On the Stacking Fault Energy of Fe-18 Pct Mn-0.6 Pct C-1.5 Pct Al Twinning-Induced Plasticity Steel

Abstract

The stacking fault energy (SFE) of Fe-18 pct Mn-0.6 pct C-1.5 pct Al twinning-induced plasticity (TWIP) steel was measured using weak-beam dark-field imaging of dissociated dislocations observed in transmission electron microscopy. The SFE was found to be 30 mJ/m 2 . A relatively wide scatter was observed in the experimentally measured partial dislocation separation of screw dislocations. It is argued that the anomalously wide partial dislocation separation is due to the interaction of point-defect pairs involving interstitial C atoms with the strain field of the partial dislocations. Internal friction (IF) experiments were carried out to detect the presence of point-defect pairs that might affect the dislocation separation, and a clear Finkelshtein-Rosin (FR) peak related to point-defect pairs involving interstitial C atoms was observed in the IF spectrum.High Mn austenitic twinning-induced plasticity (TWIP) steel is one of the most promising newly developed automotive steels owing to a superior combination of strength and ductility. The stacking fault energy (SFE) is an important material property. Its value must be within a narrow range for deformationinduced twinning to occur. The SFE is determined by composition and temperature. Remy and Pineau [1] were the first to report that the two deformation modes observed in low SFE Fe-Mn fcc alloys, the occurrence of a strain-induced c fi e martensitic transformation and deformation twinning, were controlled by the composition dependence of the SFE. The critical stacking fault energy range to achieve twinning-induced plasticity is still unclear, and no experimental SFE determination is currently available for the two TWIP steel compositions that are considered to have the most industrial potential: Fe-22 pct Mn-0.6 pct C and Fe-18 pct Mn-0.6 pct C-1.5 pct Al. Frommeyer et al. [2] indicated that whereas an SFE larger than about 25 mJ/m 2 will result in the twinning effect in a stable c phase, an SFE smaller than about 16 mJ/m 2 results in e phase formation. Allain et al. [3] mentioned a much narrower range. According to them, the SFE should be at least 19 mJ/m 2 to obtain mechanical twinning. They also mentioned that an SFE less than 10 mJ/m 2 results in strain-induced e-phase formation. Dumay et al. [4] indicate that below an SFE of 18 mJ/m 2 , twinning tends to disappear and is replaced by the formation of e-platelets. They conclude that an SFE of about 20 mJ/m 2 is needed for the best hardening rate. Finally, Jin and Lee [5] indicated that an SFE value of 33 mJ/m 2 is required to obtain twinning in Fe-18 pct Mn-0.6 pct C-1.5 pct Al. In the present contribution, the authors report the first systematic measurements of the SFE of Fe-18 pct Mn-0.6 pct C-1.5 pct Al TWIP steel. In addition, it was found that for screw dislocations, the partial dislocation separation was abnormally wide. This is believed to be caused by the same point-defect complexes that cause dynamic strain aging (DSA) in TWIP steel alloyed with carbon to stabilize the austenitic structure.In the past,...

Hydrogen Embrittlement in Al-added Twinning-induced Plasticity Steels Evaluated by Tensile Tests during Hydrogen Charging

“…It is widely accepted that deformation twinning is suppressed by the Al addition due to an increase in stacking fault energy. [16][17][18] The suppression of deformation twinning reduces local stress at grain boundaries interacting with deformation twins, preventing intergranular cracking that was reported in a previous work. 8) We believe that the quasicleavage fracture occurred because of the suppression of the intergranular fracture.…”

“…The cause of the difference also would be associated with hydrogen trap density and hydrogen distribution. The hydrogen trap density and the hydrogen distribution in boundaries such as grain and twin boundaries must change by the Al addition, because the dislocation density 15) and twin fraction [16][17][18] decrease with increasing Al content. The hydrogen in sites where brittle fracture did not occur does not contribute to the hydrogen embrittlement.…”

Strain hardening by dynamic slip band refinement in a high-Mn lightweight steel

“…where K is a geometrical factor, M ¼ 3.06 is the Taylor-factor for randomly distributed grain orientations, G ¼ 70 GPa [77,86,87] is the bulk elastic shear modulus of the alloy, b ¼ 0.26 nm is the magnitude of the Burgers vector [45,77,87] and D is the mean slip band spacing (see Fig. 6).…”

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

“…11,14) A promising approach to interpret the effects of carbon is to consider the dislocation pinning effect due to carbon diffusion/segregation. 12,15,16) An important carbon-dislocation interaction in Fe-Mn-C austenitic steels is dynamic strain aging (DSA). [17][18][19] In this study, we attempt to discuss the carbon effect on twinning in terms of dislocation pinning associated with DSA.…”

“…Then, the DSA broadens the partial dislocation separation anomalously. 15) Thus, we introduce the effect of DSA on the dislocation separation to explain the twinning behavior in the < 144 > tensile direction and at high temperature in the present Fe-Mn-C steels (4) where  ε is the strain rate; K, n, and m are empirical constants; C is the carbon concentration, Q M is the activation energy for the thermally activated process responsible for the serration, R is the gas constant, T is the deformation temperature, and ε c is the critical strain for the onset of the serration. The activation energy in Eq.…”