Scripta Materialia 57 (2007) 233–236 www.elsevier.com/locate/scriptamat

Layered nanostructured Ni with modulated hardness fabricated by surfactant-assistant electrodeposition Changdong Gu, Jianshe Lian* and Qing Jiang Key Laboratory of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Nanling Campus, Changchun 130025, China Received 23 January 2007; revised 30 March 2007; accepted 3 April 2007 Available online 17 May 2007

A layered nanostructured (NS) Ni sample, which consists of alternate layers of ultrafine and nano-sized grains, is fabricated by intermittently adding a grain refiner during electrodeposition. There exists a phenomenon of fluctuating hardness in the crosssection of the material. In addition, the mechanical characteristics of the material are located between those of ultrafine-grained Ni and those of nano-sized Ni.  2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanostructured materials; Tensile test; Electrodeposition; Ni; Layered structure

With the development of microelectromechanical systems, nanoelectromechanical systems and microand nanoscale devices, there are demands to fabricate advanced materials by easy processes. Electrodeposition is a versatile bottom-up method with a simple operation for manufacturing nanostructured (NS) materials [1–8]. By adjusting some simple electroplating parameters different NS metals/alloys with specialized microstructures can be manufactured [2,6,8,9]. Investigations of NS materials over the past two decades have shown that grain refinement to the nanometer range (<100 nm) would lead to substantial strengthening, with strength and hardness values factors of three or more greater than their conventional grain size counterparts [7–12]. However, tensile ductility of these NS metals disappointedly drops to <3% [7,11–13], which could be an intrinsic property of NS materials with uniform grain size due to the limited dislocation movement. Through controlling the grain size and its distribution, the strength and ductility of NS materials may be optimized [6,14–18]. It is known that a pure Cu with a bimodal grain size distribution, having micrometer-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains, would achieve the optimal combined mechanical properties of high strength and high ductility [17]. It is believed that the nanocrystalline

* Corresponding author. Tel./fax: +86 431 85095876; e-mail: lianjs@ jlu.edu.cn

grains provide strength and the embedded large grains stabilize the tensile deformation of the material [17]. In the bimodal grain size structure, the coarse and fine grains are distributed randomly. According to the above work [17], an interesting question arises, namely whether this mechanism could be applied for the layered materials where there are distinct grain sizes in different layers. If this were true, a material with compromised mechanical properties could be easily obtained since the manufacture of this kind of material can be fabricated by the electrodeposition technique. In this contribution, the above consideration is realized with the establishment of a new microstructure. The material consists of alternate layers with ultrafine and nano-sized grains. By a surfactant-assisted electrodeposition technique, a bulk layered NS Ni whose grain sizes for each layer are controllable is fabricated. The mechanical properties of the material are determined and compared with the same material with different grain size distributions. The layered NS Ni, with a thickness of about 2.3 mm, was fabricated by intermittently adding a grain refiner during direct-current electrodeposition from a base electrolyte containing nickel sulfate, nickel chloride and boric acid at pH 5.0 and a temperature of 50 C. At first, 1.2 g l 1 of 1,4-butenediol was added into the electrolyte to act as the grain refiner. Then 1 g l 1 of 1,4-butenediol was supplemented once every 12 h. The anode was pure Ni plate. For comparison, 18 nm grain sized Ni and ultrafine-grained (UFG) Ni, both of 300 lm

1359-6462/$ - see front matter  2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.04.005

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thickness, were manufactured, respectively, from base electrolyte with 1,4-butenediol contents of 1.2 and 0.4 g l 1. The microstructure was examined using a transmission electron microscope (TEM, H-800). Dog-bone-shaped tensile specimens with a gauge width · length of 2.0 mm · 8.0 mm and a thickness of 2.0 mm for the layered NS Ni and 250 lm for the 18 nm and the UFG Ni, respectively, were cut using a wire electrodischarging machine and then polished to a mirror-like finish surface. Tensile tests were carried out on an MTS-810 system under a strain rate of 1.04 · 10 4 s 1 at room temperature. The fracture surfaces were observed via a scanning electron microscope (SEM, JSM-5600). Hardness tests were carried on an HXD-1000 microhardness tester with a Vickers indenter, at a load of 200 g and a duration of 15 s. The indents were observed using a confocal laser scanning microscope (OLYMPUS, OLS3000). During the hardness tests, the separation between indents was initially kept at more than three times the diameter of each indent. Because the deformation process was thus unable to build up a residual dislocation network in NS electrodeposited Ni [19–22], a few indents were inserted between the former ones to enrich the results of the hardness tests. There is no residual dislocation network or no residual stress after the indentation, which is confirmed by the fact that the initial and later determined hardness values at neighboring points are similar, as shown in Figure 1. The fabrication of the layered NS Ni is attributed to the surfactant-1,4-butenediol, which is added to the electrolyte bath to restrain grain growth. In our previous studies [18,23], 1,4-butenediol was found to be an effective grain refiner. Adding a certain amount of 1,4-butenediol to the conventional electroplating Ni electrolytes causes deposits to be nanostructured. Generally, the more the 1,4-butenediol added, the smaller the grain size. The upper limit of 1,4-butenediol content is about 1.2 g l 1. Above this level, the surfactant would have a deleterious effect on the electrolyte. Meanwhile, 1,4butenediol is consumed during the deposition process. Therefore, in order to obtain continuous and uniform nano-grained deposits, 1,4-butenediol should be supplemented continuously. If it is not, the grain size of the deposits becomes larger as the surfactant is consumed. In light of this fact, we intend to prolong the supplying

period of 1,4-butenediol. As a result, electrodeposited Ni with a layered nanostructure, i.e. periodical grain size variation from small to large along the cross-section, was obtained. Figure 1 shows the microhardness of the cross-section of as-deposited Ni vs. distance from the substrate. The hardness values fluctuated periodically with increasing distance from the substrate, i.e. with increasing deposition time. The microhardness in the hard regions is about 5.0 GPa, while that in the soft regions is only about 2.8 GPa. Figure 2 presents the typical microscopic morphologies of Vickers indents of the cross-sectional deposits, and shows the periodical changes in indent size. Inset (a) is the magnified photograph corresponding to the region marked by the rectangle in the figure. Inset (b) presents a three-dimensional view of the shape of Vickers indent ‘‘2’’. Small-sized Vickers indents, like indents ‘‘1’’ and ‘‘4’’ in Figure 2, correspond to the hard layers. In contrast, the larger sized ones, typically like indent ‘‘3’’, are related to the soft layers. It is observed that there is an obvious surface uplift around the perimeter of the indent in the soft region, but not in the hard region (see insets (a) and (b)). When a Vickers indent is located on the interface of the hard and soft layers labeled ‘‘2’’, its right half has the characteristic of the indent of the soft layer (like indent ‘‘3’’) while its left half is similar to that of the hard layer (like indents ‘‘1’’ and ‘‘4’’). Figures 3a–c are typical cross-sectional TEM micrographs of the as-deposited layered NS Ni corresponding to the soft region, the hard/soft interface and the hard region, respectively. Their selected area diffraction (SAD) patterns are shown as the respectively insets. The grain size in the soft region ranges from 100 to 500 nm, while the average grain size in the hard region is about 18 nm, with a narrow grain size distribution ranging from 10 to 40 nm. At the hard/soft interface, a variation of grain size from about 80 to 200 nm on the left to about 20–40 nm on the right is observed. This gradient variation in grain size is also confirmed by the

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Figure 1. Cross-sectional hardness of as-deposited Ni vs. distance from the substrate.

Figure 2. Typical microscopic morphologies of hardness testing indents on the cross-sectional deposits. Inset (a) is a magnified photograph corresponding to the region marked by the rectangle in the figure. Inset (b) gives a three-dimensional view of the shape of Vickers indent ‘‘2’’.

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Figure 3. Cross-sectional TEM micrographs of the as-deposited layered NS Ni: (a) the soft region, (b) the interface and (c) the hard region. The insets are the corresponding SAD patterns.

SAD pattern shown in Figure 3b inset, where a mixture of regulated spots (corresponding to the coarse grains) and subtle rings (corresponding to the fine grains) is present, indicating the wide grain size distribution at the layer joint. No obvious defects (such as voids or gaps) between fine and coarse grains are found. The engineering stress–strain curves for (a) 18 nm grain sized Ni, (b) bimodal grain sized Ni [18], (c) layered NS Ni and (d) UFG Ni are shown in Figure 4. TEM micrographs of UFG Ni and 18 nm grain sized Ni have grain size distributions similar to Figures 3a and c, respectively. Their microhardness values are 3.0 and 5.1 GPa, respectively. From Figure 4, the compromising tensile properties between them are obtained for the layered NS Ni. It is believed that the hard layers of the layered NS Ni impart the high strength while the soft layers stabilize the tension to failure. However, the strength of the layered NS Ni composed of modulated layered small and large grains is slightly lower than that of the reported bimodal grain sized Ni composed of a random mixture of small and large grains [18]. The reason for this might be that the incompatible strains

Figure 4. Engineering stress–strain curves for (a) 18 nm grain sized Ni, (b) bimodal grain sized Ni [18], (c) layered NS Ni and (d) UFG Ni. The inset shows the layered NS Ni specimen for the tensile test. All Ni specimens were fabricated by direct-current electrodeposition.

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between the hard and soft layers lead to the early fracture of the layered Ni. On the plane surface of the tensile specimen, several cracks exist vertical to the tensile axis, as shown in Figure 5. The crack becomes wider when it gets closer to the fracture front of the specimen, which implies that even though the hard surface layer of the layered NS Ni is fractured, the soft interior layers would still continue to sustain the plastic deformation of the material. The hard layers fracture before the soft layers in the layered NS Ni, which should be responsible for the abovementioned slightly reduced strength compared with the bimodal grain sized Ni [18]. Furthermore, one may suspect that cracks could exist along the layer interfaces. Therefore, it could be expected that if the surface layer consists of the soft region, vertical cracks could be avoided and the corresponding mechanical properties may be improved. The fracture surface morphologies of the layered NS Ni are shown in Figure 6. Figure 6a shows typical ductile fracture morphology dimples corresponding to the soft layer Ni. As shown in Figure 6b, the typical brittle fracture morphology is related to the hard layer Ni. There is certainly a large difference between the fracture

Figure 5. Cracks occurring on the tensile fractured specimen surface of the layered NS Ni.

Figure 6. Fracture surface of the tensile layered NS Ni specimen: (a) typical ductile fracture surface in the soft region, (b) brittle fracture surfaces in the hard region and (c) fracture surface at the hard/soft interface.

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surfaces in the hard and soft layers. A typical SEM image at the hard/soft interface is shown in Figure 6c. An obvious interface between the hard and soft layers can be seen on the fracture surface. In addition, the dimple sizes also indicates the grain size distribution [7,24]. The change of dimple size shown in Figure 6c indicates that the grain size follows a gradient distribution along the depositing direction. In summary, based on surfactant-mediated electrodeposition technology, a novel layered NS Ni with modulated hardness along the cross-section was fabricated. The layered NS Ni microstructure consists of alternate layers of ultrafine and nano-sized grains. A compromise of strength and ductility was obtained in the layered NS Ni compared with the ultrafine and nano-sized grained Ni. However, its strength is slightly lower than that of the bimodal grain sized Ni composed of a random mixture of small and large grains. Further improvement in the mechanical properties of this kind of material is discussed. This work was supported by the Foundation of National Key Basic Research and Development Program No. 2004CB619301 and the Project 985-Automotive Engineering of Jilin University. [1] S.C. Tjong, H. Chen, Mater. Sci. Eng. R 45 (2004) 1. [2] U. Erb, Nanostruct. Mater. 6 (1995) 533. [3] F. Ebrahimi, G.R. Bourne, M.S. Kelly, T.E. Matthews, Nanostruct. Mater. 11 (1999) 343. [4] H. Li, F. Ebrahimi, Appl. Phys. Lett. 84 (2004) 4307. [5] L. Lu, R. Schwaiger, Z.W. Shan, M. Dao, K. Lu, S. Suresh, Acta Mater. 53 (2005) 2169.

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