- Email: [email protected]
Mo alloying
Scripta Materialia 127 (2017) 10–14
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Review Article
Simultaneously increasing both strength and ductility of Fe-Mn-C twinning-induced plasticity steel via Cr/Mo alloying Shuai Liu a,b, Lihe Qian a,b,⁎, Jiangying Meng a, Dongdong Li a,b, Penghui Ma a,b, Fucheng Zhang a,b a b
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao 066004, China
a r t i c l e
i n f o
Article history: Received 25 August 2016 Accepted 30 August 2016 Available online xxxx Keywords: Twinning-induced plasticity steel Mechanical properties Strain hardening Cr/Mo alloying
a b s t r a c t The effects of Cr/Mo alloying on the tensile deformation behavior of Fe-Mn-C twinning-induced plasticity steel were investigated. The results showed that Cr/Mo alloying increases both the strength and ductility significantly and concurrently. As compared with the premature saturation of deformation twinning in the Cr/Mo-free steel, a unique twinning behavior occurs in the Cr/Mo-alloyed steel, i.e., retarded but more persistent formation of thinner twins with straining. This unique twinning behavior gives rise to a sustained high strain-hardening rate up to higher strains, which delays the plastic instability and increases both strength and ductility simultaneously. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
In recent years, many efforts have been made to produce high-performance materials for manufacturing modern auto components with reduced weight and improved safety. This has led to the development of high manganese twinning-induced plasticity (TWIP) steels with excellent combinations of strength and ductility, stemming from deformation twinning and/or dynamic strain aging (DSA) [1–3]. TWIP steels were originally developed from Hadfield steel (Mn13) after its inventor, Sir Robert Hadfield in 1882, and later improved constantly through several strategies including compositional adjustment, grain refinement, cold-rolling and thermal treatment [4–10]. Plentiful of results are currently available on the effects of compositional adjustment on TWIP steels, and varying degrees of property improvement have been demonstrated [4–7]. However, most previous studies showed increased tensile strength or improved ductility, but seldom both in Fe-Mn-C TWIP steels alloyed with elements such as Al, Si and Cu [4–6]. As reported in Ref. [4], Al addition increases both the yield strength and total elongation by solid solution strengthening and suppressing carbide precipitation. However, Al addition also decreases the tensile strength and strain-hardening rate because of the suppressed deformation twinning, associated with the increased stacking fault energy (SFE) by Al addition. According to Lee et al. [5], Cu also increases SFE, and Cu addition tends to retard the formation of deformation twins, thus decreasing the strain-hardening rate and increasing the total elongation. On the contrary, Si is an SFE-decreasing element. Si addition increases the strain-hardening rate and the tensile strength, but decreases the ductility [6]. In addition, microalloying elements Nb, V ⁎ Corresponding author at: State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China. E-mail address: [email protected] (L. Qian).
http://dx.doi.org/10.1016/j.scriptamat.2016.08.034 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
and Ti were also attempted to change the mechanical properties of FeMn-C TWIP steels. It was shown that the addition of such microalloying elements increases the yield and tensile strengths, mainly due to solution and precipitation strengthening, but decreases the strain-hardening rate and total elongation due to the suppressed formation of deformation twins [7]. Furthermore, several studies are available, showing the potential of Cr/Mo alloying for improving the performance of TWIP steels [11–13]. It was reported that the addition of a certain amount of Cr and Mo increases the hardness and wear resistance of high manganese Hadfield steel, due to solution hardening and the formation of wear-resistant Cr/Mo carbides [11,12]. Besides, Cr addition also shows improved corrosion and oxidation resistance in high manganese TWIP steel [13]. However, the effects of Cr/Mo alloying on the tensile properties and strainhardening behavior of TWIP steel have not been reported. In this work, we prepared two Fe-Mn-C TWIP steels (one alloyed with Cr and Mo and the other without Cr/Mo alloying), and comparatively studied the tensile properties of both steels. The chemical compositions of the two steels investigated are Fe18Mn-1.0C-2Cr-1Mo and Fe-18Mn-1.0C (wt.%), referred to as Cr/Moalloyed and Cr/Mo-free steels, respectively. Both steels were melted in a vacuum induction furnace. After homogenization at 1473 K for 4 h, the ingots were hot forged into square billets. The billets were cold rolled to 4 mm thick plates with a 50% thickness reduction. The Cr/ Mo-alloyed and Cr/Mo-free steel plates were annealed for 30 min at 1413 and 1373 K, respectively, followed by water quenching. Accordingly, nearly the same grain size of ~80 μm was obtained for both steels. Tensile test specimens, with the gauge dimensions of 10 × 10 × 3 mm3, were taken from the annealed plates along the rolling direction. Tensile tests were performed at room temperature at two strain rates of
S. Liu et al. / Scripta Materialia 127 (2017) 10–14
1.5 × 10−3 and 3 × 10−5 s−1. Additional tensile tests were performed at strain rate of 1.5 × 10−3 s−1 and interrupted at various strains, for exsitu observations of microstructural evolution during tensile deformation. Substructures of the specimens deformed to different strain levels were examined using electron back scattered diffraction (EBSD) and transmission electron microscope (TEM). Fig. 1a shows the tensile stress-strain curves of the two steels at strain rate of 1.5 × 10− 3 s− 1. Obviously, the tensile curve moves up with the addition of Cr and Mo. The yield strength of the Cr/Mo-alloyed steel is 444 MPa, 30% higher than that of the Cr/Mo-free steel, 340 MPa. This is primarily attributed to the increased solid solution strengthening caused by Cr/Mo alloying. One may argue that Cr/Mo carbide precipitates may also contribute to an increase in yield strength. However, no carbides were detected in the steels by X-ray diffraction analysis and TEM observations, as shown in Fig. 4a and b, and this may rule out the contribution of precipitation strengthening. Interestingly, the Cr/Moalloyed steel shows simultaneous increases in both tensile strength and total elongation, as compared with the Cr/Mo-free steel. Additionally, Fig. 1b shows the tensile curves of the two steels at a lower strain rate of 3 × 10−5 s−1. This figure shows again that Cr/Mo alloying increases both the strength and ductility of Fe-Mn-C TWIP steel simultaneously. Furthermore, serrations are observed on the stress-strain curves of both steels at either higher or lower strain rate (Fig. 1a and b). These serrations are generally interpreted by DSA, caused primarily by the dynamic interactions between mobile dislocations and solute atoms [3, 14]. Clearly, DSA occurs in both steels. However, Cr/Mo alloying tends to weaken DSA, since the amplitude and the number of serrations are smaller for the Cr/Mo-alloyed steel than for the Cr/Mo-free steel [4,6,
10-3 s-1
1000 Cr/Mo-free
800 600 400 200 0.0
0.2
0.4 0.6 Engineering strain
(b) 1200 Strain rate 3 10-5 s-1 Engineering stress (MPa)
(a) 3500
Cr/Mo-alloyed
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Strain rate 1.5
1000
0.8
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600 400
0.4 0.6 Engineering strain
0.8
Stage II
3000
2500 Stage II
2000
1500 0.0
10-3 s-1
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0.3 0.4 0.5 True strain
(b) 3500
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Cr/Mo-free Cr/Mo-alloyed
Strain rate 1.5
1.0
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200 0.0
15]. This is probably due to the decreased carbon activity caused by Cr/Mo alloying [16]. Fig. 2a and b shows the strain-hardening rate (dσ/dε) curves of the two steels at strain rates of 1.5 × 10−3 s−1 and 3 × 10−5 s−1, respectively. All dσ/dε curves can be approximately divided into three stages. At stage I, the dσ/dε of both steels decreases in a similar way; however, the shapes of stages II and III are influenced significantly by Cr/Mo alloying at either strain rate. In particular, the range of stage II for the Cr/Mo-alloyed steel is longer than that for the Cr/Mo-free steel. At stage III, dσ/dε decreases nearly continuously for both steels; however, the decreasing rate of dσ/dε slows down considerably with Cr/Mo alloying. The longer stage II with higher dσ/dε, in conjunction with the slow decrease of dσ/dε at stage III, brings about a sustained high strain-hardening rate in the CrMo-alloyed steel, which delays the plastic instability and hence leads to simultaneous increases in tensile strength and ductility. Fig. 3 typically shows the EBSD image quality (IQ) and inverse pole figures (IPF) of the two steels deformed to strains of 0.2, 0.4 and 0.6 at the strain rate of 1.5 × 10− 3 s−1. Parallel lines are twin bundles, consisting of several individual thin twins [6]. Note that individual thin twins are not distinguishable by EBSD due to its limited resolution but identifiable by transmission electron microscopy (TEM), as will be shown in the following. At strain of 0.2, some primary and secondary twin bundles are observed in both steels (Fig. 3a and b); however, relatively fewer are observed in the Cr/Mo-alloyed steel. With increasing strain to 0.4, the amount of twin bundles increases largely in both steels (Fig. 3c and d); in contrast, for the Cr/Mo-alloyed steel, the amount of twin bundles is still slightly smaller, and secondary twin bundles remain rarely visible. This result suggests that deformation twinning is suppressed by Cr/Mo alloying at lower strains. With further increasing
Strain hardening rate (MPa)
Engineering stress (MPa)
(a) 1200
11
1.0
Fig. 1. Engineering stress-strain curves of the Cr/Mo-alloyed and -free steels at strain rates of (a) 1.5 × 10–3 s−1 and (b) 3 × 10–5 s−1. The insets show enlarged sections of the curves in the dashed black boxes.
0.6
0.7
0.8
Cr/Mo-free Cr/Mo-alloyed Stage II
3000
2500 Stage II
2000 Strain rate 3
1500 0.0
0.1
0.2
10-5 s-1
0.3 0.4 0.5 True strain
0.6
0.7
0.8
Fig. 2. Strain-hardening rate curves of the Cr/Mo-alloyed and -free steels at strain rates of (a) 1.5 × 10–3 s−1 and (b) 3 × 10–5 s−1.
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S. Liu et al. / Scripta Materialia 127 (2017) 10–14
(a)
(b)
50 µm
(c)
50 µm
(d)
50 µm
(e)
50 µm
(f)
(g)
Fraction of twinned areas (%)
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50 µm
75
111
60
TD 45
RD 30 15 0 0.0
001 Cr/Mo-free Cr/Mo-alloyed 0.2 0.4 0.6 0.8 Engineering strain
101 Tensile direction
1.0
Fig. 3. EBSD image quality (IQ) and inverse pole figures (IPF) of the two steels deformed to strains of (a, b) 0.2, (c, d) 0.4 and (e, f) 0.6 at strain rate of 1.5 × 10–3 s‐1. (a), (c) and (e) are for the Cr/Mo-free steel; (b), (d) and (f) are for the Cr/Mo-alloyed steel. (g) The evolution of twinned area fractions in the Cr/Mo-free and -alloyed steels during tensile deformation.
strain to 0.6, almost all grains in both steels are full of twin bundles (Fig. 3e and f). It is also observed that there are more intensive two-system twin bundles in the Cr/Mo-alloyed steel than in the Cr/Mo-free steel at this high strain level. Furthermore, the area fractions of twinned grains at various strains were measured according to Ref. [17], with the twinned grains determined to be those containing parallel lines of twin bundles on the EBSD figures. The twinned area fractions measured by EBSD might not be so accurate, but they may provide helpful information on the tendency of twin evolution. As is observed from Fig. 3g, deformation twinning is suppressed by Cr/Mo alloying at lower strains. However, at higher strains, twins are released steadily in the Cr/Moalloyed steel, but appear to reach premature saturation in the Cr/Mofree steel. As a result, the formation rate of twins becomes even larger in the former than in the latter steel.
Typical TEM images of the two steels deformed to fracture are shown in Fig. 4c and d. Lamellar deformation twins, as identified by the selected-area diffraction patterns in the insets, are clearly visible. Apparently, more densely-spaced and thinner twins are formed in the Cr/Mo-alloyed than in the Cr/Mo-free steel. Twin thickness and spacing (i.e. the distance between neighboring twins) were measured from the 〈110〉 zone axis orientation. The quantitative statistical results are shown in Fig. 4e and f. Apparently, the Cr/Mo-alloyed steel shows thinner and denser twin lamellae with average twin thickness of 7.3 nm and twin spacing of 9.5 nm, as compared with the Cr/Mo-free steel with average twin thickness of 25.9 nm and spacing of 20.1 nm. From the above EBSD and TEM observations, it is demonstrated that Cr/Mo alloying retards twin formation at small strains but enables a more persistent release of twins with further straining. This more
S. Liu et al. / Scripta Materialia 127 (2017) 10–14
(a)
(b)
200 nm
200 nm
(c)
(d)
100 nm 7.3 nm
Cr/Mo-alloyed Cr/Mo-free
60 40
25.9 nm
60
Cr/Mo-alloyed Cr/Mo-free
9.5 nm 20.1 nm
40 20
20 0 0
25 nm (f) 80
Number
Number
(e) 80
13
10 20 30 40 Twin thickness (nm)
50
0 0
10
20 30 40 Twin spacing (nm)
50
Fig. 4. Typical TEM microstructures of (a, c) the Cr/Mo-free and (b, d) Cr/Mo-alloyed steels. (a) and (b) are for the as-annealed condition and (c) and (d) are for deformation to fracture. Statistical distributions of (e) twin thickness and (f) twin spacing of the two steels deformed to fracture.
densely-spaced, thinner twin structure formed at higher strains provides more twin/matrix interfaces and reduces more the mean free path of dislocations [18]. It is thus considered that the retarded, but more steady and persistent release of twins up to higher strains may be the major reason for the sustained high strain-hardening rate in the Cr/Mo-alloyed steel. The underlying mechanisms for the unique twinning behavior observed in the Cr/Mo-alloyed steel could be related to the addition of alloying elements Cr and Mo. One may consider that Cr/Mo addition may have an influence on the SFE and thus on deformation twinning. Indeed, Cr and Mo were reported to decrease SFE [19,20], and a decreased SFE is expected to favor the formation of deformation twins at lower strains; however, this is inconsistent with the present observations. Another possible explanation for the observed twinning behavior is associated with DSA. As is known, enhanced DSA promotes planar dislocation slide and increases the local stress concentrations by dislocation pileups [21,22], thus being beneficial for the formation of deformation twins [2,22,23]. Recall Fig. 1a and b, DSA was weakened by Cr/Mo alloying. This is due to the decreased carbon activity and diffusivity by adding Cr and Mo, associated with the stronger interactions between
carbon and Cr/Mo atoms. The weakened DSA is therefore supposed to be responsible for the suppressed deformation twinning at lower strains in the Cr/Mo-alloyed steel. With increasing strains, dislocation density increases and concurrently, local stress concentrations increase as well. Therefore, more twin nucleation sites become available. And, for the Cr/Mo-alloyed steel, due to the existence of more non-twinned area at lower strains, it is more likely that deformation twins are formed continuously with further straining. Furthermore, twin thickness has been reported to be related to SFE [17]. The decrease in SFE caused by Cr/Mo addition may, therefore, be one possible reason for the formation of thinner twins in the Cr/Moalloyed steel. In summary, Cr/Mo alloying increases simultaneously the yield and tensile strengths and ductility of Fe-Mn-C TWIP steel. This extraordinary result is primarily attributed to the unique deformation twinning behavior occurring in the Cr/Mo-alloyed steel, i.e., retarded but more persistent formation of thinner twins with straining, in contrast with the premature saturation of twinning in the Cr/Mo-free steel. This gives rise to a sustained high strain-hardening rate up to higher strains in the Cr/Mo-alloyed steel.
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S. Liu et al. / Scripta Materialia 127 (2017) 10–14
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51171166). References [1] O. Grässel, L. Krüger, G. Frommeyer, L.W. Meyer, Int. J. Plast. 16 (2000) 1391–1409. [2] O. Bouaziz, S. Allain, C.P. Scott, P. Cugy, D. Barbier, Curr. Opin. Solid State Mater. Sci. 15 (2011) 141–168. [3] Y.N. Dastur, W.C. Leslie, Metall. Trans. A 12 (1981) 749–759. [4] J.E. Jin, Y.K. Lee, Acta Mater. 60 (2012) 1680–1688. [5] S. Lee, J. Kim, S.-J. Lee, B.C. De Cooman, Scr. Mater. 65 (2011) 1073–1076. [6] K. Jeong, J.-E. Jin, Y.-S. Jung, S. Kang, Y.-K. Lee, Acta Mater. 61 (2013) 3399–3410. [7] C. Scott, B. Remy, J.-L. Collet, A. Cael, C. Bao, F. Danoic, B. Malard, C. Curfs, Int. J. Mater. Res. 102 (2011) 538–549. [8] R. Ueji, N. Tsuchida, D. Terada, N. Tsuji, Y. Tanaka, A. Takemura, K. Kunishige, Scr. Mater. 59 (2008) 963–966.
[9] Y.F. Shen, C.H. Qiu, L. Wang, X. Sun, X.M. Zhao, L. Zuo, Mater. Sci. Eng. A 561 (2013) 329–337. [10] P. Kusakin, A. Belyakov, C. Haase, R. Kaibyshev, D.A. Molodov, Mater. Sci. Eng. A 617 (2014) 52–60. [11] L. He, Z. Jin, J. Lu, Mater. Mech. Eng. 24 (2000) 22–27. [12] L. He, Z. Jin, J. Lu, X. Chen, J. Fu, C. Sun, Iron Steel 35 (2000) 48–50. [13] X. Yuan, Y. Zhao, L. Chen, J. Northeast. Univ. 37 (2016) 184–188. [14] L. Qian, P. Guo, J. Meng, F. Zhang, J. Mater. Sci. 48 (2013) 1669–1674. [15] S. Liu, L. Qian, J. Meng, P. Ma, F. Zhang, Mater. Sci. Eng. A 639 (2015) 425–430. [16] S.-J. Lee, D.K. Matlock, C.J.V. Tyne, Scr. Mater. 64 (2011) 805–808. [17] H.K. Yang, Z.J. Zhang, Z.F. Zhang, Scr. Mater. 68 (2013) 992–995. [18] I. Gutierrez-Urrutia, D. Raabe, Scr. Mater. 66 (2012) 992–996. [19] A. Dumay, J.-P. Chateau, S. Allain, S. Migot, O. Bouaziz, Mater. Sci. Eng. A 483-84 (2008) 184–187. [20] G.R. Razavi, M.S. Rizi, H.M. Zadeh, Mater. Tech. 47 (2013) 611–614. [21] O. Bouaziz, H. Zurob, B. Chehab, J.D. Embury, S. Allain, M. Huang, Mater. Sci. Technol. 27 (2011) 707–709. [22] P. Guo, L. Qian, J. Meng, F. Zhang, L. Li, Mater. Sci. Eng. A 584 (2013) 133–142. [23] P. Franciosi, F. Tranchant, J. Vergnol, Acta Metall. Mater. 41 (1993) 1543–1550.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Review Article
Simultaneously increasing both strength and ductility of Fe-Mn-C twinning-induced plasticity steel via Cr/Mo alloying Shuai Liu a,b, Lihe Qian a,b,⁎, Jiangying Meng a, Dongdong Li a,b, Penghui Ma a,b, Fucheng Zhang a,b a b
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao 066004, China
a r t i c l e
i n f o
Article history: Received 25 August 2016 Accepted 30 August 2016 Available online xxxx Keywords: Twinning-induced plasticity steel Mechanical properties Strain hardening Cr/Mo alloying
a b s t r a c t The effects of Cr/Mo alloying on the tensile deformation behavior of Fe-Mn-C twinning-induced plasticity steel were investigated. The results showed that Cr/Mo alloying increases both the strength and ductility significantly and concurrently. As compared with the premature saturation of deformation twinning in the Cr/Mo-free steel, a unique twinning behavior occurs in the Cr/Mo-alloyed steel, i.e., retarded but more persistent formation of thinner twins with straining. This unique twinning behavior gives rise to a sustained high strain-hardening rate up to higher strains, which delays the plastic instability and increases both strength and ductility simultaneously. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
In recent years, many efforts have been made to produce high-performance materials for manufacturing modern auto components with reduced weight and improved safety. This has led to the development of high manganese twinning-induced plasticity (TWIP) steels with excellent combinations of strength and ductility, stemming from deformation twinning and/or dynamic strain aging (DSA) [1–3]. TWIP steels were originally developed from Hadfield steel (Mn13) after its inventor, Sir Robert Hadfield in 1882, and later improved constantly through several strategies including compositional adjustment, grain refinement, cold-rolling and thermal treatment [4–10]. Plentiful of results are currently available on the effects of compositional adjustment on TWIP steels, and varying degrees of property improvement have been demonstrated [4–7]. However, most previous studies showed increased tensile strength or improved ductility, but seldom both in Fe-Mn-C TWIP steels alloyed with elements such as Al, Si and Cu [4–6]. As reported in Ref. [4], Al addition increases both the yield strength and total elongation by solid solution strengthening and suppressing carbide precipitation. However, Al addition also decreases the tensile strength and strain-hardening rate because of the suppressed deformation twinning, associated with the increased stacking fault energy (SFE) by Al addition. According to Lee et al. [5], Cu also increases SFE, and Cu addition tends to retard the formation of deformation twins, thus decreasing the strain-hardening rate and increasing the total elongation. On the contrary, Si is an SFE-decreasing element. Si addition increases the strain-hardening rate and the tensile strength, but decreases the ductility [6]. In addition, microalloying elements Nb, V ⁎ Corresponding author at: State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China. E-mail address: [email protected] (L. Qian).
http://dx.doi.org/10.1016/j.scriptamat.2016.08.034 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
and Ti were also attempted to change the mechanical properties of FeMn-C TWIP steels. It was shown that the addition of such microalloying elements increases the yield and tensile strengths, mainly due to solution and precipitation strengthening, but decreases the strain-hardening rate and total elongation due to the suppressed formation of deformation twins [7]. Furthermore, several studies are available, showing the potential of Cr/Mo alloying for improving the performance of TWIP steels [11–13]. It was reported that the addition of a certain amount of Cr and Mo increases the hardness and wear resistance of high manganese Hadfield steel, due to solution hardening and the formation of wear-resistant Cr/Mo carbides [11,12]. Besides, Cr addition also shows improved corrosion and oxidation resistance in high manganese TWIP steel [13]. However, the effects of Cr/Mo alloying on the tensile properties and strainhardening behavior of TWIP steel have not been reported. In this work, we prepared two Fe-Mn-C TWIP steels (one alloyed with Cr and Mo and the other without Cr/Mo alloying), and comparatively studied the tensile properties of both steels. The chemical compositions of the two steels investigated are Fe18Mn-1.0C-2Cr-1Mo and Fe-18Mn-1.0C (wt.%), referred to as Cr/Moalloyed and Cr/Mo-free steels, respectively. Both steels were melted in a vacuum induction furnace. After homogenization at 1473 K for 4 h, the ingots were hot forged into square billets. The billets were cold rolled to 4 mm thick plates with a 50% thickness reduction. The Cr/ Mo-alloyed and Cr/Mo-free steel plates were annealed for 30 min at 1413 and 1373 K, respectively, followed by water quenching. Accordingly, nearly the same grain size of ~80 μm was obtained for both steels. Tensile test specimens, with the gauge dimensions of 10 × 10 × 3 mm3, were taken from the annealed plates along the rolling direction. Tensile tests were performed at room temperature at two strain rates of
S. Liu et al. / Scripta Materialia 127 (2017) 10–14
1.5 × 10−3 and 3 × 10−5 s−1. Additional tensile tests were performed at strain rate of 1.5 × 10−3 s−1 and interrupted at various strains, for exsitu observations of microstructural evolution during tensile deformation. Substructures of the specimens deformed to different strain levels were examined using electron back scattered diffraction (EBSD) and transmission electron microscope (TEM). Fig. 1a shows the tensile stress-strain curves of the two steels at strain rate of 1.5 × 10− 3 s− 1. Obviously, the tensile curve moves up with the addition of Cr and Mo. The yield strength of the Cr/Mo-alloyed steel is 444 MPa, 30% higher than that of the Cr/Mo-free steel, 340 MPa. This is primarily attributed to the increased solid solution strengthening caused by Cr/Mo alloying. One may argue that Cr/Mo carbide precipitates may also contribute to an increase in yield strength. However, no carbides were detected in the steels by X-ray diffraction analysis and TEM observations, as shown in Fig. 4a and b, and this may rule out the contribution of precipitation strengthening. Interestingly, the Cr/Moalloyed steel shows simultaneous increases in both tensile strength and total elongation, as compared with the Cr/Mo-free steel. Additionally, Fig. 1b shows the tensile curves of the two steels at a lower strain rate of 3 × 10−5 s−1. This figure shows again that Cr/Mo alloying increases both the strength and ductility of Fe-Mn-C TWIP steel simultaneously. Furthermore, serrations are observed on the stress-strain curves of both steels at either higher or lower strain rate (Fig. 1a and b). These serrations are generally interpreted by DSA, caused primarily by the dynamic interactions between mobile dislocations and solute atoms [3, 14]. Clearly, DSA occurs in both steels. However, Cr/Mo alloying tends to weaken DSA, since the amplitude and the number of serrations are smaller for the Cr/Mo-alloyed steel than for the Cr/Mo-free steel [4,6,
10-3 s-1
1000 Cr/Mo-free
800 600 400 200 0.0
0.2
0.4 0.6 Engineering strain
(b) 1200 Strain rate 3 10-5 s-1 Engineering stress (MPa)
(a) 3500
Cr/Mo-alloyed
Strain hardening rate (MPa)
Strain rate 1.5
1000
0.8
Cr/Mo-alloyed
600 400
0.4 0.6 Engineering strain
0.8
Stage II
3000
2500 Stage II
2000
1500 0.0
10-3 s-1
0.1
0.2
0.3 0.4 0.5 True strain
(b) 3500
Cr/Mo-free
0.2
Cr/Mo-free Cr/Mo-alloyed
Strain rate 1.5
1.0
800
200 0.0
15]. This is probably due to the decreased carbon activity caused by Cr/Mo alloying [16]. Fig. 2a and b shows the strain-hardening rate (dσ/dε) curves of the two steels at strain rates of 1.5 × 10−3 s−1 and 3 × 10−5 s−1, respectively. All dσ/dε curves can be approximately divided into three stages. At stage I, the dσ/dε of both steels decreases in a similar way; however, the shapes of stages II and III are influenced significantly by Cr/Mo alloying at either strain rate. In particular, the range of stage II for the Cr/Mo-alloyed steel is longer than that for the Cr/Mo-free steel. At stage III, dσ/dε decreases nearly continuously for both steels; however, the decreasing rate of dσ/dε slows down considerably with Cr/Mo alloying. The longer stage II with higher dσ/dε, in conjunction with the slow decrease of dσ/dε at stage III, brings about a sustained high strain-hardening rate in the CrMo-alloyed steel, which delays the plastic instability and hence leads to simultaneous increases in tensile strength and ductility. Fig. 3 typically shows the EBSD image quality (IQ) and inverse pole figures (IPF) of the two steels deformed to strains of 0.2, 0.4 and 0.6 at the strain rate of 1.5 × 10− 3 s−1. Parallel lines are twin bundles, consisting of several individual thin twins [6]. Note that individual thin twins are not distinguishable by EBSD due to its limited resolution but identifiable by transmission electron microscopy (TEM), as will be shown in the following. At strain of 0.2, some primary and secondary twin bundles are observed in both steels (Fig. 3a and b); however, relatively fewer are observed in the Cr/Mo-alloyed steel. With increasing strain to 0.4, the amount of twin bundles increases largely in both steels (Fig. 3c and d); in contrast, for the Cr/Mo-alloyed steel, the amount of twin bundles is still slightly smaller, and secondary twin bundles remain rarely visible. This result suggests that deformation twinning is suppressed by Cr/Mo alloying at lower strains. With further increasing
Strain hardening rate (MPa)
Engineering stress (MPa)
(a) 1200
11
1.0
Fig. 1. Engineering stress-strain curves of the Cr/Mo-alloyed and -free steels at strain rates of (a) 1.5 × 10–3 s−1 and (b) 3 × 10–5 s−1. The insets show enlarged sections of the curves in the dashed black boxes.
0.6
0.7
0.8
Cr/Mo-free Cr/Mo-alloyed Stage II
3000
2500 Stage II
2000 Strain rate 3
1500 0.0
0.1
0.2
10-5 s-1
0.3 0.4 0.5 True strain
0.6
0.7
0.8
Fig. 2. Strain-hardening rate curves of the Cr/Mo-alloyed and -free steels at strain rates of (a) 1.5 × 10–3 s−1 and (b) 3 × 10–5 s−1.
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(a)
(b)
50 µm
(c)
50 µm
(d)
50 µm
(e)
50 µm
(f)
(g)
Fraction of twinned areas (%)
50 µm
50 µm
75
111
60
TD 45
RD 30 15 0 0.0
001 Cr/Mo-free Cr/Mo-alloyed 0.2 0.4 0.6 0.8 Engineering strain
101 Tensile direction
1.0
Fig. 3. EBSD image quality (IQ) and inverse pole figures (IPF) of the two steels deformed to strains of (a, b) 0.2, (c, d) 0.4 and (e, f) 0.6 at strain rate of 1.5 × 10–3 s‐1. (a), (c) and (e) are for the Cr/Mo-free steel; (b), (d) and (f) are for the Cr/Mo-alloyed steel. (g) The evolution of twinned area fractions in the Cr/Mo-free and -alloyed steels during tensile deformation.
strain to 0.6, almost all grains in both steels are full of twin bundles (Fig. 3e and f). It is also observed that there are more intensive two-system twin bundles in the Cr/Mo-alloyed steel than in the Cr/Mo-free steel at this high strain level. Furthermore, the area fractions of twinned grains at various strains were measured according to Ref. [17], with the twinned grains determined to be those containing parallel lines of twin bundles on the EBSD figures. The twinned area fractions measured by EBSD might not be so accurate, but they may provide helpful information on the tendency of twin evolution. As is observed from Fig. 3g, deformation twinning is suppressed by Cr/Mo alloying at lower strains. However, at higher strains, twins are released steadily in the Cr/Moalloyed steel, but appear to reach premature saturation in the Cr/Mofree steel. As a result, the formation rate of twins becomes even larger in the former than in the latter steel.
Typical TEM images of the two steels deformed to fracture are shown in Fig. 4c and d. Lamellar deformation twins, as identified by the selected-area diffraction patterns in the insets, are clearly visible. Apparently, more densely-spaced and thinner twins are formed in the Cr/Mo-alloyed than in the Cr/Mo-free steel. Twin thickness and spacing (i.e. the distance between neighboring twins) were measured from the 〈110〉 zone axis orientation. The quantitative statistical results are shown in Fig. 4e and f. Apparently, the Cr/Mo-alloyed steel shows thinner and denser twin lamellae with average twin thickness of 7.3 nm and twin spacing of 9.5 nm, as compared with the Cr/Mo-free steel with average twin thickness of 25.9 nm and spacing of 20.1 nm. From the above EBSD and TEM observations, it is demonstrated that Cr/Mo alloying retards twin formation at small strains but enables a more persistent release of twins with further straining. This more
S. Liu et al. / Scripta Materialia 127 (2017) 10–14
(a)
(b)
200 nm
200 nm
(c)
(d)
100 nm 7.3 nm
Cr/Mo-alloyed Cr/Mo-free
60 40
25.9 nm
60
Cr/Mo-alloyed Cr/Mo-free
9.5 nm 20.1 nm
40 20
20 0 0
25 nm (f) 80
Number
Number
(e) 80
13
10 20 30 40 Twin thickness (nm)
50
0 0
10
20 30 40 Twin spacing (nm)
50
Fig. 4. Typical TEM microstructures of (a, c) the Cr/Mo-free and (b, d) Cr/Mo-alloyed steels. (a) and (b) are for the as-annealed condition and (c) and (d) are for deformation to fracture. Statistical distributions of (e) twin thickness and (f) twin spacing of the two steels deformed to fracture.
densely-spaced, thinner twin structure formed at higher strains provides more twin/matrix interfaces and reduces more the mean free path of dislocations [18]. It is thus considered that the retarded, but more steady and persistent release of twins up to higher strains may be the major reason for the sustained high strain-hardening rate in the Cr/Mo-alloyed steel. The underlying mechanisms for the unique twinning behavior observed in the Cr/Mo-alloyed steel could be related to the addition of alloying elements Cr and Mo. One may consider that Cr/Mo addition may have an influence on the SFE and thus on deformation twinning. Indeed, Cr and Mo were reported to decrease SFE [19,20], and a decreased SFE is expected to favor the formation of deformation twins at lower strains; however, this is inconsistent with the present observations. Another possible explanation for the observed twinning behavior is associated with DSA. As is known, enhanced DSA promotes planar dislocation slide and increases the local stress concentrations by dislocation pileups [21,22], thus being beneficial for the formation of deformation twins [2,22,23]. Recall Fig. 1a and b, DSA was weakened by Cr/Mo alloying. This is due to the decreased carbon activity and diffusivity by adding Cr and Mo, associated with the stronger interactions between
carbon and Cr/Mo atoms. The weakened DSA is therefore supposed to be responsible for the suppressed deformation twinning at lower strains in the Cr/Mo-alloyed steel. With increasing strains, dislocation density increases and concurrently, local stress concentrations increase as well. Therefore, more twin nucleation sites become available. And, for the Cr/Mo-alloyed steel, due to the existence of more non-twinned area at lower strains, it is more likely that deformation twins are formed continuously with further straining. Furthermore, twin thickness has been reported to be related to SFE [17]. The decrease in SFE caused by Cr/Mo addition may, therefore, be one possible reason for the formation of thinner twins in the Cr/Moalloyed steel. In summary, Cr/Mo alloying increases simultaneously the yield and tensile strengths and ductility of Fe-Mn-C TWIP steel. This extraordinary result is primarily attributed to the unique deformation twinning behavior occurring in the Cr/Mo-alloyed steel, i.e., retarded but more persistent formation of thinner twins with straining, in contrast with the premature saturation of twinning in the Cr/Mo-free steel. This gives rise to a sustained high strain-hardening rate up to higher strains in the Cr/Mo-alloyed steel.
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