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Grain growth and mechanical properties of electrodeposited nanocrystalline nickel–4.4mass% phosphorus alloy
Materials Science and Engineering A358 (2003) 76 /83 www.elsevier.com/locate/msea
Grain growth and mechanical properties of electrodeposited nanocrystalline nickel 4.4mass% phosphorus alloy /
Shigeaki Kobayashi *, Youhei Kashikura 1 Department of Materials Engineering, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogayaku, Yokohama 240-8501, Japan Received 2 December 2002; received in revised form 17 March 2003; accepted 4 April 2003
Abstract The relationship between microstructural evolution and mechanical properties in Ni /4.4mass% P alloy was systematically examined. As-electrodeposited Ni /4.4mass% P alloy consists of grains having an average grain size of 37 nm and texture strongly oriented to (1 1 1). Phosphorus is entirely in solid solution. Heat treatment induces precipitation of Ni3P phase. Rapid grain growth occurs only at the initial stage of heat treatment, and saturated at all the annealing temperatures, even at temperatures higher than 873 K (0.51Tm). Grain growth exponents n in Ni and Ni3P phases were found to be 4.5 /5.9 and 4.9 /5.4, respectively. These values are close to those for grain boundary diffusion-controlled growth. Activation energies for grain growth in Ni and Ni3P phases were estimated as 210 and 180 kJ mol 1, respectively. The nanocrystalline and ultrafine-grained specimens (about 40 /400 nm) fractured prior to yielding, while the larger grained specimens ( /400 nm) showed plastic deformation. Fracture strength and fracture strain depend not only on grain size but also on connectivity of Ni3P grains, where cracks were preferentially nucleated. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline nickel /phosphorus alloy; Grain growth; Grain morphology; Microhardness; Fracture strength; Fracture strain
1. Introduction Electrodeposited nickel /phosphorus (Ni /P) alloys have found applications to underlying layer of electronic devices and magnetic disks. As P content increases during electrodeposition, alloy structure changes from fine-grained to nanocrystalline, and eventually becomes amorphous [1]. In particular, nanocrystalline Ni /P alloys that obtained at low P contents (less than about 5 mass% P) have superior hardness of HV660 [2,3]. The hardness of these alloys is increased up to HV990 by annealing owing to precipitation of hard Ni3P phase [2]. Thus, annealed Ni /P alloys are expected to use as wearresistant coatings. However, Ni /P alloys show so severe brittleness that their workability is substantially limited. Improvement of the brittleness in Ni /P alloys is an important aspect for realizing widespread applications. Evaluation of mechanical properties that related to
* Corresponding author. Tel./fax: /81-45-339-3845. E-mail address: [email protected] (S. Kobayashi). 1 Graduate student of Yokohama National University.
microstructural evolution is thought to be important for improving ductility of Ni /P alloys. To date, there are some reports about microstructural evolution in Ni /P alloys with low P contents. The thermal stability of Ni /P alloys [4 /9] is higher than that of pure Ni [10 /13] because of solute-induced drag effect. Klement et al. [11] demonstrated that the grain growth in pure Ni with an initial grain size of 10 and 20 nm occurs even at 353 K (0.2Tm, where Tm is the melting point). The grain structure in Ni /1.2mass% P alloy has been shown to be stable at temperatures up to 633 K (0.38Tm) [8]. Grain structure stability of Ni /P alloys is improved by increasing P content. The grain growth behavior of nanocrystalline Ni /P alloys at lower temperatures (B/773 K) has been actively investigated from the viewpoint of evaluation of thermal stability, but it is thought that the observation of microstructural evolution at higher temperatures is also important for microstructure control. In relation to the mechanical properties of nanocrystalline Ni /P alloys, the relationship between grain size and hardness has been investigated [3,14]. The negative
0921-5093/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(03)00285-5
S. Kobayashi, Y. Kashikura / Materials Science and Engineering A358 (2003) 76 /83
slope of the Hall /Petch relationship, i.e., material softens with decreasing grain size, has been found at the case of grain smaller than a certain size (nanometer range), whereas coarser grain sizes exhibit the usual Hall /Petch relationship (positive slope). Little research has been carried out on other mechanical properties of nanocrystalline Ni /P alloys at room temperature, such as fracture and ductility. Sui et al. [15] investigated that the fracture of Ni /20at.% P (Ni / 11.7mass% P) alloy with ultrafine grains ranging from 5 to 100 nm. These specimens did not show plastic deformation, and both the fracture stress and fracture strain were increasing with decreasing grain size. The behavior of fracture stress and fracture strain was discussed from volume fraction and density of interfaces. However, the crack nucleation sites and crack propagation pass have been not clarified. In addition, the effects of grain orientation distribution, grain boundaries, and second phase on mechanical properties of nanocrystalline materials have not been understood. In particular, second phase precipitates when an electrodeposited Ni /P alloy is heat-treated and the mechanical properties are expected to change dramatically by the precipitation. In this work, electrodeposited Ni /4.4mass% P alloy was used as a starting material, because nanocrystalline structure and superior hardness were obtained at low P content range (less than about 5 mass%). In order to reveal the microstructure effective to improve ductility, the microstructural evolution and preferential site of crack nucleation in Ni /4.4mass% P alloy were investigated.
2. Experimental A nanocrystalline Ni /P alloy was produced by the electrodeposition onto a Ti substrate that was carried out by use of an electrolytic bath of 150 g l 1 nickel sulfate, 45 g l 1 nickel chloride, 80 g l 1 phosphoric acid and 4 g l 1 phosphorous acid, at pH 1.2, 338 K, and a current density of 2.0 mA mm 2 for 10.8 ks. The electrodeposited Ni /P alloy has a thickness of approximately 300 mm and a density of about 8.2 g cm 3 and contains 4.4 mass% P. Ni /P alloy sheet was mechanically stripped from the Ti substrate. The specimens were heat-treated at 873 K (0.51Tm), 973 K (0.56Tm) or 1073 K (0.62Tm) for 60 s/7.2 ks in an evacuated (1 /10 1 Pa) silica tube and cooled by aircooling to room temperature. For observation of microstructure, the mechanically polished specimens were electropolished in an electrolytic solution of 6 vol.% perchloric acid, 15 vol.% methanol and 79 vol.% acetic acid at 280 K, at a current density of 4 mA mm 2 for 15 s. An X-ray diffractometer (Rigaku JDX-3530) with monochromatic Cu Ka
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radiation and an electron back-scattered diffraction pattern (EBSP) analysis [16,17] on a scanning electron microscope (HITACHI S-4200 FE-SEM) was employed to analyze the orientation and phase change by heat treatment. A Vickers microhardness tester was employed to measure the microhardness of specimens of various microstructures developed by heat treatment, at an indentation load of 0.49 N for 15 s. The indentation left on the specimen was about 2/5 mm in depth, which is sufficiently shallow in relation to specimen thickness (300 mm). Five measurements were averaged in order to determine the hardness of each specimen. The bending tests were performed on the as-electrodeposited and heat-treated specimens measuring 40 mm in length, 6 mm in width and 0.2 mm in thick by use of a three-point bending jig with gauge length of 15 mm, at a cross-head speed of 0.04 mm s 1 and room temperature. Three measurements were performed in order to determine the bending stress and fracture strain of each specimen. The tests were stopped when the specimen reached a bending angle of 908 (about 2% strain).
3. Results and discussion 3.1. Initial microstructure of electrodeposited nanocrystalline Ni /4.4mass% P alloy Fig. 1 shows the XRD pattern for an as-electrodeposited specimen. Grain size is 37 nm, as determined from the full-width at half-maximum of (1 1 1) reflection by Scherrer equation. The surface of the as-electrodeposited specimen is strongly oriented around (1 1 1), indicating a preference for the planes with the lowest surface free energy to lie in the plane of the specimen. According to the g-plot showing the orientation dependence of the surface free energy for f.c.c. structure based on a bonds-broken model [18], the surface free energy exhibits a minimum value of 0.7 at {1 1 1}, when the energy of {0 1 2} is assumed to be unity.
Fig. 1. X-ray diffraction pattern of nanocrystalline Ni /4.4mass% P as-electrodeposited.
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Although Ni /P phase diagram [19] shows that the solubility limit of P into Ni matrix is 0.17 mass% and that Ni3P phase precipitates in alloys of high P content, Ni3P did not precipitate in the as-electrodeposited Ni / 4.4mass% P alloy. Boylan et al. [5] reported that the aselectrodeposited Ni /1.2mass% P alloy is a supersaturated solid solution that is stable up to 473 K. Moreover, in electroless-plated Ni /3.6at.% P (Ni /1.9mass% P) alloy as well, all the P is in solid solution [20]. This work showed that in the electrodeposited Ni /4.4mass% P alloy, P is dissolved into Ni matrix in an amount of about 25 times the solubility limit. Hentschel et al. [20] clarified, by atom-probe field-ion microscopy (APFIM), that the P atoms in the electrodeposited Ni /P alloy segregate at the grain boundary of Ni matrix. Since the volume fraction of grain boundary layers becomes remarkably high when grain size is refined to less than 100 nm [21], the excess P atoms might segregate at grain boundaries of Ni matrix. 3.2. Evolution of microstructures in nanocrystalline Ni / 4.4mass% P alloy
to (2 0 0), (2 2 0) and (3 1 1) planes. The tendency of precipitation of Ni3P phase and nucleation of Ni phase in various orientations were hardly changed, even for specimens that had been subjected to further heat treatment for 7.2 ks. Figs. 3(a) /(c) show the SEM micrographs of specimens heat-treated at 873, 973 and 1073 K for 60 s, respectively. We can see that the heat-treated specimens are composed of two phases, which appear as black and white grains. EBSPs show that the black and white grains are Ni and Ni3P grains, respectively. Moreover, both the Ni and Ni3P phases have equiaxed grain structure. As shown in Fig. 3(a), the Ni and Ni3P grains were rapidly grown to an average grain size of about 150 nm by heat treatment at 873 K for 60 s. The Ni and Ni3P grains form respective grain clusters where some grains in the same phase are connected together as shown in Figs. 3(a) /(c). The number of grains consisting clusters become smaller with increasing heat-treatment temperature and time. The interface energies of Ni3P/Ni3P and Ni3P/Ni interface are 0.111 and 0.155 J m 2, respectively [6]. Therefore, these grain
Figs. 2(a) /(c) show the XRD patterns of specimens that had been heat-treated at 873, 973 and 1073 K for 60 s, respectively. In all the heat-treated specimens, reflections of Ni3P phase are observed at the short heattreatment time of 60 s. For Ni phase, the observed reflections correspond not only to (1 1 1)-plane, but also
Fig. 2. X-ray diffraction patterns of Ni /4.4mass% P heat-treated at (a) 873 K for 60 s, (b) 973 K for 60 s and (c) 1073 K for 60 s.
Fig. 3. Microstructures of Ni /4.4mass% P alloy heat-treated at (a) 873 K for 60 s, (b) 973 K for 60 s and (c) 1073 K for 60 s.
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Fig. 4. Inverse pole-figure of Ni /4.4mass% P alloy heat-treated at 1073 K for 120 s.
clusters are probably formed in order to lower total boundary energy in the specimen. Fig. 4 shows the inverse pole-figure of Ni phase of the specimen that had been heat-treated at 1073 K for 120 s, as obtained by OIM analysis. Ni phase is weakly oriented to (1 1 1)-plane after heat treatment, whereas in the as-electrodeposited specimen shown in Fig. 1, Ni grains are strongly oriented to (1 1 1)-plane. The random orientation distribution of Ni phase is thought to be caused by nucleation and growth of Ni grains with various surface orientations that accompanied with precipitation of Ni3P phase. Figs. 5(a), (b) and (c) show the relationship between grain size and heat-treatment time at the temperatures of 873, 973 and 1073 K, respectively. In all three cases, the Ni grains grow rapidly at the initial stage of heat treatment, i.e., within 60 s, and after heat treatment for 600 s grain growth immediately reaches steady state by heat treatment. At 60 s, Ni3P grains are rapidly precipitated and grow to about the same order of grain size as Ni grains. Heat treatment beyond 600 s results in negligible growth of Ni3P grains. Heat-treatment temperature affects grain growth, especially at the initial stage; higher temperature induces faster grain growth at the initial stage. Moreover, although in specimens heattreated at 873 K Ni and Ni3P grains are of similar size, in specimens treated at higher temperature Ni grains become larger than Ni3P grains. Longer heat treatment was found to result in scattering of grain size of both the Ni and Ni3P phases, within the range 800 /3300 nm. Fig. 6 shows the grain size distribution of Ni and Ni3P grains in the specimen heattreated at 1073 K for 7.2 ks. Both phases show normal distribution of grain size, although the distributions are widely scattered. In general, nanocrystalline materials have unstable microstructure. Remarkable grain growth has been reported to occur even at 353 K (0.20Tm) in nanocrystalline Ni [11] and at 633 K (0.37Tm) in nanocrystalline Ni /1.2mass% P alloy [8]. In the nanocrystalline Ni / 4.4mass% P alloy of this work, grain growth was relatively slow even at 873 K (0.51Tm). Thus, higher content of P is shown to give higher stability to microstructure of Ni /P alloys.
Fig. 5. Relationship between grain size and heat-treatment time at (a) 873 K, (b) 973 K and (c) 1073 K.
3.3. Grain growth kinetics in nanocrystalline Ni / 4.4mass% P alloy Fig. 7 shows, in a log /log plot, the relationship between heat-treatment time and grain size of Ni and Ni3P phases, respectively, at temperatures ranging from 873 to 1073 K. Heat-treatment time is limited to 600 s before grain growth rate becomes negligibly small because the grain size of Ni /4.4mass% P alloy tend to be asymptotic towards a constant value. The slopes of the straight lines in Fig. 7 represent grain growth exponent n. The n values in Ni and Ni3P phases are determined as 4.5 /5.0 and 4.9 /5.4, respectively. These n values were close to the values of 4.0 /5.0 for grain boundary diffusion-controlled growth in duplex alloy [22]. The n value for Ni and Ni3P phases decreases with increasing heat-treatment temperature. The tendency towards lower n value at higher temperatures has been often reported for the other polycrystalline and nanocrystalline materials [23,24]. The temperature dependence of n value in this work is likely caused by
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Fig. 8. Plots of ln(d n d0n ) versus 1/T for Ni and Ni3P phases in Ni / 4.4mass% P alloy.
Fig. 6. Grain size distribution of Ni and Ni3P phases in Ni /4.4mass% P alloy heat-treated at 1073 K for 7.2 ks.
Fig. 8 shows the temperature dependence of grain growth. From the slopes of these plots, the activation energies for grain growth in Ni and Ni3P phases are estimated to be QNi /210 kJ mol 1 and QNi3P /180 kJ mol 1, respectively. The activation energy for selfdiffusion and grain boundary self-diffusion in Ni is about 280 kJ mol 1 [26] and 110 /200 kJ mol 1 [27], respectively. Obtained activation energies were slightly larger than that for grain boundary self-diffusion in Ni. The larger activation energies were probably induced by duplex microstructure of Ni and Ni3P phases. Lu et al. [6] reported that the activation energy for grain growth in Ni /20at.% P (Ni /11.7mass% P) alloy composed of Ni of 9 nm in grain size and Ni3P of 13 nm in grain size. The activation energies for these two phases were found to be 182 and 156 kJ mol 1, respectively, i.e., the activation energy of Ni3P phase is lower than that of Ni phase. The alloy employed in this work exhibits a similar tendency.
3.4. Relationship between grain structure and mechanical properties of Ni /4.4mass% P alloys Fig. 7. Plots of ln(d /d0) versus ln t for Ni and Ni3P phases in Ni / 4.4mass% P alloy heat-treated at 873, 973 and 1073 K.
weakening of grain boundary pinning by coarsening Ni3P precipitates at higher temperature. In Ni /1.2mass% P alloy having an initial grain size of 5 nm, n is estimated to be 3.0 for the temperature range 573 /673 K [25]. Although the heat-treatment temperature in this work (873 /1073 K) is higher than that of the employed in the above study, a higher value of n is derived. Since the specimens employed in this work have higher P content, grain growth must be strongly inhibited by the higher volume fraction of Ni3P precipitates.
Fig. 9 shows the grain size dependence of stress / strain curves obtained by the three-point bending test. The as-electrodeposited Ni /4.4mass% P alloy is fractured prior to yielding. Heat-treated specimens having grain sizes of 140/280 nm are also fractured prior to yielding. In these specimens, fracture stress increases slightly with decreasing grain size. These specimens show an elastic strain of less than 1.0% to fracture, which is smaller than that of the as-electrodeposited specimen. When the grains grow to about 420 nm, the specimen yields and undergoes slight plastic deformation. Specimens of larger grain size show larger plastic strain. When grain size reaches about 650 nm, specimen fracture does not occur.
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Fig. 9. Grain size dependence of stress /strain curves of Ni /4.4mass% P alloy.
Fig. 10 shows the grain size dependence of Vickers hardness, fracture stress and fracture strain as determined by the three-point bending tests for the fractured specimens. The Hall /Petch relationship can be recognized between Vickers hardness and grain size in the heat-treated specimens. There is no definite grain size dependence of fracture stress. Although none of the specimens fractured by the Vickers hardness tests, the brittle fracture was occurred at bending tests. Therefore, the grain size dependence of fracture stress is not definite. The as-electrodeposited specimen having no
Ni3P precipitation deviated from the relationship between hardness and grain size found in heat-treated twophase Ni /P alloys. The as-electrodeposited specimen exhibits an elastic strain of about 1.2% to fracture. The heat-treated specimens having grain sizes less than 400 nm were fractured by a small strain of about 0.9%, which is almost constant regardless of grain size. These specimens did not show plastic deformation. However, in specimens of grain size exceeding 400 nm, fracture strain is found to increase drastically with plastic deformation. According to Sui et al. [15], Ni /11.7mass% P alloy having grain size of 5 /100 nm was fractured prior to yielding. On the other hand, it was found that Ni / 4.4mass% P alloy having grain size more than 400 nm shows plastic deformation. The discrepancy of these results was probably caused by difference of grain size and P content. It is important for improving ductility of Ni /P alloy to reveal effect of grain size and P content on fracture. 3.5. Inhibition of fracture in Ni/4.4mass% P alloy by microstructural control
Fig. 10. Grain size dependence of Vickers hardness, fracture stress and fracture strain of Ni /4.4mass% P alloy.
In order to reveal the microstructure dominating the fracture strain, fracture surfaces formed by three-point bending tests are observed. Fig. 11(a) is an example of a micrograph taken near the fracture surface subjected to tensile stress and Fig. 11(b) is a schematic illustration of the surrounding area in the micrograph. The micrograph shows that the fracture in Ni /4.4mass% P alloy was caused by remarkable grain boundary fracture. The Ni/Ni3P and Ni3P/Ni3P boundaries were the preferential sites of crack nucleation shown in Fig. 11(b). The cracks nucleated at these boundaries are thought to have propagated and connected with increasing strain, resulting in fracture. Therefore, the number of Ni3P grains consisting Ni3P grain clusters is thought to be the dominant factor of fracture in Ni /P alloy, because the number of Ni3P grains consisting grain clusters is
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Figs. 11(a) and (b), crack nucleation preferentially occurred at Ni/Ni3P and Ni3P/Ni3P boundaries. Therefore, the specimens with higher N value were fractured at smaller strains. In contrast, specimens with lower N value are thought to show larger fracture strain, because of shorter propagation paths. Higher N value in the specimens of smaller grain size is explained by interface energies between Ni/Ni3P and Ni3P/Ni3P boundaries. As discussed in Section 3.2, since the energy of Ni/Ni3P boundary is about 40% higher than that of Ni3P/Ni3P boundary [6], Ni3P grains are connected to each other in order to lower the total boundary energy of materials. In the larger grained specimens, since the density of grain and phase boundaries becomes lower, total boundary energy should decrease even though Ni/Ni3P boundaries are more common. Therefore, N value in smaller grained specimens is thought to be greater than that in larger grained specimens. The results of this work reveal that the grain size and N value mainly dominates the fracture of heat-treated Ni /4.4mass% P alloy. Improvement of ductility of Ni / P alloy, with low P content that has been considered as remarkable brittleness at room temperature, is attained by lowering N value accompanied with coarsening grains. Fig. 11. (a) SEM micrograph and (b) schematic illustration near fracture surface.
4. Conclusions connected with connectivity of Ni/Ni3P and Ni3P/Ni3P boundaries. Fig. 12 shows the relationship between average number of Ni3P grains consisting grain clusters (connectivity of Ni3P grains, N ) and grain size in heattreated Ni /P specimens. N value decreases sensitively from 7 to 4 when grain size is increased from 140 to 400 nm. Moreover, the decrease in N value becomes small in specimens having grain sizes greater than 400 nm. As shown in Fig. 9, the specimens having grain sizes less than 400 nm were fractured prior to yielding. These specimens have higher N value. The higher N value induced longer propagation paths, because, as shown in
Fig. 12. Relationship between connectivity of Ni3P grains and grain size in heat-treated Ni /4.4mass% P alloy.
In order to determine the optimal grain structure for improving strength and ductility of electrodeposited Ni /4.4mass% P alloy, effects of heat-treatment on grain growth and mechanical properties were investigated. The following conclusions were obtained. (1) As-electrodeposited Ni /4.4mass% P alloy is in solid solution, and Ni grains are strongly oriented to (1 1 1). (2) Precipitation of Ni3P phase and rapid grain growth occurs in the initial stage of heat treatment, and further grain growth is negligibly small, even in the case where heat-treatment temperature is above 873 K (0.51Tm). (3) The values of grain growth exponent in Ni and Ni3P phases are n /4.5 /5.9 and 4.9 /5.4, respectively. These values are close to the values for grain boundary diffusion-controlled growth in duplex alloy. The activation energies for grain growth in Ni and Ni3P phases are estimated to be 210 and 180 kJ mol 1, respectively. (4) While the nanocrystalline and ultrafine-grained specimens does not show plastic deformation, the larger grained specimens (/400 nm) show plastic deformation. Bending stress and fracture strain in heat-treated Ni /4.4mass% P two-phase alloys are controlled not only by grain size, but also by the connectivity of Ni3P grains. Large fracture strain with plastic strain was
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obtained by lowering connectivity of Ni3P grains accompanied with coarsening grains.
Acknowledgements The authors would like to thank Prof. Takao Yakou, Yokohama National University, for his useful comments and advice. The authors heartily thank Mr. Kouji Yamashita, a student at Yokohama National University, for his assistance in this work. Also, the authors are grateful to Prof. Tadao Watanabe and Sadahiro Tsurekawa, Tohoku University, for supporting SEM/EBSP measurements.
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Grain growth and mechanical properties of electrodeposited nanocrystalline nickel 4.4mass% phosphorus alloy /
Shigeaki Kobayashi *, Youhei Kashikura 1 Department of Materials Engineering, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogayaku, Yokohama 240-8501, Japan Received 2 December 2002; received in revised form 17 March 2003; accepted 4 April 2003
Abstract The relationship between microstructural evolution and mechanical properties in Ni /4.4mass% P alloy was systematically examined. As-electrodeposited Ni /4.4mass% P alloy consists of grains having an average grain size of 37 nm and texture strongly oriented to (1 1 1). Phosphorus is entirely in solid solution. Heat treatment induces precipitation of Ni3P phase. Rapid grain growth occurs only at the initial stage of heat treatment, and saturated at all the annealing temperatures, even at temperatures higher than 873 K (0.51Tm). Grain growth exponents n in Ni and Ni3P phases were found to be 4.5 /5.9 and 4.9 /5.4, respectively. These values are close to those for grain boundary diffusion-controlled growth. Activation energies for grain growth in Ni and Ni3P phases were estimated as 210 and 180 kJ mol 1, respectively. The nanocrystalline and ultrafine-grained specimens (about 40 /400 nm) fractured prior to yielding, while the larger grained specimens ( /400 nm) showed plastic deformation. Fracture strength and fracture strain depend not only on grain size but also on connectivity of Ni3P grains, where cracks were preferentially nucleated. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline nickel /phosphorus alloy; Grain growth; Grain morphology; Microhardness; Fracture strength; Fracture strain
1. Introduction Electrodeposited nickel /phosphorus (Ni /P) alloys have found applications to underlying layer of electronic devices and magnetic disks. As P content increases during electrodeposition, alloy structure changes from fine-grained to nanocrystalline, and eventually becomes amorphous [1]. In particular, nanocrystalline Ni /P alloys that obtained at low P contents (less than about 5 mass% P) have superior hardness of HV660 [2,3]. The hardness of these alloys is increased up to HV990 by annealing owing to precipitation of hard Ni3P phase [2]. Thus, annealed Ni /P alloys are expected to use as wearresistant coatings. However, Ni /P alloys show so severe brittleness that their workability is substantially limited. Improvement of the brittleness in Ni /P alloys is an important aspect for realizing widespread applications. Evaluation of mechanical properties that related to
* Corresponding author. Tel./fax: /81-45-339-3845. E-mail address: [email protected] (S. Kobayashi). 1 Graduate student of Yokohama National University.
microstructural evolution is thought to be important for improving ductility of Ni /P alloys. To date, there are some reports about microstructural evolution in Ni /P alloys with low P contents. The thermal stability of Ni /P alloys [4 /9] is higher than that of pure Ni [10 /13] because of solute-induced drag effect. Klement et al. [11] demonstrated that the grain growth in pure Ni with an initial grain size of 10 and 20 nm occurs even at 353 K (0.2Tm, where Tm is the melting point). The grain structure in Ni /1.2mass% P alloy has been shown to be stable at temperatures up to 633 K (0.38Tm) [8]. Grain structure stability of Ni /P alloys is improved by increasing P content. The grain growth behavior of nanocrystalline Ni /P alloys at lower temperatures (B/773 K) has been actively investigated from the viewpoint of evaluation of thermal stability, but it is thought that the observation of microstructural evolution at higher temperatures is also important for microstructure control. In relation to the mechanical properties of nanocrystalline Ni /P alloys, the relationship between grain size and hardness has been investigated [3,14]. The negative
0921-5093/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(03)00285-5
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slope of the Hall /Petch relationship, i.e., material softens with decreasing grain size, has been found at the case of grain smaller than a certain size (nanometer range), whereas coarser grain sizes exhibit the usual Hall /Petch relationship (positive slope). Little research has been carried out on other mechanical properties of nanocrystalline Ni /P alloys at room temperature, such as fracture and ductility. Sui et al. [15] investigated that the fracture of Ni /20at.% P (Ni / 11.7mass% P) alloy with ultrafine grains ranging from 5 to 100 nm. These specimens did not show plastic deformation, and both the fracture stress and fracture strain were increasing with decreasing grain size. The behavior of fracture stress and fracture strain was discussed from volume fraction and density of interfaces. However, the crack nucleation sites and crack propagation pass have been not clarified. In addition, the effects of grain orientation distribution, grain boundaries, and second phase on mechanical properties of nanocrystalline materials have not been understood. In particular, second phase precipitates when an electrodeposited Ni /P alloy is heat-treated and the mechanical properties are expected to change dramatically by the precipitation. In this work, electrodeposited Ni /4.4mass% P alloy was used as a starting material, because nanocrystalline structure and superior hardness were obtained at low P content range (less than about 5 mass%). In order to reveal the microstructure effective to improve ductility, the microstructural evolution and preferential site of crack nucleation in Ni /4.4mass% P alloy were investigated.
2. Experimental A nanocrystalline Ni /P alloy was produced by the electrodeposition onto a Ti substrate that was carried out by use of an electrolytic bath of 150 g l 1 nickel sulfate, 45 g l 1 nickel chloride, 80 g l 1 phosphoric acid and 4 g l 1 phosphorous acid, at pH 1.2, 338 K, and a current density of 2.0 mA mm 2 for 10.8 ks. The electrodeposited Ni /P alloy has a thickness of approximately 300 mm and a density of about 8.2 g cm 3 and contains 4.4 mass% P. Ni /P alloy sheet was mechanically stripped from the Ti substrate. The specimens were heat-treated at 873 K (0.51Tm), 973 K (0.56Tm) or 1073 K (0.62Tm) for 60 s/7.2 ks in an evacuated (1 /10 1 Pa) silica tube and cooled by aircooling to room temperature. For observation of microstructure, the mechanically polished specimens were electropolished in an electrolytic solution of 6 vol.% perchloric acid, 15 vol.% methanol and 79 vol.% acetic acid at 280 K, at a current density of 4 mA mm 2 for 15 s. An X-ray diffractometer (Rigaku JDX-3530) with monochromatic Cu Ka
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radiation and an electron back-scattered diffraction pattern (EBSP) analysis [16,17] on a scanning electron microscope (HITACHI S-4200 FE-SEM) was employed to analyze the orientation and phase change by heat treatment. A Vickers microhardness tester was employed to measure the microhardness of specimens of various microstructures developed by heat treatment, at an indentation load of 0.49 N for 15 s. The indentation left on the specimen was about 2/5 mm in depth, which is sufficiently shallow in relation to specimen thickness (300 mm). Five measurements were averaged in order to determine the hardness of each specimen. The bending tests were performed on the as-electrodeposited and heat-treated specimens measuring 40 mm in length, 6 mm in width and 0.2 mm in thick by use of a three-point bending jig with gauge length of 15 mm, at a cross-head speed of 0.04 mm s 1 and room temperature. Three measurements were performed in order to determine the bending stress and fracture strain of each specimen. The tests were stopped when the specimen reached a bending angle of 908 (about 2% strain).
3. Results and discussion 3.1. Initial microstructure of electrodeposited nanocrystalline Ni /4.4mass% P alloy Fig. 1 shows the XRD pattern for an as-electrodeposited specimen. Grain size is 37 nm, as determined from the full-width at half-maximum of (1 1 1) reflection by Scherrer equation. The surface of the as-electrodeposited specimen is strongly oriented around (1 1 1), indicating a preference for the planes with the lowest surface free energy to lie in the plane of the specimen. According to the g-plot showing the orientation dependence of the surface free energy for f.c.c. structure based on a bonds-broken model [18], the surface free energy exhibits a minimum value of 0.7 at {1 1 1}, when the energy of {0 1 2} is assumed to be unity.
Fig. 1. X-ray diffraction pattern of nanocrystalline Ni /4.4mass% P as-electrodeposited.
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Although Ni /P phase diagram [19] shows that the solubility limit of P into Ni matrix is 0.17 mass% and that Ni3P phase precipitates in alloys of high P content, Ni3P did not precipitate in the as-electrodeposited Ni / 4.4mass% P alloy. Boylan et al. [5] reported that the aselectrodeposited Ni /1.2mass% P alloy is a supersaturated solid solution that is stable up to 473 K. Moreover, in electroless-plated Ni /3.6at.% P (Ni /1.9mass% P) alloy as well, all the P is in solid solution [20]. This work showed that in the electrodeposited Ni /4.4mass% P alloy, P is dissolved into Ni matrix in an amount of about 25 times the solubility limit. Hentschel et al. [20] clarified, by atom-probe field-ion microscopy (APFIM), that the P atoms in the electrodeposited Ni /P alloy segregate at the grain boundary of Ni matrix. Since the volume fraction of grain boundary layers becomes remarkably high when grain size is refined to less than 100 nm [21], the excess P atoms might segregate at grain boundaries of Ni matrix. 3.2. Evolution of microstructures in nanocrystalline Ni / 4.4mass% P alloy
to (2 0 0), (2 2 0) and (3 1 1) planes. The tendency of precipitation of Ni3P phase and nucleation of Ni phase in various orientations were hardly changed, even for specimens that had been subjected to further heat treatment for 7.2 ks. Figs. 3(a) /(c) show the SEM micrographs of specimens heat-treated at 873, 973 and 1073 K for 60 s, respectively. We can see that the heat-treated specimens are composed of two phases, which appear as black and white grains. EBSPs show that the black and white grains are Ni and Ni3P grains, respectively. Moreover, both the Ni and Ni3P phases have equiaxed grain structure. As shown in Fig. 3(a), the Ni and Ni3P grains were rapidly grown to an average grain size of about 150 nm by heat treatment at 873 K for 60 s. The Ni and Ni3P grains form respective grain clusters where some grains in the same phase are connected together as shown in Figs. 3(a) /(c). The number of grains consisting clusters become smaller with increasing heat-treatment temperature and time. The interface energies of Ni3P/Ni3P and Ni3P/Ni interface are 0.111 and 0.155 J m 2, respectively [6]. Therefore, these grain
Figs. 2(a) /(c) show the XRD patterns of specimens that had been heat-treated at 873, 973 and 1073 K for 60 s, respectively. In all the heat-treated specimens, reflections of Ni3P phase are observed at the short heattreatment time of 60 s. For Ni phase, the observed reflections correspond not only to (1 1 1)-plane, but also
Fig. 2. X-ray diffraction patterns of Ni /4.4mass% P heat-treated at (a) 873 K for 60 s, (b) 973 K for 60 s and (c) 1073 K for 60 s.
Fig. 3. Microstructures of Ni /4.4mass% P alloy heat-treated at (a) 873 K for 60 s, (b) 973 K for 60 s and (c) 1073 K for 60 s.
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Fig. 4. Inverse pole-figure of Ni /4.4mass% P alloy heat-treated at 1073 K for 120 s.
clusters are probably formed in order to lower total boundary energy in the specimen. Fig. 4 shows the inverse pole-figure of Ni phase of the specimen that had been heat-treated at 1073 K for 120 s, as obtained by OIM analysis. Ni phase is weakly oriented to (1 1 1)-plane after heat treatment, whereas in the as-electrodeposited specimen shown in Fig. 1, Ni grains are strongly oriented to (1 1 1)-plane. The random orientation distribution of Ni phase is thought to be caused by nucleation and growth of Ni grains with various surface orientations that accompanied with precipitation of Ni3P phase. Figs. 5(a), (b) and (c) show the relationship between grain size and heat-treatment time at the temperatures of 873, 973 and 1073 K, respectively. In all three cases, the Ni grains grow rapidly at the initial stage of heat treatment, i.e., within 60 s, and after heat treatment for 600 s grain growth immediately reaches steady state by heat treatment. At 60 s, Ni3P grains are rapidly precipitated and grow to about the same order of grain size as Ni grains. Heat treatment beyond 600 s results in negligible growth of Ni3P grains. Heat-treatment temperature affects grain growth, especially at the initial stage; higher temperature induces faster grain growth at the initial stage. Moreover, although in specimens heattreated at 873 K Ni and Ni3P grains are of similar size, in specimens treated at higher temperature Ni grains become larger than Ni3P grains. Longer heat treatment was found to result in scattering of grain size of both the Ni and Ni3P phases, within the range 800 /3300 nm. Fig. 6 shows the grain size distribution of Ni and Ni3P grains in the specimen heattreated at 1073 K for 7.2 ks. Both phases show normal distribution of grain size, although the distributions are widely scattered. In general, nanocrystalline materials have unstable microstructure. Remarkable grain growth has been reported to occur even at 353 K (0.20Tm) in nanocrystalline Ni [11] and at 633 K (0.37Tm) in nanocrystalline Ni /1.2mass% P alloy [8]. In the nanocrystalline Ni / 4.4mass% P alloy of this work, grain growth was relatively slow even at 873 K (0.51Tm). Thus, higher content of P is shown to give higher stability to microstructure of Ni /P alloys.
Fig. 5. Relationship between grain size and heat-treatment time at (a) 873 K, (b) 973 K and (c) 1073 K.
3.3. Grain growth kinetics in nanocrystalline Ni / 4.4mass% P alloy Fig. 7 shows, in a log /log plot, the relationship between heat-treatment time and grain size of Ni and Ni3P phases, respectively, at temperatures ranging from 873 to 1073 K. Heat-treatment time is limited to 600 s before grain growth rate becomes negligibly small because the grain size of Ni /4.4mass% P alloy tend to be asymptotic towards a constant value. The slopes of the straight lines in Fig. 7 represent grain growth exponent n. The n values in Ni and Ni3P phases are determined as 4.5 /5.0 and 4.9 /5.4, respectively. These n values were close to the values of 4.0 /5.0 for grain boundary diffusion-controlled growth in duplex alloy [22]. The n value for Ni and Ni3P phases decreases with increasing heat-treatment temperature. The tendency towards lower n value at higher temperatures has been often reported for the other polycrystalline and nanocrystalline materials [23,24]. The temperature dependence of n value in this work is likely caused by
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Fig. 8. Plots of ln(d n d0n ) versus 1/T for Ni and Ni3P phases in Ni / 4.4mass% P alloy.
Fig. 6. Grain size distribution of Ni and Ni3P phases in Ni /4.4mass% P alloy heat-treated at 1073 K for 7.2 ks.
Fig. 8 shows the temperature dependence of grain growth. From the slopes of these plots, the activation energies for grain growth in Ni and Ni3P phases are estimated to be QNi /210 kJ mol 1 and QNi3P /180 kJ mol 1, respectively. The activation energy for selfdiffusion and grain boundary self-diffusion in Ni is about 280 kJ mol 1 [26] and 110 /200 kJ mol 1 [27], respectively. Obtained activation energies were slightly larger than that for grain boundary self-diffusion in Ni. The larger activation energies were probably induced by duplex microstructure of Ni and Ni3P phases. Lu et al. [6] reported that the activation energy for grain growth in Ni /20at.% P (Ni /11.7mass% P) alloy composed of Ni of 9 nm in grain size and Ni3P of 13 nm in grain size. The activation energies for these two phases were found to be 182 and 156 kJ mol 1, respectively, i.e., the activation energy of Ni3P phase is lower than that of Ni phase. The alloy employed in this work exhibits a similar tendency.
3.4. Relationship between grain structure and mechanical properties of Ni /4.4mass% P alloys Fig. 7. Plots of ln(d /d0) versus ln t for Ni and Ni3P phases in Ni / 4.4mass% P alloy heat-treated at 873, 973 and 1073 K.
weakening of grain boundary pinning by coarsening Ni3P precipitates at higher temperature. In Ni /1.2mass% P alloy having an initial grain size of 5 nm, n is estimated to be 3.0 for the temperature range 573 /673 K [25]. Although the heat-treatment temperature in this work (873 /1073 K) is higher than that of the employed in the above study, a higher value of n is derived. Since the specimens employed in this work have higher P content, grain growth must be strongly inhibited by the higher volume fraction of Ni3P precipitates.
Fig. 9 shows the grain size dependence of stress / strain curves obtained by the three-point bending test. The as-electrodeposited Ni /4.4mass% P alloy is fractured prior to yielding. Heat-treated specimens having grain sizes of 140/280 nm are also fractured prior to yielding. In these specimens, fracture stress increases slightly with decreasing grain size. These specimens show an elastic strain of less than 1.0% to fracture, which is smaller than that of the as-electrodeposited specimen. When the grains grow to about 420 nm, the specimen yields and undergoes slight plastic deformation. Specimens of larger grain size show larger plastic strain. When grain size reaches about 650 nm, specimen fracture does not occur.
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Fig. 9. Grain size dependence of stress /strain curves of Ni /4.4mass% P alloy.
Fig. 10 shows the grain size dependence of Vickers hardness, fracture stress and fracture strain as determined by the three-point bending tests for the fractured specimens. The Hall /Petch relationship can be recognized between Vickers hardness and grain size in the heat-treated specimens. There is no definite grain size dependence of fracture stress. Although none of the specimens fractured by the Vickers hardness tests, the brittle fracture was occurred at bending tests. Therefore, the grain size dependence of fracture stress is not definite. The as-electrodeposited specimen having no
Ni3P precipitation deviated from the relationship between hardness and grain size found in heat-treated twophase Ni /P alloys. The as-electrodeposited specimen exhibits an elastic strain of about 1.2% to fracture. The heat-treated specimens having grain sizes less than 400 nm were fractured by a small strain of about 0.9%, which is almost constant regardless of grain size. These specimens did not show plastic deformation. However, in specimens of grain size exceeding 400 nm, fracture strain is found to increase drastically with plastic deformation. According to Sui et al. [15], Ni /11.7mass% P alloy having grain size of 5 /100 nm was fractured prior to yielding. On the other hand, it was found that Ni / 4.4mass% P alloy having grain size more than 400 nm shows plastic deformation. The discrepancy of these results was probably caused by difference of grain size and P content. It is important for improving ductility of Ni /P alloy to reveal effect of grain size and P content on fracture. 3.5. Inhibition of fracture in Ni/4.4mass% P alloy by microstructural control
Fig. 10. Grain size dependence of Vickers hardness, fracture stress and fracture strain of Ni /4.4mass% P alloy.
In order to reveal the microstructure dominating the fracture strain, fracture surfaces formed by three-point bending tests are observed. Fig. 11(a) is an example of a micrograph taken near the fracture surface subjected to tensile stress and Fig. 11(b) is a schematic illustration of the surrounding area in the micrograph. The micrograph shows that the fracture in Ni /4.4mass% P alloy was caused by remarkable grain boundary fracture. The Ni/Ni3P and Ni3P/Ni3P boundaries were the preferential sites of crack nucleation shown in Fig. 11(b). The cracks nucleated at these boundaries are thought to have propagated and connected with increasing strain, resulting in fracture. Therefore, the number of Ni3P grains consisting Ni3P grain clusters is thought to be the dominant factor of fracture in Ni /P alloy, because the number of Ni3P grains consisting grain clusters is
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Figs. 11(a) and (b), crack nucleation preferentially occurred at Ni/Ni3P and Ni3P/Ni3P boundaries. Therefore, the specimens with higher N value were fractured at smaller strains. In contrast, specimens with lower N value are thought to show larger fracture strain, because of shorter propagation paths. Higher N value in the specimens of smaller grain size is explained by interface energies between Ni/Ni3P and Ni3P/Ni3P boundaries. As discussed in Section 3.2, since the energy of Ni/Ni3P boundary is about 40% higher than that of Ni3P/Ni3P boundary [6], Ni3P grains are connected to each other in order to lower the total boundary energy of materials. In the larger grained specimens, since the density of grain and phase boundaries becomes lower, total boundary energy should decrease even though Ni/Ni3P boundaries are more common. Therefore, N value in smaller grained specimens is thought to be greater than that in larger grained specimens. The results of this work reveal that the grain size and N value mainly dominates the fracture of heat-treated Ni /4.4mass% P alloy. Improvement of ductility of Ni / P alloy, with low P content that has been considered as remarkable brittleness at room temperature, is attained by lowering N value accompanied with coarsening grains. Fig. 11. (a) SEM micrograph and (b) schematic illustration near fracture surface.
4. Conclusions connected with connectivity of Ni/Ni3P and Ni3P/Ni3P boundaries. Fig. 12 shows the relationship between average number of Ni3P grains consisting grain clusters (connectivity of Ni3P grains, N ) and grain size in heattreated Ni /P specimens. N value decreases sensitively from 7 to 4 when grain size is increased from 140 to 400 nm. Moreover, the decrease in N value becomes small in specimens having grain sizes greater than 400 nm. As shown in Fig. 9, the specimens having grain sizes less than 400 nm were fractured prior to yielding. These specimens have higher N value. The higher N value induced longer propagation paths, because, as shown in
Fig. 12. Relationship between connectivity of Ni3P grains and grain size in heat-treated Ni /4.4mass% P alloy.
In order to determine the optimal grain structure for improving strength and ductility of electrodeposited Ni /4.4mass% P alloy, effects of heat-treatment on grain growth and mechanical properties were investigated. The following conclusions were obtained. (1) As-electrodeposited Ni /4.4mass% P alloy is in solid solution, and Ni grains are strongly oriented to (1 1 1). (2) Precipitation of Ni3P phase and rapid grain growth occurs in the initial stage of heat treatment, and further grain growth is negligibly small, even in the case where heat-treatment temperature is above 873 K (0.51Tm). (3) The values of grain growth exponent in Ni and Ni3P phases are n /4.5 /5.9 and 4.9 /5.4, respectively. These values are close to the values for grain boundary diffusion-controlled growth in duplex alloy. The activation energies for grain growth in Ni and Ni3P phases are estimated to be 210 and 180 kJ mol 1, respectively. (4) While the nanocrystalline and ultrafine-grained specimens does not show plastic deformation, the larger grained specimens (/400 nm) show plastic deformation. Bending stress and fracture strain in heat-treated Ni /4.4mass% P two-phase alloys are controlled not only by grain size, but also by the connectivity of Ni3P grains. Large fracture strain with plastic strain was
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obtained by lowering connectivity of Ni3P grains accompanied with coarsening grains.
Acknowledgements The authors would like to thank Prof. Takao Yakou, Yokohama National University, for his useful comments and advice. The authors heartily thank Mr. Kouji Yamashita, a student at Yokohama National University, for his assistance in this work. Also, the authors are grateful to Prof. Tadao Watanabe and Sadahiro Tsurekawa, Tohoku University, for supporting SEM/EBSP measurements.
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