Deformation Twinning Behavior of Twinning-induced Plasticity Steels with Different Carbon Concentrations – Part 1: Atomic Force Microscopy and Electron Backscatter Diffraction Measurements

Abstract

The deformation twinning behavior in Fe-17Mn-0.6C, Fe-17Mn-0.8C, and Fe-18Mn-1.2C (wt.%) twinning-induced plasticity (TWIP) steels was investigated by atomic force microscopy (AFM) and electron backscatter diffraction pattern (EBSD) analyses. AFM-based surface relief analysis combined with EBSD measurements was employed to determine the active twinning direction and deformation twin fraction for specific crystallographic orientations. The addition of carbon is known to increase the stacking fault energy. The < 111 > tensile orientation grains revealed suppression of deformation twinning with increasing carbon concentration; however, the deformation twin fraction in the < 144 > tensile orientation did not change as a function of the carbon concentration. These results imply that another factor in addition to stacking-fault-energy-based criteria is required to interpret the deformation twinning behavior of carbonadded TWIP steels.KEY WORDS: surface relief analysis; deformation twinning; austenitic steel; atomic force microscopy (AFM); electron backscatter diffraction (EBSD).

Deformation Twinning Behavior of Twinning-induced Plasticity Steels with Different Carbon Concentrations – Part 2: Proposal of Dynamic-strain-aging-assisted Deformation Twinning

“…In the previous paper, 7) we reported the following facts with respect to the effects of carbon on deformation twinning behavior:…”

“…4(a) and 4(b). ε-martensite and α'-martensite were not detected in the Fe-17Mn-0.6, Fe-17Mn-0.8C, and Fe-18Mn-1.2C steels after a 10% tensile deformation at ambient temperature 7) despite the appearance of the serrations at less than 10% tensile strain (Fig. 1).…”

“…In the present study, we discuss the carbon effects based on the twinning behavior at different crystallographic orientations and deformation temperatures in the Fe-Mn-C twinning-induced plasticity (TWIP) steels observed in the previous 7) and present studies. Then, we propose a new concept of the necessary conditions for deformation twinning in terms of the dislocation pinning effect (particularly from the viewpoint of DSA), which explains the characteristics of deformation twinning in Fe-Mn-C austenitic steels consisting of a low critical stress and strain for deformation twinning and wide deformation temperature and stacking fault energy ranges for deformation twinning.…”

“…In terms of stacking fault energy, it increases with increasing carbon concentration in high Mn austenitic steels when the concentration exceeds 0.3% wt%. 5,6) In a previous study, 7) the total amount of deformation twins and local twin fraction in < 111 > tensile orientations decreased with increasing carbon content in Fe-Mn-C austenitic steels. Namely, the carbon concentration dependence of the deformation twinning behavior was confirmed to be the same as the conventional stacking fault energy dependence.…”

“…However, in < 144 > tensile orientations, the deformation twin fraction did not exhibit an exceptional carbon concentration dependence. 7) Namely, the conventional understanding in terms of stacking fault energy does not provide a comprehensive mechanism to explain the carbon concentration dependence of the deformation twinning behavior.…”