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Simultaneously improving the strength and ductility of coarse-grained Hadfield steel with increasing strain rate
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Scripta Materialia 66 (2012) 431–434 www.elsevier.com/locate/scriptamat
Simultaneously improving the strength and ductility of coarse-grained Hadfield steel with increasing strain rate F.C. Liu, Z.N. Yang, C.L. Zheng and F.C. Zhang⇑ State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, People’s Republic of China Received 19 October 2011; revised 1 December 2011; accepted 2 December 2011 Available online 10 December 2011
In general, the strength of coarse-grained metals rises with increases in the strain rate, while the ductility decreases. However, the strength and ductility of Hadfield steel increased simultaneously with an increase in the strain rate. A maximum elongation of 64% combined with true strength of 1596 MPa was obtained at a high strain rate of 1 101 s 1. This is associated with the fact that Hadfield steel exhibits more strain-induced deformation twins at higher strain rates. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Steel; Deformation twin; Strain hardening; Strength; Ductility
Simultaneous improvement of the strength and ductility of metals has been pursued for a long time because these properties are attractive for advanced structural applications [1,2]. Over the past two decades, many efforts has been devoted to driving the grain sizes of bulk metals down into the nanometer regime for the purpose of increasing the strength of the metals [3], based on the well-known Hall–Petch relationship [4]. While these nanostructured materials offer high strength compared to their conventional coarse-grained counterparts, their ductility is often inadequate. Recently, several strategies aimed at improving the poor ductility of the metals have been reported [5–7]. Most of these approaches focused on tailoring the nanoscale features present in the nanostructured materials. Despite some encouraging reports, the improvements in ductility remain quite limited. Improving the strength and ductility of metals simultaneously remains a challenge. In addition to microstructural refinement, one could also change the deformation parameters to increase the strength and ductility of coarse-grained metals. For example, the behaviour of increasing elongation and strength with decreasing tensile temperature in austenitic cryogenic Fe–Mn–Al steels was previously reported [8]. Researchers also showed that the strain-hardening rate increased with increasing strain rate and/or decreasing
⇑ Corresponding
author. Tel.: +86 335 8063949; fax: +86 335 8074568; e-mail: [email protected]
temperature in nanostructured metals, and as a result, so did the total tensile strain [9,10]. In general, the strength of coarse-grained materials rises with an increase in the strain rate, while the ductility decreases. However, Wu et al. [11] observed an interesting phenomenon, namely that the ductility of a superaustenitic stainless steel increases significantly with increasing strain rate due to the enhanced strain-induced e martensite transformation at higher strain rates. The phenomena of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) have been widely observed in high-manganese steels which contain 15–30% Mn and additions of Al and Si of about 2–6% (wt.%). These steels exhibit both high strength and ductility combined with high energy absorption capacity as well as good forming properties due to the gradual transformation of austenite to martensite in the TRIP steel and the gradual formation of deformation twins in the TWIP steel during deformation [12,13]. Grassel et al. [13] showed that both TRIP and TWIP steels exhibited total elongations of more than 80% at low strain rates, although a reduction in failure strain with increasing strain rate was observed in both steels. Austenitic Hadfield steel, which contains 10–14% Mn and 1.0–1.4% C (wt.%), possesses high wear resistance, high toughness and a high strain-hardening rate in polycrystalline form. Owing to these properties it has been widely used to manufacture excavators, crusher jaws, grinding mill liners and railway crossings. Hadfield steel has a low stacking fault energy (SFE, 23 mJ m–2) [14],
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.12.005
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F. C. Liu et al. / Scripta Materialia 66 (2012) 431–434
Figure 1. Tensile properties of Hadfield steel at various strain rates: (a) engineering stress–strain curves; (b) correlations between the strength and elongation with the strain rate; (c) true stress–strain curves; and (d) variation of strain-hardening exponent with true strain.
which promotes the formation of deformation twins. Early studies [15,16] demonstrated that twinning is activated in polycrystalline Hadfield steel, particularly at strains of the order of 5–10%. In addition to strain, the strain rates also have an important effect on deformation twinning [17,18]. However, the effect of strain rate on the mechanical properties and microstructural evolution of Hadfield steel is still poorly understood. If the formation of deformation twins and/or the strain-hardening rate were enhanced at high strain rates, it would be possible to achieve a combination of high strength and ductility. Therefore, the structurally related deformation behaviour of Hadfield steel was investigated over a wide strain rate range. The aim was to research the possibility of achieving enhanced ductility with a gradual formation of deformation twins at high strain rates in Hadfield steel. The chemical composition (in wt.%) of the commercial Hadfield steel used here was 1.2 C, 12.4 Mn, 0.60 Si, 0.016 S and 0.022 P, with the remainder being Fe. A uniform austenitic microstructure was achieved by a water toughening process: the specimens were heated to 1050 °C and then held at that temperature for 1 h followed by quenching in water. Tensile specimens with a gauge length of 30 mm and a diameter of 6 mm were machined from the austenitic Hadfield steel. Room-temperature and high-temperature tensile tests were conducted using a Gleeble 3500 test machine in the strain-rate range of 1 10 4–1 101 s 1. Five specimens were subjected to tensile test at each strain rate. The samples which were cut from the gauge region of the deformed tensile specimens were lightly electropolished to produce a strain-free surface parallel to the tensile axis. Electron backscatter diffraction (EBSD) orientation maps were obtained using a Zeiss Supra 55 operated at 20 kV and interfaced to an HKL Channel EBSD system. Kikuchi patterns were obtained automatically at steps of 0.03 and 1 lm. Fractographs of failed tensile samples were observed by scanning electron microscopy (SEM) with a Hitachi S-4800 operating at 15 kV.
Figure 1a shows the stress–strain curves of Hadfield steels deformed at various strain rates. Both the ultimate tensile strength (UTS) and elongation conspicuously increased with increasing strain rate. The summarized UTS, yield strength (YS) and elongation are plotted in Figure 1b. The YS increased from 419 to 447 MPa when the strain rate was increased from 1 10 4 to 1 10 2 s 1, and then decreased to 388 MPa with a further increase in the strain rate to 1 101 s 1. Both the UTS and elongation increased gently when the strain rate was increased from 1 10 4 to 1 10 1 s 1 and then exhibited a sharp increase when the strain rate was increased from 1 10 1 to 1 100 s 1. A maximum elongation of 64% combined with the highest tensile strength of 973 MPa was obtained at a strain rate of 1 101 s 1. This trend violates the normal condition that the strength of materials increases with an increase in the strain rate while the ductility decreases. The true stress–strain curves of Hadfield steel showed that the highest true stress of 1596 MPa was achieved at the strain rate of 1 101 s 1 (Fig. 1c). The variation of strain-hardening exponents (n) with true strain for the Hadfield steel deformed at various strain rates is summarized in Figure 1d. For the samples deformed at high strain rates, high values of n were observed at various strains. This corresponded well with the observation that elongation increased with increases in the strain rate. Figure 2 shows the typical failed tensile specimens deformed at different strain rates. All the specimens showed uniform elongation without observable necking. This indicated that the high ductility of the Hadfield steel originated from its pre-necking elongation. Figure 3 shows typical fractographs of Hadfield steels after tensile failure. The dominant fracture behaviour of the specimens which failed at a strain rate of 1 10 4 s 1 was intergranular fracture. When the strain rate was increased to 1 10 1 s 1, the characteristics of both intergranular and transgranular cracks were observed and the areas of microdimples became relatively
F. C. Liu et al. / Scripta Materialia 66 (2012) 431–434
Figure 2. Tensile specimens pulled to failure at various strain rates.
larger. By comparison, the fracture surface of the sample which failed at strain rates of 1 100 and 1 101 s 1 was characterized by transgranular cracks with microdimples and some large dimples. These suggested that with increasing strain rate, the displayed rupture tendency changed from intergranular cracking to transgranular cracking to ductile rupture. To understand the enhanced ductility at high strain rates, it is necessary to investigate the microstructural features that evolved at various strain rates. Figure 4 shows the EBSD maps of the deformed Hadfield steels. The black and white lines represent the high-angle grain boundaries (HAGBs, grain boundary orientation angle > 15°) and low-angle grain boundaries (LAGBs, grain boundary orientation angle < 15°), respectively. LAGBs are observed in most of the grains while deformation twins appeared in some of the grains in the Hadfield steel which was subjected to tensile deformation, indicating that deformation was inhomogeneous in the tensile test. In comparison, a higher density of deformation twins appeared in the Hadfield steel which was deformed at a strain rate of 1 101 s 1 than in the sample deformed at a strain rate of 1 10 4 s 1 when the samples were deformed to a strain of 0.23 (Fig. 4a and c). Some twin intersections are evident in the sample
433
deformed at a strain rate of 1 101 s 1. High-resolution EBSD maps obtained at a step of 0.03 lm are shown in Figure 4b and d. The thin twins which can be observed in the band contrast maps were not revealed in the EBSD maps due to the resolution limit. These demonstrated the existence of deformation twins of the order of tens and hundreds of nanometers in size. Furthermore, Figure 4d shows that the twin layers in Figure 4c should be twin packs consisting of thin twins which help to increase the density of twin boundaries. These correspond well with the findings of previous studies showing that increasing the strain rate facilitated the formation of deformation twins during plastic deformation [17]. When the strain was increased further to 0.49 at a strain rate of 1 101 s 1, the density of both the deformation twins and LAGBs increased significantly (Fig. 4e and f). The thickness of some deformation twins increased to 10 lm. Lu and coworkers [17] showed that with an increase in strain, many deformation twins formed and the average twin layer thickness decreased when Cu–Zn alloy was subjected to dynamic plastic deformation at liquid nitrogen temperature. Twin growth was retarded by the formation of new twins during compressed deformation in their work. Although new deformation twins were inserted in the matrix with increasing strain, the twin layer thickness increased significantly in the Hadfield steels. It is likely that twin growth differs depending on the tension and compression conditions. High plastic strain exerted a large amount of strain energy on the material. In the samples deformed at high strain rates, there was less time for the heat to dissipate to the surroundings, leading to a significant increase in temperature. The highest surface temperature was measured to be 120 °C for the specimens deformed at 1 101 s 1 but 25 °C for the specimens deformed at 1 10 4 s 1. The high temperature facilitated the dislocation motion, cross-slip and interaction, which is beneficial for the ductility. In order to evaluate the effect of temperature on ductility, the Hadfield steel was subjected to tensile testing at 1 10 4 s 1 and 120 °C.
Figure 3. The fracture surfaces of Hadfield steels were pulled to failure at (a) 1 10
4
s 1, (b) 1 10
1
s 1, (c) 1 100 s
1
and (d) 1 101 s 1.
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steel. These gradually increasing twin boundaries subdivide the grains and therefore increase the number of barriers to dislocation motion and the value of n (Fig. 1d). The high values of n at a high strain rate, representing a strong ability of the material to retard necking, delayed necking to rupture and then resulted in a high elongation. The strength and ductility of Hadfield steel could be simultaneously increased with increasing strain rate due to the enhanced TWIP. This marked property is beneficial to the widespread use of Hadfield steel in fields where high-strain-rate deformation is required. Furthermore, in the selection of metals used in high-strain-rate deformation, more attention should be paid to materials which exhibit TWIP at high strain rates. In summary, the higher the strain rates and strains applied, the greater the number of strain-induced deformation twins that formed. The twin boundaries subdivided the grains and acted as strong barriers to subsequent dislocation motion, and thereby increased the value of the strain-hardening exponents. Therefore, the strength and ductility of Hadfield steel were simultaneously increased with increasing strain rate. Figure 4. EBSD maps for Hadfield steel deformed to (a and b) a strain of 0.23 at a strain rate of 1 10 4 s 1, (c and d) a strain of 0.23 at a strain rate of 1 101 s 1, and (e and f) a strain of 0.49 at a strain rate of 1 101 s 1.
The ductility increased from 26% to 30% when the temperature increased from 25 to 120 °C for the specimens deformed at 1 10 4 s 1. This indicated that the exceptionally high elongation achieved at 1 101 s 1 cannot be ascribed mainly to the temperature increment. The deformation mechanisms responsible for the high strain-hardening rate in Hadfield steel include interactions between dislocations and dipoles of C and Mn atoms [19,20], interactions between dislocations and twins [7,14,21], and interactions between twinning systems [22]. Generally all three deformation mechanisms are active during deformation but the contribution of each one varied according to strain, strain rate and deformation temperature. The present study showed that the contribution of twins increased with increasing strain rate and strain in Hadfield steel. This is consistent with the researches which showed that high strain rates often lead to twinning in face-centred cubic metals such as Cu and Ni which should not twin under normal deformation conditions because of their high stacking fault energy [23] and with the report which suggested that increasing either the strain or the strain rate would facilitate the formation of deformation twins [17]. Under a high strain rate, the increasing temperature is unfavourable to twin formation, but this drawback was completely overcome by the high strain rate which facilitated the formation of deformation twins in the Hadfield steel. Strength and ductility are two of the most important mechanical properties for structural materials. However, they are often mutually exclusive [24]. This is also true for most of the metals deformed at high strain rates, which usually leads to high strength but disappointingly low ductility. In this investigation, the density of twin boundaries increased with strain and strain rate in Hadfield
This work was supported by the National Science Foundation for Distinguished Young Scholars of China (No. 50925522), the Foundation for Innovative Research Groups of the National Science Foundation of China (No. 50821001), and the Hebei Province Scientific Committee of China (Nos. E2009001632 and 10165138P). [1] I. Maxwell, A. Hellawell, Acta Metall. 23 (1975) 230. [2] R.Z. Valiev, R. Korznikov, R. Muliukov, Mater. Sci. Eng. A 168 (1993) 141. [3] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427. [4] S. Cheng, J.A. Spenser, W.W. Milligan, Acta Mater. 51 (2003) 4505. [5] L. Lu, Y.F. Shen, X.H. Chen, L.H. Qian, K. Lu, Science 304 (2004) 422. [6] Y.M. Wang, E. Ma, Acta Mater. 52 (2004) 1699. [7] W.L. Li, N.R. Tao, K. Lu, Science 331 (2011) 1587. [8] Y.G. Kim, C.Y. Lim, Metall. Trans. A 19 (1988) 1925. [9] Y.M. Wang, E. Ma, Appl. Phys. Lett. 83 (2003) 3165. [10] Y.M. Wang, Adv. Mater. 16 (2004) 328. [11] C.C. Wu, S.H. Wang, C.Y. Chen, J.R. Yang, P.K. Chiu, J. Fang, Scripta Mater. 56 (2007) 717. [12] K. Sato, M. Ichinose, Y. Hirotsu, Y. Inoue, ISIJ Int. 29 (1989) 868. [13] O. Grassel, L. Kruger, G. Frommeter, L.W. Meyer, Int. J. Plast. 16 (2000) 1391. [14] I. Karaman, H. Sehitoglu, K. Gall, Y.I. Chumlyakov, H.J. Maier, Acta Mater. 48 (2000) 1345. [15] Z. Nishiyama, M. Oka, H. Nakagawa, Trans. Jpn. Inst. Metals 6 (1965) 88. [16] K.S. Raghavan, A.S. Sastri, M.J. Marcinkowski, Trans. TMS-AIME 245 (1969) 1569. [17] G.H. Xiao, N.R. Tao, K. Lu, Scripta Mater. 59 (2008) 975. [18] C. Zener, J. Hollomon, J. Appl. Phys. 15 (1944) 22. [19] Y.N. Dastur, W.C. Leslie, Metall. Trans. A 12 (1981) 749. [20] W. Owen, M. Grujicic, Acta Mater. 47 (1998) 111. [21] I. Karaman, H. Sehitoglu, A.J. Beaudoin, Y.I. Chumlyakov, H.J. Maier, C.N. Tome, Acta Mater. 48 (2000) 2031. [22] C. Efstathiou, H. Sehitoglu, Acta Mater. 58 (2010) 1479. [23] Y.S. Li, N.R. Tao, K. Lu, Acta Mater. 56 (2008) 230. [24] R.Z. Valiev, Nature 419 (2002) 887.
Scripta Materialia 66 (2012) 431–434 www.elsevier.com/locate/scriptamat
Simultaneously improving the strength and ductility of coarse-grained Hadfield steel with increasing strain rate F.C. Liu, Z.N. Yang, C.L. Zheng and F.C. Zhang⇑ State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, People’s Republic of China Received 19 October 2011; revised 1 December 2011; accepted 2 December 2011 Available online 10 December 2011
In general, the strength of coarse-grained metals rises with increases in the strain rate, while the ductility decreases. However, the strength and ductility of Hadfield steel increased simultaneously with an increase in the strain rate. A maximum elongation of 64% combined with true strength of 1596 MPa was obtained at a high strain rate of 1 101 s 1. This is associated with the fact that Hadfield steel exhibits more strain-induced deformation twins at higher strain rates. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Steel; Deformation twin; Strain hardening; Strength; Ductility
Simultaneous improvement of the strength and ductility of metals has been pursued for a long time because these properties are attractive for advanced structural applications [1,2]. Over the past two decades, many efforts has been devoted to driving the grain sizes of bulk metals down into the nanometer regime for the purpose of increasing the strength of the metals [3], based on the well-known Hall–Petch relationship [4]. While these nanostructured materials offer high strength compared to their conventional coarse-grained counterparts, their ductility is often inadequate. Recently, several strategies aimed at improving the poor ductility of the metals have been reported [5–7]. Most of these approaches focused on tailoring the nanoscale features present in the nanostructured materials. Despite some encouraging reports, the improvements in ductility remain quite limited. Improving the strength and ductility of metals simultaneously remains a challenge. In addition to microstructural refinement, one could also change the deformation parameters to increase the strength and ductility of coarse-grained metals. For example, the behaviour of increasing elongation and strength with decreasing tensile temperature in austenitic cryogenic Fe–Mn–Al steels was previously reported [8]. Researchers also showed that the strain-hardening rate increased with increasing strain rate and/or decreasing
⇑ Corresponding
author. Tel.: +86 335 8063949; fax: +86 335 8074568; e-mail: [email protected]
temperature in nanostructured metals, and as a result, so did the total tensile strain [9,10]. In general, the strength of coarse-grained materials rises with an increase in the strain rate, while the ductility decreases. However, Wu et al. [11] observed an interesting phenomenon, namely that the ductility of a superaustenitic stainless steel increases significantly with increasing strain rate due to the enhanced strain-induced e martensite transformation at higher strain rates. The phenomena of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) have been widely observed in high-manganese steels which contain 15–30% Mn and additions of Al and Si of about 2–6% (wt.%). These steels exhibit both high strength and ductility combined with high energy absorption capacity as well as good forming properties due to the gradual transformation of austenite to martensite in the TRIP steel and the gradual formation of deformation twins in the TWIP steel during deformation [12,13]. Grassel et al. [13] showed that both TRIP and TWIP steels exhibited total elongations of more than 80% at low strain rates, although a reduction in failure strain with increasing strain rate was observed in both steels. Austenitic Hadfield steel, which contains 10–14% Mn and 1.0–1.4% C (wt.%), possesses high wear resistance, high toughness and a high strain-hardening rate in polycrystalline form. Owing to these properties it has been widely used to manufacture excavators, crusher jaws, grinding mill liners and railway crossings. Hadfield steel has a low stacking fault energy (SFE, 23 mJ m–2) [14],
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.12.005
432
F. C. Liu et al. / Scripta Materialia 66 (2012) 431–434
Figure 1. Tensile properties of Hadfield steel at various strain rates: (a) engineering stress–strain curves; (b) correlations between the strength and elongation with the strain rate; (c) true stress–strain curves; and (d) variation of strain-hardening exponent with true strain.
which promotes the formation of deformation twins. Early studies [15,16] demonstrated that twinning is activated in polycrystalline Hadfield steel, particularly at strains of the order of 5–10%. In addition to strain, the strain rates also have an important effect on deformation twinning [17,18]. However, the effect of strain rate on the mechanical properties and microstructural evolution of Hadfield steel is still poorly understood. If the formation of deformation twins and/or the strain-hardening rate were enhanced at high strain rates, it would be possible to achieve a combination of high strength and ductility. Therefore, the structurally related deformation behaviour of Hadfield steel was investigated over a wide strain rate range. The aim was to research the possibility of achieving enhanced ductility with a gradual formation of deformation twins at high strain rates in Hadfield steel. The chemical composition (in wt.%) of the commercial Hadfield steel used here was 1.2 C, 12.4 Mn, 0.60 Si, 0.016 S and 0.022 P, with the remainder being Fe. A uniform austenitic microstructure was achieved by a water toughening process: the specimens were heated to 1050 °C and then held at that temperature for 1 h followed by quenching in water. Tensile specimens with a gauge length of 30 mm and a diameter of 6 mm were machined from the austenitic Hadfield steel. Room-temperature and high-temperature tensile tests were conducted using a Gleeble 3500 test machine in the strain-rate range of 1 10 4–1 101 s 1. Five specimens were subjected to tensile test at each strain rate. The samples which were cut from the gauge region of the deformed tensile specimens were lightly electropolished to produce a strain-free surface parallel to the tensile axis. Electron backscatter diffraction (EBSD) orientation maps were obtained using a Zeiss Supra 55 operated at 20 kV and interfaced to an HKL Channel EBSD system. Kikuchi patterns were obtained automatically at steps of 0.03 and 1 lm. Fractographs of failed tensile samples were observed by scanning electron microscopy (SEM) with a Hitachi S-4800 operating at 15 kV.
Figure 1a shows the stress–strain curves of Hadfield steels deformed at various strain rates. Both the ultimate tensile strength (UTS) and elongation conspicuously increased with increasing strain rate. The summarized UTS, yield strength (YS) and elongation are plotted in Figure 1b. The YS increased from 419 to 447 MPa when the strain rate was increased from 1 10 4 to 1 10 2 s 1, and then decreased to 388 MPa with a further increase in the strain rate to 1 101 s 1. Both the UTS and elongation increased gently when the strain rate was increased from 1 10 4 to 1 10 1 s 1 and then exhibited a sharp increase when the strain rate was increased from 1 10 1 to 1 100 s 1. A maximum elongation of 64% combined with the highest tensile strength of 973 MPa was obtained at a strain rate of 1 101 s 1. This trend violates the normal condition that the strength of materials increases with an increase in the strain rate while the ductility decreases. The true stress–strain curves of Hadfield steel showed that the highest true stress of 1596 MPa was achieved at the strain rate of 1 101 s 1 (Fig. 1c). The variation of strain-hardening exponents (n) with true strain for the Hadfield steel deformed at various strain rates is summarized in Figure 1d. For the samples deformed at high strain rates, high values of n were observed at various strains. This corresponded well with the observation that elongation increased with increases in the strain rate. Figure 2 shows the typical failed tensile specimens deformed at different strain rates. All the specimens showed uniform elongation without observable necking. This indicated that the high ductility of the Hadfield steel originated from its pre-necking elongation. Figure 3 shows typical fractographs of Hadfield steels after tensile failure. The dominant fracture behaviour of the specimens which failed at a strain rate of 1 10 4 s 1 was intergranular fracture. When the strain rate was increased to 1 10 1 s 1, the characteristics of both intergranular and transgranular cracks were observed and the areas of microdimples became relatively
F. C. Liu et al. / Scripta Materialia 66 (2012) 431–434
Figure 2. Tensile specimens pulled to failure at various strain rates.
larger. By comparison, the fracture surface of the sample which failed at strain rates of 1 100 and 1 101 s 1 was characterized by transgranular cracks with microdimples and some large dimples. These suggested that with increasing strain rate, the displayed rupture tendency changed from intergranular cracking to transgranular cracking to ductile rupture. To understand the enhanced ductility at high strain rates, it is necessary to investigate the microstructural features that evolved at various strain rates. Figure 4 shows the EBSD maps of the deformed Hadfield steels. The black and white lines represent the high-angle grain boundaries (HAGBs, grain boundary orientation angle > 15°) and low-angle grain boundaries (LAGBs, grain boundary orientation angle < 15°), respectively. LAGBs are observed in most of the grains while deformation twins appeared in some of the grains in the Hadfield steel which was subjected to tensile deformation, indicating that deformation was inhomogeneous in the tensile test. In comparison, a higher density of deformation twins appeared in the Hadfield steel which was deformed at a strain rate of 1 101 s 1 than in the sample deformed at a strain rate of 1 10 4 s 1 when the samples were deformed to a strain of 0.23 (Fig. 4a and c). Some twin intersections are evident in the sample
433
deformed at a strain rate of 1 101 s 1. High-resolution EBSD maps obtained at a step of 0.03 lm are shown in Figure 4b and d. The thin twins which can be observed in the band contrast maps were not revealed in the EBSD maps due to the resolution limit. These demonstrated the existence of deformation twins of the order of tens and hundreds of nanometers in size. Furthermore, Figure 4d shows that the twin layers in Figure 4c should be twin packs consisting of thin twins which help to increase the density of twin boundaries. These correspond well with the findings of previous studies showing that increasing the strain rate facilitated the formation of deformation twins during plastic deformation [17]. When the strain was increased further to 0.49 at a strain rate of 1 101 s 1, the density of both the deformation twins and LAGBs increased significantly (Fig. 4e and f). The thickness of some deformation twins increased to 10 lm. Lu and coworkers [17] showed that with an increase in strain, many deformation twins formed and the average twin layer thickness decreased when Cu–Zn alloy was subjected to dynamic plastic deformation at liquid nitrogen temperature. Twin growth was retarded by the formation of new twins during compressed deformation in their work. Although new deformation twins were inserted in the matrix with increasing strain, the twin layer thickness increased significantly in the Hadfield steels. It is likely that twin growth differs depending on the tension and compression conditions. High plastic strain exerted a large amount of strain energy on the material. In the samples deformed at high strain rates, there was less time for the heat to dissipate to the surroundings, leading to a significant increase in temperature. The highest surface temperature was measured to be 120 °C for the specimens deformed at 1 101 s 1 but 25 °C for the specimens deformed at 1 10 4 s 1. The high temperature facilitated the dislocation motion, cross-slip and interaction, which is beneficial for the ductility. In order to evaluate the effect of temperature on ductility, the Hadfield steel was subjected to tensile testing at 1 10 4 s 1 and 120 °C.
Figure 3. The fracture surfaces of Hadfield steels were pulled to failure at (a) 1 10
4
s 1, (b) 1 10
1
s 1, (c) 1 100 s
1
and (d) 1 101 s 1.
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F. C. Liu et al. / Scripta Materialia 66 (2012) 431–434
steel. These gradually increasing twin boundaries subdivide the grains and therefore increase the number of barriers to dislocation motion and the value of n (Fig. 1d). The high values of n at a high strain rate, representing a strong ability of the material to retard necking, delayed necking to rupture and then resulted in a high elongation. The strength and ductility of Hadfield steel could be simultaneously increased with increasing strain rate due to the enhanced TWIP. This marked property is beneficial to the widespread use of Hadfield steel in fields where high-strain-rate deformation is required. Furthermore, in the selection of metals used in high-strain-rate deformation, more attention should be paid to materials which exhibit TWIP at high strain rates. In summary, the higher the strain rates and strains applied, the greater the number of strain-induced deformation twins that formed. The twin boundaries subdivided the grains and acted as strong barriers to subsequent dislocation motion, and thereby increased the value of the strain-hardening exponents. Therefore, the strength and ductility of Hadfield steel were simultaneously increased with increasing strain rate. Figure 4. EBSD maps for Hadfield steel deformed to (a and b) a strain of 0.23 at a strain rate of 1 10 4 s 1, (c and d) a strain of 0.23 at a strain rate of 1 101 s 1, and (e and f) a strain of 0.49 at a strain rate of 1 101 s 1.
The ductility increased from 26% to 30% when the temperature increased from 25 to 120 °C for the specimens deformed at 1 10 4 s 1. This indicated that the exceptionally high elongation achieved at 1 101 s 1 cannot be ascribed mainly to the temperature increment. The deformation mechanisms responsible for the high strain-hardening rate in Hadfield steel include interactions between dislocations and dipoles of C and Mn atoms [19,20], interactions between dislocations and twins [7,14,21], and interactions between twinning systems [22]. Generally all three deformation mechanisms are active during deformation but the contribution of each one varied according to strain, strain rate and deformation temperature. The present study showed that the contribution of twins increased with increasing strain rate and strain in Hadfield steel. This is consistent with the researches which showed that high strain rates often lead to twinning in face-centred cubic metals such as Cu and Ni which should not twin under normal deformation conditions because of their high stacking fault energy [23] and with the report which suggested that increasing either the strain or the strain rate would facilitate the formation of deformation twins [17]. Under a high strain rate, the increasing temperature is unfavourable to twin formation, but this drawback was completely overcome by the high strain rate which facilitated the formation of deformation twins in the Hadfield steel. Strength and ductility are two of the most important mechanical properties for structural materials. However, they are often mutually exclusive [24]. This is also true for most of the metals deformed at high strain rates, which usually leads to high strength but disappointingly low ductility. In this investigation, the density of twin boundaries increased with strain and strain rate in Hadfield
This work was supported by the National Science Foundation for Distinguished Young Scholars of China (No. 50925522), the Foundation for Innovative Research Groups of the National Science Foundation of China (No. 50821001), and the Hebei Province Scientific Committee of China (Nos. E2009001632 and 10165138P). [1] I. Maxwell, A. Hellawell, Acta Metall. 23 (1975) 230. [2] R.Z. Valiev, R. Korznikov, R. Muliukov, Mater. Sci. Eng. A 168 (1993) 141. [3] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427. [4] S. Cheng, J.A. Spenser, W.W. Milligan, Acta Mater. 51 (2003) 4505. [5] L. Lu, Y.F. Shen, X.H. Chen, L.H. Qian, K. Lu, Science 304 (2004) 422. [6] Y.M. Wang, E. Ma, Acta Mater. 52 (2004) 1699. [7] W.L. Li, N.R. Tao, K. Lu, Science 331 (2011) 1587. [8] Y.G. Kim, C.Y. Lim, Metall. Trans. A 19 (1988) 1925. [9] Y.M. Wang, E. Ma, Appl. Phys. Lett. 83 (2003) 3165. [10] Y.M. Wang, Adv. Mater. 16 (2004) 328. [11] C.C. Wu, S.H. Wang, C.Y. Chen, J.R. Yang, P.K. Chiu, J. Fang, Scripta Mater. 56 (2007) 717. [12] K. Sato, M. Ichinose, Y. Hirotsu, Y. Inoue, ISIJ Int. 29 (1989) 868. [13] O. Grassel, L. Kruger, G. Frommeter, L.W. Meyer, Int. J. Plast. 16 (2000) 1391. [14] I. Karaman, H. Sehitoglu, K. Gall, Y.I. Chumlyakov, H.J. Maier, Acta Mater. 48 (2000) 1345. [15] Z. Nishiyama, M. Oka, H. Nakagawa, Trans. Jpn. Inst. Metals 6 (1965) 88. [16] K.S. Raghavan, A.S. Sastri, M.J. Marcinkowski, Trans. TMS-AIME 245 (1969) 1569. [17] G.H. Xiao, N.R. Tao, K. Lu, Scripta Mater. 59 (2008) 975. [18] C. Zener, J. Hollomon, J. Appl. Phys. 15 (1944) 22. [19] Y.N. Dastur, W.C. Leslie, Metall. Trans. A 12 (1981) 749. [20] W. Owen, M. Grujicic, Acta Mater. 47 (1998) 111. [21] I. Karaman, H. Sehitoglu, A.J. Beaudoin, Y.I. Chumlyakov, H.J. Maier, C.N. Tome, Acta Mater. 48 (2000) 2031. [22] C. Efstathiou, H. Sehitoglu, Acta Mater. 58 (2010) 1479. [23] Y.S. Li, N.R. Tao, K. Lu, Acta Mater. 56 (2008) 230. [24] R.Z. Valiev, Nature 419 (2002) 887.