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On the coupled temperature–strain rate sensitivity of ultrafine-grained interstitial-free steel
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Scripta Materialia 63 (2010) 544–547 www.elsevier.com/locate/scriptamat
On the coupled temperature–strain rate sensitivity of ultrafinegrained interstitial-free steel D. Canadinc,a,* T. Niendorf b and H.J. Maierb a
Koc University, Advanced Materials Group, Department of Mechanical Engineering, Sariyer, Istanbul 34450, Turkey b University of Paderborn, Lehrstuhl f. Werkstoffkunde (Materials Science), Paderborn 33095, Germany Received 1 April 2010; revised 15 May 2010; accepted 19 May 2010 Available online 2 June 2010
The coupled temperature–strain rate sensitivity of ultrafine-grained interstitial-free steel was investigated. The current findings indicate that the grain boundary misorientation angle distribution dictates the observed increased strain rate sensitivity and strain hardening rate at elevated temperatures. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ultrafine-grained material; Strain rate sensitivity; Steel; High temperature
The digital image correlation (DIC) technique was utilized in conjunction with mechanical experiments carried out at different strain rates and temperatures in order to study the coupled strain rate sensitivity (SRS) and temperature effects on the compressive deformation response of ultrafine-grained (UFG) interstitial-free (IF) steel. The results shed light onto the role of grain boundary character on the strain rate and temperature sensitivities of the UFG IF steel. Nanocrystalline and UFG materials have received considerable attention within the last decade owing to their improved properties as compared to conventional coarse-grained (CG) materials [1–4]. The scientific efforts in this field are currently moving from the investigation of fundamental properties of these classes of materials towards their utilization in applications. This, in turn, raises new issues, such as large-scale production and fatigue of the manufactured components. Various studies have focused on the cyclic deformation response of UFG materials, addressing the fatigue-related issues surrounding these alloys [4–12]. Manufacturing, on the other hand, has been mostly limited to the introduction of the UFG microstructure by means of severe plastic deformation [13–17], yet a significant amount of scientific work is still needed prior to the safe utility of UFG materials in applications. Specifically, depending on the manufacturing process, * Corresponding author. Tel.: +90 212 338 1891; fax: +90 212 338 1548; e-mail: [email protected]
materials might undergo complex plastic deformation states, prompting simultaneous consideration of several factors, including various stress states, processing temperature and SRS. Thus, a deep understanding of these phenomena is essential both for the successful production of components made of UFG materials and to ensure their reliability during service. These issues, and especially the SRS, have come into consideration for UFG alloys only very recently [18–28]. Most of the work has been limited to the exploration of the uniaxial deformation response at various strain rates, and the focus was placed onto the room temperature (RT) behavior in the case of body-centered cubic (bcc) alloys. Specifically, a significant softening has been reported for slower rates of deformation in UFG materials, whereas such an effect was absent in the CG counterpart. The observed phenomenon has mostly been argued to stem from grain coarsening due to the instability of the grain boundaries. However, other reasons, such as grain boundary sliding, thermally activated climb controlled annihilation of dislocations at the grain boundaries, dynamic strain aging and mean free path of dislocations smaller than the UFG grain size, have also been forwarded [18–28]. Thus, a clear cause for the observed behavior has not been identified to date. Furthermore, considering the manufacturing processes, it is more than likely for SRS and temperature effects to coexist, and a thorough study investigating the coupled effects of SRS and temperature has not been forwarded for UFG alloys with a bcc structure yet.
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.05.026
D. Canadinc et al. / Scripta Materialia 63 (2010) 544–547
The current work was undertaken with the motivation of gaining an insight into the coupled temperature–SRS influence on the deformation response of UFG alloys and uncovering the underlying cause(s). Thus, the SRS of a commercial grade ferritic IF steel with a low carbon content [4] was investigated at various temperatures. In particular, the SRS of UFG IF steel, which has been previously shown to demonstrate excellent cyclic stability both at RT and elevated temperatures [4,11,12], was studied through a set of strain rate jump experiments. For this purpose, the CG IF steel was subjected to route 8BC equal channel angular pressing (ECAP), resulting in a UFG microstructure [4]. Miniature cubic samples with dimensions of 4 mm 4 mm 8 mm were extracted from the ECAP-processed billet, then subjected to strain rate jump experiments under compressive loading. A second set of UFG samples were first heat treated at 600 °C for an hour, then underwent the same experimental program in order to establish the SRS of the same material in CG condition. Both the CG and UFG IF steel samples were subjected to uniaxial compressive loading at RT and 100, 200, 300 and 400 °C, where the strain rate varied between 6 106 and 6 102 s1 (Fig. 1). The softening concomitant with temperature is evident for both CG and UFG microstructures, being more pronounced for
545
the latter. Even though the strength levels are higher for the UFG material, as expected, the amount of softening increases drastically as compared with the CG IF steel, and a notable increase in SRS is also present. Specifically, as the temperature increases, a strain rate jump to a lower strain rate brings about larger drops in the magnitude of the flow stress and an increase in the strain hardening coefficients in UFG IF steel samples, as evidenced by the experimental results (Fig. 1). Such a notable change in strain hardening or stress levels, however, is not present for the CG material. In situ DIC results shown in Figure 1 indicate that, for the same strain rate jump from 6 104 to 6 103 s1, the increase in temperature yields a high degree of localization of strain fields at similar global strain values in the UFG condition (Fig. 1). This implies that dislocations possess higher energy due to the increased strain rate, and locally glide longer distances, bringing about the increased local strains [29]. This implied increase in the mean free path of dislocations is usually unexpected in a UFG microstructure, where the grain size is notably small, restricting the mean free path of dislocations. It is well known that ECAP processing, especially along efficient routes, such as 8BC, induces a larger volume fraction of high angle grain boundaries (HAGBs)
Figure 1. SRS experiments carried out on CG and UFG IF steel samples at various temperatures, accompanied by in situ DIC.
546
D. Canadinc et al. / Scripta Materialia 63 (2010) 544–547
in the microstructure [1–17]. The electron backscatter diffraction (EBSD) analyses carried out on the UFG microstructures in the current work prove that this holds true for the current material (Fig. 2), where a majority of the grain boundary misorientation angles (GBMAs) are initially (at RT) larger than 10°, and thus classified as HAGBs [30]. However, a significant number of low angle grain boundaries (LAGBs) are still present in the microstructure. As the temperature increases, most of the LAGBs yield to grain coarsening in the UFG material during deformation, increasing the relative volume fraction of HAGBs, which remain stable (Fig. 3). This not only results in a microstructure that features larger grains, but also brings about a locally increased mean free path of dislocations, which explains the locally increased strains at elevated temperatures evidenced by DIC results (Fig. 1). Furthermore, since the HAGBs resist grain coarsening, more dislocations are stored at the HAGBs, contributing to the increased strain hardening rates observed at elevated temperatures in UFG IF steel (Fig. 1). This also explains the increase in SRS concomitant with temperature, where dislocations moving at higher speeds (owing to the added thermal energy) disrupt more LAGBs at higher strain rates, leading to higher strain hardening coefficients as compared with the CG counterparts. It should be noted that the description provided in the schematic of Figure 3 simplifies the experimental GBMA distributions obtained from EBSD measure-
Figure 2. GBMA distribution for the UFG IF steel at various temperatures.
Figure 3. Schematic illustrating the microstructure–temperature–SRS relationship in UFG IF steel.
ments obtained on samples tested at various temperatures (Fig. 2), and is representative of the general trend. The EBSD measurements shown here (Fig. 2) were carried out at RT following the strain rate jump experiments. Thus, even though representative of the behavior, dynamic recovery that has possibly taken place despite the fast cooling might have affected the results and yielded a slightly different trend than the actual one in a few cases. The current findings on the SRS of UFG IF steel with a bcc structure are consistent with the literature, such that a material with a very similar bcc structure, the airon, exhibits increased SRS at elevated temperatures [31,32]. The influence of temperature on SRS in the CG counterparts of both materials, however, is rather limited, as evidenced by the much smaller differences in stresses recorded at the same strain rates and temperatures (Fig. 1 and Refs. [31,32]). Furthermore, for both materials, increased rate of strain hardening is evident as the temperature increases and the strain rate decreases. However, in contradiction to the results reported in Refs. [31,32], the UFG IF steel demonstrates moderate SRS at RT (Fig. 1). The alloying elements, especially titanium, are mainly responsible for this effect, such that the alloying elements form carbides at the grain boundaries, stabilizing the grain boundaries [4], which restricts the effect of SRS to moderate levels in the absence of temperature influence. Compared to face-centered cubic UFG alloys, such as aluminum [18,31,32], the UFG IF steel demonstrates a similar trend in SRS at various temperatures, such that the SRS becomes more significant as the temperature increases and the strain rate decreases (Fig. 1 and Ref. [18]). However, a notable difference exists between the two materials, such that the rate of strain hardening increases with temperature at slower strain rates for the UFG steel and work hardening is observed at all strain rates, whereas this hardening is replaced by softening in the UFG Al at intermediate strain rates of 104 and 105 s1 [18]. This change from hardening to softening at intermediate strain rates in UFG Al is a clear indicator of dynamic strain aging (DSA) in this material [31,32]. However, neither the strain rate jump experiments nor the DIC results indicate that DSA governs the coupled temperature–SRS sensitivity of UFG IF steel. It should be noted that calculation of SRS values from experiments, where no steady state condition is achieved, could be critical. Furthermore, as the materials do not show pronounced saturation state at particular temperatures, only instantaneous SRS values can be determined: M i ¼ ddrlni e_ , where the subscript i denotes instantaneous, and r and e represent stress and strain, respectively. However, it is important that the SRS values are obtained for identical microstructures, and thus the instantaneous SRS values can be calculated immediately after the strain rate change during the strain rate jump experiments. The other option to ensure identical microstructures would be calculating the steady-state responses, which would be expected to yield the same trend, i.e. the current UFG material exhibiting more SRS at higher temperatures. In summary, the current study presents a thorough comparison of the compressive deformation response
D. Canadinc et al. / Scripta Materialia 63 (2010) 544–547
of UFG and CG IF steel under the coupled influence of temperature and strain rate. The current results obtained by utilizing various experimental techniques demonstrate that SRS becomes more pronounced in IF steel with a UFG microstructure. Furthermore, a significant increase in the strain hardening rate is prevalent at elevated temperatures for lower strain rates. The observed behavior is attributed to the instability of LAGBs, which cannot resist deformation and give way to grain coarsening, thereby increasing the mean free path of dislocations. This, together with the added energy due to elevated temperatures, results in a higher rate of storage of dislocation at HAGBs that remain stable, and thus higher rates of strain hardening. This study was funded by Deutsche Forschungsgemeinschaft, within the Research Unit Program “Mechanische Eigenschaften und Grenzfla¨chen ultrafeinko¨rniger Werkstoffe”. The authors are grateful to Dr. Ibrahim Karaman of Texas A&M University for the ECAP processing of the samples. Ms. Nicole Schult is thanked for her help with the experiments. [1] R.Z. Valiev, A.V. Korznikov, R.R. Mulyukov, Mater. Sci. Eng. A 168 (1993) 141. [2] S.X. McFadden, R.S. Mishra, R.Z. Valiev, A.P. Zhilyaev, A.K. Mukherjee, Nature 398 (1999) 684. [3] Y. Wang, M. Chen, F. Zhou, E. Ma, Nature 419 (2002) 912. [4] T. Niendorf, D. Canadinc, H.J. Maier, I. Karaman, S.G. Sutter, Int. J. Mater. Res. 97 (2006) 1328. [5] H.J. Maier, P. Gabor, N. Gupta, I. Karaman, M. Haouaoui, Int. J. Fatigue 28 (2006) 243. [6] H.J. Maier, P. Gabor, I. Karaman, Mater. Sci. Eng. A 410–411 (2005) 457. [7] H.W. Ho¨ppel, M. Kautz, C. Xu, M. Murashkin, T.G. Langdon, R.Z. Valiev, H. Mughrabi, Int. J. Fatigue 28 (2006) 1001. [8] H. Mughrabi, H.W. Ho¨ppel, M. Kautz, Scripta Mater. 51 (2004) 807. [9] H.K. Kim, M.I. Choi, C.S. Chung, D.H. Shin, Mater. Sci. Eng. A 340 (2003) 243.
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[10] S.V.S. Narayana Murty, S. Torizuka, K. Nagai, T. Kitai, Y. Kogo, Scripta Mater. 53 (2005) 763. [11] T. Niendorf, D. Canadinc, H.J. Maier, I. Karaman, Int. J. Fatigue 30 (2008) 426. [12] T. Niendorf, H.J. Maier, D. Canadinc, I. Karaman, Mater. Sci. Eng. A 503 (2009) 160. [13] A.P. Zhilyaev, G.V. Nurislamova, B.-K. Kim, M.D. Baro´, J.A. Szpunar, T.G. Langdon, Acta Mater. 51 (2003) 753. [14] Y. Saito, H. Utsunomiya, N. Tsuji, T. Sakai, Acta Mater. 47 (1999) 579. [15] J. Huang, Y.T. Zhu, D.J. Alexander, X. Liao, T.C. Lowe, R.J. Asaro, Mater. Sci. Eng. A 371 (2004) 35. [16] V.M. Segal, Mater. Sci. Eng. A 197 (1995) 157. [17] Y.T. Zhu, T.C. Lowe, Mater. Sci. Eng. A 291 (2000) 46. [18] J. May, H.W. Ho¨ppel, M. Go¨ken, Scripta Mater. 53 (2005) 189. [19] I. Sabirov, M.R. Barnett, Y. Estrin, P.D. Hodgson, Scripta Mater. 61 (2009) 181. [20] B. Farrokh, A.S. Khan, Int. J. Plast. 25 (2009) 715. [21] W. Blum, X.H. Zeng, Acta Mater. 57 (2009) 1966. [22] R. Kapoor, J.B. Singh, J.K. Chakravartty, Mater. Sci. Eng. A 496 (2008) 308. [23] R. Tejedor, R. Rodriguez-Baracaldo, J.A. Benito, J. Caro, J.M. Cabrera, Scripta Mater. 59 (2008) 631. [24] A. Mishra, M. Martin, N.N. Thadhani, B.K. Kad, E.A. Kenik, M.A. Meyers, Acta Mater. 56 (2008) 2770. [25] P. Cavaliere, Physica B 403 (2008) 569. [26] M. Martin, A. Mishra, M.A. Meyers, N.N. Thadhani, Mater. Sci. Eng. A 464 (2007) 202. [27] Q. Wei, S. Cheng, K.T. Ramesh, E. Ma, Mater. Sci. Eng. A 381 (2004) 71. [28] Y.J. Li, R. Valiev, W. Blum, Mater. Sci. Eng. A 410–411 (2005) 451. [29] D. Canadinc, C. Efstathiou, H. Sehitoglu, Scripta Mater. 59 (2008) 1103. [30] D. Canadinc, H.J. Maier, M. Haouaoui, I. Karaman, Scripta Mater. 58 (2008) 307. [31] H.W. Ho¨ppel, J. May, P. Eisenlohr, M. Go¨ken, Z. Metallkd. 96 (2005) 566. [32] J. May, H.W. Ho¨ppel, M. Go¨ken, Mater. Sci. Forum 503–504 (2006) 781.
Scripta Materialia 63 (2010) 544–547 www.elsevier.com/locate/scriptamat
On the coupled temperature–strain rate sensitivity of ultrafinegrained interstitial-free steel D. Canadinc,a,* T. Niendorf b and H.J. Maierb a
Koc University, Advanced Materials Group, Department of Mechanical Engineering, Sariyer, Istanbul 34450, Turkey b University of Paderborn, Lehrstuhl f. Werkstoffkunde (Materials Science), Paderborn 33095, Germany Received 1 April 2010; revised 15 May 2010; accepted 19 May 2010 Available online 2 June 2010
The coupled temperature–strain rate sensitivity of ultrafine-grained interstitial-free steel was investigated. The current findings indicate that the grain boundary misorientation angle distribution dictates the observed increased strain rate sensitivity and strain hardening rate at elevated temperatures. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ultrafine-grained material; Strain rate sensitivity; Steel; High temperature
The digital image correlation (DIC) technique was utilized in conjunction with mechanical experiments carried out at different strain rates and temperatures in order to study the coupled strain rate sensitivity (SRS) and temperature effects on the compressive deformation response of ultrafine-grained (UFG) interstitial-free (IF) steel. The results shed light onto the role of grain boundary character on the strain rate and temperature sensitivities of the UFG IF steel. Nanocrystalline and UFG materials have received considerable attention within the last decade owing to their improved properties as compared to conventional coarse-grained (CG) materials [1–4]. The scientific efforts in this field are currently moving from the investigation of fundamental properties of these classes of materials towards their utilization in applications. This, in turn, raises new issues, such as large-scale production and fatigue of the manufactured components. Various studies have focused on the cyclic deformation response of UFG materials, addressing the fatigue-related issues surrounding these alloys [4–12]. Manufacturing, on the other hand, has been mostly limited to the introduction of the UFG microstructure by means of severe plastic deformation [13–17], yet a significant amount of scientific work is still needed prior to the safe utility of UFG materials in applications. Specifically, depending on the manufacturing process, * Corresponding author. Tel.: +90 212 338 1891; fax: +90 212 338 1548; e-mail: [email protected]
materials might undergo complex plastic deformation states, prompting simultaneous consideration of several factors, including various stress states, processing temperature and SRS. Thus, a deep understanding of these phenomena is essential both for the successful production of components made of UFG materials and to ensure their reliability during service. These issues, and especially the SRS, have come into consideration for UFG alloys only very recently [18–28]. Most of the work has been limited to the exploration of the uniaxial deformation response at various strain rates, and the focus was placed onto the room temperature (RT) behavior in the case of body-centered cubic (bcc) alloys. Specifically, a significant softening has been reported for slower rates of deformation in UFG materials, whereas such an effect was absent in the CG counterpart. The observed phenomenon has mostly been argued to stem from grain coarsening due to the instability of the grain boundaries. However, other reasons, such as grain boundary sliding, thermally activated climb controlled annihilation of dislocations at the grain boundaries, dynamic strain aging and mean free path of dislocations smaller than the UFG grain size, have also been forwarded [18–28]. Thus, a clear cause for the observed behavior has not been identified to date. Furthermore, considering the manufacturing processes, it is more than likely for SRS and temperature effects to coexist, and a thorough study investigating the coupled effects of SRS and temperature has not been forwarded for UFG alloys with a bcc structure yet.
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.05.026
D. Canadinc et al. / Scripta Materialia 63 (2010) 544–547
The current work was undertaken with the motivation of gaining an insight into the coupled temperature–SRS influence on the deformation response of UFG alloys and uncovering the underlying cause(s). Thus, the SRS of a commercial grade ferritic IF steel with a low carbon content [4] was investigated at various temperatures. In particular, the SRS of UFG IF steel, which has been previously shown to demonstrate excellent cyclic stability both at RT and elevated temperatures [4,11,12], was studied through a set of strain rate jump experiments. For this purpose, the CG IF steel was subjected to route 8BC equal channel angular pressing (ECAP), resulting in a UFG microstructure [4]. Miniature cubic samples with dimensions of 4 mm 4 mm 8 mm were extracted from the ECAP-processed billet, then subjected to strain rate jump experiments under compressive loading. A second set of UFG samples were first heat treated at 600 °C for an hour, then underwent the same experimental program in order to establish the SRS of the same material in CG condition. Both the CG and UFG IF steel samples were subjected to uniaxial compressive loading at RT and 100, 200, 300 and 400 °C, where the strain rate varied between 6 106 and 6 102 s1 (Fig. 1). The softening concomitant with temperature is evident for both CG and UFG microstructures, being more pronounced for
545
the latter. Even though the strength levels are higher for the UFG material, as expected, the amount of softening increases drastically as compared with the CG IF steel, and a notable increase in SRS is also present. Specifically, as the temperature increases, a strain rate jump to a lower strain rate brings about larger drops in the magnitude of the flow stress and an increase in the strain hardening coefficients in UFG IF steel samples, as evidenced by the experimental results (Fig. 1). Such a notable change in strain hardening or stress levels, however, is not present for the CG material. In situ DIC results shown in Figure 1 indicate that, for the same strain rate jump from 6 104 to 6 103 s1, the increase in temperature yields a high degree of localization of strain fields at similar global strain values in the UFG condition (Fig. 1). This implies that dislocations possess higher energy due to the increased strain rate, and locally glide longer distances, bringing about the increased local strains [29]. This implied increase in the mean free path of dislocations is usually unexpected in a UFG microstructure, where the grain size is notably small, restricting the mean free path of dislocations. It is well known that ECAP processing, especially along efficient routes, such as 8BC, induces a larger volume fraction of high angle grain boundaries (HAGBs)
Figure 1. SRS experiments carried out on CG and UFG IF steel samples at various temperatures, accompanied by in situ DIC.
546
D. Canadinc et al. / Scripta Materialia 63 (2010) 544–547
in the microstructure [1–17]. The electron backscatter diffraction (EBSD) analyses carried out on the UFG microstructures in the current work prove that this holds true for the current material (Fig. 2), where a majority of the grain boundary misorientation angles (GBMAs) are initially (at RT) larger than 10°, and thus classified as HAGBs [30]. However, a significant number of low angle grain boundaries (LAGBs) are still present in the microstructure. As the temperature increases, most of the LAGBs yield to grain coarsening in the UFG material during deformation, increasing the relative volume fraction of HAGBs, which remain stable (Fig. 3). This not only results in a microstructure that features larger grains, but also brings about a locally increased mean free path of dislocations, which explains the locally increased strains at elevated temperatures evidenced by DIC results (Fig. 1). Furthermore, since the HAGBs resist grain coarsening, more dislocations are stored at the HAGBs, contributing to the increased strain hardening rates observed at elevated temperatures in UFG IF steel (Fig. 1). This also explains the increase in SRS concomitant with temperature, where dislocations moving at higher speeds (owing to the added thermal energy) disrupt more LAGBs at higher strain rates, leading to higher strain hardening coefficients as compared with the CG counterparts. It should be noted that the description provided in the schematic of Figure 3 simplifies the experimental GBMA distributions obtained from EBSD measure-
Figure 2. GBMA distribution for the UFG IF steel at various temperatures.
Figure 3. Schematic illustrating the microstructure–temperature–SRS relationship in UFG IF steel.
ments obtained on samples tested at various temperatures (Fig. 2), and is representative of the general trend. The EBSD measurements shown here (Fig. 2) were carried out at RT following the strain rate jump experiments. Thus, even though representative of the behavior, dynamic recovery that has possibly taken place despite the fast cooling might have affected the results and yielded a slightly different trend than the actual one in a few cases. The current findings on the SRS of UFG IF steel with a bcc structure are consistent with the literature, such that a material with a very similar bcc structure, the airon, exhibits increased SRS at elevated temperatures [31,32]. The influence of temperature on SRS in the CG counterparts of both materials, however, is rather limited, as evidenced by the much smaller differences in stresses recorded at the same strain rates and temperatures (Fig. 1 and Refs. [31,32]). Furthermore, for both materials, increased rate of strain hardening is evident as the temperature increases and the strain rate decreases. However, in contradiction to the results reported in Refs. [31,32], the UFG IF steel demonstrates moderate SRS at RT (Fig. 1). The alloying elements, especially titanium, are mainly responsible for this effect, such that the alloying elements form carbides at the grain boundaries, stabilizing the grain boundaries [4], which restricts the effect of SRS to moderate levels in the absence of temperature influence. Compared to face-centered cubic UFG alloys, such as aluminum [18,31,32], the UFG IF steel demonstrates a similar trend in SRS at various temperatures, such that the SRS becomes more significant as the temperature increases and the strain rate decreases (Fig. 1 and Ref. [18]). However, a notable difference exists between the two materials, such that the rate of strain hardening increases with temperature at slower strain rates for the UFG steel and work hardening is observed at all strain rates, whereas this hardening is replaced by softening in the UFG Al at intermediate strain rates of 104 and 105 s1 [18]. This change from hardening to softening at intermediate strain rates in UFG Al is a clear indicator of dynamic strain aging (DSA) in this material [31,32]. However, neither the strain rate jump experiments nor the DIC results indicate that DSA governs the coupled temperature–SRS sensitivity of UFG IF steel. It should be noted that calculation of SRS values from experiments, where no steady state condition is achieved, could be critical. Furthermore, as the materials do not show pronounced saturation state at particular temperatures, only instantaneous SRS values can be determined: M i ¼ ddrlni e_ , where the subscript i denotes instantaneous, and r and e represent stress and strain, respectively. However, it is important that the SRS values are obtained for identical microstructures, and thus the instantaneous SRS values can be calculated immediately after the strain rate change during the strain rate jump experiments. The other option to ensure identical microstructures would be calculating the steady-state responses, which would be expected to yield the same trend, i.e. the current UFG material exhibiting more SRS at higher temperatures. In summary, the current study presents a thorough comparison of the compressive deformation response
D. Canadinc et al. / Scripta Materialia 63 (2010) 544–547
of UFG and CG IF steel under the coupled influence of temperature and strain rate. The current results obtained by utilizing various experimental techniques demonstrate that SRS becomes more pronounced in IF steel with a UFG microstructure. Furthermore, a significant increase in the strain hardening rate is prevalent at elevated temperatures for lower strain rates. The observed behavior is attributed to the instability of LAGBs, which cannot resist deformation and give way to grain coarsening, thereby increasing the mean free path of dislocations. This, together with the added energy due to elevated temperatures, results in a higher rate of storage of dislocation at HAGBs that remain stable, and thus higher rates of strain hardening. This study was funded by Deutsche Forschungsgemeinschaft, within the Research Unit Program “Mechanische Eigenschaften und Grenzfla¨chen ultrafeinko¨rniger Werkstoffe”. The authors are grateful to Dr. Ibrahim Karaman of Texas A&M University for the ECAP processing of the samples. Ms. Nicole Schult is thanked for her help with the experiments. [1] R.Z. Valiev, A.V. Korznikov, R.R. Mulyukov, Mater. Sci. Eng. A 168 (1993) 141. [2] S.X. McFadden, R.S. Mishra, R.Z. Valiev, A.P. Zhilyaev, A.K. Mukherjee, Nature 398 (1999) 684. [3] Y. Wang, M. Chen, F. Zhou, E. Ma, Nature 419 (2002) 912. [4] T. Niendorf, D. Canadinc, H.J. Maier, I. Karaman, S.G. Sutter, Int. J. Mater. Res. 97 (2006) 1328. [5] H.J. Maier, P. Gabor, N. Gupta, I. Karaman, M. Haouaoui, Int. J. Fatigue 28 (2006) 243. [6] H.J. Maier, P. Gabor, I. Karaman, Mater. Sci. Eng. A 410–411 (2005) 457. [7] H.W. Ho¨ppel, M. Kautz, C. Xu, M. Murashkin, T.G. Langdon, R.Z. Valiev, H. Mughrabi, Int. J. Fatigue 28 (2006) 1001. [8] H. Mughrabi, H.W. Ho¨ppel, M. Kautz, Scripta Mater. 51 (2004) 807. [9] H.K. Kim, M.I. Choi, C.S. Chung, D.H. Shin, Mater. Sci. Eng. A 340 (2003) 243.
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