Direct monitoring of twinning/detwinning in a TWIP steel under reversed cyclic loading

In situ tensile and reversed cyclic tests were run on a TWIP steel in SEM, with High-Resolution Digital Image Correlation (HR-DIC) measurements of the plastic strain field few selected grains prone to twinning, spatial resolution between 150 250 nm, or under an AFM, surface steps height at emerging deformation twins. Evidences detwinning upon load reversal, as well quantitative data twinning/detwinning/retwinning obtained. Detwinning retwinning, which often only partial, spite fully loading, did not seem start onset stress reversal. It required sufficient variation stress, close twinning (estimated 400–475 MPa) absolute value, so that mechanical hysteresis local twinned fraction occurred. Primary secondary along same plane, inducing axial strains opposite directions, also allowed some accommodate strain. Under fixed amplitude (±500 MPa), twin all monitored saturated values 0.5 3.5%, from 2nd cycle, while (±0.5%), it increased ratchetting way during whole hardening stage, reaching 0.5–5%. both cases, however, accommodated by twinning/detwinning, reached 0.35–0.42% 1st decreased down less than 0.05% after 100 1000 cycles.