On the first direct observation of de-twinning in a twinning-induced plasticity steel

Abstract

Electron back-scattering diffraction was used to track the microstructure evolution of a fully annealed Fe-24Mn-3Al-2Si-1Ni-0.06C TWinning Induced Plasticity (TWIP) steel during interrupted reverse (tensioncompression) loading. Direct observation of the same selected area revealed that all deformation twins formed during forward tension loading (0.128 true strain) were removed upon subsequent reverse compression loading (0.031 true strain). Consequently, the present study provides the first unambiguous experimental evidence of de-twinning during the reverse loading of a polycrystalline TWIP steel. The reverse loading behaviour was simulated by a dislocation-based hardening model embedded in the Visco-Plastic Self-Consistent (VPSC) polycrystal framework taking into account the accumulation and annihilation of dislocations and back-stress effects. The model has been extended to account for the processes of twinning and de-twinning, as well as the twin barrier effect under load reversal. A new formulation based on the changes in the dislocation mean free path is proposed to track twin lamellae generation/annihilation throughout deformation along with its associated effect on hardening.

Experimental and Self-Consistent Modeling Study of De-twinning in a Twinning-Induced Plasticity Steel

“…Along with the experimental observation of de-twinning in the above study [7], the tensioncompression loading behavior was successfully simulated using a dislocation-based hardening model embedded in the visco-plastic self-consistent (VPSC) polycrystal framework [8][9]. The hardening model is based on the Rauch-Gracio-Barlat-Vincze (RGBV) [10] continuum plasticity model developed to simulate the mechanical response under strain path changes; building on the concept that a subset of dislocations generated during loading in one direction can reverse their glide direction and progressively recombine upon load reversal.…”

“…[7] is also applied in this study, the model formulation is not reproduced here in order to ensure brevity in the text. Briefly speaking, the model is based on the RGBV-VPSC framework [11][12][13] and has been modified to take into account the twinning barrier effect (as deformation twins act as strong obstacles to dislocation glide) and the occurrence of de-twinning under strain path change [7]. While the inclusion formalism of the VPSC model inherently accounts for the intergranular stresses effect, the intragranular stresses are captured via empirical back stress equations introduced for both slip and twinning systems.…”

“…[6] suggested that de-twinning could be taking place upon load reversal due to the reversibility of slip along the 〈112〉 direction. To verify the occurrence of de-twinning (i.e., the disappearance of twinned regions as the twins are reoriented back to their parent grains), electron backscattering diffraction (EBSD) was applied in a follow-up study to track microstructure evolution in a region of interest during interrupted reverse (tension-compression) loading of the same TWIP steel [7].…”

“…Thereafter, Wen et al extended the crystallographic RGBV model to predict the mechanical response associated with tension reloading of AZ31 Mg alloy [12] and cyclic shear tests of low-carbon steel [13]. The model was further modified in order to account for the processes of twinning and de-twinning, as well as the back stress and twin barrier effects under tension-compression loading of TWIP steel [7].…”

“…To this end, EBSD is applied to track microstructure evolution in a region of interest during interrupted compression-tension loading. The modified RGBV-VPSC model [7] is also used in the present work to further validate our formulation of the twinning back-stress and the twin barrier effect introduced to capture the processes of twinning and de-twinning (or twin lamellae generation and annihilation) during load reversal.…”