Comparison of twinning evolution with work hardening ability in twinning-induced plasticity steel under different strain rates H.K. Yang, Z.J. Zhang, Z.F. Zhang n Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China article info Article history: Received 12 October 2014 Received in revised form 7 November 2014 Accepted 11 November 2014 Available online 18 November 2014 Keywords: Twinning-induced plasticity (TWIP) steel Strength Elongation Twin boundary Strain rate abstract For Fe–22Mn–0.6C (wt%) twinning-induced plasticity (TWIP) steel with grain sizes of 35 and 170 μm, both the ultimate tensile strength and uniform elongation decrease with increasing the tensile strain rates (from 10 4 to 10 0 s 1 ). The density of twin boundary (DTB) is presumed to be the major factor influencing mechanical properties. Accordingly, a parameter 2/(t þs)(t and s represent the thickness and spacing of deformation twins), also the inverse of the mean free path for dislocation slip, is proposed to characterize the planar-oriented twinned structure in infinite grain size (D»t Es). It also evaluates the effect of DTB on the work hardening ability and ductility of the TWIP steel. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Owing to its strong work hardening ability, the twinning- induced plasticity (TWIP) steel exhibits high strength (greater than 800 MPa) and good ductility (uniform elongation up to 80%) at room temperature [1,2]. Numerous studies have reported that this superior mechanical property derives from the interaction between dislocations and deformation twins (DTs) [3–5]. Specifi- cally, the sessile dislocations, observed by Idrissi, contribute to reach the critical stress inside the thin twins [5,6]. In addition, the DTs can refine grain size and hinder dislocation motion during deformation, exhibiting dynamic Hall–Petch effect accordingly [7,8]. Generally, the DTs can influence the overall mechanical property. Recently, Yang [3] and Gutierrez-Urrutia et al. [9] found that the Al in TWIP steel could coarsen the DTs, which reduces the fraction of twinned grains and shifts the onset of mechanical twinning to much higher loads by increasing the stacking fault energy (SFE). Also Steinmetz et al. [10] confirmed through simula- tion and experiment that an increased temperature above 300 1C delayed the onset of deformation twinning. So the DTs become relatively difficult to form by increasing the Al content or tem- perature. In general, the characteristics of DTs (including fraction of DTs or twinned grains, thickness and spacing of DTs) can be adjusted by changing the SFE or deformation temperature, which finally affects the mechanical property of TWIP steels [3,10,11]. However, these researches omitted the facts that Al in TWIP steel may bring solution strengthening, and high temperature often leads to deformation softening. Depending on our previous study [12], the distribution of DTs can be adjusted by altering the strain rates. Additionally, the maximal temperature rise was evaluated to be 95 1C during high strain rate test [13]. This temperature rise can be neglected in comparison with the melting point (more than 1300 1C [14]), especially considering the high thermal stability of the DTs [15]. Hence, the direct relation between DTs and work hardening can be evaluated by adjusting the tensile strain rate, without the interferences from the solution strengthening or the high temperature softening. Meanwhile, a parameter named density of TB (DTB) was proposed to evaluate the effect of DTs on the work hardening in Fe–22Mn–0.6C (wt%) TWIP steel with two grain sizes under different strain rates. 2. Experiments The ingot of Fe–22Mn–0.6C (wt%, denoted FeMnC) TWIP steel was melted in a vacuum induction furnace and austenitized at 1150 1C for 2 h. Then the ingot was forged to two bars with a final sectional dimension of 25  25 mm 2 over 850 1C. After cooled in air, these two bars were annealed at 1000 1C for 30 min and 1200 1C for 120 min, respectively. The mean grain sizes of the two bars were 35 μm(fine grain, denoted FG) and 170 μm (coarse grain, denoted CG) observed under an optical microscope (OM). Dog-bone tensile specimens were spark cut along the axial direction with 15  3  3 mm 3 gauge dimension. Tensile tests were Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2014.11.031 0921-5093/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ86 24 23971043. E-mail address: zhfzhang@imr.ac.cn (Z.F. Zhang). Materials Science & Engineering A 622 (2015) 184–188