority of plastic strains, not simple strain partitioning between γ and B2. Because of this, from the onset of yielding to end of tensile deformation, those γ-grains, in contrast to left γ-grains and B2 phase, exhibit a maximal increment in KAM values. This serves as a solid evidence of the generation of geometrically necessary dislocations to accommodate strain gradient near γ/B2 phase boundaries. It turns out that hetero-deformation due to plastic incompatibility induces the operation of back stresses, leading to both strengthening and strain hardening as well. Finally, a microstructure-based model was developed to calculate back stress which was well consistent with experimentally measured back stress.