In situ identification of elastic–plastic strain distribution in a microalloyed transformation induced plasticity steel using digital image correlation

• In-situ digital image correlation (DIC) method was applied to study inhomogeneous deformation of TRIP steel.
• The DIC results show that the yield point is controlled by stress-assisted martensite transformation.
• DIC results show that the high elongation to fracture in present steel is controlled by strain-induced martensitic transformation.
• DIC is an appropriate technique to distinguish the onset of TRIP through stress or strain induced martensite formation.

A non-contact strain measurement technique, based on an in-situ digital image correlation (DIC) method in association with magnetic martensite point measurement (Feritoscopy testing) was applied to study inhomogeneous deformation corresponding to martensitic transformation of a microalloyed low carbon transformation induced plasticity steel during tensile straining. The progress of inhomogeneous deformation is traced by the strain maps. The microstructural observation is used to validate the DIC results. The experimental steel shows continuous yielding with a high true fracture strength of 1410±10 MPa at 25 °C along with the lack of tensile necking. The DIC results show that the yield point is controlled by stress-assisted martensite transformation, which in turn induces the strain inhomogeneity. The latter starts prior to the yield point after straining to 0.016. The microstructural evolution reveals the ε-martensite is obtained through stress-assisted martensite formation. After yielding, thanks to the strain-induced martensite transformation, the deformation inhomogeneity in strain maps is increased with strain, corresponding to increasing the volume fraction of martensite. The results suggest that the continuous yielding and initial strain hardening is controlled by stress-assisted martensite formation while the higher total elongation to fracture (80%) and the tensile necking behavior is mainly influenced by the strain-induced martensite transformation.