Microstructural origins of energy storage during plastic deformation of 310S TWIP steel

The microstructural mechanisms governing energy storage during plastic deformation of twinning-induced plasticity (TWIP) steels remain insufficiently understood, particularly under conditions of strain localization. This study provides a crystallographic-scale interpretation of energy storage in 310S TWIP steel exhibiting complex deformation mechanisms. Electron backscatter diffraction (EBSD) was used to characterize the evolution of local crystallographic orientation and microtexture during uniaxial tensile deformation using two complementary approaches: tracking the same surface region at successive strain levels and analysing regions corresponding to known local plastic strain. Deformation was initially dominated by dislocation slip, while twinning activity increased significantly beyond an equivalent plastic strain of approximately 0.3. Progressive deformation produced pronounced lattice rotations and the development of a dual-fibre texture consisting of a dominant 111 parallel to RD component and a secondary 100 parallel to RD component associated with deformation twinning. Correlation with previously quantified energy storage behaviour obtained from coupled digital image correlation and infrared thermography measurements reveals that intensified twinning and texture evolution in strain-localized regions are accompanied by a marked reduction in the energy storage rate. The results indicate that twin-matrix refinement and lattice rotation progressively reduce the material’s capacity to store deformation energy and create favourable conditions for shear-band-mediated deformation.

💡 Research Summary

This paper investigates the microstructural origins of energy storage during plastic deformation of the austenitic 310S TWIP (twinning‑induced plasticity) steel, linking crystallographic texture evolution to previously measured thermomechanical energy conversion. The alloy, with a high nickel content, exhibits a stacking‑fault energy (SFE) of 35–43 mJ m⁻², placing it in the intermediate‑SFE regime where both dislocation slip and mechanical twinning are active. Two complementary EBSD strategies were employed. The first tracked the same surface region through successive loading‑unloading cycles, allowing direct observation of orientation changes, lattice rotations, and twin nucleation as strain increased. The second approach correlated DIC‑derived equivalent plastic strain maps with EBSD measurements taken from distinct regions representing low, moderate, and high local strains, including through‑thickness cross‑sections to ensure bulk representativeness.

Initial microstructure consists of equiaxed grains (~21 µm) with a weak 〈111〉‖rolling‑direction (RD) texture and abundant annealing twins. At low strains (εp < 0.3) deformation is dominated by dislocation slip; grain reference orientation deviation (GROD) values remain modest (≈7–9°) and the energy‑storage rate Z, defined as dE_storage/dεp, is high (≈0.4). This stage corresponds to storage of energy mainly in low‑energy dislocation structures (LEDS).

When the equivalent plastic strain reaches about 0.3, the Schmid factor for the {111}⟨112⟩ twinning system becomes comparable to that for {111}⟨110⟩ slip, triggering rapid twin formation. Twinning introduces a secondary 〈100〉‖RD fiber, producing a dual‑fiber texture (dominant 〈111〉‖RD plus secondary 〈100〉‖RD). Grain‑scale analysis shows two contrasting behaviours: grains favouring slip (high slip Schmid factor) experience extensive lattice rotation (GROD up to 35°) and fragment into sub‑grains, whereas grains favouring twinning develop lamellar twin‑matrix structures with limited rotation.

In regions where strain localizes (the necking zone), twin‑matrix refinement and pronounced lattice rotation dramatically reduce Z. Surface maps of Z reveal a drop from ~0.4 at early stages to <0.1 during localization, and just before fracture Z becomes negative, indicating that the heat released exceeds the incremental plastic work and previously stored energy is being liberated. The zero‑crossing of Z coincides with the Considére criterion, i.e., the onset of rapid softening.

The evolution of texture is central to this energy behaviour. The emergence of the 〈100〉‖RD component, together with large lattice rotations, creates incompatibilities that favour the nucleation and propagation of shear bands (strain‑localization bands). These bands provide pathways for rapid energy dissipation and are associated with the observed decline in stored energy.

Overall, the study demonstrates that in 310S TWIP steel energy storage is initially governed by dislocation slip but is progressively suppressed as mechanical twinning becomes dominant, the texture evolves, and lattice rotations intensify. Twin‑matrix refinement and shear‑band formation act synergistically to convert stored elastic energy into heat, explaining the marked reduction in Z during strain localization. These insights clarify the microstructural basis of the exceptional combination of high ductility and energy absorption in TWIP steels and offer guidance for tailoring microstructures to optimise impact‑resistance and formability.