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Origin of low-temperature shoulder internal friction peak of Snoek-Köster peak in a medium carbon high alloyed steel
Solid State Communications 195 (2014) 31–34
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Origin of low-temperature shoulder internal friction peak of Snoek-Köster peak in a medium carbon high alloyed steel Xianwen Lu a, Mingjiang Jin a,n, Hongshan Zhao a, Wei Li a, Xuejun Jin b,n a Institute of Advanced Steel and Materials, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China b State Key Lab of Metal Matrix Composite, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2014 Received in revised form 17 June 2014 Accepted 23 June 2014 by X.C. Shen Available online 2 July 2014
A distinct internal friction peak located at the low-temperature shoulder of Snoek-Köster peak (LTS-SK) was found in Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel and its evolution with respect to various aging treatments was investigated. The LTS-SK internal friction peak was found to occur when aged below 373 K. TEM observation confirmed that the ε-carbide precipitated beyond 373 K, providing an evidence that the LTS-SK peak cannot be caused by ε-carbide precipitation. The corresponding evolution on the S-K peak and thermoelectric power (TEP) illustrated that the carbon content in the solid solution decreases due to carbon atoms segregation on the surrounding dislocations during low-temperature aging. The origin of the LTS-SK peak is likely attributed to the interaction between the carbon atoms and twin boundaries in martensite. & 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Medium carbon high alloyed steel D. Carbon atom segregation E. Internal friction
1. Introduction Migration of carbon atoms in steels, such as segregation on dislocations, carbide precipitation from matrix and partitioning from supersaturated martensite to austenite, plays an important role in securing the feasible microstructures for the expected mechanical properties. The characterization of carbon migration is one of the key issues in steels research. Many analyzing techniques, such as atom probe tomography (APT), transmission electron microscope (TEM), X-ray diffraction (XRD) and internal friction (IF), were introduced for such analyses [1–4]. Among them, IF measurement with high sensitivity is considered a unique method available for determining the interstitial or dissolved carbon atoms in bcc structural phases in steels, such as ferrite and martensite. Mechanical spectroscopy of steels between RT and 600 K has been frequently reported in previous works [5,6]. Two typical relaxation-type internal friction peaks, one called Snoek peak and the other Snoek-Köster peak (S-K peak), were systematically studied. The intensity of the Snoek peak reflects the quantity of the interstitial atoms in the bcc phase. The S-K peak is believed to relate to the interactions between the mobile foreign interstitial
n
Corresponding authors. Tel./fax: þ86 21 54745560. E-mail addresses: [email protected] (X. Lu), [email protected] (M. Jin), [email protected] (H. Zhao), [email protected] (W. Li), [email protected] (X. Jin). http://dx.doi.org/10.1016/j.ssc.2014.06.018 0038-1098/& 2014 Elsevier Ltd. All rights reserved.
atoms and the dislocations stress field [5]. Meanwhile, an exotic internal friction peak, located at the low-temperature shoulder of the S-K peak (LTS-SK), has been observed in some high carbon or medium carbon high alloyed steels [7–12]. The origin of this peak was usually interpreted as: (1) formation of transition carbides results in reducing of the interstitial carbon content and pinning of the mobile dislocations, and then leading to a decrease of the rising IF background [7–9]; (2) stress-induced movement of coherent interfaces between ε-carbides and matrix [10]; (3) stress-induced atomic movement at twin interfaces [11]. In summary, most of the explanations figured it as a precipitation-related internal friction peak, but there is lack of clear evidence. Recently, an LTS-SK peak was also observed in the mechanical spectra of a Q&P model steel (Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel), which is intentionally designed for the study of kinetics of carbon partitioning with a considerable lower martensitic transformation temperature [13]. In this paper, the origin of the LTS-SK internal friction peak is proposed through the evolutionary trends of the S-K internal friction peak, thermoelectric power measurement and microstructure analysis.
2. Experimental procedures The chemical composition of the steels used in the present study is Fe–0.39C–9.8Ni–1.56Si–2.0Mn (wt%) with MS temperature of 403 K. An ingot was hot-rolled to a plate with a thickness of
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X. Lu et al. / Solid State Communications 195 (2014) 31–34
20 mm after annealing at 1473 K for 3 h. Then the specimens were austenited at 1003 K for 30 min followed by water quenching (298 K), noted as as-WQ. The as-WQ specimen contains about 70 vol% martensite and 30 vol.% austenite measured by magnetization methods proposed by Zhao et al. [14]. Low-frequency internal friction measurements were carried out by the free decay method with an inverted torsion pendulum at about 2 Hz, at a heating rate of 1 K/min and an amplitude of 20 10 6. The dimensions of the samples are 1 3 60 mm3.With increasing temperature, the logarithmic decrement (δ) was measured. The as-received data were fitted for quantitative analysis. The fitting process of temperature-dependent internal friction Q 1 ðTÞ is as follows [15]: Z 1 1 Q 1 ðTÞ ¼ pffiffiffiffi∑ Δi expð ω2 Þsechðxi þ βi ωÞ dω þ Q B 1 ðTÞ 2 π i 1
Fig. 1. (Color online) Internal friction spectra of specimens in as-WQ state and followed by aging at different temperature.
ð1Þ where β is the relative peak width, ω is the angular frequency of the stress sequence, xi ¼ lnð2π f τmi Þ, f is the frequency, τmi is the relaxation time of a relaxation process. The background internal friction Q B 1 ðTÞ was according to the following equation: Q B 1 ðTÞ ¼ a þ bexpðc=TÞ
ð2Þ
in which a, b and c are constants. The microstructure of the specimens was studied by transmission electron microscopy (JEM-2100F TEM). The thermoelectric power (TEP) test was carried out using the LSR-3 SEEBECK measurement of LINSEIS. The specimen for the TEP test is rectangular with dimensions of 2 4 20 mm3. The actual temperature gradient between two detected points is about 8 K.
3. Results and discussion The temperature-dependent low-frequency internal frictions Q 1 of the Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel in various aged states during heating from 300 K to 573 K were characterized, as shown in Fig. 1. Two peaks, marked as P1 and P2, were observed only in the internal friction spectrum of the as-WQ sample. P1 is the so-called LTS-SK peak. The nature of P2 was confirmed as a frequency-dependent anelastic S-K-like peak in a previous paper [13]. After the steel was aged at 373 K for 1 h, the intensity of P2 peak was slightly enhanced and the peak temperature remained stable. As the steel was aged at higher temperatures of 473 K and 573 K, the height of P2 peak decreased as well as the peak also moved to higher temperatures. As the height of the S-K peak is confirmed proportional to the dislocation density [6], its decreasing implies that the dislocation density reduced after aging at 473 K and 573 K. Meanwhile, the peak temperature moved to higher temperatures, indicating that the precipitated carbides seem to form in the aging process and pin the dislocations. The significant change is that the P1 peak was found to disappear after being aged at 373 K for 1 h. Therefore, it can be inferred that precipitation transformation and the reduction of dislocation density occurred as aged at temperature beyond 373 K, before which a certain structural change associated with the disappearance of P1 internal friction peak must have had completed. The microstructures of the Fe–0.39C–9.8Ni–1.56Si–2.0Mn specimens were observed by TEM to further certify the relation between the mechanical spectrum and microstructures. Results show that all samples consisted of dislocation-type lath martensite, twin-type martensite, blocky and film-like austenite. Fig. 2(a) and (b) shows the bright field and dark field micrographs of the as-WQ specimen, respectively. A typical microstructure of twin martensite was observed while no precipitate can be distinguished from the diffraction pattern. The microstructure of the specimen aged at 373 K for 1 h is shown in
Fig. 2(c) and (d). Again, no precipitate was observed both in austenite and in twin martensite. It indicates that carbides hardly formed during aging at 373 K, which is in agreement with the previous inference. Fig. 2(e) and (f) shows the microstructures of the specimen aged at 473 K for 30 min. It can be seen that the transitional carbide (ε) precipitated in martensite as aged at above 473 K, which is in accordance with the evolution of the S-K peak observed in Fig. 1. Microstructural analyses on the aged Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel further proved that the evolution of the LTS-SK peak occurred prior to the formation of ε carbide. These TEM results provide a direct evidence to verify that the origin of the LTS-SK peak cannot be caused by ε-carbide precipitation. In order to clarify the characteristics of the LTS-SK peak that evolved during low-temperature aging (below 373 K), mechanical spectra of the specimens aged at 373 K with different times are illustrated in Fig. 3(a). It was observed that the intensity of the LTSSK peak, marked as P1, decreased gradually with increasing aging time and disappeared at 60 min. Meanwhile, the intensity of the S-K peak, marked as P2, was found to increase with increasing aging time. The S-K peak is confirmed to relate to the interactions between the mobile foreign interstitial atoms and the dislocations stress field [5]. To our knowledge, the dislocation density in steels should not increase during aging. So, the promotion of the S-K internal friction peak would result from increasing of the interstitial atoms nearby the dislocations. The inset figure in Fig. 3 (a) shows the fitting curves of the corresponding mechanical spectra, which were obtained using Eq. (1). The heights of the LTS-SK peak and the S-K peak as a function of aging time at 373 K are further presented clearly. The evolution of the two types of IF peaks with aging time shows an opposite trend. As we know, the increasing intensity of the S-K peak results from the content of interstitial carbon atoms segregated at a neighboring area of dislocations [16]. Therefore, the fading of the LTS-SK peak with low-temperature aging would be associated with the depletion of interstitial carbon atoms during the segregation process. Fig. 3(b) shows the mechanical spectra of the Fe–0.39C–9.8Ni– 1.56Si–2.0Mn steels aged at room temperature for 10 h, 30 days and 1 year. With increasing aging time, the height of the P1 peak was also found to decrease. The fitting curve is shown in the inset of Fig. 3(b). The evolution of both the LTS-SK and S-K peaks during aging at room temperature is in agreement with the results observed in the specimens aged at 373 K. It is worth noting that the fitted peak temperature of the LTS-SK peak slightly increases with aging time. As we know, besides the occurrence of carbon segregation, a considerable isothermal martensite would form as the specimen was held at RT for a very long time. This additional structural change may cause the increase of the LTS-SK peak temperature.
X. Lu et al. / Solid State Communications 195 (2014) 31–34
33
Fig. 2. (Color online) TEM micrographs of (a) BF image of as-WQ specimen, (b) DF image of (a), showing the twin martensite and inserted SAD pattern, (c) BF image of the specimens aged at 373 K for 1 h, (d) DF image of (c) and inserted SAD pattern of the twin martensite, (e) BF image of the specimens aged at 473 K for 30 min, (f) DF image of (e), showing the ε carbide and inserted SAD pattern of the ε carbide and martensite.
Fig. 3. Internal friction spectra of as-WQ and various aged specimens at temperature (a) 373 K and (b) room temperature, measured in inverted torsion pendulum. The inset shows the fitting results of the corresponding internal friction spectra.
After low-temperature aging (RT and 373 K), the height of the LTS-SK peak will decrease gradually and even disappear. The decrease of the LTS-SK peak is inferred to be associated with the
carbon atom segregation. The thermoelectric power (TEP) is considered very sensitive to the microstructural evolution in steel, especially to the carbon content in solid solution [17]. So a TEP
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steels with low carbon content, concentration of carbon in solid solution can be estimated from a measurement of the height of the Snoek peak, with a proportional relationship. However, a Snoek peak is neither expected nor observed in medium or high carbon steels with tetragonal martensite, based on the calculations by Johnson [22]. Fortunately, the evolution of the LTS-SK peak is proved associated with the change of the interstitial carbon atoms. It indicates that this IF peak would be a useful simple way to predict the evolution of the carbon content in solid solution state, such as the carbon atoms segregation or cluster formation in high carbon or medium carbon high alloyed steels.
4. Conclusions Fig. 4. (Color online) TEP of WQ specimens after aging at 373 K for different times, measured at room temperature.
measurement was used to verify the process of carbon atom segregation in the Fe–0.39C–1.56Si–2Mn–9.84Ni steel during aging at 373 K. Fig. 4 shows the TEP value (ΔS) of the specimens as a function of aging time. It was observed that with increasing aging time, ΔS increases dramatically in the first stage and then becomes stable after being aged for about 60 min. The result indicates that the carbon content in solid solution decreased during aging at 373 K in the first 60 min [17,18]. It further confirmed that during aging at 373 K, the solid-dissolved carbon atoms will segregate to the nearby defects. Evolution of the S-K peak and TEP results illustrated that the only microstructural evolution involved in low-temperature aging (below 373 K) is the carbon atom segregation, which leads to the decrease of the LTS-SK peak. As mentioned before, the LTS-SK peak was only observed in high carbon or medium carbon high alloyed steels, in which quite a portion of the martensite is twin type [8,11,12]. In this paper, the present results further show that the LTS-SK peak was only observed in the specimens with twin-type martensite and a considerable content of carbon atoms was in solid solution. It can be reasonably deduced that the LTS-SK internal friction peak might be triggered by the interaction of twin boundary with the carbon atoms in solid solution. There have been few previous literature on an internal friction peak related to the interaction of twin with the interstitial atoms in twin-type martensite steel [11]. However, two internal friction peaks associated with the interaction between the H atoms and twin martensite in Ti–Ni-based shape memory alloys were reported recently. One is called “broad peak” that located at around 250 K, which was considered caused by a hydrogen dragging effect on the twin motion. [19,20] The other one is called “150 K peak”, which shows a non-thermally activated nature [21]. The “150 K” peak was considered presumably caused by hysteretic depinning processes of dislocation or twin boundaries from the weak pinning point (H atoms). Based on the similar mechanism, it could be inferred that the LTS-SK peak may originate from the interaction of carbon atoms with the twin boundaries in twin martensite under cyclic applied stress. The mechanism of this relaxation peak needs further study. For the increasingly sophisticated microstructure controlling in AHSS, it is crucial to understand the interaction of interstitial atoms with the substructure. Internal friction measurement is certainly a unique and effective technique to characterize the movement of the interstitial carbon atoms in martensite. For the
In summary, the mechanical spectra of the Fe–0.39C–1.56Si– 2Mn–9.84Ni steel in various aged states were investigated. Results show that the evolution of the LTS-SK internal friction peak occurred as aged at temperatures not higher than 373 K, which is prior to the carbide precipitation. The corresponding evolutions of the S-K peak and TEP illustrate that the solid dissolved carbon content decreased due to carbon atoms segregating to the surrounding dislocations when aged below 373 K. The origin of the internal friction peak located at the low-temperature shoulder of the S-K peak is likely attributed to the interaction between the carbon atoms and twin boundaries in martensite.
Acknowledgments The authors are grateful for partial financial support from the National Natural Science Foundation of China (51174251), the National Basic Research Program of China (Nos. 2011CB706604 and 2010CB630803). They would also like to thank Prof. L. Magalas for useful discussion. References [1] M. Gouné, F. Danoix, S. Allain, O. Bouaziz, Scr. Mater. 68 (2013) 1004–1007. [2] K. Taylor, G. Olson, M. Cohen, J. Sande, Metall. Mater. Trans. A 20 (1989) 2749–2765. [3] P. Chen, P. Winchell, Metall. Mater. Trans. A 11 (1980) 1333–1339. [4] Y. Liu, Acta Metall. 41 (1993) 3277–3287. [5] L. Magalas, Acta Metall. Sin. 39 (2003) 1145–1152. [6] L. Magalas, Le J. Phys. IV 6 (1996) 163–172. [7] R. Bagramov, D. Mari, W. Benoit, Philos. Mag. A 81 (2001) 2797–2808. [8] R. Martin, I. Tkalcec, D. Mari, R. Schaller, Philos. Mag. 88 (2008) 2907–2920. [9] J. Hoyos, A. Ghilarducci, H. Salva, J. VŽlez, Proc. Mater. Sci. 1 (2012) 185–190. [10] T.S. Ke, M. Yingliang, Acta Phys. Sin. 11 (1955) 479–492. [11] G. Klems, R. Miner, F. Hultgren, R. Gibala, Metall. Mater. Trans. A 7 (1976) 839–849. [12] S. Li, L. Deng, X. Wu, H. Wang, Y. Min, Mater. Sci. Eng., A 527 (2010) 6899–6903. [13] X. Lu, H. Liu, X. Jin, J. Alloys Compd. 577S (2013) s72–s75. [14] L. Zhao, N. Van Dijk, E. Brück, J. Sietsma, S. Van der Zwaag, Mater. Sci. Eng., A 313 (2001) 145–152. [15] Y. Lixi, F. Qianfeng, Acta Metall. Sin. 34 (1998) 1016–1020. [16] S. Li, N. Min, J. Li, X. Wu, C. Li, L. Tang, Mater. Sci. Eng., A 575 (2013) 51–60. [17] D. Benkirat, P. Merle, R. Borrelly, Acta Metall. 36 (1988) 613–620. [18] V. Massardier, N. Lavaire, M. Soler, J. Merlin, Scr. Mater. 50 (2004) 1435–1439. [19] G. Fan, Y. Zhou, K. Otsuka, X. Ren, K. Nakamura, T. Ohba, T. Suzuki, I. Yoshida, F. Yin, Acta Mater. 54 (2006) 5221–5229. [20] G.L. Fan, Z.Q. Li, D. Zhang, X.B. Ren, Mater. Sci. Forum 687 (2011) 533–538. [21] F. Mazzolai, A. Biscarini, B. Coluzzi, G. Mazzolai, E. Villa, A. Tuissi, Acta Mater. 55 (2007) 4243–4252. [22] R. Johnson, Acta Metall 13 (1965) 1259–1262.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Origin of low-temperature shoulder internal friction peak of Snoek-Köster peak in a medium carbon high alloyed steel Xianwen Lu a, Mingjiang Jin a,n, Hongshan Zhao a, Wei Li a, Xuejun Jin b,n a Institute of Advanced Steel and Materials, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China b State Key Lab of Metal Matrix Composite, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2014 Received in revised form 17 June 2014 Accepted 23 June 2014 by X.C. Shen Available online 2 July 2014
A distinct internal friction peak located at the low-temperature shoulder of Snoek-Köster peak (LTS-SK) was found in Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel and its evolution with respect to various aging treatments was investigated. The LTS-SK internal friction peak was found to occur when aged below 373 K. TEM observation confirmed that the ε-carbide precipitated beyond 373 K, providing an evidence that the LTS-SK peak cannot be caused by ε-carbide precipitation. The corresponding evolution on the S-K peak and thermoelectric power (TEP) illustrated that the carbon content in the solid solution decreases due to carbon atoms segregation on the surrounding dislocations during low-temperature aging. The origin of the LTS-SK peak is likely attributed to the interaction between the carbon atoms and twin boundaries in martensite. & 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Medium carbon high alloyed steel D. Carbon atom segregation E. Internal friction
1. Introduction Migration of carbon atoms in steels, such as segregation on dislocations, carbide precipitation from matrix and partitioning from supersaturated martensite to austenite, plays an important role in securing the feasible microstructures for the expected mechanical properties. The characterization of carbon migration is one of the key issues in steels research. Many analyzing techniques, such as atom probe tomography (APT), transmission electron microscope (TEM), X-ray diffraction (XRD) and internal friction (IF), were introduced for such analyses [1–4]. Among them, IF measurement with high sensitivity is considered a unique method available for determining the interstitial or dissolved carbon atoms in bcc structural phases in steels, such as ferrite and martensite. Mechanical spectroscopy of steels between RT and 600 K has been frequently reported in previous works [5,6]. Two typical relaxation-type internal friction peaks, one called Snoek peak and the other Snoek-Köster peak (S-K peak), were systematically studied. The intensity of the Snoek peak reflects the quantity of the interstitial atoms in the bcc phase. The S-K peak is believed to relate to the interactions between the mobile foreign interstitial
n
Corresponding authors. Tel./fax: þ86 21 54745560. E-mail addresses: [email protected] (X. Lu), [email protected] (M. Jin), [email protected] (H. Zhao), [email protected] (W. Li), [email protected] (X. Jin). http://dx.doi.org/10.1016/j.ssc.2014.06.018 0038-1098/& 2014 Elsevier Ltd. All rights reserved.
atoms and the dislocations stress field [5]. Meanwhile, an exotic internal friction peak, located at the low-temperature shoulder of the S-K peak (LTS-SK), has been observed in some high carbon or medium carbon high alloyed steels [7–12]. The origin of this peak was usually interpreted as: (1) formation of transition carbides results in reducing of the interstitial carbon content and pinning of the mobile dislocations, and then leading to a decrease of the rising IF background [7–9]; (2) stress-induced movement of coherent interfaces between ε-carbides and matrix [10]; (3) stress-induced atomic movement at twin interfaces [11]. In summary, most of the explanations figured it as a precipitation-related internal friction peak, but there is lack of clear evidence. Recently, an LTS-SK peak was also observed in the mechanical spectra of a Q&P model steel (Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel), which is intentionally designed for the study of kinetics of carbon partitioning with a considerable lower martensitic transformation temperature [13]. In this paper, the origin of the LTS-SK internal friction peak is proposed through the evolutionary trends of the S-K internal friction peak, thermoelectric power measurement and microstructure analysis.
2. Experimental procedures The chemical composition of the steels used in the present study is Fe–0.39C–9.8Ni–1.56Si–2.0Mn (wt%) with MS temperature of 403 K. An ingot was hot-rolled to a plate with a thickness of
32
X. Lu et al. / Solid State Communications 195 (2014) 31–34
20 mm after annealing at 1473 K for 3 h. Then the specimens were austenited at 1003 K for 30 min followed by water quenching (298 K), noted as as-WQ. The as-WQ specimen contains about 70 vol% martensite and 30 vol.% austenite measured by magnetization methods proposed by Zhao et al. [14]. Low-frequency internal friction measurements were carried out by the free decay method with an inverted torsion pendulum at about 2 Hz, at a heating rate of 1 K/min and an amplitude of 20 10 6. The dimensions of the samples are 1 3 60 mm3.With increasing temperature, the logarithmic decrement (δ) was measured. The as-received data were fitted for quantitative analysis. The fitting process of temperature-dependent internal friction Q 1 ðTÞ is as follows [15]: Z 1 1 Q 1 ðTÞ ¼ pffiffiffiffi∑ Δi expð ω2 Þsechðxi þ βi ωÞ dω þ Q B 1 ðTÞ 2 π i 1
Fig. 1. (Color online) Internal friction spectra of specimens in as-WQ state and followed by aging at different temperature.
ð1Þ where β is the relative peak width, ω is the angular frequency of the stress sequence, xi ¼ lnð2π f τmi Þ, f is the frequency, τmi is the relaxation time of a relaxation process. The background internal friction Q B 1 ðTÞ was according to the following equation: Q B 1 ðTÞ ¼ a þ bexpðc=TÞ
ð2Þ
in which a, b and c are constants. The microstructure of the specimens was studied by transmission electron microscopy (JEM-2100F TEM). The thermoelectric power (TEP) test was carried out using the LSR-3 SEEBECK measurement of LINSEIS. The specimen for the TEP test is rectangular with dimensions of 2 4 20 mm3. The actual temperature gradient between two detected points is about 8 K.
3. Results and discussion The temperature-dependent low-frequency internal frictions Q 1 of the Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel in various aged states during heating from 300 K to 573 K were characterized, as shown in Fig. 1. Two peaks, marked as P1 and P2, were observed only in the internal friction spectrum of the as-WQ sample. P1 is the so-called LTS-SK peak. The nature of P2 was confirmed as a frequency-dependent anelastic S-K-like peak in a previous paper [13]. After the steel was aged at 373 K for 1 h, the intensity of P2 peak was slightly enhanced and the peak temperature remained stable. As the steel was aged at higher temperatures of 473 K and 573 K, the height of P2 peak decreased as well as the peak also moved to higher temperatures. As the height of the S-K peak is confirmed proportional to the dislocation density [6], its decreasing implies that the dislocation density reduced after aging at 473 K and 573 K. Meanwhile, the peak temperature moved to higher temperatures, indicating that the precipitated carbides seem to form in the aging process and pin the dislocations. The significant change is that the P1 peak was found to disappear after being aged at 373 K for 1 h. Therefore, it can be inferred that precipitation transformation and the reduction of dislocation density occurred as aged at temperature beyond 373 K, before which a certain structural change associated with the disappearance of P1 internal friction peak must have had completed. The microstructures of the Fe–0.39C–9.8Ni–1.56Si–2.0Mn specimens were observed by TEM to further certify the relation between the mechanical spectrum and microstructures. Results show that all samples consisted of dislocation-type lath martensite, twin-type martensite, blocky and film-like austenite. Fig. 2(a) and (b) shows the bright field and dark field micrographs of the as-WQ specimen, respectively. A typical microstructure of twin martensite was observed while no precipitate can be distinguished from the diffraction pattern. The microstructure of the specimen aged at 373 K for 1 h is shown in
Fig. 2(c) and (d). Again, no precipitate was observed both in austenite and in twin martensite. It indicates that carbides hardly formed during aging at 373 K, which is in agreement with the previous inference. Fig. 2(e) and (f) shows the microstructures of the specimen aged at 473 K for 30 min. It can be seen that the transitional carbide (ε) precipitated in martensite as aged at above 473 K, which is in accordance with the evolution of the S-K peak observed in Fig. 1. Microstructural analyses on the aged Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel further proved that the evolution of the LTS-SK peak occurred prior to the formation of ε carbide. These TEM results provide a direct evidence to verify that the origin of the LTS-SK peak cannot be caused by ε-carbide precipitation. In order to clarify the characteristics of the LTS-SK peak that evolved during low-temperature aging (below 373 K), mechanical spectra of the specimens aged at 373 K with different times are illustrated in Fig. 3(a). It was observed that the intensity of the LTSSK peak, marked as P1, decreased gradually with increasing aging time and disappeared at 60 min. Meanwhile, the intensity of the S-K peak, marked as P2, was found to increase with increasing aging time. The S-K peak is confirmed to relate to the interactions between the mobile foreign interstitial atoms and the dislocations stress field [5]. To our knowledge, the dislocation density in steels should not increase during aging. So, the promotion of the S-K internal friction peak would result from increasing of the interstitial atoms nearby the dislocations. The inset figure in Fig. 3 (a) shows the fitting curves of the corresponding mechanical spectra, which were obtained using Eq. (1). The heights of the LTS-SK peak and the S-K peak as a function of aging time at 373 K are further presented clearly. The evolution of the two types of IF peaks with aging time shows an opposite trend. As we know, the increasing intensity of the S-K peak results from the content of interstitial carbon atoms segregated at a neighboring area of dislocations [16]. Therefore, the fading of the LTS-SK peak with low-temperature aging would be associated with the depletion of interstitial carbon atoms during the segregation process. Fig. 3(b) shows the mechanical spectra of the Fe–0.39C–9.8Ni– 1.56Si–2.0Mn steels aged at room temperature for 10 h, 30 days and 1 year. With increasing aging time, the height of the P1 peak was also found to decrease. The fitting curve is shown in the inset of Fig. 3(b). The evolution of both the LTS-SK and S-K peaks during aging at room temperature is in agreement with the results observed in the specimens aged at 373 K. It is worth noting that the fitted peak temperature of the LTS-SK peak slightly increases with aging time. As we know, besides the occurrence of carbon segregation, a considerable isothermal martensite would form as the specimen was held at RT for a very long time. This additional structural change may cause the increase of the LTS-SK peak temperature.
X. Lu et al. / Solid State Communications 195 (2014) 31–34
33
Fig. 2. (Color online) TEM micrographs of (a) BF image of as-WQ specimen, (b) DF image of (a), showing the twin martensite and inserted SAD pattern, (c) BF image of the specimens aged at 373 K for 1 h, (d) DF image of (c) and inserted SAD pattern of the twin martensite, (e) BF image of the specimens aged at 473 K for 30 min, (f) DF image of (e), showing the ε carbide and inserted SAD pattern of the ε carbide and martensite.
Fig. 3. Internal friction spectra of as-WQ and various aged specimens at temperature (a) 373 K and (b) room temperature, measured in inverted torsion pendulum. The inset shows the fitting results of the corresponding internal friction spectra.
After low-temperature aging (RT and 373 K), the height of the LTS-SK peak will decrease gradually and even disappear. The decrease of the LTS-SK peak is inferred to be associated with the
carbon atom segregation. The thermoelectric power (TEP) is considered very sensitive to the microstructural evolution in steel, especially to the carbon content in solid solution [17]. So a TEP
34
X. Lu et al. / Solid State Communications 195 (2014) 31–34
steels with low carbon content, concentration of carbon in solid solution can be estimated from a measurement of the height of the Snoek peak, with a proportional relationship. However, a Snoek peak is neither expected nor observed in medium or high carbon steels with tetragonal martensite, based on the calculations by Johnson [22]. Fortunately, the evolution of the LTS-SK peak is proved associated with the change of the interstitial carbon atoms. It indicates that this IF peak would be a useful simple way to predict the evolution of the carbon content in solid solution state, such as the carbon atoms segregation or cluster formation in high carbon or medium carbon high alloyed steels.
4. Conclusions Fig. 4. (Color online) TEP of WQ specimens after aging at 373 K for different times, measured at room temperature.
measurement was used to verify the process of carbon atom segregation in the Fe–0.39C–1.56Si–2Mn–9.84Ni steel during aging at 373 K. Fig. 4 shows the TEP value (ΔS) of the specimens as a function of aging time. It was observed that with increasing aging time, ΔS increases dramatically in the first stage and then becomes stable after being aged for about 60 min. The result indicates that the carbon content in solid solution decreased during aging at 373 K in the first 60 min [17,18]. It further confirmed that during aging at 373 K, the solid-dissolved carbon atoms will segregate to the nearby defects. Evolution of the S-K peak and TEP results illustrated that the only microstructural evolution involved in low-temperature aging (below 373 K) is the carbon atom segregation, which leads to the decrease of the LTS-SK peak. As mentioned before, the LTS-SK peak was only observed in high carbon or medium carbon high alloyed steels, in which quite a portion of the martensite is twin type [8,11,12]. In this paper, the present results further show that the LTS-SK peak was only observed in the specimens with twin-type martensite and a considerable content of carbon atoms was in solid solution. It can be reasonably deduced that the LTS-SK internal friction peak might be triggered by the interaction of twin boundary with the carbon atoms in solid solution. There have been few previous literature on an internal friction peak related to the interaction of twin with the interstitial atoms in twin-type martensite steel [11]. However, two internal friction peaks associated with the interaction between the H atoms and twin martensite in Ti–Ni-based shape memory alloys were reported recently. One is called “broad peak” that located at around 250 K, which was considered caused by a hydrogen dragging effect on the twin motion. [19,20] The other one is called “150 K peak”, which shows a non-thermally activated nature [21]. The “150 K” peak was considered presumably caused by hysteretic depinning processes of dislocation or twin boundaries from the weak pinning point (H atoms). Based on the similar mechanism, it could be inferred that the LTS-SK peak may originate from the interaction of carbon atoms with the twin boundaries in twin martensite under cyclic applied stress. The mechanism of this relaxation peak needs further study. For the increasingly sophisticated microstructure controlling in AHSS, it is crucial to understand the interaction of interstitial atoms with the substructure. Internal friction measurement is certainly a unique and effective technique to characterize the movement of the interstitial carbon atoms in martensite. For the
In summary, the mechanical spectra of the Fe–0.39C–1.56Si– 2Mn–9.84Ni steel in various aged states were investigated. Results show that the evolution of the LTS-SK internal friction peak occurred as aged at temperatures not higher than 373 K, which is prior to the carbide precipitation. The corresponding evolutions of the S-K peak and TEP illustrate that the solid dissolved carbon content decreased due to carbon atoms segregating to the surrounding dislocations when aged below 373 K. The origin of the internal friction peak located at the low-temperature shoulder of the S-K peak is likely attributed to the interaction between the carbon atoms and twin boundaries in martensite.
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