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

Abstract

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

Cyclic hardening/softening and deformation mechanisms of a twip steel under reversed loading

“…Both components of the flow stress slightly decrease during the softening stage, whose origin is discussed, based on TEM observations at peak stress amplitude, or after softening. The present measurements and TEM observations, combined with those from a previous study of twinning/detwinning kinetics in push-pull on the same steel [29], suggest that under cyclic loading, mechanical twinning cannot be responsible for significant kinematic hardening of intragranular nature (or "dynamic Hall-Petch effect"), but rather contributes to a back stress of intergranular origin.…”

“…5a) would also be consistent with the lower twin fraction under cyclic loading. Furthermore, during reversed loading, repeated twinning/detwinning, or alternating twinning on two systems of the same twinning plane, in tension and compression, respectively were observed in most monitored grains [29],…”

“…In parallel with this classical approach, the deformation mechanisms during cyclic loading, and in particular twinning/detwinning were monitored at grain-scale by tests run in situ, either in a SEM with high-resolution digital image correlation, or under an AFM. The results of these observations have been reported in detail in [29] and will not be repeated here, but referred to in the discussion.…”

“…5a) also deserves some comments. As mentioned in the introduction, the evolution of the twinned fraction, during in situ pushpull tests run on the present TWIP steel, under fixed stress amplitude or fixed plastic strain amplitude, under an Atomic Force Microscope (AFM) was measured in a few, well oriented grains [29]. Under fixed stress amplitude (/2= 500 MPa), the twin fraction in all monitored grains was significant after the first ramp, due to nearly 5% monotonic plastic strain, but saturated already from the 2 nd cycle, while under fixed plastic strain amplitude (p/2 = 0.5%), it increased in a ratchetting way during the whole cyclic hardening stage, and stopped increasing at the onset of cyclic softening.…”

Study of work-hardening behavior of high manganese steel during compression

“…For materials with SFE>50 mJ m −2 , the formation of twinning was strongly suppressed, and microbands are formed by planar glide in order to accommodate plastic deformation. Otherwise, The loading modes [7], temperature [8], grain size [9,10], grain orientation [11] and strain rate, all can influence the activation of deformation twinning [12,13].…”