- Email: [email protected]
Dynamic mechanical properties of the Fe–30Mn–3Si–4Al TWIP steel after different heat treatments
Materials Science and Engineering A 530 (2011) 426–431
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Dynamic mechanical properties of the Fe–30Mn–3Si–4Al TWIP steel after different heat treatments Zhi-ping Xiong a , Xue-ping Ren a,∗ , Wei-ping Bao b , Shu-xia Li a , Hai-tao Qu a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China ASSAB Tooling (Beijing) Co. Ltd., Beijing 100176, China
a r t i c l e
i n f o
Article history: Received 19 January 2011 Received in revised form 8 September 2011 Accepted 27 September 2011 Available online 4 October 2011 Keywords: TWIP steel Dynamic compression Heat treatment Strain rate
a b s t r a c t The dynamic mechanical properties of Fe–30Mn–3Si–4Al TWIP steel were studied using a splitHopkinson pressure bar at room temperature with strain rates between 700 and 5000 s−1 . It was found that the effect of heat treatments on the dynamic mechanical properties is limited. With increasing the strain rate from 700 to 2500 s−1 , the strain hardening dominates the process. When the strain rate is further increased to 5000 s−1 , the effect of strain softening is active. The density of deformation twins increases with the strain rate, and then decreases a little with further increasing the strain rate. The deformation behavior is attributed to the opposite effects of pulse duration and impact pressure. © 2011 Elsevier B.V. All rights reserved.
1. Introduction TWIP (Twinning Induced Plasticity) steel has attracted considerate attention recently since it has excellent tensile strength–ductility combination [1–6]. When the Mn is up to 25 wt.%, Al is more than 3 wt.%, and Si is between 2 and 3 wt.%, the austenitic steel exhibits TWIP effect. The development of the automotive industry accelerates this trend. The strength and the elongation of TWIP steel can be up to 600–1100 MPa and 60–95%. The energy-absorption capacity can reach 0.5 J/mm3 at 20 ◦ C, which is almost two times higher than conventional deep punching steel. Additionally, the TWIP steel has no brittleness transition at low temperature. Till now, most studies are focused on static properties, such as the tensile properties [5,7–9], fatigue behaviors [10–12], welding properties [13], alloying effects [14,15], and deformation mechanism [16,17]. Dynamic mechanical properties play an important role in describing the mechanical properties of materials as well. However, the report of dynamic mechanical properties of TWIP steel was relatively few [18,19], and the strain rate is relatively low, from 10−5 to 1000 s−1 . We face versatile kinds of dynamic events in our daily life, the actual engineering projects, military and scientific research, like car accidents, the collisions between airplanes and birds, explosive, and so on. The actual strain rate in the above examples is much higher. So the dynamic mechanical properties of TWIP steel under
∗ Corresponding author. Tel.: +86 13611252065; fax: +86 1062334559. E-mail address: [email protected] (X.-p. Ren). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.09.106
higher strain rates are requested. In the present study, the dynamic mechanical properties of TWIP steel were investigated using a splitHopkinson pressure bar (SHPB) with higher strain rates at room temperature, and the effect of heat treatments on dynamic mechanical properties was analyzed as well. This study will shed light on the selection of heat treatment conditions for TWIP steel with both static and dynamic mechanical properties.
2. Experimental The TWIP steel used in the present study was prepared by induction-melting in an argon atmosphere, and the chemical compositions of the steel are listed in Table 1. The ingots were solution treated at 1100 ◦ C for 2 h, and then forged into bars of around 30 mm in diameter at 1100 ◦ C (Sample A). The samples under examinations were cut along the bar axis using wire-electrode cutting method. Our previous study has found that heat treatment temperature and duration both affect the plasticity of Fe–30Mn–3Si–4Al TWIP steel significantly under static tensile testing conditions [20]. In the present study, three kinds of heat treated samples were selected to study the dynamic mechanical properties, and compared to the as-forged samples (Sample A). Sample B was annealed at 950 ◦ C for 15 min and then water quenched, corresponding to the lowest elongation under static condition. The sample that was annealed at 1000 ◦ C for 60 min and then water quenched (Sample D) corresponds to the largest elongation. The elongation of the sample that was heat treated at 1000 ◦ C for 45 min and then cooled to room temperature within the furnace (Sample C), is between them [20].
Z.-p. Xiong et al. / Materials Science and Engineering A 530 (2011) 426–431
The microstructure was characterized using an optical microscopy (OM) after mechanical polishing and etching in a 5% HNO3 alcohol solution. The dynamic mechanical properties were conducted using a split-Hopkinson pressure bar (SHPB) at room temperature, as Fig. 1 shows. We have considered the dispersion correction during the design of Hopkinson setup, where the ratio of incident radius/stress pulse width r/ ≤ 0.1. In this case, the dispersion correction can be neglected. The strain rate ranges from 700 to 5000 s−1 . The samples were cut into a cylinder of 6 mm in diameter and 5 mm in length, and the samples were polished using silicon carbide papers before tests. To reduce the friction force during experiments, a high pressure lubricant was put on the loading surface of the samples. The speed of the striker bar, which inputs the energy to the incident bar and the sample, was controlled through adjusting the gas pressure in the gas gun. The signals of incident wave εi (t), reflection wave εr (t), and the transmission wave εt (t) were recorded during experiments, and a typical wave form is presented in Fig. 2. The average compression strain ε, strain rate ε˙ and stress of the samples can be obtained based on one-dimensional elastic wave propagation theory by [21];
⎧ A ⎪ (t) = Eεt (t) ⎪ ⎪ A0 ⎪ ⎨ t ε(t) = −
2C
(ε (t) − εt (t)) dt
l0 0 i ⎪ ⎪ ⎪ ⎪ 2C ⎩ ε(t) ˙ (εi (t) − εt (t)) =−
(1)
l0
where C is the longitudinal wave velocity, l0 the effective gage length of the samples, E the Young’s modulus of the split bar, while A and A0 are the cross-sectional areas of the split bar and the samples, respectively. For each condition, at least three samples were conducted, and the average stain–stress curves were obtained
427
Table 1 Chemical compositions of the Fe–30Mn–3Si–4Al TWIP steel (wt.%). Mn
Si
Al
C
P
S
Fe
29.5
3.50
2.94
0.06
0.006
0.005
Balance
through the above analysis, where the standard deviation is added in Table 2. 3. Results and discussion 3.1. Dynamic mechanical properties The microstructure of samples before deformation is shown in Fig. 3. The annealing twins are observed in all samples irrespective of heat treatment history. The as-forged samples (Sample A) contain coarse austenite grains, where a few fibrous carbides are along the grain boundaries. After heat treatments, the samples consist of the coarse austenite grains with annealing twins, and the carbides have dissolved in the austenite grains. With increasing the annealing temperature and duration, the austenite grain size increases, and the amount of annealing twins increases, as Fig. 3b–d shows [20]. The strain rate affects on the dynamic mechanical behavior of Fe–30Mn–3Si–4Al TWIP steel. The true strain–stress relationship obtained from the dynamic mechanical experiments is presented in Fig. 4, and the obtained dynamic mechanical parameters are listed in Table 2. First of all, the shape of the true stress–strain curves is similar for the samples that underwent different heat treatments, implying that the effect of heat treatments on the dynamic mechanical properties of Fe–30Mn–3Si–4Al TWIP steel is limited. Additionally, the true stress–strain curves can be divided
Fig. 1. The sketch of the split Hopkinson pressure bar (SHPB) testing system: (1) gas gun, (2) strike bar, (3) strain gage, (4) incident bar, (5) sample, (6) transmission bar, (7) momentum trap bar, (8) buffer, (9) amplifier, (10) computer.
Fig. 2. Waveform of dynamic mechanical test with strain rate as 2500 s−1 for samples heat treated at 1000 ◦ C for 45 min, and then cooled to room temperature within furnace.
428
Z.-p. Xiong et al. / Materials Science and Engineering A 530 (2011) 426–431
Table 2 The dynamic properties of the Fe–30Mn–3Si–4Al TWIP steel. Heat treatment
˙ s−1 ε,
0.8 , MPa
εmax
A
As-forged
683 2342 5008
376(±3) 288(±2) 428(±5)
0.113 0.286 0.284
B
950 ◦ C/15 min, water quenched
762 2410 5031
463(±3) 230(±2) 409(±6)
C
1000 ◦ C/45 min, cooled within furnace
698 2374 5046
D
1000 ◦ C/60 min, water quenched
701 2453 5108
Sample
, MPa
T, ◦ C
2 , MPa
749(±4) 1029(±3) 872(±6)
273 741 444
17.79 54.85 52.42
515(±2) 449(±2) 506(±3)
0.127 0.296 0.290
722(±3) 957(±2) 859(±5)
259 727 450
19.67 53.87 51.14
521(±2) 452(±3) 496(±5)
466(±2) 359(±2) 412(±7)
0.116 0.286 0.293
731(±4) 949(±6) 864(±6)
265 590 452
18.40 52.15 52.24
531(±1) 446(±3) 506(±4)
401(±2) 274(±2) 436(±4)
0.119 0.302 0.297
715(±3) 954(±3) 854(±7)
314 680 418
17.86 54.72 52.83
491(±2) 434(±2) 501(±5)
into elastic and strain-hardening stages, and the inflection points indicate the yield stress. In the initial stage of dynamic mechanical experiments, both the incident and the transmission waves increase with time, implying that the deformation is inhomogeneous (Fig. 2), so the yield stress is taken in a larger strain, 0.8% [21], as listed in Table 2. The yield stress is comparable for the four samples under the same strain rate, which is attributed to the similar microstructure, as Fig. 3 shows. The yield stress decreases with increasing austenite grain size in TWIP steel [22], which increases the TWIP effect. The strain rate affects the yield stress, which decreases with increasing the strain rate from 700 to 2500 s−1 , and then increases with further increasing to 5000 s−1 . The stress 2 was also extracted, which is listed in Table 2 as well. Interestingly, the trend of stress 2 with strain rate is similar with that of stress 0.8 , indicating that the stress 0.8 can represent the deformation behavior.
max , MPa
The strain rate affects on the maximum stress max of Fe–30Mn–3Si–4Al TWIP steel as well, as listed in Table 2. The maximum stress max increases when the strain rate increases from 700 to 2500 s−1 . With further increasing the strain rate to 5000 s−1 , the maximum stress max decreases a little. The stress difference between the maximum stress max and the yield stress 0.8 reflects the strain hardening effect, as listed in Table 2. For Sample D, when the strain rate increases from 700 to 2500 s−1 , the stress difference increases from 314 MPa to 680 MPa. With further increasing the strain rate to 5000 s−1 , the stress difference decreases to 418 MPa. A common empirical description of the strain hardening is expressed as [23],
true = kεntrue
(2)
Fig. 3. Microstructures of the Fe–30Mn–3Si–4Al TWIP steel before deformation: (a) the as-forged sample, and after heat treatment at (b) 950 ◦ C for 15 min and then water quenched, (c) 1000 ◦ C for 45 min and then cooled to room temperature within furnace, and (d) 1000 ◦ C for 60 min and then water quenched.
Z.-p. Xiong et al. / Materials Science and Engineering A 530 (2011) 426–431
429
Fig. 4. The dynamic mechanical behavior of the Fe–30Mn–3Si–4Al TWIP steel after different heat treatments: (a) as-forged, (b) 950 ◦ C for 15 min and then water quenched, (c) 1000 ◦ C for 45 min and then cooled to room temperature within furnace, and (d) 1000 ◦ C for 60 min and then water quenched.
where true and εtrue are, respectively, the true stress and strain, and k is the strength coefficient. Thus, the strain hardening exponent n can be obtained by n = dln true /dln εtrue . The strain hardening exponent n variation with true strain Sample D is shown in Fig. 5, and other samples have similar behaviors. The exponent n increases with true strain when the strain rate is set as 700 s−1 . When the strain rate increases to 2500 s−1 , the exponent n increases first, and then keeps almost constant, indicating that the effect of strain
softening is active. When the strain rate is further increased to 5000 s−1 , the exponent n increases first, and then decreases with true strain, implying that the strain softening dominates the process.
3.2. Adiabatic temperature rise Temperature and microstructure are the two dominant factors to affect the deformation behavior of materials. In dynamic experiments, temperature rising is significant and it is analyzed in the following. With increasing the strain rate, plastic deformation transits from an isothermal process to a quasi-adiabatic process or an adiabatic process. Localization and instability of plastic deformation increase the temperature. Adiabatic temperature rise T can be calculated as [4], T =
Fig. 5. The strain hardening exponent n as a function of true strain for the Fe–30Mn–3Si–4Al TWIP steel after annealing at 1000 ◦ C for 60 min and then water quenched.
ˇ Cp
εmax
dε
(3)
0
where is the density of the material (7.8 × 103 kg/m3 ), Cp is the specific heat (4.9 × 10−4 kJ/kg/◦ C), and ˇ the proportion of plastic work converted to heat, which is approximately taken as 1 [23]. The adiabatic temperature rise with different strain rates is listed in Table 2. Under the same strain rate, the adiabatic temperature rise in different samples is comparable. The adiabatic temperature rise increases significantly, from around 18 ◦ C to 54 ◦ C, when the strain rate increases from 700 to 2500 s−1 . However, the adiabatic temperature rise has almost no change with further increasing the strain rate to 5000 s−1 . So other factors are active as well except the adiabatic temperature rise.
430
Z.-p. Xiong et al. / Materials Science and Engineering A 530 (2011) 426–431
Fig. 6. The deformation structures of the Fe–30Mn–3Si–4Al TWIP steel after annealing at 1000 ◦ C for 60 min and then water quenched (Sample D): (a) 700 s−1 , (b) 2500 s−1 , and (c) 5000 s−1 .
3.3. Microstructure observation Microstructure is another factor to affect the strain hardening behavior. A couple of samples were cut from different region and polished to conduct the microstructure observation for each condition. Fig. 6 shows the typical deformation microstructure of Sample D with different strain rates, and other samples have similar behaviors. When the strain rate is 700 s−1 , a small amount of annealing twins still exists after deformation, and the density of deformation twins is low, whose spacing is large. When the strain rate is increased to 2500 s−1 , as Fig. 6b shows, the deformation twins are homogeneous and the density is high, whose spacing is thin. With further increasing the strain rate to 5000 s−1 , the density of deformation twins decreases a little, and the deformation twins are intersected each other, indicating that multi twinning systems are active. There are multiple twinning systems in many grains at 5000 s−1 while there are fewer grains containing deformation twins than those at 2500 s−1 as shown in Fig. 6. Through observation of some samples, it can be found that the twin density at 5000 s−1 is smaller than that at 2500 s−1 . It is worthy to note that it is impossible to obtain the microstructure image at the yield strain point, ε0.8 , during the SHPB test. However, it is an informational estimate through comparing the microstructure at the end of the SHPB test, which can provide some basic information on the deformation mechanism during the process. The density of deformation twins affects the hardening behavior of Fe–30Mn–3Si–4Al TWIP steel. The formation of twins can induce the slip system which is not in the favorable resolved stress direction [22], which reduces the flow stress, and increases the interaction between dislocation and twins, the stacking faults and twins, and even the twins and twins. All these increase the strain hardening effect. Additionally, the formation of twins reduces grain size as well [7], which reduces the slip length of dislocation and increases the strain-hardening rate. As Fig. 6 shows, the density of
twins is highest when the strain rate is 2500 s−1 , resulting in the maximum flow stress. With further increasing the strain rate to 5000 s−1 , the density of twins decreases, implying that the effect of strain softening is introduced. 3.4. Effects of pulse duration and impact pressure The pulse duration and impact pressure affect the twin density of steels [23–25], and the strain hardening effect. The pulse duration of Fe–30Mn–3Si–4Al TWIP steel under different strain rates is summarized in Fig. 7. Under the same strain rate, the pulse duration is comparable for different samples, which leads to their similar mechanical properties (Fig. 4). The pulse duration decreases
Fig. 7. Pulse duration of the Fe–30Mn–3Si–4Al TWIP steel at different strain rates.
Z.-p. Xiong et al. / Materials Science and Engineering A 530 (2011) 426–431
with increasing the strain rate, which reduces the deformation duration and restrains the nucleation and growth of twins. On the other hand, the impact pressure increases with strain rate, which increases the nucleation and growth of twins [26]. The opposite effects of pulse duration and impact pressure result in the reduction of twin density when strain rate increases from 2500 to 5000 s−1 , as Fig. 6 shows. The impact pressure affects the twin stress significantly which has been demonstrated at very high rates such as in shock compression [24,25,27]. In order to make the discussion clearer and more persuasion, the same argument is taken into account in Hopkinson experiments, which is needed to be proofed in future. It is generally accepted that the nucleation stress of twinning is critical, and the growth stress is only a fraction of the nucleation stress [27]. When the strain rate increases from 700 to 2500 s−1 , the yield stress decreases dramatically (Fig. 4 and Table 2), implying that the impact pressure has exceeded the critical nucleation stress of Fe–30Mn–3Si–4Al TWIP steel when the strain rate is around 700 s−1 . With further increasing the strain rate to 5000 s−1 , multi twinning systems are active, as stated in Section 3.3, which results in an increase in the yield stress (Fig. 4 and Table 2). The deformation behavior of Fe–30Mn–3Si–4Al TWIP steel, especially the strain hardening exponent n, is affected by a couple of factors. When the true strain is less than 7%, the strain hardening exponent n is the largest for the samples deformed with strain rate as 700 s−1 , and that from 2500 s−1 is the smallest. In this strain range, the interaction between deformation twinning is limited. When the strain rate is 700 s−1 , the impact pressure is smallest and the adiabatic temperature rise is only around 18–20 ◦ C, resulting the largest exponent n. For the samples deformed with 5000 s−1 , the multi twinning systems are active, which increases its exponent n. For the samples deformed with 2500 s−1 , when the true strain is between 7% and 13%, the interaction of deformation twinning or the density of deformation twins dominates the process. As stated in Section 3.3, the density of deformation twins is highest for the samples deformed with 2500 s−1 , which increases the exponent n significantly in this strain range. After that, the softening and hardening is almost balanced, leading to a saturation of exponent n. For the sample deformed with 5000 s−1 , the softening and hardening is balanced when the strain is about 8%. After that, the exponent n decreases a little with further increasing strain (Fig. 5), which is possibly related to the shortest impact duration. 4. Conclusions The dynamic mechanical properties of Fe–30Mn–3Si–4Al TWIP steel were investigated using a split-Hopkinson pressure bar at room temperature with strain rates between 700 and 5000 s−1 . The following observations were made: (1) The effect of heat treatments on the dynamic mechanical properties of Fe–30Mn–3Si–4Al TWIP steel is limited although heat treatments affect the static mechanical properties significantly.
431
(2) The stress–strain curves can be divided into two stages: the initial elastic deformation and strain hardening stages. The yield stress, 0.8 , decreases with the strain rate from 700 to 2500 s−1 , and then increases with further increasing the strain rate to 5000 s−1 . (3) The density of deformation twins increases with the strain rate variation from 700 to 2500 s−1 , and then it decreases a little with further increasing the strain rate to 5000 s−1 . (4) The strain hardening effect dominates the process when the strain rate changes from 700 to 2500 s−1 . The strain softening effect is active with further increasing the strain rate to 5000 s−1 . This deformation behavior is attributed to the reduction of pulse duration and increase of impact pressure with increasing the strain rate from 700 to 5000 s−1 . (5) The adiabatic temperature rise T increases significantly when the strain rate increases from 700 to 2500 s−1 , and then it keeps almost constant with further increasing to 5000 s−1 . References [1] O. Grassel, G. Frommeyer, C. Derder, H. Hofmann, J. Phys. IV 7 (1997) 383–388. [2] Z.L. Mi, D. Tang, H.T. Jiang, Y.J. Dai, S.S. Li, Int. J. Miner. Metall. Mater. 16 (2009) 154–158. [3] G. Dini, R. Ueji, A. Najafizadeh, S.M. Monir-Vaghefi, Mater. Sci. Eng. A 527 (2010) 2759–2763. [4] S. Kang, Y.-S. Jung, J.-H. Jun, Y.-K. Lee, Mater. Sci. Eng. A 527 (2010) 745–751. [5] R. Ueji, N. Tsuchida, D. Terada, N. Tsuji, Y. Tanaka, A. Takemura, K. Kunishige, Scr. Mater. 59 (2008) 963–966. [6] O. Grassel, L. Kruger, G. Frommeyer, L.W. Meyer, Int. J. Plast. 16 (2000) 1391–1409. [7] D. Barbier, N. Gey, S. Allain, N. Bozzolo, M. Humbert, Mater. Sci. Eng. A 500 (2009) 196–206. [8] J.D. Yoo, S.W. Hwang, K.-T. Park, Mater. Sci. Eng. A 508 (2009) 234–240. [9] A.S. Hamada, L.P. Karjalainen, Mater. Sci. Eng. A 528 (2011) 1819–1827. [10] T. Niendorf, C. Lotze, D. Canadinc, A. Frehn, H.J. Maier, Mater. Sci. Eng. A 499 (2009) 518–524. [11] T. Niendorf, F. Rubitschek, H.J. Maier, J. Niendorf, H.A. Richard, A. Frehn, Mater. Sci. Eng. A 527 (2010) 2412–2417. [12] A.S. Hamada, L.P. Karjalainen, Mater. Sci. Eng. A 527 (2010) 5715–5722. [13] L. Mujica, S. Weber, H. Pinto, C. Thomy, F. Vollertsen, Mater. Sci. Eng. A 527 (2010) 2071–2078. [14] B.X. Huang, X.D. Wang, Y.H. Rong, L. Wang, L. Jin, Mater. Sci. Eng. A 438 (2006) 306–311. [15] M. Abbasi, S. Kheirandish, Y. Kharrazi, J. Hejazi, Mater. Sci. Eng. A 513–514 (2009) 72–76. [16] T.A. Lebedkina, M.A. Lebyodkin, J.P. Chateau, A. Jacques, S. Allain, Mater. Sci. Eng. A 519 (2009) 147–154. [17] A. Soulami, K.S. Choi, Y.F. Shen, W.N. Liu, X. Sun, M.A. Khaleel, Mater. Sci. Eng. A 528 (2011) 1402–1408. [18] D.Z. Li, Y.H. Wei, C.Y. Liu, L.F. Hou, D.F. Liu, X.Z. Jin, J. Iron Steel Res. Int. 17 (2010) 67–73. [19] R.-g. Xiong, R.-y. Fu, Y. Su, Q. Li, X.-c. Wei, L. Li, J. Iron Steel Res. Int. 16 (2009), 81–86, 21. [20] W.P. Bao, Y.J. Zhao, L.W. Xu, Z.P. Xiong, X.P. Ren, Heat Treatment Met. 35 (2010) 33–37. [21] Y.L. Li, T. Suo, W.G. Guo, R. Hu, J.S. Li, H.Z. Fu, Explosion Shock Waves 25 (2005) 487–492. [22] I. Gutierrez-Urrutia, S. Zaefferer, D. Raabe, Mater. Sci. Eng. A 527 (2010) 3552–3560. [23] M.A. Meyers, Dynamic Behavior of Materials, John Wiley & Sons, Inc., New York, 1994. [24] A.R. Champion, R.W. Rohde, J. Appl. Phys. 41 (1970) 2213. [25] L.E. Murr, K.P. Staudhammer, Mater. Sci. Eng. 20 (1975) 35–46. [26] L.E. Murr, in: M.A. Meyers, L.E. Murr (Eds.), Shock Waves and High-Strain-Rate Phenomena in Metals, Plenum, New York, 1981, p. 607. [27] M.A. Meyers, O. Vohringer, V.A. Lubarda, Acta Mater. 49 (2001) 4025–4039.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Dynamic mechanical properties of the Fe–30Mn–3Si–4Al TWIP steel after different heat treatments Zhi-ping Xiong a , Xue-ping Ren a,∗ , Wei-ping Bao b , Shu-xia Li a , Hai-tao Qu a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China ASSAB Tooling (Beijing) Co. Ltd., Beijing 100176, China
a r t i c l e
i n f o
Article history: Received 19 January 2011 Received in revised form 8 September 2011 Accepted 27 September 2011 Available online 4 October 2011 Keywords: TWIP steel Dynamic compression Heat treatment Strain rate
a b s t r a c t The dynamic mechanical properties of Fe–30Mn–3Si–4Al TWIP steel were studied using a splitHopkinson pressure bar at room temperature with strain rates between 700 and 5000 s−1 . It was found that the effect of heat treatments on the dynamic mechanical properties is limited. With increasing the strain rate from 700 to 2500 s−1 , the strain hardening dominates the process. When the strain rate is further increased to 5000 s−1 , the effect of strain softening is active. The density of deformation twins increases with the strain rate, and then decreases a little with further increasing the strain rate. The deformation behavior is attributed to the opposite effects of pulse duration and impact pressure. © 2011 Elsevier B.V. All rights reserved.
1. Introduction TWIP (Twinning Induced Plasticity) steel has attracted considerate attention recently since it has excellent tensile strength–ductility combination [1–6]. When the Mn is up to 25 wt.%, Al is more than 3 wt.%, and Si is between 2 and 3 wt.%, the austenitic steel exhibits TWIP effect. The development of the automotive industry accelerates this trend. The strength and the elongation of TWIP steel can be up to 600–1100 MPa and 60–95%. The energy-absorption capacity can reach 0.5 J/mm3 at 20 ◦ C, which is almost two times higher than conventional deep punching steel. Additionally, the TWIP steel has no brittleness transition at low temperature. Till now, most studies are focused on static properties, such as the tensile properties [5,7–9], fatigue behaviors [10–12], welding properties [13], alloying effects [14,15], and deformation mechanism [16,17]. Dynamic mechanical properties play an important role in describing the mechanical properties of materials as well. However, the report of dynamic mechanical properties of TWIP steel was relatively few [18,19], and the strain rate is relatively low, from 10−5 to 1000 s−1 . We face versatile kinds of dynamic events in our daily life, the actual engineering projects, military and scientific research, like car accidents, the collisions between airplanes and birds, explosive, and so on. The actual strain rate in the above examples is much higher. So the dynamic mechanical properties of TWIP steel under
∗ Corresponding author. Tel.: +86 13611252065; fax: +86 1062334559. E-mail address: [email protected] (X.-p. Ren). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.09.106
higher strain rates are requested. In the present study, the dynamic mechanical properties of TWIP steel were investigated using a splitHopkinson pressure bar (SHPB) with higher strain rates at room temperature, and the effect of heat treatments on dynamic mechanical properties was analyzed as well. This study will shed light on the selection of heat treatment conditions for TWIP steel with both static and dynamic mechanical properties.
2. Experimental The TWIP steel used in the present study was prepared by induction-melting in an argon atmosphere, and the chemical compositions of the steel are listed in Table 1. The ingots were solution treated at 1100 ◦ C for 2 h, and then forged into bars of around 30 mm in diameter at 1100 ◦ C (Sample A). The samples under examinations were cut along the bar axis using wire-electrode cutting method. Our previous study has found that heat treatment temperature and duration both affect the plasticity of Fe–30Mn–3Si–4Al TWIP steel significantly under static tensile testing conditions [20]. In the present study, three kinds of heat treated samples were selected to study the dynamic mechanical properties, and compared to the as-forged samples (Sample A). Sample B was annealed at 950 ◦ C for 15 min and then water quenched, corresponding to the lowest elongation under static condition. The sample that was annealed at 1000 ◦ C for 60 min and then water quenched (Sample D) corresponds to the largest elongation. The elongation of the sample that was heat treated at 1000 ◦ C for 45 min and then cooled to room temperature within the furnace (Sample C), is between them [20].
Z.-p. Xiong et al. / Materials Science and Engineering A 530 (2011) 426–431
The microstructure was characterized using an optical microscopy (OM) after mechanical polishing and etching in a 5% HNO3 alcohol solution. The dynamic mechanical properties were conducted using a split-Hopkinson pressure bar (SHPB) at room temperature, as Fig. 1 shows. We have considered the dispersion correction during the design of Hopkinson setup, where the ratio of incident radius/stress pulse width r/ ≤ 0.1. In this case, the dispersion correction can be neglected. The strain rate ranges from 700 to 5000 s−1 . The samples were cut into a cylinder of 6 mm in diameter and 5 mm in length, and the samples were polished using silicon carbide papers before tests. To reduce the friction force during experiments, a high pressure lubricant was put on the loading surface of the samples. The speed of the striker bar, which inputs the energy to the incident bar and the sample, was controlled through adjusting the gas pressure in the gas gun. The signals of incident wave εi (t), reflection wave εr (t), and the transmission wave εt (t) were recorded during experiments, and a typical wave form is presented in Fig. 2. The average compression strain ε, strain rate ε˙ and stress of the samples can be obtained based on one-dimensional elastic wave propagation theory by [21];
⎧ A ⎪ (t) = Eεt (t) ⎪ ⎪ A0 ⎪ ⎨ t ε(t) = −
2C
(ε (t) − εt (t)) dt
l0 0 i ⎪ ⎪ ⎪ ⎪ 2C ⎩ ε(t) ˙ (εi (t) − εt (t)) =−
(1)
l0
where C is the longitudinal wave velocity, l0 the effective gage length of the samples, E the Young’s modulus of the split bar, while A and A0 are the cross-sectional areas of the split bar and the samples, respectively. For each condition, at least three samples were conducted, and the average stain–stress curves were obtained
427
Table 1 Chemical compositions of the Fe–30Mn–3Si–4Al TWIP steel (wt.%). Mn
Si
Al
C
P
S
Fe
29.5
3.50
2.94
0.06
0.006
0.005
Balance
through the above analysis, where the standard deviation is added in Table 2. 3. Results and discussion 3.1. Dynamic mechanical properties The microstructure of samples before deformation is shown in Fig. 3. The annealing twins are observed in all samples irrespective of heat treatment history. The as-forged samples (Sample A) contain coarse austenite grains, where a few fibrous carbides are along the grain boundaries. After heat treatments, the samples consist of the coarse austenite grains with annealing twins, and the carbides have dissolved in the austenite grains. With increasing the annealing temperature and duration, the austenite grain size increases, and the amount of annealing twins increases, as Fig. 3b–d shows [20]. The strain rate affects on the dynamic mechanical behavior of Fe–30Mn–3Si–4Al TWIP steel. The true strain–stress relationship obtained from the dynamic mechanical experiments is presented in Fig. 4, and the obtained dynamic mechanical parameters are listed in Table 2. First of all, the shape of the true stress–strain curves is similar for the samples that underwent different heat treatments, implying that the effect of heat treatments on the dynamic mechanical properties of Fe–30Mn–3Si–4Al TWIP steel is limited. Additionally, the true stress–strain curves can be divided
Fig. 1. The sketch of the split Hopkinson pressure bar (SHPB) testing system: (1) gas gun, (2) strike bar, (3) strain gage, (4) incident bar, (5) sample, (6) transmission bar, (7) momentum trap bar, (8) buffer, (9) amplifier, (10) computer.
Fig. 2. Waveform of dynamic mechanical test with strain rate as 2500 s−1 for samples heat treated at 1000 ◦ C for 45 min, and then cooled to room temperature within furnace.
428
Z.-p. Xiong et al. / Materials Science and Engineering A 530 (2011) 426–431
Table 2 The dynamic properties of the Fe–30Mn–3Si–4Al TWIP steel. Heat treatment
˙ s−1 ε,
0.8 , MPa
εmax
A
As-forged
683 2342 5008
376(±3) 288(±2) 428(±5)
0.113 0.286 0.284
B
950 ◦ C/15 min, water quenched
762 2410 5031
463(±3) 230(±2) 409(±6)
C
1000 ◦ C/45 min, cooled within furnace
698 2374 5046
D
1000 ◦ C/60 min, water quenched
701 2453 5108
Sample
, MPa
T, ◦ C
2 , MPa
749(±4) 1029(±3) 872(±6)
273 741 444
17.79 54.85 52.42
515(±2) 449(±2) 506(±3)
0.127 0.296 0.290
722(±3) 957(±2) 859(±5)
259 727 450
19.67 53.87 51.14
521(±2) 452(±3) 496(±5)
466(±2) 359(±2) 412(±7)
0.116 0.286 0.293
731(±4) 949(±6) 864(±6)
265 590 452
18.40 52.15 52.24
531(±1) 446(±3) 506(±4)
401(±2) 274(±2) 436(±4)
0.119 0.302 0.297
715(±3) 954(±3) 854(±7)
314 680 418
17.86 54.72 52.83
491(±2) 434(±2) 501(±5)
into elastic and strain-hardening stages, and the inflection points indicate the yield stress. In the initial stage of dynamic mechanical experiments, both the incident and the transmission waves increase with time, implying that the deformation is inhomogeneous (Fig. 2), so the yield stress is taken in a larger strain, 0.8% [21], as listed in Table 2. The yield stress is comparable for the four samples under the same strain rate, which is attributed to the similar microstructure, as Fig. 3 shows. The yield stress decreases with increasing austenite grain size in TWIP steel [22], which increases the TWIP effect. The strain rate affects the yield stress, which decreases with increasing the strain rate from 700 to 2500 s−1 , and then increases with further increasing to 5000 s−1 . The stress 2 was also extracted, which is listed in Table 2 as well. Interestingly, the trend of stress 2 with strain rate is similar with that of stress 0.8 , indicating that the stress 0.8 can represent the deformation behavior.
max , MPa
The strain rate affects on the maximum stress max of Fe–30Mn–3Si–4Al TWIP steel as well, as listed in Table 2. The maximum stress max increases when the strain rate increases from 700 to 2500 s−1 . With further increasing the strain rate to 5000 s−1 , the maximum stress max decreases a little. The stress difference between the maximum stress max and the yield stress 0.8 reflects the strain hardening effect, as listed in Table 2. For Sample D, when the strain rate increases from 700 to 2500 s−1 , the stress difference increases from 314 MPa to 680 MPa. With further increasing the strain rate to 5000 s−1 , the stress difference decreases to 418 MPa. A common empirical description of the strain hardening is expressed as [23],
true = kεntrue
(2)
Fig. 3. Microstructures of the Fe–30Mn–3Si–4Al TWIP steel before deformation: (a) the as-forged sample, and after heat treatment at (b) 950 ◦ C for 15 min and then water quenched, (c) 1000 ◦ C for 45 min and then cooled to room temperature within furnace, and (d) 1000 ◦ C for 60 min and then water quenched.
Z.-p. Xiong et al. / Materials Science and Engineering A 530 (2011) 426–431
429
Fig. 4. The dynamic mechanical behavior of the Fe–30Mn–3Si–4Al TWIP steel after different heat treatments: (a) as-forged, (b) 950 ◦ C for 15 min and then water quenched, (c) 1000 ◦ C for 45 min and then cooled to room temperature within furnace, and (d) 1000 ◦ C for 60 min and then water quenched.
where true and εtrue are, respectively, the true stress and strain, and k is the strength coefficient. Thus, the strain hardening exponent n can be obtained by n = dln true /dln εtrue . The strain hardening exponent n variation with true strain Sample D is shown in Fig. 5, and other samples have similar behaviors. The exponent n increases with true strain when the strain rate is set as 700 s−1 . When the strain rate increases to 2500 s−1 , the exponent n increases first, and then keeps almost constant, indicating that the effect of strain
softening is active. When the strain rate is further increased to 5000 s−1 , the exponent n increases first, and then decreases with true strain, implying that the strain softening dominates the process.
3.2. Adiabatic temperature rise Temperature and microstructure are the two dominant factors to affect the deformation behavior of materials. In dynamic experiments, temperature rising is significant and it is analyzed in the following. With increasing the strain rate, plastic deformation transits from an isothermal process to a quasi-adiabatic process or an adiabatic process. Localization and instability of plastic deformation increase the temperature. Adiabatic temperature rise T can be calculated as [4], T =
Fig. 5. The strain hardening exponent n as a function of true strain for the Fe–30Mn–3Si–4Al TWIP steel after annealing at 1000 ◦ C for 60 min and then water quenched.
ˇ Cp
εmax
dε
(3)
0
where is the density of the material (7.8 × 103 kg/m3 ), Cp is the specific heat (4.9 × 10−4 kJ/kg/◦ C), and ˇ the proportion of plastic work converted to heat, which is approximately taken as 1 [23]. The adiabatic temperature rise with different strain rates is listed in Table 2. Under the same strain rate, the adiabatic temperature rise in different samples is comparable. The adiabatic temperature rise increases significantly, from around 18 ◦ C to 54 ◦ C, when the strain rate increases from 700 to 2500 s−1 . However, the adiabatic temperature rise has almost no change with further increasing the strain rate to 5000 s−1 . So other factors are active as well except the adiabatic temperature rise.
430
Z.-p. Xiong et al. / Materials Science and Engineering A 530 (2011) 426–431
Fig. 6. The deformation structures of the Fe–30Mn–3Si–4Al TWIP steel after annealing at 1000 ◦ C for 60 min and then water quenched (Sample D): (a) 700 s−1 , (b) 2500 s−1 , and (c) 5000 s−1 .
3.3. Microstructure observation Microstructure is another factor to affect the strain hardening behavior. A couple of samples were cut from different region and polished to conduct the microstructure observation for each condition. Fig. 6 shows the typical deformation microstructure of Sample D with different strain rates, and other samples have similar behaviors. When the strain rate is 700 s−1 , a small amount of annealing twins still exists after deformation, and the density of deformation twins is low, whose spacing is large. When the strain rate is increased to 2500 s−1 , as Fig. 6b shows, the deformation twins are homogeneous and the density is high, whose spacing is thin. With further increasing the strain rate to 5000 s−1 , the density of deformation twins decreases a little, and the deformation twins are intersected each other, indicating that multi twinning systems are active. There are multiple twinning systems in many grains at 5000 s−1 while there are fewer grains containing deformation twins than those at 2500 s−1 as shown in Fig. 6. Through observation of some samples, it can be found that the twin density at 5000 s−1 is smaller than that at 2500 s−1 . It is worthy to note that it is impossible to obtain the microstructure image at the yield strain point, ε0.8 , during the SHPB test. However, it is an informational estimate through comparing the microstructure at the end of the SHPB test, which can provide some basic information on the deformation mechanism during the process. The density of deformation twins affects the hardening behavior of Fe–30Mn–3Si–4Al TWIP steel. The formation of twins can induce the slip system which is not in the favorable resolved stress direction [22], which reduces the flow stress, and increases the interaction between dislocation and twins, the stacking faults and twins, and even the twins and twins. All these increase the strain hardening effect. Additionally, the formation of twins reduces grain size as well [7], which reduces the slip length of dislocation and increases the strain-hardening rate. As Fig. 6 shows, the density of
twins is highest when the strain rate is 2500 s−1 , resulting in the maximum flow stress. With further increasing the strain rate to 5000 s−1 , the density of twins decreases, implying that the effect of strain softening is introduced. 3.4. Effects of pulse duration and impact pressure The pulse duration and impact pressure affect the twin density of steels [23–25], and the strain hardening effect. The pulse duration of Fe–30Mn–3Si–4Al TWIP steel under different strain rates is summarized in Fig. 7. Under the same strain rate, the pulse duration is comparable for different samples, which leads to their similar mechanical properties (Fig. 4). The pulse duration decreases
Fig. 7. Pulse duration of the Fe–30Mn–3Si–4Al TWIP steel at different strain rates.
Z.-p. Xiong et al. / Materials Science and Engineering A 530 (2011) 426–431
with increasing the strain rate, which reduces the deformation duration and restrains the nucleation and growth of twins. On the other hand, the impact pressure increases with strain rate, which increases the nucleation and growth of twins [26]. The opposite effects of pulse duration and impact pressure result in the reduction of twin density when strain rate increases from 2500 to 5000 s−1 , as Fig. 6 shows. The impact pressure affects the twin stress significantly which has been demonstrated at very high rates such as in shock compression [24,25,27]. In order to make the discussion clearer and more persuasion, the same argument is taken into account in Hopkinson experiments, which is needed to be proofed in future. It is generally accepted that the nucleation stress of twinning is critical, and the growth stress is only a fraction of the nucleation stress [27]. When the strain rate increases from 700 to 2500 s−1 , the yield stress decreases dramatically (Fig. 4 and Table 2), implying that the impact pressure has exceeded the critical nucleation stress of Fe–30Mn–3Si–4Al TWIP steel when the strain rate is around 700 s−1 . With further increasing the strain rate to 5000 s−1 , multi twinning systems are active, as stated in Section 3.3, which results in an increase in the yield stress (Fig. 4 and Table 2). The deformation behavior of Fe–30Mn–3Si–4Al TWIP steel, especially the strain hardening exponent n, is affected by a couple of factors. When the true strain is less than 7%, the strain hardening exponent n is the largest for the samples deformed with strain rate as 700 s−1 , and that from 2500 s−1 is the smallest. In this strain range, the interaction between deformation twinning is limited. When the strain rate is 700 s−1 , the impact pressure is smallest and the adiabatic temperature rise is only around 18–20 ◦ C, resulting the largest exponent n. For the samples deformed with 5000 s−1 , the multi twinning systems are active, which increases its exponent n. For the samples deformed with 2500 s−1 , when the true strain is between 7% and 13%, the interaction of deformation twinning or the density of deformation twins dominates the process. As stated in Section 3.3, the density of deformation twins is highest for the samples deformed with 2500 s−1 , which increases the exponent n significantly in this strain range. After that, the softening and hardening is almost balanced, leading to a saturation of exponent n. For the sample deformed with 5000 s−1 , the softening and hardening is balanced when the strain is about 8%. After that, the exponent n decreases a little with further increasing strain (Fig. 5), which is possibly related to the shortest impact duration. 4. Conclusions The dynamic mechanical properties of Fe–30Mn–3Si–4Al TWIP steel were investigated using a split-Hopkinson pressure bar at room temperature with strain rates between 700 and 5000 s−1 . The following observations were made: (1) The effect of heat treatments on the dynamic mechanical properties of Fe–30Mn–3Si–4Al TWIP steel is limited although heat treatments affect the static mechanical properties significantly.
431
(2) The stress–strain curves can be divided into two stages: the initial elastic deformation and strain hardening stages. The yield stress, 0.8 , decreases with the strain rate from 700 to 2500 s−1 , and then increases with further increasing the strain rate to 5000 s−1 . (3) The density of deformation twins increases with the strain rate variation from 700 to 2500 s−1 , and then it decreases a little with further increasing the strain rate to 5000 s−1 . (4) The strain hardening effect dominates the process when the strain rate changes from 700 to 2500 s−1 . The strain softening effect is active with further increasing the strain rate to 5000 s−1 . This deformation behavior is attributed to the reduction of pulse duration and increase of impact pressure with increasing the strain rate from 700 to 5000 s−1 . (5) The adiabatic temperature rise T increases significantly when the strain rate increases from 700 to 2500 s−1 , and then it keeps almost constant with further increasing to 5000 s−1 . References [1] O. Grassel, G. Frommeyer, C. Derder, H. Hofmann, J. Phys. IV 7 (1997) 383–388. [2] Z.L. Mi, D. Tang, H.T. Jiang, Y.J. Dai, S.S. Li, Int. J. Miner. Metall. Mater. 16 (2009) 154–158. [3] G. Dini, R. Ueji, A. Najafizadeh, S.M. Monir-Vaghefi, Mater. Sci. Eng. A 527 (2010) 2759–2763. [4] S. Kang, Y.-S. Jung, J.-H. Jun, Y.-K. Lee, Mater. Sci. Eng. A 527 (2010) 745–751. [5] R. Ueji, N. Tsuchida, D. Terada, N. Tsuji, Y. Tanaka, A. Takemura, K. Kunishige, Scr. Mater. 59 (2008) 963–966. [6] O. Grassel, L. Kruger, G. Frommeyer, L.W. Meyer, Int. J. Plast. 16 (2000) 1391–1409. [7] D. Barbier, N. Gey, S. Allain, N. Bozzolo, M. Humbert, Mater. Sci. Eng. A 500 (2009) 196–206. [8] J.D. Yoo, S.W. Hwang, K.-T. Park, Mater. Sci. Eng. A 508 (2009) 234–240. [9] A.S. Hamada, L.P. Karjalainen, Mater. Sci. Eng. A 528 (2011) 1819–1827. [10] T. Niendorf, C. Lotze, D. Canadinc, A. Frehn, H.J. Maier, Mater. Sci. Eng. A 499 (2009) 518–524. [11] T. Niendorf, F. Rubitschek, H.J. Maier, J. Niendorf, H.A. Richard, A. Frehn, Mater. Sci. Eng. A 527 (2010) 2412–2417. [12] A.S. Hamada, L.P. Karjalainen, Mater. Sci. Eng. A 527 (2010) 5715–5722. [13] L. Mujica, S. Weber, H. Pinto, C. Thomy, F. Vollertsen, Mater. Sci. Eng. A 527 (2010) 2071–2078. [14] B.X. Huang, X.D. Wang, Y.H. Rong, L. Wang, L. Jin, Mater. Sci. Eng. A 438 (2006) 306–311. [15] M. Abbasi, S. Kheirandish, Y. Kharrazi, J. Hejazi, Mater. Sci. Eng. A 513–514 (2009) 72–76. [16] T.A. Lebedkina, M.A. Lebyodkin, J.P. Chateau, A. Jacques, S. Allain, Mater. Sci. Eng. A 519 (2009) 147–154. [17] A. Soulami, K.S. Choi, Y.F. Shen, W.N. Liu, X. Sun, M.A. Khaleel, Mater. Sci. Eng. A 528 (2011) 1402–1408. [18] D.Z. Li, Y.H. Wei, C.Y. Liu, L.F. Hou, D.F. Liu, X.Z. Jin, J. Iron Steel Res. Int. 17 (2010) 67–73. [19] R.-g. Xiong, R.-y. Fu, Y. Su, Q. Li, X.-c. Wei, L. Li, J. Iron Steel Res. Int. 16 (2009), 81–86, 21. [20] W.P. Bao, Y.J. Zhao, L.W. Xu, Z.P. Xiong, X.P. Ren, Heat Treatment Met. 35 (2010) 33–37. [21] Y.L. Li, T. Suo, W.G. Guo, R. Hu, J.S. Li, H.Z. Fu, Explosion Shock Waves 25 (2005) 487–492. [22] I. Gutierrez-Urrutia, S. Zaefferer, D. Raabe, Mater. Sci. Eng. A 527 (2010) 3552–3560. [23] M.A. Meyers, Dynamic Behavior of Materials, John Wiley & Sons, Inc., New York, 1994. [24] A.R. Champion, R.W. Rohde, J. Appl. Phys. 41 (1970) 2213. [25] L.E. Murr, K.P. Staudhammer, Mater. Sci. Eng. 20 (1975) 35–46. [26] L.E. Murr, in: M.A. Meyers, L.E. Murr (Eds.), Shock Waves and High-Strain-Rate Phenomena in Metals, Plenum, New York, 1981, p. 607. [27] M.A. Meyers, O. Vohringer, V.A. Lubarda, Acta Mater. 49 (2001) 4025–4039.