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Relationship between grain boundary relaxation strengthening and orientation in electrodeposited bulk nanocrystalline Ni alloys
Materials Letters 205 (2017) 211–214
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Relationship between grain boundary relaxation strengthening and orientation in electrodeposited bulk nanocrystalline Ni alloys Isao Matsui a,⇑, Mizuki Kanetake b, Hiroki Mori b, Yorinobu Takigawa b, Kenji Higashi b a b
Structural Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan Department of Materials Science, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan
a r t i c l e
i n f o
Article history: Received 29 May 2017 Received in revised form 18 June 2017 Accepted 19 June 2017 Available online 21 June 2017 Keywords: Electrodeposition Nanocrystalline materials Grain boundaries
a b s t r a c t Many studies have been performed to better understand the Hall–Petch effect at the nanometer scale. Hardening can be caused not only by a reduction of the grain size, but also through the relaxation of the nonequilibrium grain boundary structure. Despite considerable effort, there is still a large discrepancy among the available data for the strength values of nanocrystalline metals because of the difficulty in quantitatively evaluating the state of grain boundary relaxation. In this study, we used electrodeposited bulk nanocrystalline Ni–Fe and Ni–W alloys to develop a better predictive method of the grain boundary relaxation behavior. Relatively low-temperature thermal treatment resulted in grain boundary relaxation and thus increased the hardness by 0.07–0.74 GPa. We found that the increase in hardness decreased with increasing orientation index for the (2 0 0) plane. Specifically, we concluded that electrodeposited Ni alloys with an orientation index for the (2 0 0) plane greater than 3.0 do not exhibit grain boundary relaxation strengthening, because these alloys do not have a nonequilibrium grain boundary structure even in the as-deposited state. The relationship also enables the prediction of the grain boundary relaxation state of electrodeposited bulk nanocrystalline Ni alloys. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction When the grain size is reduced below 100 nm, grain boundaries begin to account for a large volume fraction of the crystal structure of a material [1]. In such nanocrystalline materials, the grain boundary structure has a key determining role in the mechanical properties in addition to the grain size. Much attention has been focused on studying the atomic structure of grain boundaries in these materials. Several studies [2–4] have reported that nanocrystalline metals often contain nonequilibrium grain boundaries with excess dislocations, misfit regions, or excess free volume in their as-prepared states. Low-temperature annealing has been shown to release excess defects without any measurable change in grain size or texture; this is termed grain boundary relaxation [2]. Grain boundary relaxation has been reported to increase the hardness [5] and tensile strength [6], which is thought to be the result of a reduction in the number of dislocation sources [5]. In recent studies using electrodeposited nanocrystalline Ni and Ni alloys as a model system, the effect of grain size on the strength has been investigated [7,8]. Unfortunately, these studies were generally unable to separate the effects of the state of grain boundary ⇑ Corresponding author. E-mail address: [email protected] (I. Matsui). http://dx.doi.org/10.1016/j.matlet.2017.06.094 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
relaxation and the grain size. This is because it is practically difficult to quantitatively evaluate the grain boundary relaxation state. Thus, the development of a convenient method to predict the grain boundary relaxation state would provide a deeper understanding of the hardening effect in nanocrystalline metals. In this study, we examined the effect of grain boundary relaxation on the hardness of electrodeposited bulk nanocrystalline Ni–Fe and Ni–W alloys towards this goal. We prepared a set of 39 samples by electrodeposition. The hardness of these alloys was measured in the as-deposited state and after annealing to investigate the grain boundary relaxation strengthening behavior. We found that the grain boundary relaxation strengthening could be estimated on the basis of the orientation index of the (2 0 0) plane.
2. Experimental procedure A set of 39 bulk nanocrystalline Ni alloys was prepared. All alloys were synthesized using an electrodeposition technique that is described elsewhere [9–12]. The experimental setup for electrodeposition is described in Ref. [13]. The samples were deposited onto Cu substrates of commercial purity. Electrodeposition for Ni– Fe alloys was performed at a current density of 10–30 mA cm2, bath temperature of 50–55 °C, and pH of 2.0–2.2. Electrodeposition
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I. Matsui et al. / Materials Letters 205 (2017) 211–214
Fig. 1. Representative XRD patterns (a,b) and STEM images (c,d) of electrodeposited bulk nanocrystalline (a,c) Ni–Fe alloys and (b,d) Ni–W alloys.
for Ni–W alloys was performed at a current density of 30– 40 mA cm2, bath temperature of 50–60 °C, and pH of 4.0. The pH values of the solutions during electrodeposition were maintained by adding drops of 1.0 mol L1 sulfamic acid and 5.0 mol L1 sodium hydroxide. After electrodeposition, the following analyses were conducted. The Fe and W content of the electrodeposits was determined by energy-dispersive X-ray spectrometry (EDS) using a scanning electron microscope (HITACHI S-4800). To calculate the orientation index [14] and estimate the grain size, X-ray diffraction (XRD, RIGAKU Ultimate IV) was performed using Cu Ka radiation. Transmission electron microscopy (TEM) specimens were prepared by ion milling and were examined using a JEOL ARM-200FC (Cscorrected) transmission electron microscope, operated at 200 kV for microstructure observation. To evaluate the hardness of the electrodeposits, micro-Vickers hardness tests were conducted using a load of 500 g for 10 s. Each reported data point represents the average of at least 12 indentations. To demonstrate grain boundary relaxation strengthening, all electrodeposited samples
were annealed at 200 °C for 2 h, after which the hardness was measured again.
3. Results and discussion The electrodeposited Ni–Fe and Ni–W nanocrystalline alloys contained 45–60 at.% Fe and 2–5 at.% W, respectively. To estimate the grain size, we conducted XRD analysis. Representative XRD patterns of bulk nanocrystalline Ni–Fe and Ni–W alloys are shown in Fig. 1a and b, respectively. All patterns are indicative of a single face-centered cubic (fcc) structure. The grain sizes of each sample were estimated from the (111) diffraction peak width using the Scherrer equation. The grain sizes of the Ni–W and Ni–Fe alloys ranged 13–16 nm and 17–24 nm, respectively. We also confirmed the grain sizes by TEM observation; representative scanning transmission electron microscope (STEM) images of bulk nanocrystalline Ni–Fe and Ni–W alloys are shown in Fig. 1c and d, respectively. These images reveal that the nanocrystalline structure of
213
I. Matsui et al. / Materials Letters 205 (2017) 211–214 Table 1 Measurement values for bulk nanocrystalline Ni alloys in the as-deposited state and after annealing at 200 °C. Sample
Grain size (nm)
N200
Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W
15 14 15 16 15 15 14 13 13 15 14 14 13 15 15 24 22 21 20 20 21 20 19 20 20 19 19 18 19 17 18 18 18 19 18 19 18 19 20
1.27 1.64 0.70 0.63 0.59 0.61 0.70 0.70 0.76 1.19 0.54 0.75 1.00 0.67 0.65 2.59 2.24 1.84 1.58 1.39 2.08 1.74 1.66 2.33 1.21 1.87 1.46 0.82 1.39 0.85 1.15 0.85 1.47 0.79 1.31 2.25 1.90 0.96 1.77
Fig. 2. Change in the hardness after annealing at 200 °C as a function of the orientation index for the (200) plane for electrodeposited bulk nanocrystalline Ni– W and Ni–Fe alloys, along with the linear regression line.
the Ni–Fe and Ni–W alloys mainly consisted of grains around 15 and 30 nm in size, respectively. In both alloys, the grain sizes observed in the STEM images are comparable to the sizes esti-
Hardness (GPa) As-deposited state
After annealing
4.46 4.19 4.29 4.36 4.27 4.20 4.14 4.00 4.03 4.14 4.23 4.66 4.96 4.21 4.27 4.14 4.52 4.89 4.93 5.08 4.81 5.08 5.19 4.94 5.29 5.11 5.11 5.51 5.21 5.45 5.50 5.43 5.45 5.41 5.36 5.08 5.17 5.00 4.92
4.75 4.40 4.80 4.79 4.81 4.72 4.65 4.53 4.55 4.67 4.96 5.10 5.25 4.72 4.76 4.21 4.71 5.15 5.22 5.39 5.10 5.35 5.54 5.20 5.64 5.39 5.77 5.98 5.71 5.88 6.04 5.77 5.85 5.99 5.79 5.30 5.49 5.28 5.12
mated from the XRD peak widths. It is worth mentioning that grain-size measurements using X-ray line broadening are considered to be accurate for grain sizes less than approximately 30 nm [15,16]. The results of the hardness tests for the bulk nanocrystalline Ni alloys in the as-deposited state and after annealing at 200 °C for 2 h are summarized in Table 1. The hardness values of the Ni–Fe and Ni–W alloys in the as-deposited state were 4.00–4.96 GPa and 4.14–5.51 GPa, respectively. Annealing increased the hardness of the Ni–W and Ni–Fe alloys to 4.40–5.25 GPa and 4.21–6.04 GPa, respectively. Thus, the change in the hardness caused by annealing was 0.21–0.74 GPa (5.0%–17.4%) and 0.07–0.57 GPa (1.6%–10.6%) for the Ni–Fe and Ni–W alloys, respectively. Low temperature annealing also would promote segregation of light elements [17] or form precipitates [18]. On the other hand, in the case of electrodeposited nanocrystalline Ni and Ni alloys, the segregation of impurities, such as carbon and sulfur, hardly occurs at low annealing temperature at approximately 200 °C [5,6]. In addition, Renk et al. [19] conducted detailed study, based on the mechanical and microstructural characterization by atom probe tomography, to clarify the concern of whether solute segregation at the boundaries can account for the hardening behavior by annealing. Their results have revealed that the hardening phenomenon on nanocrystalline metals is apparently not related to solute segregation. Based on the above, the increase in the hardness of Ni alloys for this study was caused to the grain boundary relaxation. The variation in the grain boundary relaxation
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I. Matsui et al. / Materials Letters 205 (2017) 211–214
strengthening among samples is caused by the different relaxation states of the grain boundaries in the as-deposited state [11]. We found that the grain boundary relaxation strengthening could be predicted according to the orientation index of the (2 0 0) plane, N200. The change in the hardness after annealing at 200 °C is shown as a function of N200 for the Ni–Fe and Ni–W alloys in Fig. 2, in which the strengthening induced by grain boundary relaxation decreases with increasing N200. This trend demonstrates that the grain boundary relaxation strengthening can be estimated by simple XRD analysis. In electrodeposited Ni and its alloys, the orientation is strongly affected by the growth mode during electrodeposition [20,21]. A strong (2 0 0) orientation reflects a freelateral growth mode [20], for which there is less inhibition of growth by adsorbed hydrogen and gaseous hydrogen [21]. At a result, the formation of a nonequilibrium grain boundary structure would be suppressed. A linear fit was computed for all data in Fig. 2 using the least squares method. The fit is expressed as follows:
DHVð%Þ ¼ 4:769 N200 þ 14:317
ð1Þ
where DHV is the change in the hardness. According to Eq. (1), electrodeposited nanocrystalline Ni alloys with N200 greater than 3.0 do not exhibit grain boundary relaxation strengthening. On the basis of this estimation, alloys with N200 greater than 3.0 can be considered to contain no nonequilibrium grain boundary structures, even in the as-deposited state. 4. Conclusions We electrodeposited bulk nanocrystalline Ni–Fe and Ni–W alloys and examined the effects of low-temperature thermal treatment, which resulted in grain boundary relaxation, on the hardness. Annealing at 200 °C increased the hardness of all electrodeposited alloys (by 0.07–0.74 GPa) as a result of grain boundary relaxation. Thus, the as-deposited states of the alloys had different states of grain boundary relaxation, which can be attributed to differences in the electrodeposition conditions. We found that grain boundary relaxation strengthening was related to the orientation index of the (2 0 0) plane: strengthening decreased from 0.74 to 0.07 GPa with an increase in the orientation index of the (2 0 0) plane from 0.54 to 2.59. This trend allowed us to the conclude that electrodeposited Ni alloys with orientation index for the (2 0 0) plane of approximately 3.0 or higher do not contain
nonequilibrium grain boundary structures even in the asdeposited state. In addition, the results of this study indicate that the grain boundary relaxation state of electrodeposited Ni alloys can easily be estimated by XRD analysis. Acknowledgments A part of this study was also supported by NIMS microstructural characterization platform (NMCP) as a program of ‘‘Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors would like thank Prof. T. Yamamoto (Department of Quantum Engineering, Nagoya University) in NMCP for his TEM observations. References [1] G. Palumbo, S.J. Thorpe, K.T. Aust, Scr. Metall. Mater. 24 (1990) 1347–1350. [2] D. Jang, M. Atzmon, J. Appl. Phys. 99 (2006) 083504. [3] S. Ranganathan, R. Divakar, V.S. Raghunathan, Scripta Mater. 44 (2001) 1169– 1174. [4] X.L. Wu, Y.T. Zhu, Appl. Phys. Lett. 89 (2006) 031922. [5] T.J. Rupert, J.R. Trelewicz, C.A. Schuh, J. Mater. Res. 27 (2012) 1285–1294. [6] Y.M. Wang, S. Cheng, Q.M. Wei, E. Ma, T.G. Nieh, A. Hamza, Scripta Mater. 51 (2004) 1023–1028. [7] I. Matsui, T. Uesugi, Y. Takigawa, K. Higashi, Acta Mater. 61 (2013) 3360–3369. [8] C.A. Schuh, T.G. Nieh, H. Iwasaki, Acta Mater. 51 (2003) 431–443. [9] I. Matsui, T. Kawakatsu, Y. Takigawa, T. Uesugi, K. Higashi, Mater. Lett. 116 (2014) 71–74. [10] H. Mori, I. Matsui, Y. Takigawa, T. Uesugi, K. Higashi, Mater. Lett. 175 (2016) 86–88. [11] I. Matsui, H. Mori, T. Kawakatsu, Y. Takigawa, T. Uesugi, K. Higashi, Mater. Sci. Eng. A 607 (2014) 505–510. [12] I. Matsui, Y. Takigawa, T. Uesugi, K. Higashi, Mater. Sci. Eng. A 578 (2013) 318– 322. [13] I. Matsui, Mater. Jpn. 55 (2016) 166–170. [14] J.D. Giallonardo, U. Erb, K.T. Aust, G. Palumbo, Philos. Mag. 91 (2011) 4594– 4605. [15] C.C. Koch, R.O. Scattergood, M. Saber, H. Kotan, J. Mater. Res. 28 (2013) 1785– 1791. [16] K. Tanaka, Y. Koike, K. Sano, H. Tanaka, S. Machiya, T. Shobu, H. Kimachi, J. Soc. Mater. Sci. Jpn. 64 (2015) 528–535. [17] B.B. Straumal, S.V. Dobatkin, A.O. Rodin, S.G. Protasova, A.A. Mazilkin, D. Goll, B. Baretzky, Adv. Eng. Mater. 13 (2011) 463–469. [18] B. Straumal, R. Valiev, O. Kogtenkova, P. Zieba, T. Czeppe, E. Bielanska, M. Faryna, Acta Mater. 56 (2008) 6123–6131. [19] O. Renk, A. Hohenwarter, K. Eder, K.S. Kormout, J.M. Cairney, R. Pippan, Scripta Mater. 95 (2015) 27–30. [20] J. Amblard, I. Epelboin, M. Froment, G. Maurin, J. Appl. Electrochem. 9 (1979) 233–242. [21] I. Matsui, Y. Takigawa, D. Yokoe, T. Kato, T. Uesugi, K. Higashi, Mater. Trans. 55 (2014) 1859–1866.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Relationship between grain boundary relaxation strengthening and orientation in electrodeposited bulk nanocrystalline Ni alloys Isao Matsui a,⇑, Mizuki Kanetake b, Hiroki Mori b, Yorinobu Takigawa b, Kenji Higashi b a b
Structural Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan Department of Materials Science, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan
a r t i c l e
i n f o
Article history: Received 29 May 2017 Received in revised form 18 June 2017 Accepted 19 June 2017 Available online 21 June 2017 Keywords: Electrodeposition Nanocrystalline materials Grain boundaries
a b s t r a c t Many studies have been performed to better understand the Hall–Petch effect at the nanometer scale. Hardening can be caused not only by a reduction of the grain size, but also through the relaxation of the nonequilibrium grain boundary structure. Despite considerable effort, there is still a large discrepancy among the available data for the strength values of nanocrystalline metals because of the difficulty in quantitatively evaluating the state of grain boundary relaxation. In this study, we used electrodeposited bulk nanocrystalline Ni–Fe and Ni–W alloys to develop a better predictive method of the grain boundary relaxation behavior. Relatively low-temperature thermal treatment resulted in grain boundary relaxation and thus increased the hardness by 0.07–0.74 GPa. We found that the increase in hardness decreased with increasing orientation index for the (2 0 0) plane. Specifically, we concluded that electrodeposited Ni alloys with an orientation index for the (2 0 0) plane greater than 3.0 do not exhibit grain boundary relaxation strengthening, because these alloys do not have a nonequilibrium grain boundary structure even in the as-deposited state. The relationship also enables the prediction of the grain boundary relaxation state of electrodeposited bulk nanocrystalline Ni alloys. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction When the grain size is reduced below 100 nm, grain boundaries begin to account for a large volume fraction of the crystal structure of a material [1]. In such nanocrystalline materials, the grain boundary structure has a key determining role in the mechanical properties in addition to the grain size. Much attention has been focused on studying the atomic structure of grain boundaries in these materials. Several studies [2–4] have reported that nanocrystalline metals often contain nonequilibrium grain boundaries with excess dislocations, misfit regions, or excess free volume in their as-prepared states. Low-temperature annealing has been shown to release excess defects without any measurable change in grain size or texture; this is termed grain boundary relaxation [2]. Grain boundary relaxation has been reported to increase the hardness [5] and tensile strength [6], which is thought to be the result of a reduction in the number of dislocation sources [5]. In recent studies using electrodeposited nanocrystalline Ni and Ni alloys as a model system, the effect of grain size on the strength has been investigated [7,8]. Unfortunately, these studies were generally unable to separate the effects of the state of grain boundary ⇑ Corresponding author. E-mail address: [email protected] (I. Matsui). http://dx.doi.org/10.1016/j.matlet.2017.06.094 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
relaxation and the grain size. This is because it is practically difficult to quantitatively evaluate the grain boundary relaxation state. Thus, the development of a convenient method to predict the grain boundary relaxation state would provide a deeper understanding of the hardening effect in nanocrystalline metals. In this study, we examined the effect of grain boundary relaxation on the hardness of electrodeposited bulk nanocrystalline Ni–Fe and Ni–W alloys towards this goal. We prepared a set of 39 samples by electrodeposition. The hardness of these alloys was measured in the as-deposited state and after annealing to investigate the grain boundary relaxation strengthening behavior. We found that the grain boundary relaxation strengthening could be estimated on the basis of the orientation index of the (2 0 0) plane.
2. Experimental procedure A set of 39 bulk nanocrystalline Ni alloys was prepared. All alloys were synthesized using an electrodeposition technique that is described elsewhere [9–12]. The experimental setup for electrodeposition is described in Ref. [13]. The samples were deposited onto Cu substrates of commercial purity. Electrodeposition for Ni– Fe alloys was performed at a current density of 10–30 mA cm2, bath temperature of 50–55 °C, and pH of 2.0–2.2. Electrodeposition
212
I. Matsui et al. / Materials Letters 205 (2017) 211–214
Fig. 1. Representative XRD patterns (a,b) and STEM images (c,d) of electrodeposited bulk nanocrystalline (a,c) Ni–Fe alloys and (b,d) Ni–W alloys.
for Ni–W alloys was performed at a current density of 30– 40 mA cm2, bath temperature of 50–60 °C, and pH of 4.0. The pH values of the solutions during electrodeposition were maintained by adding drops of 1.0 mol L1 sulfamic acid and 5.0 mol L1 sodium hydroxide. After electrodeposition, the following analyses were conducted. The Fe and W content of the electrodeposits was determined by energy-dispersive X-ray spectrometry (EDS) using a scanning electron microscope (HITACHI S-4800). To calculate the orientation index [14] and estimate the grain size, X-ray diffraction (XRD, RIGAKU Ultimate IV) was performed using Cu Ka radiation. Transmission electron microscopy (TEM) specimens were prepared by ion milling and were examined using a JEOL ARM-200FC (Cscorrected) transmission electron microscope, operated at 200 kV for microstructure observation. To evaluate the hardness of the electrodeposits, micro-Vickers hardness tests were conducted using a load of 500 g for 10 s. Each reported data point represents the average of at least 12 indentations. To demonstrate grain boundary relaxation strengthening, all electrodeposited samples
were annealed at 200 °C for 2 h, after which the hardness was measured again.
3. Results and discussion The electrodeposited Ni–Fe and Ni–W nanocrystalline alloys contained 45–60 at.% Fe and 2–5 at.% W, respectively. To estimate the grain size, we conducted XRD analysis. Representative XRD patterns of bulk nanocrystalline Ni–Fe and Ni–W alloys are shown in Fig. 1a and b, respectively. All patterns are indicative of a single face-centered cubic (fcc) structure. The grain sizes of each sample were estimated from the (111) diffraction peak width using the Scherrer equation. The grain sizes of the Ni–W and Ni–Fe alloys ranged 13–16 nm and 17–24 nm, respectively. We also confirmed the grain sizes by TEM observation; representative scanning transmission electron microscope (STEM) images of bulk nanocrystalline Ni–Fe and Ni–W alloys are shown in Fig. 1c and d, respectively. These images reveal that the nanocrystalline structure of
213
I. Matsui et al. / Materials Letters 205 (2017) 211–214 Table 1 Measurement values for bulk nanocrystalline Ni alloys in the as-deposited state and after annealing at 200 °C. Sample
Grain size (nm)
N200
Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–Fe Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W Ni–W
15 14 15 16 15 15 14 13 13 15 14 14 13 15 15 24 22 21 20 20 21 20 19 20 20 19 19 18 19 17 18 18 18 19 18 19 18 19 20
1.27 1.64 0.70 0.63 0.59 0.61 0.70 0.70 0.76 1.19 0.54 0.75 1.00 0.67 0.65 2.59 2.24 1.84 1.58 1.39 2.08 1.74 1.66 2.33 1.21 1.87 1.46 0.82 1.39 0.85 1.15 0.85 1.47 0.79 1.31 2.25 1.90 0.96 1.77
Fig. 2. Change in the hardness after annealing at 200 °C as a function of the orientation index for the (200) plane for electrodeposited bulk nanocrystalline Ni– W and Ni–Fe alloys, along with the linear regression line.
the Ni–Fe and Ni–W alloys mainly consisted of grains around 15 and 30 nm in size, respectively. In both alloys, the grain sizes observed in the STEM images are comparable to the sizes esti-
Hardness (GPa) As-deposited state
After annealing
4.46 4.19 4.29 4.36 4.27 4.20 4.14 4.00 4.03 4.14 4.23 4.66 4.96 4.21 4.27 4.14 4.52 4.89 4.93 5.08 4.81 5.08 5.19 4.94 5.29 5.11 5.11 5.51 5.21 5.45 5.50 5.43 5.45 5.41 5.36 5.08 5.17 5.00 4.92
4.75 4.40 4.80 4.79 4.81 4.72 4.65 4.53 4.55 4.67 4.96 5.10 5.25 4.72 4.76 4.21 4.71 5.15 5.22 5.39 5.10 5.35 5.54 5.20 5.64 5.39 5.77 5.98 5.71 5.88 6.04 5.77 5.85 5.99 5.79 5.30 5.49 5.28 5.12
mated from the XRD peak widths. It is worth mentioning that grain-size measurements using X-ray line broadening are considered to be accurate for grain sizes less than approximately 30 nm [15,16]. The results of the hardness tests for the bulk nanocrystalline Ni alloys in the as-deposited state and after annealing at 200 °C for 2 h are summarized in Table 1. The hardness values of the Ni–Fe and Ni–W alloys in the as-deposited state were 4.00–4.96 GPa and 4.14–5.51 GPa, respectively. Annealing increased the hardness of the Ni–W and Ni–Fe alloys to 4.40–5.25 GPa and 4.21–6.04 GPa, respectively. Thus, the change in the hardness caused by annealing was 0.21–0.74 GPa (5.0%–17.4%) and 0.07–0.57 GPa (1.6%–10.6%) for the Ni–Fe and Ni–W alloys, respectively. Low temperature annealing also would promote segregation of light elements [17] or form precipitates [18]. On the other hand, in the case of electrodeposited nanocrystalline Ni and Ni alloys, the segregation of impurities, such as carbon and sulfur, hardly occurs at low annealing temperature at approximately 200 °C [5,6]. In addition, Renk et al. [19] conducted detailed study, based on the mechanical and microstructural characterization by atom probe tomography, to clarify the concern of whether solute segregation at the boundaries can account for the hardening behavior by annealing. Their results have revealed that the hardening phenomenon on nanocrystalline metals is apparently not related to solute segregation. Based on the above, the increase in the hardness of Ni alloys for this study was caused to the grain boundary relaxation. The variation in the grain boundary relaxation
214
I. Matsui et al. / Materials Letters 205 (2017) 211–214
strengthening among samples is caused by the different relaxation states of the grain boundaries in the as-deposited state [11]. We found that the grain boundary relaxation strengthening could be predicted according to the orientation index of the (2 0 0) plane, N200. The change in the hardness after annealing at 200 °C is shown as a function of N200 for the Ni–Fe and Ni–W alloys in Fig. 2, in which the strengthening induced by grain boundary relaxation decreases with increasing N200. This trend demonstrates that the grain boundary relaxation strengthening can be estimated by simple XRD analysis. In electrodeposited Ni and its alloys, the orientation is strongly affected by the growth mode during electrodeposition [20,21]. A strong (2 0 0) orientation reflects a freelateral growth mode [20], for which there is less inhibition of growth by adsorbed hydrogen and gaseous hydrogen [21]. At a result, the formation of a nonequilibrium grain boundary structure would be suppressed. A linear fit was computed for all data in Fig. 2 using the least squares method. The fit is expressed as follows:
DHVð%Þ ¼ 4:769 N200 þ 14:317
ð1Þ
where DHV is the change in the hardness. According to Eq. (1), electrodeposited nanocrystalline Ni alloys with N200 greater than 3.0 do not exhibit grain boundary relaxation strengthening. On the basis of this estimation, alloys with N200 greater than 3.0 can be considered to contain no nonequilibrium grain boundary structures, even in the as-deposited state. 4. Conclusions We electrodeposited bulk nanocrystalline Ni–Fe and Ni–W alloys and examined the effects of low-temperature thermal treatment, which resulted in grain boundary relaxation, on the hardness. Annealing at 200 °C increased the hardness of all electrodeposited alloys (by 0.07–0.74 GPa) as a result of grain boundary relaxation. Thus, the as-deposited states of the alloys had different states of grain boundary relaxation, which can be attributed to differences in the electrodeposition conditions. We found that grain boundary relaxation strengthening was related to the orientation index of the (2 0 0) plane: strengthening decreased from 0.74 to 0.07 GPa with an increase in the orientation index of the (2 0 0) plane from 0.54 to 2.59. This trend allowed us to the conclude that electrodeposited Ni alloys with orientation index for the (2 0 0) plane of approximately 3.0 or higher do not contain
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