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Roles of grain boundary microstructure in high-cycle fatigue of electrodeposited nanocrystalline Ni–P alloy
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Scripta Materialia 61 (2009) 1032–1035 www.elsevier.com/locate/scriptamat
Roles of grain boundary microstructure in high-cycle fatigue of electrodeposited nanocrystalline Ni–P alloy Shigeaki Kobayashi,a,* Akiyuki Kamataa,1 and Tadao Watanabeb,2 a
Department of Mechanical Engineering, Faculty of Engineering, Ashikaga Institute of Technology, 268-1 Omae, Ashikaga, Tochigi 326-8558, Japan b Key Laboratory for Anisotropy and Texture of Materials, Northeastern University, Shenyang 110004, China Received 25 May 2009; revised 3 August 2009; accepted 14 August 2009 Available online 19 August 2009
The fatigue properties and the morphology of the specimen surface and fracture surface were investigated in electrodeposited nanocrystalline Ni–P alloy subjected to high-cycle fatigue. The nanocrystalline specimens, which have a {0 0 1} texture and high fractions of low-angle and R3 boundaries, show a higher fatigue limit than Ni polycrystals with conventional grain size. The change in specimen surface and fracture surface by cyclic deformation is discussed in connection with grain boundary microstructure to reveal the mechanism of fatigue fracture in nanocrystalline materials. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanocrystalline material; Fatigue; Grain boundary microstructure; Fracture surface; Crack nucleation
Nanocrystalline metals and alloys have great potential as structural materials for microelectromechanical system (MEMS) devices, because of their excellent high strength as predicted by the Hall–Petch relationship. It has been reported that the grain size of electrodeposited Ni–P alloy decreases with increasing P content, eventually leading to the amorphous state [1]. Ni–P alloys with a P content of <2 wt.% have an advantage for practical applications as a structural material for MEMS devices, because, according to our findings, these alloys have a high hardness of 600HV and plasticity of the order of 10% elongation. Knowledge of fatigue is particularly important for structural engineering materials, and the understanding of fatigue and fracture of nanocrystalline materials has received increasing attention [2–9]. Hanlon et al. reported that the fatigue life of pure Ni was improved by nanocrystallization [3]. To date, discussion of the fatigue property of nanocrystalline materials has mostly focused only on the effect of grain refinement. The roles of other microstructural factors, such as texture and grain boundary microstructure, have been little studied. Recently, the present authors revealed that intergranular
* Corresponding author. Tel.: +81 284 62 0605; fax: +81 284 62 9802; e-mail: [email protected] 1 Present address: Sumitomo Metal Industries, Ltd., Japan. 2 Formerly, Tohoku University, Sendai, Japan.
fatigue cracks in polycrystalline Al nucleated predominantly at random boundaries, while fatigue cracks were never observed at low-angle boundaries [10]. The low-R coincidence site lattice (CSL) boundaries showed higher resistance to fatigue crack nucleation than random boundaries [10]. Thus the effects of grain boundary microstructure on fatigue and fracture in nanocrystalline materials may be more significant than those in conventional microcrystalline materials, because the density of grain boundaries becomes remarkably high in nanocrystalline materials. This work was performed to investigate the fatigue fracture process in nanocrystalline materials in connection with grain boundary microstructure in electrodeposited nanocrystalline Ni–P alloy. The formation of surface damage, which is closely related to fatigue crack nucleation, and the morphological features of the fracture surface under cyclic stress were observed and discussed on the basis of field emission scanning electron microscopy (FE-SEM)/electron backscatter diffraction (EBSD) analyses of the grain boundary microstructure. A nanocrystalline Ni–2.0 wt.% P alloy was produced by electrodeposition using an electrolytic bath of 150 g l 1 nickel sulfate, 45 g l 1 nickel chloride, 80 g l 1 phosphoric acid and 0.5 g l 1 phosphorous acid with pH 1.5 at 338 K and a current density of 2.0 mA mm 2 for 3 h. Flat dog-bone-shaped fatigue test specimens with gage zone dimensions of 5.0 mm long 2.0 mm wide 0.2 mm thick were cut out from the electrodeposited
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.08.021
S. Kobayashi et al. / Scripta Materialia 61 (2009) 1032–1035
sheets mechanically stripped from the Ti substrate. X-ray diffraction (XRD) with monochromatic Cu Ka radiation was utilized to analyze the microstructure of the initial specimen and post-fatigue fractured specimen. An automated FE-SEM/EBSD/orientation imaging microscopy (OIM) set-up (TSL Inc.) was applied to quantitatively analyze the grain boundary microstructures. Fatigue tests were carried out by using a servohydraulic machine in air at room temperature. The sinusoidal load was applied at stress amplitudes of 324–540 MPa at a stress ratio of 0.1 and a frequency of 10 Hz. The maximum stress ranged from 0.6ry to 1.0ry, where ry is the yield strength, determined as 720–1200 MPa. Figure 1a–d shows an OIM micrograph with the inverse pole figure, the grain boundary characterization (GBC) micrograph, the grain size distribution and the grain boundary character distribution for the initial specimen, respectively. The OIM micrograph shows individual grain orientations in distinct colors, corresponding to those indicated in the stereo-triangle of the inverse pole figure. In the GBC micrograph, low-angle (2°–15°), R3, other R and random boundaries are shown by thick grey lines, thick black lines, black lines and thin black lines, respectively. From the OIM micrograph, it can be seen that the surface orientation of the initial specimen strongly localizes around {0 0 1}. The other grains tend to have surface orientations around {1 1 2}, {1 2 3} and {1 1 0}. As shown in Figure 1c, the {0 0 1} grains had grain sizes ranging from 10 to 180 nm and an average grain size of 56 nm. The grain sizes of the {0 0 1} grains were more widely spread than for the grains with other surface orientations. It was found that the initial specimen had a very high fraction of low-angle and R3 boundaries, as shown in Figure 1d. The fractions of low-angle and R3 boundaries were 20% and 36%, almost 10 and 20 times higher than for a ran-
Figure 1. Grain boundary microstructures of electrodeposited nanocrystalline Ni–2.0 wt.% P alloy characterized by FE-SEM/EBSD/OIM analysis: (a) EBSD/OIM micrograph; (b) grain boundary characterization micrograph; (c) grain size distribution; and (d) grain boundary character distribution.
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dom polycrystal, respectively. The fraction of R9 boundaries was moderately high, probably because of multiple twinning of R3. Judging from the OIM and GBC micrographs, R3 boundaries tend to surround fine grains with {1 1 2} and {1 2 3} surfaces. These twin boundaries may be of non-equilibrium state, because these R3-related boundaries showed sluggish and irregular shape. Figure 2 shows the relationship between the stress amplitudes and the logarithm of the number of cycles to fracture for the electrodeposited nanocrystalline Ni– 2.0 wt.% P specimens. The data obtained from electrodeposited nanocrystalline Ni with an average grain size in the range 20–40 nm [3] and electrodeposited microcrystalline Ni with columnar grain structure [11] are shown together in Figure 2. The fatigue limit for nanocrystalline Ni and microcrystalline Ni was about 400 and 180 MPa, respectively [3,11]. The fatigue limit for the nanocrystalline Ni–P alloy in the present work was about 360 MPa. Hence, this value of fatigue limit is 2 times higher than that for the microcrystalline pure Ni reported by others [11], but slightly lower than that reported for nanocrystalline pure Ni [3]. Thus, it is evident that the fatigue limit of polycrystalline materials is improved by grain refinement to the nanocrystalline level. It is worth noting here that the dynamic fatigue limit of the present nanocrystalline Ni–P alloy was 2 times higher than that of microcrystalline Ni, but the static tensile strength and hardness of the nanocrystalline metals become 3–10 times as high as those of the polycrystalline metals with conventional grain size [4,7,12–14]. We suggest that a change in grain boundary microstructure and surface morphology under cyclic stress may affect the moderate fatigue limit of nanocrystalline Ni–P alloy. Figure 3a is a SEM micrograph of the surface of fatigued specimen. Figure 3b and c are the higher-magnification images of areas (i) and (ii), respectively, in Figure 3a. The stress direction is horizontal in the micrographs. The morphology of fatigued surface can be classified into two distinct types. The area (ii) around the crack source showed a brittle fracture mode. The specimen surface was rather flat and wavey microcracks were observed in area (ii). The nucleation of these microcracks was found to be the source of fatigue fracture. On the other hand, the surrounding area (i) showed a ductile tensile fracture mode. Steps which probably resulted from the formation of shear bands were observed. It seems that these shear steps were formed by sliding along the mutual grain boundaries like cooperative grain boundary sliding by shear of groups of grains re-
Figure 2. S–N curve of electrodeposited nanocrystalline Ni–2.0 wt.% P alloy subjected to cyclic deformation via stress amplitudes of 324– 540 MPa at a frequency of 10 Hz at room temperature.
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S. Kobayashi et al. / Scripta Materialia 61 (2009) 1032–1035
Figure 3. SEM micrographs of specimen surface of electrodeposited nanocrystalline Ni–2.0 wt.% P alloy after high-cycle fatigue test: (a) the specimen surface around the fracture surface; (b and c) highmagnification images of areas (i) and (ii), respectively.
ported for superplastic deformation of fine-grained materials [15]. The spacing of neighboring shear steps was 2.0– 5.0 lm. Several experimental studies of the formation of shear bands during cyclic deformation have recently been reported for nanocrystalline [16] and ultrafinegrained [17–19] metals. The formation of shear bands induces short fatigue life due to severe strain localization [19]. In the surrounding area (i), the surface cracks nucleated and propagated along the shear steps. It is suggested that the shear steps play a role as the preferential site for the stress concentration and subsequent crack nucleation near the specimen surface during cyclic deformation. Table 1 gives the average grain size, the surface orientation and the nature of the observed phase evaluated by XRD measurements. The average grain size calculated by means of the Scherrer equation [20] was 60 nm for the fatigued specimen, although the dislocation density is increased by applied cyclic stress. The average grain size for the initial specimen obtained from XRD analysis was 49 nm, which is in good agreement with the value of 45 nm obtained from FE-SEM/EBSD analysis. The results of XRD measurements suggest that the grain growth likely occurred under cyclic stress. It has often been reported that grain growth can occur even at room temperature in nanocrystalline pure metals under the
influence of static stress [21–24]. Until recently, however, the studies on the stress-induced grain growth under cyclic deformation in nanocrystalline materials have been scarce [2,8], although the grain growth in electrodeposited microcrystalline Cu under cyclic deformation has been reported [25]. Quite recently, Yang et al. have reported the occurrence of grain growth under cyclic loading in nanocrystalline Ni–Fe alloy [8]. A noticeable change in grain size from tens to hundreds of nanometers was observed near the fatigue crack. Unfortunately, a complete understanding of the mechanisms of room temperature grain growth under cyclic stress in nanocrystalline materials has not yet been obtained. Figure 4a shows the fracture surface of the fatigued specimen cyclically deformed at a stress amplitude of 400 MPa. Figure 4b–d are the higher-magnification images of areas corresponding to (i), (ii) and (iii), respectively, in Figure 4a. The morphologies of fracture surface were classified into three types. The central area (i) close to the crack source is suggested to be associated with brittle fracture mode. This area looks planar and the fracture propagates toward the peripheral area of the specimen. Area (ii) adjacent to area (i) shows a striation pattern of characteristic of fatigue fracture. The width of striations ranged from 0.5 to 1.0 lm. The peripheral areas (iii) and (iv) are characterized as dimple patterns typical of tensile ductile fracture, as shown in Figure 4d. The size of dimples ranged from 0.5 to 3.0 lm. In previous studies, the observed sizes of striations and dimples were larger than the average grain size in the pre- and post-fatigued specimens. The sizes of striations and dimples were similar to those sizes in the fatigue-fractured nanocrystalline metals reported by other researchers [6,8]. Until now, the reason for the mismatch between grain size and morphological features of the fracture surface has not been fully understood. The
Table 1. Average grain size, grain orientation distribution and phase for the initial and fatigued specimens measured by XRD analyses. Specimen condition
Grain size (nm)
Orientation
Phase
Initial Fatigued
48 60
(2 0 0) (2 0 0)
Ni Ni
Figure 4. SEM micrographs of fracture surface: (a) the whole fracture surface; (b–d) high-magnification images of areas corresponding to (i), (ii) and (iii), respectively.
S. Kobayashi et al. / Scripta Materialia 61 (2009) 1032–1035
present authors have previously revealed that the slip bands can continuously transfer across the low-angle boundaries and that low-R boundaries show a higher resistance to fatigue crack nucleation [10]. From these findings, we suggest that the {0 0 1}-oriented grain cluster with very high fraction of low-angle (20%) and low-R (45%) boundaries may deform like a single crystal on the micrometer scale. Therefore, striations and dimples on the micrometer scale might be formed in nanocrystalline Ni–P alloy. Moreover, the effect of nanocrystallization on the dynamic fatigue limit may become less than that on the static strength predicted by the Hall–Petch relationship, because the nanocrystalline specimen was deformed for a group of grains in {0 0 1}-oriented grain clusters in the present nanocrystalline Ni–P alloy. In summary, the fatigue and fracture associated with grain boundary microstructure and morphology change by high-cycle fatigue were investigated using electrodeposited nanocrystalline Ni–2.0 wt.% P alloy. The nanocrystalline specimens with {0 0 1} texture showed a high fraction of low-angle and R3 boundaries and a fatigue limit of 360 MPa. This fatigue limit was found to be higher than that in polycrystalline Ni with conventional grain size. The surface features of fatigued specimen were classified into two different morphologies. The central area was characterized as associated with brittle fracture and the adjacent area as ductile tensile fracture, resulting in the formation of step patterns due to shear banding. It was also found that cyclic deformation-induced grain growth occurred. The present work has revealed that the fatigue fracture in nanocrystalline metals is strongly affected by grain boundary microstructure, not simply by the grain size. The authors would like to express their hearty thanks to Professor Tsurekawa of Kumamoto University for the provision of the EBSD/OIM analyses. The authors are also grateful to Professor S. Tobe and Associate Professor Y. Ando of Ashikaga Institute of Technology, for their support with XRD measurements. [1] G. McMahon, U. Erb, J. Mater. Sci. Lett. 8 (1989) 865. [2] A.B. Witney, P.G. Sanders, J.R. Weertman, J.A. Eastman, Scripta Metall. Mater. 33 (1995) 2025.
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[3] T. Hanlon, Y.-N. Kwon, S. Suresh, Scripta Mater. 49 (2003) 675. [4] K.S. Kumar, H. Van Swygenhoven, S. Suresh, Acta Mater. 51 (2003) 5743. [5] B. Moser, T. Hanlon, K.S. Kumar, S. Suresh, Scripta Mater. 54 (2006) 1151. [6] S. Cheng, A.D. Stoica, X.-L. Wang, G.Y. Wang, H. Choo, P.K. Liaw, Scripta Mater. 57 (2007) 217. [7] M. Dao, L. Lu, R.J. Asaro, J.T.M. De Hosson, E. Ma, Acta Mater. 55 (2007) 4041. [8] Y. Yang, B. Imasogie, G.J. Fan, P.K. Liaw, W.O. Soboyejo, Metall. Mater. Trans. 39A (2008) 1145. [9] S. Cheng, J. Xie, A.D. Stoica, X.-L. Wang, J.A. Horton, D.W. Brown, H. Choo, P.K. Liaw, Acta Mater. 57 (2009) 1272. [10] S. Kobayashi, T. Inomata, H. Kobayashi, S. Tsurekawa, T. Watanabe, J. Mater. Sci. 43 (2008) 3792. [11] J. Aktaa, J.Th. Reszat, M. Walter, K. Bade, K.J. Hemker, Scripta Mater. 52 (2005) 1217. [12] F. Dalla Torre, H. Van Swygenhoven, M. Victoria, Acta Mater. 50 (2002) 3957. [13] M. Dao, L. Lu, Y.F. Shen, S. Suresh, Acta Mater. 54 (2006) 5421. [14] F. Ebrahimi, G.R. Bourne, M.S. Kelly, T.E. Matthews, NanoStructured Mater. 11 (1999) 343. [15] M.G. Zelin, N.A. Krasilnikov, R.Z. Valiev, M.W. Grabski, H.S. Yang, A.K. Mukherjee, Acta Metall. Mater. 42 (1994) 119. [16] J. Xie, X. Wu, Y. Hong, Scripta Mater. 57 (2007) 5. [17] M. Goto, S.Z. Han, T. Yakushiji, C.Y. Lim, S.S. Kim, Scripta Mater. 54 (2006) 2101. [18] M. Goto, S.Z. Han, S.S. Kim, N. Kawagoishi, C.Y. Lim, Scripta Mater. 57 (2007) 293. [19] M.K. Wong, W.P. Kao, J.T. Lui, C.P. Chang, P.W. Kao, Acta Mater. 55 (2007) 715. [20] B.D. Cullity, Elements of X-ray Diffraction, 2nd edn., Addison-Wesley, Reading, MA, 1978. [21] D.S. Gianola, S. Van Petegem, M. Legros, S. Brandstetter, H. Van Swygenhoven, K.J. Hemker, Acta Mater. 54 (2006) 2253. [22] G.J. Fan, L.F. Fu, H. Choo, P.K. Liaw, N.D. Browning, Acta Mater. 54 (2006) 4781. [23] P.L. Gai, K. Zhang, J. Weertman, Scripta Mater. 56 (2007) 25. [24] I.A. Ovid’ko, A.G. Sheinerman, E.C. Aifantis, Acta Mater. 56 (2008) 2718. [25] M. Suzuki, H. Takahashi, Z. Metallkde. 57 (1966) 484.
Scripta Materialia 61 (2009) 1032–1035 www.elsevier.com/locate/scriptamat
Roles of grain boundary microstructure in high-cycle fatigue of electrodeposited nanocrystalline Ni–P alloy Shigeaki Kobayashi,a,* Akiyuki Kamataa,1 and Tadao Watanabeb,2 a
Department of Mechanical Engineering, Faculty of Engineering, Ashikaga Institute of Technology, 268-1 Omae, Ashikaga, Tochigi 326-8558, Japan b Key Laboratory for Anisotropy and Texture of Materials, Northeastern University, Shenyang 110004, China Received 25 May 2009; revised 3 August 2009; accepted 14 August 2009 Available online 19 August 2009
The fatigue properties and the morphology of the specimen surface and fracture surface were investigated in electrodeposited nanocrystalline Ni–P alloy subjected to high-cycle fatigue. The nanocrystalline specimens, which have a {0 0 1} texture and high fractions of low-angle and R3 boundaries, show a higher fatigue limit than Ni polycrystals with conventional grain size. The change in specimen surface and fracture surface by cyclic deformation is discussed in connection with grain boundary microstructure to reveal the mechanism of fatigue fracture in nanocrystalline materials. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanocrystalline material; Fatigue; Grain boundary microstructure; Fracture surface; Crack nucleation
Nanocrystalline metals and alloys have great potential as structural materials for microelectromechanical system (MEMS) devices, because of their excellent high strength as predicted by the Hall–Petch relationship. It has been reported that the grain size of electrodeposited Ni–P alloy decreases with increasing P content, eventually leading to the amorphous state [1]. Ni–P alloys with a P content of <2 wt.% have an advantage for practical applications as a structural material for MEMS devices, because, according to our findings, these alloys have a high hardness of 600HV and plasticity of the order of 10% elongation. Knowledge of fatigue is particularly important for structural engineering materials, and the understanding of fatigue and fracture of nanocrystalline materials has received increasing attention [2–9]. Hanlon et al. reported that the fatigue life of pure Ni was improved by nanocrystallization [3]. To date, discussion of the fatigue property of nanocrystalline materials has mostly focused only on the effect of grain refinement. The roles of other microstructural factors, such as texture and grain boundary microstructure, have been little studied. Recently, the present authors revealed that intergranular
* Corresponding author. Tel.: +81 284 62 0605; fax: +81 284 62 9802; e-mail: [email protected] 1 Present address: Sumitomo Metal Industries, Ltd., Japan. 2 Formerly, Tohoku University, Sendai, Japan.
fatigue cracks in polycrystalline Al nucleated predominantly at random boundaries, while fatigue cracks were never observed at low-angle boundaries [10]. The low-R coincidence site lattice (CSL) boundaries showed higher resistance to fatigue crack nucleation than random boundaries [10]. Thus the effects of grain boundary microstructure on fatigue and fracture in nanocrystalline materials may be more significant than those in conventional microcrystalline materials, because the density of grain boundaries becomes remarkably high in nanocrystalline materials. This work was performed to investigate the fatigue fracture process in nanocrystalline materials in connection with grain boundary microstructure in electrodeposited nanocrystalline Ni–P alloy. The formation of surface damage, which is closely related to fatigue crack nucleation, and the morphological features of the fracture surface under cyclic stress were observed and discussed on the basis of field emission scanning electron microscopy (FE-SEM)/electron backscatter diffraction (EBSD) analyses of the grain boundary microstructure. A nanocrystalline Ni–2.0 wt.% P alloy was produced by electrodeposition using an electrolytic bath of 150 g l 1 nickel sulfate, 45 g l 1 nickel chloride, 80 g l 1 phosphoric acid and 0.5 g l 1 phosphorous acid with pH 1.5 at 338 K and a current density of 2.0 mA mm 2 for 3 h. Flat dog-bone-shaped fatigue test specimens with gage zone dimensions of 5.0 mm long 2.0 mm wide 0.2 mm thick were cut out from the electrodeposited
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.08.021
S. Kobayashi et al. / Scripta Materialia 61 (2009) 1032–1035
sheets mechanically stripped from the Ti substrate. X-ray diffraction (XRD) with monochromatic Cu Ka radiation was utilized to analyze the microstructure of the initial specimen and post-fatigue fractured specimen. An automated FE-SEM/EBSD/orientation imaging microscopy (OIM) set-up (TSL Inc.) was applied to quantitatively analyze the grain boundary microstructures. Fatigue tests were carried out by using a servohydraulic machine in air at room temperature. The sinusoidal load was applied at stress amplitudes of 324–540 MPa at a stress ratio of 0.1 and a frequency of 10 Hz. The maximum stress ranged from 0.6ry to 1.0ry, where ry is the yield strength, determined as 720–1200 MPa. Figure 1a–d shows an OIM micrograph with the inverse pole figure, the grain boundary characterization (GBC) micrograph, the grain size distribution and the grain boundary character distribution for the initial specimen, respectively. The OIM micrograph shows individual grain orientations in distinct colors, corresponding to those indicated in the stereo-triangle of the inverse pole figure. In the GBC micrograph, low-angle (2°–15°), R3, other R and random boundaries are shown by thick grey lines, thick black lines, black lines and thin black lines, respectively. From the OIM micrograph, it can be seen that the surface orientation of the initial specimen strongly localizes around {0 0 1}. The other grains tend to have surface orientations around {1 1 2}, {1 2 3} and {1 1 0}. As shown in Figure 1c, the {0 0 1} grains had grain sizes ranging from 10 to 180 nm and an average grain size of 56 nm. The grain sizes of the {0 0 1} grains were more widely spread than for the grains with other surface orientations. It was found that the initial specimen had a very high fraction of low-angle and R3 boundaries, as shown in Figure 1d. The fractions of low-angle and R3 boundaries were 20% and 36%, almost 10 and 20 times higher than for a ran-
Figure 1. Grain boundary microstructures of electrodeposited nanocrystalline Ni–2.0 wt.% P alloy characterized by FE-SEM/EBSD/OIM analysis: (a) EBSD/OIM micrograph; (b) grain boundary characterization micrograph; (c) grain size distribution; and (d) grain boundary character distribution.
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dom polycrystal, respectively. The fraction of R9 boundaries was moderately high, probably because of multiple twinning of R3. Judging from the OIM and GBC micrographs, R3 boundaries tend to surround fine grains with {1 1 2} and {1 2 3} surfaces. These twin boundaries may be of non-equilibrium state, because these R3-related boundaries showed sluggish and irregular shape. Figure 2 shows the relationship between the stress amplitudes and the logarithm of the number of cycles to fracture for the electrodeposited nanocrystalline Ni– 2.0 wt.% P specimens. The data obtained from electrodeposited nanocrystalline Ni with an average grain size in the range 20–40 nm [3] and electrodeposited microcrystalline Ni with columnar grain structure [11] are shown together in Figure 2. The fatigue limit for nanocrystalline Ni and microcrystalline Ni was about 400 and 180 MPa, respectively [3,11]. The fatigue limit for the nanocrystalline Ni–P alloy in the present work was about 360 MPa. Hence, this value of fatigue limit is 2 times higher than that for the microcrystalline pure Ni reported by others [11], but slightly lower than that reported for nanocrystalline pure Ni [3]. Thus, it is evident that the fatigue limit of polycrystalline materials is improved by grain refinement to the nanocrystalline level. It is worth noting here that the dynamic fatigue limit of the present nanocrystalline Ni–P alloy was 2 times higher than that of microcrystalline Ni, but the static tensile strength and hardness of the nanocrystalline metals become 3–10 times as high as those of the polycrystalline metals with conventional grain size [4,7,12–14]. We suggest that a change in grain boundary microstructure and surface morphology under cyclic stress may affect the moderate fatigue limit of nanocrystalline Ni–P alloy. Figure 3a is a SEM micrograph of the surface of fatigued specimen. Figure 3b and c are the higher-magnification images of areas (i) and (ii), respectively, in Figure 3a. The stress direction is horizontal in the micrographs. The morphology of fatigued surface can be classified into two distinct types. The area (ii) around the crack source showed a brittle fracture mode. The specimen surface was rather flat and wavey microcracks were observed in area (ii). The nucleation of these microcracks was found to be the source of fatigue fracture. On the other hand, the surrounding area (i) showed a ductile tensile fracture mode. Steps which probably resulted from the formation of shear bands were observed. It seems that these shear steps were formed by sliding along the mutual grain boundaries like cooperative grain boundary sliding by shear of groups of grains re-
Figure 2. S–N curve of electrodeposited nanocrystalline Ni–2.0 wt.% P alloy subjected to cyclic deformation via stress amplitudes of 324– 540 MPa at a frequency of 10 Hz at room temperature.
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S. Kobayashi et al. / Scripta Materialia 61 (2009) 1032–1035
Figure 3. SEM micrographs of specimen surface of electrodeposited nanocrystalline Ni–2.0 wt.% P alloy after high-cycle fatigue test: (a) the specimen surface around the fracture surface; (b and c) highmagnification images of areas (i) and (ii), respectively.
ported for superplastic deformation of fine-grained materials [15]. The spacing of neighboring shear steps was 2.0– 5.0 lm. Several experimental studies of the formation of shear bands during cyclic deformation have recently been reported for nanocrystalline [16] and ultrafinegrained [17–19] metals. The formation of shear bands induces short fatigue life due to severe strain localization [19]. In the surrounding area (i), the surface cracks nucleated and propagated along the shear steps. It is suggested that the shear steps play a role as the preferential site for the stress concentration and subsequent crack nucleation near the specimen surface during cyclic deformation. Table 1 gives the average grain size, the surface orientation and the nature of the observed phase evaluated by XRD measurements. The average grain size calculated by means of the Scherrer equation [20] was 60 nm for the fatigued specimen, although the dislocation density is increased by applied cyclic stress. The average grain size for the initial specimen obtained from XRD analysis was 49 nm, which is in good agreement with the value of 45 nm obtained from FE-SEM/EBSD analysis. The results of XRD measurements suggest that the grain growth likely occurred under cyclic stress. It has often been reported that grain growth can occur even at room temperature in nanocrystalline pure metals under the
influence of static stress [21–24]. Until recently, however, the studies on the stress-induced grain growth under cyclic deformation in nanocrystalline materials have been scarce [2,8], although the grain growth in electrodeposited microcrystalline Cu under cyclic deformation has been reported [25]. Quite recently, Yang et al. have reported the occurrence of grain growth under cyclic loading in nanocrystalline Ni–Fe alloy [8]. A noticeable change in grain size from tens to hundreds of nanometers was observed near the fatigue crack. Unfortunately, a complete understanding of the mechanisms of room temperature grain growth under cyclic stress in nanocrystalline materials has not yet been obtained. Figure 4a shows the fracture surface of the fatigued specimen cyclically deformed at a stress amplitude of 400 MPa. Figure 4b–d are the higher-magnification images of areas corresponding to (i), (ii) and (iii), respectively, in Figure 4a. The morphologies of fracture surface were classified into three types. The central area (i) close to the crack source is suggested to be associated with brittle fracture mode. This area looks planar and the fracture propagates toward the peripheral area of the specimen. Area (ii) adjacent to area (i) shows a striation pattern of characteristic of fatigue fracture. The width of striations ranged from 0.5 to 1.0 lm. The peripheral areas (iii) and (iv) are characterized as dimple patterns typical of tensile ductile fracture, as shown in Figure 4d. The size of dimples ranged from 0.5 to 3.0 lm. In previous studies, the observed sizes of striations and dimples were larger than the average grain size in the pre- and post-fatigued specimens. The sizes of striations and dimples were similar to those sizes in the fatigue-fractured nanocrystalline metals reported by other researchers [6,8]. Until now, the reason for the mismatch between grain size and morphological features of the fracture surface has not been fully understood. The
Table 1. Average grain size, grain orientation distribution and phase for the initial and fatigued specimens measured by XRD analyses. Specimen condition
Grain size (nm)
Orientation
Phase
Initial Fatigued
48 60
(2 0 0) (2 0 0)
Ni Ni
Figure 4. SEM micrographs of fracture surface: (a) the whole fracture surface; (b–d) high-magnification images of areas corresponding to (i), (ii) and (iii), respectively.
S. Kobayashi et al. / Scripta Materialia 61 (2009) 1032–1035
present authors have previously revealed that the slip bands can continuously transfer across the low-angle boundaries and that low-R boundaries show a higher resistance to fatigue crack nucleation [10]. From these findings, we suggest that the {0 0 1}-oriented grain cluster with very high fraction of low-angle (20%) and low-R (45%) boundaries may deform like a single crystal on the micrometer scale. Therefore, striations and dimples on the micrometer scale might be formed in nanocrystalline Ni–P alloy. Moreover, the effect of nanocrystallization on the dynamic fatigue limit may become less than that on the static strength predicted by the Hall–Petch relationship, because the nanocrystalline specimen was deformed for a group of grains in {0 0 1}-oriented grain clusters in the present nanocrystalline Ni–P alloy. In summary, the fatigue and fracture associated with grain boundary microstructure and morphology change by high-cycle fatigue were investigated using electrodeposited nanocrystalline Ni–2.0 wt.% P alloy. The nanocrystalline specimens with {0 0 1} texture showed a high fraction of low-angle and R3 boundaries and a fatigue limit of 360 MPa. This fatigue limit was found to be higher than that in polycrystalline Ni with conventional grain size. The surface features of fatigued specimen were classified into two different morphologies. The central area was characterized as associated with brittle fracture and the adjacent area as ductile tensile fracture, resulting in the formation of step patterns due to shear banding. It was also found that cyclic deformation-induced grain growth occurred. The present work has revealed that the fatigue fracture in nanocrystalline metals is strongly affected by grain boundary microstructure, not simply by the grain size. The authors would like to express their hearty thanks to Professor Tsurekawa of Kumamoto University for the provision of the EBSD/OIM analyses. The authors are also grateful to Professor S. Tobe and Associate Professor Y. Ando of Ashikaga Institute of Technology, for their support with XRD measurements. [1] G. McMahon, U. Erb, J. Mater. Sci. Lett. 8 (1989) 865. [2] A.B. Witney, P.G. Sanders, J.R. Weertman, J.A. Eastman, Scripta Metall. Mater. 33 (1995) 2025.
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