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Stacking fault energy and tensile deformation behavior of high-carbon twinning-induced plasticity steels: Effect of Cu addition
Materials and Design 45 (2013) 518–523
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Stacking fault energy and tensile deformation behavior of high-carbon twinning-induced plasticity steels: Effect of Cu addition Xian Peng a,⇑, Dingyi Zhu a, Zhenming Hu b, Weifa Yi a, Haijun Liu a, Mingjie Wang a,c a
College of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian 350108, PR China Fujian San Ming Iron and Steel Group Co., Sanming, Fujian 365000, PR China c College of Materials Science and Engineering, Fujian University of Technology, Fuzhou, Fujian 350108, PR China b
a r t i c l e
i n f o
Article history: Received 13 July 2012 Accepted 6 September 2012 Available online 29 September 2012 Keywords: Fe–Mn–Cu–C steel Austenitic Stacking fault energy Twinning Mechanical behavior
a b s t r a c t Three experimental fully austenitic high-carbon twinning-induced plasticity (TWIP) steel grades were produced and the stacking fault energy (SFE) was investigated based on the thermodynamic modeling approach. The SFE of Fe–20Mn–xCu–1.3C (x = 0, 1.5 and 3.0) steels varied from 24.36 to 28.74 mJ m2 at room temperature. In order to study the correlation between the SFE and the mechanical behavior of TWIP steels, tensile tests were performed at room temperature and the deformed microstructures were examined at different strain levels by transmission electron microscopy. The Cu additions resulted in a remarkable increase in total elongation without a slight loss of tensile strength. In addition, the critical strain for serration start on the tensile stress–strain curves (i.e. required strain to generate mechanical twinning) was found to increase with increasing Cu content. Transmission electron microscope (TEM) observations also indicated that the occurrence of mechanical twinning was suppressed by increasing the Cu addition. The strain hardening mechanism and the superior ductility in deformation are dominated by the interaction of twins and dislocations. The mechanical behavior of TWIP steels is related to the Cu addition, the SFE, the interaction of twins and dislocations. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction High-manganese twinning-induced plasticity (TWIP) steels have an extraordinary combination of high strength and high ductility, which is attributed to the high rate of work hardening resulting from the formation of mechanical twins [1,2]. The TWIP effect can be regarded as a dynamical Hall–Petch effect as the twinned crystal planes act as strong obstacles. The occurrence of TWIP effect is strongly related to the stacking fault energy (SFE) [3,4], which is mainly determined by the chemical composition and the deformation temperature [5–7]. Frommeyer et al. [2] reported that a low SFE (6 16 mJ m2 ) resulted in e phase formation, while a high SFE (P 25 mJ m2 ) resulted in twinning effect in a stable austenitic phase. Allain et al. [8] calculated that mechanical twinning can occur for stacking fault energies (SFEs) between 12 and 35 mJ m2, whereas the micro-twins are replaced by e-martensitic transformation if SFE is below 18 mJ m2. In addition, twinning was reported to occur at SFEs roughly 25 6 C 6 65 mJ m2 [9]. Curtze and Kuokkala [10] indicated that when SFE is larger than 45 mJ m2, plasticity and strain hardening are mainly related to the dislocations slip. ⇑ Corresponding author. Tel.: +86 591 22866540; fax: +86 591 22866537. E-mail addresses: [email protected], [email protected] (X. Peng). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.09.014
Recently, numerous studies have been carried out on the TWIP steels based on Fe–Mn–C alloy, especially the Fe–Mn–Al–C system has received much attention [7,11–13]. It was found that Al additions lead to a lower strain hardening and decrease the frequency of formation of mechanical twins in austenite owing to an increase in the stacking fault energy. Cu might have a potential to achieve better strength-ductility balance due to its precipitation in iron. The effect of Cu addition on the mechanical properties of transformation-induced plasticity (TRIP)/TWIP steel is gradually receiving much attention. Recently, Lee et al. [14] investigated the influence of Cu addition on the mechanical behavior of Fe–12%Mn–0.7%C– 1.0%Al steel. The addition of Cu was found to influence the type of serrations on the stress–strain curve. Early investigations by Kim and co-workers [15] on the cold-rolled low-carbon TRIP steel showed an increase in the strength and ductility as the increased austenite stability by adding Cu. Consequently, knowledge of the tensile behavior and plasticity mechanism of high-carbon TWIP steel by adding Cu is essential to better understand, although it has been reported that a slight increment of SFE with few percent of Cu in addition [16]. The primary aim of the present investigation is to study the deformation behavior of TWIP steels characterized by extensive deformation twins, and also to understand better the correlation between SFE and plasticity mechanism of fully austenitic high-carbon steels. For this purpose, Cu was added to the Fe–20Mn–1.3C
X. Peng et al. / Materials and Design 45 (2013) 518–523
base composition. The SFE of the materials was calculated according to the thermodynamic modeling approach proposed by Curtze et al. [10,17] based on the study of Olson and Cohen [18]. In addition, the tensile characteristics and microstructure development of the steels were analyzed. These properties are discussed in respect of deformation mechanisms, stacking fault energy and the influence of Cu on them.
In the equation, DGc?e is the molar Gibbs energy of the austenite to e-martensite phase transformation c ? e, q is the molar surface density along the f1 1 1g planes. rc/e is the energy per surface unit of the f1 1 1g interface between e and c phases and it is always taken between 5 and 15 mJ m2 [18]. The molar surface density q can be expressed using the lattice parameter a and Avogadro’s constant N as
pffiffiffi
q ¼ 4= 3a2 N
2. Experimental procedure
519
ð2Þ
c?e
The ingots of three grades TWIP steels with the different Cu contents, i.e. Fe–20Mn–1.3C, Fe–20Mn–1.5Cu–1.3C and Fe– 20Mn–3.0Cu–1.3C in wt.% (the 0 Cu, 1.5 Cu and 3.0 Cu TWIP steel, respectively), were prepared by vacuum induction melting. After austenite annealed at 1050 °C for 4.5 h, the cast ingots were hot rolled to the plate of 3 mm thickness: hot rolling temperature was 900 °C. The hot-rolled sheets were cold rolled to 1.2 mm thick sheets and subsequently solution treated at 1050 °C for 4.5 h for 20 min prior to water quenched. The chemical compositions of the three steels are listed in Table 1. The tensile tests were performed according to Chinese Standard GB/T 228-2002 [19]. The tensile samples were cut out of the sheets parallel to the transverse direction and conducted at room temperature by applying a strain rate of 104 s1. Phase identification of un-deformed and deformed tensile samples was carried using X-ray diffraction (XRD, Cu Ka1, D/MAX Ultima III) with voltage 40 kV, current 30 mA. X-ray patterns were taken by measuring 2h from 40° to 100°, at a step size of 0.02° and dwell time of 1 s per step. Specimens for transmission electron microscope (TEM) were taken from the center part of tensile specimens along the longitudinal direction (e = 11%, 20%, 40% and 73%). Prior to the TEM observations, small disks 3 mm in diameter were machined by electrical discharge machining form the strained samples. Electron transparent areas were obtained by twin-jet electro-polisher with a solution of 5% HClO4 and 95% acetic acid at room temperature with an applied potential of 20 V. The foils were subsequently thinned using a Gatan Precision Ion Polishing system at 4 keV. TEM observations of samples were performed in a FEI Tecnai G2 F20 transmission electron microscope operating at an accelerated voltage of 200 kV.
DG was calculated using a regular solution model, which accounts for each element’s contribution to the Gibbs energy, the first-order excess free energies and magnetic contributions [10,17]. The detailed description of this model can be found in Refs. [10,16]. In addition, all of the three terms are the function of the molar fractions of the pure alloying elements and temperature. The relevant thermodynamic parameters needed for calculating DGc?e were acquired from Refs. [8,10,16,17,21]. In the present study, the molar surface q was calculated according to Eq. (2), where the lattice parameter a was determined from X-ray diffraction (XRD) measurements on the TWIP steel grades using Cu Ka radiation. The variation in the determined lattice parameter between the studied TWIP steel grades was of the order of Da = ±0.01 Å, which is considered small enough to be neglected, and therefore its influence on cSFE is negligible. The same average value a = 3.63 Å was used for all TWIP steel grades. The c/e interfacial energy was assumed to be rc/e = 8 mJ m2, according to Allain et al. [8], who studied a comparable chemical composition. According to these thermodynamic data, the values
3. Results and discussion 3.1. SFE calculations The thermodynamic model applied for the stacking fault energy calculations was mainly based on the thermodynamic formulas and thermodynamic parameters for each composition of the experimental steels. The SFE is mainly determined by the chemical composition and temperature. In face-centered cubic (fcc) structures, twinning can be modeled by stacking faults (SFs) extending in parallel adjacent dense planes. A stacking fault can be considered as two atomic layers of e-martensite within the dense planes. Accordingly, the SFE of fcc alloys can be calculated using the following equation proposed by Hirth [20]:
cSFE ¼ 2qDGc!e þ 2rc=e
ð1Þ
Table 1 Chemical composition in weight percent of the TWIP steel used in this study. Grade
C
Mn
Cu
S
Fe
Fe–20Mn–1.3C Fe–20Mn–1.5Cu–1.3C Fe–20Mn–3.0Cu–1.3C
1.30 1.31 1.31
19.84 19.87 19.87
– 1.50 3.04
0.006 0.006 0.006
Bal. Bal. Bal.
Fig. 1. (a) XRD profiles of the three steels prior to deformation. (b) XRD profiles of three steels after deformation at room temperature.
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Fig. 2. (a) Engineering stress–strain curves of the present steels (strain rate = 104 s1). The enlarged section of the stress–strain curves in the inset shows that the critical strain for serrations formation depends on the Cu content. (b) True stress–strain curves of the present steels, showing the strain dependence of the work hardening rate.
Table 2 The mechanical characteristics of the present TWIP steels at room temperature. Steels
Yield stress (MPa)
Ultimate tensile strength (MPa)
Elongation at fracture (%)
Fe–20Mn–1.3C Fe–20Mn– 1.5Cu–1.3C Fe–20Mn– 3.0Cu–1.3C
470 504
1353 1284
61.2 73.0
538
1256
77.6
obtained for SFE of the steels 0 Cu, 1.5 Cu and 3.0 Cu at room temperature are 24.36, 26.56 and 28.74 mJ m2, respectively. Considering the previous studies [2,8], the present SFE of the steels of these steels is in a suitable range for enhancing mechanical twinning during plastic deformation. 3.2. Tensile characteristics In the initial state before deformation at room temperature, Xray analysis (Fig. 1a) revealed that all steels were fully austenitic after solution treatment followed by water quenching. The microstructure of the annealed sample before tensile test was fully
austenitic, and the mean grain size of all the steels is 41 ± 2 lm, when annealing twin boundaries were considered in the grain size measurement. The tensile tests were performed up to fracture at room temperature. Samples are analyzed after tensile testing by X-ray diffraction there exists only austenitic in the steels, and thus, no transformation occurs (Fig. 1b). As results from the tensile tests performed at the average strain rate 104 s1, the typical stress–strain curves and work hardening curves of the Cu-added Fe–20Mn–1.3C TWIP steel and the Cu–free Fe–20Mn–1.3C TWIP steel are displayed in Fig. 2. It is clearly observed that the Cu alloying has distinct influence on the yield strength, the strain hardening rate and ductility. From the engineering stress–strain curves in Fig. 2a, it is seen that regularly serrated flow occurred in all TWIP steels, but the critical strain where the serrations on the stress–strain curve start, increased from 1.5% to 3.3% after the addition of 3 wt.% Cu. According to Yü [22], the occurrence of serrations on the stress–strain curves means the startup of deformation twins and twins frequently nucleate in the high-stress concentrated region. In addition, the stress needed for twins to nucleate is far more than that for twins grow or the dislocation glide. Once the twin appears the stress–strain curves will exhibit an abrupt drop. The critical
X. Peng et al. / Materials and Design 45 (2013) 518–523
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The true stress–strain curves of the three TWIP steels and the variation of the work hardening rate with strain are showed in Fig. 2b. As seen in Fig. 2b, all TWIP steels showed continuous yielding followed by extensive work hardening rate and the work hardening behavior of the three TWIP steels is different from each other. At room temperature and a low strain rate (104 s1), the work hardening rate is highest for the 0 Cu TWIP steel with the lowest SFE for strains lower than 45%. In the case of the 0 Cu TWIP steel, the work hardening rate increased from 3000 MPa at a true strain 0.05 to 3500 MPa at a strain close to necking. The work hardening rate of the 3.0 Cu TWIP steel gradually decreased up to a true strain of 0.2 and then remained nearly unchanged with 2800 MPa up to necking. The excellent work harden-ability of the 0 Cu TWIP steel is considered to result in its higher tensile strength (TS) than 3.0 Cu TWIP steel despite its lower yield strength (YS). The higher strain hardening rate of the 0 Cu steel must be related to its lower SFE. The different work hardening behavior of three TWIP steels suggests a connection between the deformation mode, the Cu content and the SFE. The mechanical characteristics of present TWIP steels are shown in Table 2. The YS was high in the order of the 3.0 Cu steel, 1.5 Cu steel and 0 Cu steel, and it is true for elongation at fracture. For instance, at room temperature, the YS values are 470 MPa, 504 MPa and 538 MPa for the 0 Cu, 1.5 Cu and 3.0 Cu steel, respectively. The small difference between the values can be suggested to be the result from the solid solution strengthening effect of Cu additions. However, due to the little difference between the Cu atom radius (0.127 nm) and Fe atom radius (0.126 nm), which did not make an appreciable solution strengthening. However, the TS of the 0 Cu TWIP steel was higher than that of 3.0 Cu TWIP steel. Both of the reduction of TS and the increase of uniform elongation with increasing Cu content are attributable to the increase of SFE. In addition, the interaction of twins and dislocations may be beneficial for achieving the superior ductility. Fig. 3. Optical micrographs of the Fe–20Mn–1.3C and Fe–20Mn–3.0Cu–1.3C TWIP steels strained at 11% after full recrystallization anneal.
strain for serration start on the tensile stress–strain curves becomes higher with increasing the Cu content suggested that the kinetics of the twinning generation was suppressed by Cu addition, which is favored to the dislocation glide. Moreover, this different behavior also means the critical stress for mechanical twinning increasing with the Cu addition or SFE. The result is consistent with the conclusion of Park et al. [23], which indicated that the critical stress for mechanical twinning becomes higher with increasing the SFE. This different behavior indicates a connection between the strain harden-ability, SFE and the strengthening associated with the TWIP effect.
3.3. Microstructures The typical optical microstructures of specimens of the Fe– 20Mn–1.3C and Fe–20Mn–3.0Cu–1.3C TWIP steels tensile strained at 11% are shown in Fig. 3a and b. The microstructure of the deformed Fe–20Mn–1.3C TWIP steel shows a large number of mechanical twins were formed (Fig. 3a). However, mechanical twins were not observed in the Fe–20Mn–3.0Cu–1.3C TWIP steel simple (Fig. 3b). The influence of the Cu addition on twin density in the both steels can be seen by comparing Fig. 3a and b, and the reductions in number of twins with increasing the Cu addition. However, optical microscopy is too simple to characterize the kind of structures, so that the observations are only qualitative.
Fig. 4. The TEM micrographs showing the microstructures of the Fe–20Mn–1.3C and Fe–20Mn–3.0Cu–1.3C TWIP steels corresponding to Fig. 3.
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The TEM micrographs corresponding to Fig. 3 are presented in Fig. 4. As seen in Fig. 4, unlike the optical observation, it is difficult to observe well-developed mechanical twins, probably due to the limited area. In Fig. 4a, the developed mechanical twins can be observed in the 0 Cu sample deformed to 11%. Unlike the 0 Cu steel, dislocations with high density were distributed throughout the matrix of the 3.0 Cu steel at 11% strain. It implies that the formation of twins is evidently dependent on the Cu content. We have calculated that the SFE of the Fe–20Mn–1.3C TWIP steel increased from 24.36 to 28.74 mJ m2 on addition of 3.0 wt.% Cu. So that the SFE of Fe–20Mn–1.3C TWIP steel increased by the Cu additions and that resulted in delaying the formation of mechanical twins. 3.4. Deformed microstructures of Fe–20Mn–3.0Cu–1.3C TWIP steel Since the 3.0 Cu TWIP steel exhibited the extended ductility of 77%, the additional TEM observation was made on the sample deformed to 11%, 20%, 40% and 73% strain. Microstructures of the Fe– 20Mn–3.0Cu–1.3C TWIP steel deformed to different strain levels are presented in Fig. 5. As shown in Fig. 5a, dislocations with the high density throughout the matrix at 11% strain. Dislocation activity always is viewed as a precursor to twinning [24,25]. At low strain stage, the twinning deformation will start once the stress concentration which is provided by piled-up dislocations surpasses the critical stress for deformation twins. Otherwise, slip will start once more. As seen in Fig. 5b, mechanical twins were observed up to 20% strain. This is mainly due to the higher SFE of the 3.0 Cu steel leading to the increase of the critical stress for mechanical twinning. Fig. 5c and d are the TEM microstructure of the 3.0 Cu steel strained to 40% and 73%, respectively. It is clearly observed that the deformation twinning acts as obstacles for the subsequent movement of dislocations (just like grain boundaries) [26,27], and there are interactions between twins and dislocations (Fig. 5c). Fig. 5d shows massive deformation twins, the thin and straight
deformation twins with the thickness of a few ten nanometers are observed. It indicates that the strain hardening mechanism and the superior ductility in deformation are mainly dominated by the interaction of twins and dislocations.
4. Conclusion Based on the results of the present investigation, the main conclusions can be drawn as follows: (1) Three fully austenitic experimental high-carbon TWIP steel grades, differing in their content of Cu were produced and the stacking fault energies of the TWIP steels increased from 24.36 to 28.74 mJ m2 on the addition of 3.0 wt.% Cu. (2) According to the XRD analysis, all the TWIP steels consisted of austenite, even after the tensile test the specimens were fully composed of austenite without martensite. (3) The yield strength and elongation of 3.0 Cu-containing Fe– 20Mn–1.3C TWIP steel were 538 MPa and 77.6%, while those of Cu free steel were 470 MPa and 61.2% with a slight difference of tensile strength (from 1353 MPa to 1256 MPa), showing the superior tensile properties of Cu-containing TWIP steel. (4) The TEM observations indicated that the occurrence of mechanical twinning was suppressing or delaying with increasing the Cu addition. The higher the SFE is, the higher the critical strain for serration start on the tensile stress– strain curves becomes. In a word, the kinetics of the twin formation is apparently dependent on the Cu content. (5) During the tensile deformation, the contribution from the interaction of twins and dislocations associated with strain hardening mechanism may be beneficial for achieving the superior ductility in TWIP steels.
Fig. 5. TEM micrographs showing microstructures of Fe–20Mn–3.0Cu–1.3C steel developed at different strain levels.
X. Peng et al. / Materials and Design 45 (2013) 518–523
Acknowledgements The authors are thankful to the Industry-University Cooperation Major Program of the Science and Technology Department of Fujian Province (Project: 2011H6012) and the key program of the Science and Technology Department of Fujian Province (Project: 2011H0001), for sponsoring this research work. Special thanks are due to Professor ZHU Ding-yi, Laboratory Technician LU Hong, Senior Engineer DUI Wei-zhen and Dr. DING Xiao-kun, for their kind help in experiment instruction, tensile tests and TEM test, respectively.
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[10] Curtze S, Kuokkala VT. Dependence of tensile deformation behavior of TWIP steels on stacking fault energy, temperature and strain rate. Acta Mater 2010;58:5129–41. [11] Chin KG, Kang CY, Shin SY, Hong S, et al. Effect of Al addition on deformation and fracture mechanisms in two high manganese TWIP steels. Mater Sci Eng A 2011;528:2922–8. [12] So KH, Kim JS, Chun TS, Park KT, Lee YK, Lee CS. Hydrogen delayed fracture properties and internal hydrogen behavior of a Fe–18Mn–1.5Al–0.6C TWIP steel. ISIJ Int 2009;49:1952–9. [13] Kang S, Jung YS, Jun JH, Lee YK. Effects of recrystallization annealing temperature on carbide precipitation, microstructure, and mechanical properties in Fe–18Mn–0.6C–1.5Al TWIP steel. Mater Sci Eng A 2010;527:745–51. [14] Lee S, Kim J, Lee SJ, et al. Effect of Cu addition on the mechanical behavior of austenitic twinning-induced plasticity steel. Scr Mater 2011;65:1073–6. [15] Kim SJ, Lee CG, Lee TH, Oh CS. Effect of Cu, Cr and Ni on mechanical properties of 0.15 wt.% C TRIP-aided cole rolled steels. Scr Mater 2003;48:539–44. [16] Dumay A, Chateau JP, Allain S, et al. Influence of addition elements on the stacking-fault energy and mechanical properties of an austenitic Fe–Mn–C steel. Mater Sci Eng A 2008;483–484:184–7. [17] Curtze S, Kuokkala VT, Oikari A, Talonen J. Thermodynamic modeling of the stacking fault energy of austenitic steels. Acta Mater 2011;59:1068–76. [18] Olson GB, Cohen M. A general mechanism of martensitic nucleation: Part I. General concepts and the FCC ? HCP transformation. Metall Trans A 1976;7:1897–904. [19] GB/T 228-2202, Metallic materials-tensile testing at ambient temperature. China; 2002. [20] Hirth JP. Thermodynamics of stacking faults. Metall Mater Trans B 1970;1:2367–74. [21] Dinsdale AT. SGTE data for pure elements. Calphad 1991;15(4):317–425. [22] Yü Y. Metallographic principle. 1st ed. Beijing: Metallurgical Industry Publishing House; 1999 [chapter 8, in Chinese]. [23] Park KT, Chin KG, Han SH, Hwang SW, Choi K, Lee CS. Stacking fault energy and plastic deformation of fully austenitic high manganese steels: effect of Al addition. Mater Sci Eng A 2010;527:3651–61. [24] Karaman I, Sehitoglu H, Gall K, Chumlyakov YI. Deformation of single crystal hadfield steel by twinning and slip. Acta Mater 2000;48:1345–59. [25] Ding H, Ding H, Song D, Tang Z, et al. Strain hardening behavior of a TRIP/TWIP steel with 18.8% Mn. Mater Sci Eng A 2011;528:868–73. [26] Serra A, Bacon DJ, Pond RC. Twins are barriers to basal slip in hexagonal-closepacked metals. Metall Mater Trans A 2002;33A:809–12. [27] Dini G, Najafizadeh A, Ueji R, Monir-Vaghefi SM. Tensile deformation behavior of high manganese austenitic steel: the role of grain size. Mater Des 2010;31:3395–402.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Stacking fault energy and tensile deformation behavior of high-carbon twinning-induced plasticity steels: Effect of Cu addition Xian Peng a,⇑, Dingyi Zhu a, Zhenming Hu b, Weifa Yi a, Haijun Liu a, Mingjie Wang a,c a
College of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian 350108, PR China Fujian San Ming Iron and Steel Group Co., Sanming, Fujian 365000, PR China c College of Materials Science and Engineering, Fujian University of Technology, Fuzhou, Fujian 350108, PR China b
a r t i c l e
i n f o
Article history: Received 13 July 2012 Accepted 6 September 2012 Available online 29 September 2012 Keywords: Fe–Mn–Cu–C steel Austenitic Stacking fault energy Twinning Mechanical behavior
a b s t r a c t Three experimental fully austenitic high-carbon twinning-induced plasticity (TWIP) steel grades were produced and the stacking fault energy (SFE) was investigated based on the thermodynamic modeling approach. The SFE of Fe–20Mn–xCu–1.3C (x = 0, 1.5 and 3.0) steels varied from 24.36 to 28.74 mJ m2 at room temperature. In order to study the correlation between the SFE and the mechanical behavior of TWIP steels, tensile tests were performed at room temperature and the deformed microstructures were examined at different strain levels by transmission electron microscopy. The Cu additions resulted in a remarkable increase in total elongation without a slight loss of tensile strength. In addition, the critical strain for serration start on the tensile stress–strain curves (i.e. required strain to generate mechanical twinning) was found to increase with increasing Cu content. Transmission electron microscope (TEM) observations also indicated that the occurrence of mechanical twinning was suppressed by increasing the Cu addition. The strain hardening mechanism and the superior ductility in deformation are dominated by the interaction of twins and dislocations. The mechanical behavior of TWIP steels is related to the Cu addition, the SFE, the interaction of twins and dislocations. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction High-manganese twinning-induced plasticity (TWIP) steels have an extraordinary combination of high strength and high ductility, which is attributed to the high rate of work hardening resulting from the formation of mechanical twins [1,2]. The TWIP effect can be regarded as a dynamical Hall–Petch effect as the twinned crystal planes act as strong obstacles. The occurrence of TWIP effect is strongly related to the stacking fault energy (SFE) [3,4], which is mainly determined by the chemical composition and the deformation temperature [5–7]. Frommeyer et al. [2] reported that a low SFE (6 16 mJ m2 ) resulted in e phase formation, while a high SFE (P 25 mJ m2 ) resulted in twinning effect in a stable austenitic phase. Allain et al. [8] calculated that mechanical twinning can occur for stacking fault energies (SFEs) between 12 and 35 mJ m2, whereas the micro-twins are replaced by e-martensitic transformation if SFE is below 18 mJ m2. In addition, twinning was reported to occur at SFEs roughly 25 6 C 6 65 mJ m2 [9]. Curtze and Kuokkala [10] indicated that when SFE is larger than 45 mJ m2, plasticity and strain hardening are mainly related to the dislocations slip. ⇑ Corresponding author. Tel.: +86 591 22866540; fax: +86 591 22866537. E-mail addresses: [email protected], [email protected] (X. Peng). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.09.014
Recently, numerous studies have been carried out on the TWIP steels based on Fe–Mn–C alloy, especially the Fe–Mn–Al–C system has received much attention [7,11–13]. It was found that Al additions lead to a lower strain hardening and decrease the frequency of formation of mechanical twins in austenite owing to an increase in the stacking fault energy. Cu might have a potential to achieve better strength-ductility balance due to its precipitation in iron. The effect of Cu addition on the mechanical properties of transformation-induced plasticity (TRIP)/TWIP steel is gradually receiving much attention. Recently, Lee et al. [14] investigated the influence of Cu addition on the mechanical behavior of Fe–12%Mn–0.7%C– 1.0%Al steel. The addition of Cu was found to influence the type of serrations on the stress–strain curve. Early investigations by Kim and co-workers [15] on the cold-rolled low-carbon TRIP steel showed an increase in the strength and ductility as the increased austenite stability by adding Cu. Consequently, knowledge of the tensile behavior and plasticity mechanism of high-carbon TWIP steel by adding Cu is essential to better understand, although it has been reported that a slight increment of SFE with few percent of Cu in addition [16]. The primary aim of the present investigation is to study the deformation behavior of TWIP steels characterized by extensive deformation twins, and also to understand better the correlation between SFE and plasticity mechanism of fully austenitic high-carbon steels. For this purpose, Cu was added to the Fe–20Mn–1.3C
X. Peng et al. / Materials and Design 45 (2013) 518–523
base composition. The SFE of the materials was calculated according to the thermodynamic modeling approach proposed by Curtze et al. [10,17] based on the study of Olson and Cohen [18]. In addition, the tensile characteristics and microstructure development of the steels were analyzed. These properties are discussed in respect of deformation mechanisms, stacking fault energy and the influence of Cu on them.
In the equation, DGc?e is the molar Gibbs energy of the austenite to e-martensite phase transformation c ? e, q is the molar surface density along the f1 1 1g planes. rc/e is the energy per surface unit of the f1 1 1g interface between e and c phases and it is always taken between 5 and 15 mJ m2 [18]. The molar surface density q can be expressed using the lattice parameter a and Avogadro’s constant N as
pffiffiffi
q ¼ 4= 3a2 N
2. Experimental procedure
519
ð2Þ
c?e
The ingots of three grades TWIP steels with the different Cu contents, i.e. Fe–20Mn–1.3C, Fe–20Mn–1.5Cu–1.3C and Fe– 20Mn–3.0Cu–1.3C in wt.% (the 0 Cu, 1.5 Cu and 3.0 Cu TWIP steel, respectively), were prepared by vacuum induction melting. After austenite annealed at 1050 °C for 4.5 h, the cast ingots were hot rolled to the plate of 3 mm thickness: hot rolling temperature was 900 °C. The hot-rolled sheets were cold rolled to 1.2 mm thick sheets and subsequently solution treated at 1050 °C for 4.5 h for 20 min prior to water quenched. The chemical compositions of the three steels are listed in Table 1. The tensile tests were performed according to Chinese Standard GB/T 228-2002 [19]. The tensile samples were cut out of the sheets parallel to the transverse direction and conducted at room temperature by applying a strain rate of 104 s1. Phase identification of un-deformed and deformed tensile samples was carried using X-ray diffraction (XRD, Cu Ka1, D/MAX Ultima III) with voltage 40 kV, current 30 mA. X-ray patterns were taken by measuring 2h from 40° to 100°, at a step size of 0.02° and dwell time of 1 s per step. Specimens for transmission electron microscope (TEM) were taken from the center part of tensile specimens along the longitudinal direction (e = 11%, 20%, 40% and 73%). Prior to the TEM observations, small disks 3 mm in diameter were machined by electrical discharge machining form the strained samples. Electron transparent areas were obtained by twin-jet electro-polisher with a solution of 5% HClO4 and 95% acetic acid at room temperature with an applied potential of 20 V. The foils were subsequently thinned using a Gatan Precision Ion Polishing system at 4 keV. TEM observations of samples were performed in a FEI Tecnai G2 F20 transmission electron microscope operating at an accelerated voltage of 200 kV.
DG was calculated using a regular solution model, which accounts for each element’s contribution to the Gibbs energy, the first-order excess free energies and magnetic contributions [10,17]. The detailed description of this model can be found in Refs. [10,16]. In addition, all of the three terms are the function of the molar fractions of the pure alloying elements and temperature. The relevant thermodynamic parameters needed for calculating DGc?e were acquired from Refs. [8,10,16,17,21]. In the present study, the molar surface q was calculated according to Eq. (2), where the lattice parameter a was determined from X-ray diffraction (XRD) measurements on the TWIP steel grades using Cu Ka radiation. The variation in the determined lattice parameter between the studied TWIP steel grades was of the order of Da = ±0.01 Å, which is considered small enough to be neglected, and therefore its influence on cSFE is negligible. The same average value a = 3.63 Å was used for all TWIP steel grades. The c/e interfacial energy was assumed to be rc/e = 8 mJ m2, according to Allain et al. [8], who studied a comparable chemical composition. According to these thermodynamic data, the values
3. Results and discussion 3.1. SFE calculations The thermodynamic model applied for the stacking fault energy calculations was mainly based on the thermodynamic formulas and thermodynamic parameters for each composition of the experimental steels. The SFE is mainly determined by the chemical composition and temperature. In face-centered cubic (fcc) structures, twinning can be modeled by stacking faults (SFs) extending in parallel adjacent dense planes. A stacking fault can be considered as two atomic layers of e-martensite within the dense planes. Accordingly, the SFE of fcc alloys can be calculated using the following equation proposed by Hirth [20]:
cSFE ¼ 2qDGc!e þ 2rc=e
ð1Þ
Table 1 Chemical composition in weight percent of the TWIP steel used in this study. Grade
C
Mn
Cu
S
Fe
Fe–20Mn–1.3C Fe–20Mn–1.5Cu–1.3C Fe–20Mn–3.0Cu–1.3C
1.30 1.31 1.31
19.84 19.87 19.87
– 1.50 3.04
0.006 0.006 0.006
Bal. Bal. Bal.
Fig. 1. (a) XRD profiles of the three steels prior to deformation. (b) XRD profiles of three steels after deformation at room temperature.
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Fig. 2. (a) Engineering stress–strain curves of the present steels (strain rate = 104 s1). The enlarged section of the stress–strain curves in the inset shows that the critical strain for serrations formation depends on the Cu content. (b) True stress–strain curves of the present steels, showing the strain dependence of the work hardening rate.
Table 2 The mechanical characteristics of the present TWIP steels at room temperature. Steels
Yield stress (MPa)
Ultimate tensile strength (MPa)
Elongation at fracture (%)
Fe–20Mn–1.3C Fe–20Mn– 1.5Cu–1.3C Fe–20Mn– 3.0Cu–1.3C
470 504
1353 1284
61.2 73.0
538
1256
77.6
obtained for SFE of the steels 0 Cu, 1.5 Cu and 3.0 Cu at room temperature are 24.36, 26.56 and 28.74 mJ m2, respectively. Considering the previous studies [2,8], the present SFE of the steels of these steels is in a suitable range for enhancing mechanical twinning during plastic deformation. 3.2. Tensile characteristics In the initial state before deformation at room temperature, Xray analysis (Fig. 1a) revealed that all steels were fully austenitic after solution treatment followed by water quenching. The microstructure of the annealed sample before tensile test was fully
austenitic, and the mean grain size of all the steels is 41 ± 2 lm, when annealing twin boundaries were considered in the grain size measurement. The tensile tests were performed up to fracture at room temperature. Samples are analyzed after tensile testing by X-ray diffraction there exists only austenitic in the steels, and thus, no transformation occurs (Fig. 1b). As results from the tensile tests performed at the average strain rate 104 s1, the typical stress–strain curves and work hardening curves of the Cu-added Fe–20Mn–1.3C TWIP steel and the Cu–free Fe–20Mn–1.3C TWIP steel are displayed in Fig. 2. It is clearly observed that the Cu alloying has distinct influence on the yield strength, the strain hardening rate and ductility. From the engineering stress–strain curves in Fig. 2a, it is seen that regularly serrated flow occurred in all TWIP steels, but the critical strain where the serrations on the stress–strain curve start, increased from 1.5% to 3.3% after the addition of 3 wt.% Cu. According to Yü [22], the occurrence of serrations on the stress–strain curves means the startup of deformation twins and twins frequently nucleate in the high-stress concentrated region. In addition, the stress needed for twins to nucleate is far more than that for twins grow or the dislocation glide. Once the twin appears the stress–strain curves will exhibit an abrupt drop. The critical
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The true stress–strain curves of the three TWIP steels and the variation of the work hardening rate with strain are showed in Fig. 2b. As seen in Fig. 2b, all TWIP steels showed continuous yielding followed by extensive work hardening rate and the work hardening behavior of the three TWIP steels is different from each other. At room temperature and a low strain rate (104 s1), the work hardening rate is highest for the 0 Cu TWIP steel with the lowest SFE for strains lower than 45%. In the case of the 0 Cu TWIP steel, the work hardening rate increased from 3000 MPa at a true strain 0.05 to 3500 MPa at a strain close to necking. The work hardening rate of the 3.0 Cu TWIP steel gradually decreased up to a true strain of 0.2 and then remained nearly unchanged with 2800 MPa up to necking. The excellent work harden-ability of the 0 Cu TWIP steel is considered to result in its higher tensile strength (TS) than 3.0 Cu TWIP steel despite its lower yield strength (YS). The higher strain hardening rate of the 0 Cu steel must be related to its lower SFE. The different work hardening behavior of three TWIP steels suggests a connection between the deformation mode, the Cu content and the SFE. The mechanical characteristics of present TWIP steels are shown in Table 2. The YS was high in the order of the 3.0 Cu steel, 1.5 Cu steel and 0 Cu steel, and it is true for elongation at fracture. For instance, at room temperature, the YS values are 470 MPa, 504 MPa and 538 MPa for the 0 Cu, 1.5 Cu and 3.0 Cu steel, respectively. The small difference between the values can be suggested to be the result from the solid solution strengthening effect of Cu additions. However, due to the little difference between the Cu atom radius (0.127 nm) and Fe atom radius (0.126 nm), which did not make an appreciable solution strengthening. However, the TS of the 0 Cu TWIP steel was higher than that of 3.0 Cu TWIP steel. Both of the reduction of TS and the increase of uniform elongation with increasing Cu content are attributable to the increase of SFE. In addition, the interaction of twins and dislocations may be beneficial for achieving the superior ductility. Fig. 3. Optical micrographs of the Fe–20Mn–1.3C and Fe–20Mn–3.0Cu–1.3C TWIP steels strained at 11% after full recrystallization anneal.
strain for serration start on the tensile stress–strain curves becomes higher with increasing the Cu content suggested that the kinetics of the twinning generation was suppressed by Cu addition, which is favored to the dislocation glide. Moreover, this different behavior also means the critical stress for mechanical twinning increasing with the Cu addition or SFE. The result is consistent with the conclusion of Park et al. [23], which indicated that the critical stress for mechanical twinning becomes higher with increasing the SFE. This different behavior indicates a connection between the strain harden-ability, SFE and the strengthening associated with the TWIP effect.
3.3. Microstructures The typical optical microstructures of specimens of the Fe– 20Mn–1.3C and Fe–20Mn–3.0Cu–1.3C TWIP steels tensile strained at 11% are shown in Fig. 3a and b. The microstructure of the deformed Fe–20Mn–1.3C TWIP steel shows a large number of mechanical twins were formed (Fig. 3a). However, mechanical twins were not observed in the Fe–20Mn–3.0Cu–1.3C TWIP steel simple (Fig. 3b). The influence of the Cu addition on twin density in the both steels can be seen by comparing Fig. 3a and b, and the reductions in number of twins with increasing the Cu addition. However, optical microscopy is too simple to characterize the kind of structures, so that the observations are only qualitative.
Fig. 4. The TEM micrographs showing the microstructures of the Fe–20Mn–1.3C and Fe–20Mn–3.0Cu–1.3C TWIP steels corresponding to Fig. 3.
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The TEM micrographs corresponding to Fig. 3 are presented in Fig. 4. As seen in Fig. 4, unlike the optical observation, it is difficult to observe well-developed mechanical twins, probably due to the limited area. In Fig. 4a, the developed mechanical twins can be observed in the 0 Cu sample deformed to 11%. Unlike the 0 Cu steel, dislocations with high density were distributed throughout the matrix of the 3.0 Cu steel at 11% strain. It implies that the formation of twins is evidently dependent on the Cu content. We have calculated that the SFE of the Fe–20Mn–1.3C TWIP steel increased from 24.36 to 28.74 mJ m2 on addition of 3.0 wt.% Cu. So that the SFE of Fe–20Mn–1.3C TWIP steel increased by the Cu additions and that resulted in delaying the formation of mechanical twins. 3.4. Deformed microstructures of Fe–20Mn–3.0Cu–1.3C TWIP steel Since the 3.0 Cu TWIP steel exhibited the extended ductility of 77%, the additional TEM observation was made on the sample deformed to 11%, 20%, 40% and 73% strain. Microstructures of the Fe– 20Mn–3.0Cu–1.3C TWIP steel deformed to different strain levels are presented in Fig. 5. As shown in Fig. 5a, dislocations with the high density throughout the matrix at 11% strain. Dislocation activity always is viewed as a precursor to twinning [24,25]. At low strain stage, the twinning deformation will start once the stress concentration which is provided by piled-up dislocations surpasses the critical stress for deformation twins. Otherwise, slip will start once more. As seen in Fig. 5b, mechanical twins were observed up to 20% strain. This is mainly due to the higher SFE of the 3.0 Cu steel leading to the increase of the critical stress for mechanical twinning. Fig. 5c and d are the TEM microstructure of the 3.0 Cu steel strained to 40% and 73%, respectively. It is clearly observed that the deformation twinning acts as obstacles for the subsequent movement of dislocations (just like grain boundaries) [26,27], and there are interactions between twins and dislocations (Fig. 5c). Fig. 5d shows massive deformation twins, the thin and straight
deformation twins with the thickness of a few ten nanometers are observed. It indicates that the strain hardening mechanism and the superior ductility in deformation are mainly dominated by the interaction of twins and dislocations.
4. Conclusion Based on the results of the present investigation, the main conclusions can be drawn as follows: (1) Three fully austenitic experimental high-carbon TWIP steel grades, differing in their content of Cu were produced and the stacking fault energies of the TWIP steels increased from 24.36 to 28.74 mJ m2 on the addition of 3.0 wt.% Cu. (2) According to the XRD analysis, all the TWIP steels consisted of austenite, even after the tensile test the specimens were fully composed of austenite without martensite. (3) The yield strength and elongation of 3.0 Cu-containing Fe– 20Mn–1.3C TWIP steel were 538 MPa and 77.6%, while those of Cu free steel were 470 MPa and 61.2% with a slight difference of tensile strength (from 1353 MPa to 1256 MPa), showing the superior tensile properties of Cu-containing TWIP steel. (4) The TEM observations indicated that the occurrence of mechanical twinning was suppressing or delaying with increasing the Cu addition. The higher the SFE is, the higher the critical strain for serration start on the tensile stress– strain curves becomes. In a word, the kinetics of the twin formation is apparently dependent on the Cu content. (5) During the tensile deformation, the contribution from the interaction of twins and dislocations associated with strain hardening mechanism may be beneficial for achieving the superior ductility in TWIP steels.
Fig. 5. TEM micrographs showing microstructures of Fe–20Mn–3.0Cu–1.3C steel developed at different strain levels.
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Acknowledgements The authors are thankful to the Industry-University Cooperation Major Program of the Science and Technology Department of Fujian Province (Project: 2011H6012) and the key program of the Science and Technology Department of Fujian Province (Project: 2011H0001), for sponsoring this research work. Special thanks are due to Professor ZHU Ding-yi, Laboratory Technician LU Hong, Senior Engineer DUI Wei-zhen and Dr. DING Xiao-kun, for their kind help in experiment instruction, tensile tests and TEM test, respectively.
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