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Mechanical properties of nickel silicon carbide nanocomposites
Materials Science and Engineering A328 (2002) 137– 146 www.elsevier.com/locate/msea
Mechanical properties of nickel silicon carbide nanocomposites A.F. Zimmerman a, G. Palumbo b, K.T. Aust a, U. Erb a,* a
Department of Materials Science and Engineering, Uni6ersity of Toronto, Toronto, ON, Canada, MS5 3E4 b Integran Technologies, 1 Meridian Rd., Toronto, ON, Canada, M9W 4Z6 Received 13 March 2001; received in revised form 11 June 2001
Abstract Nanocomposite materials consisting of a nanocrystalline Ni matrix (grain size 10 – 15 nm) reinforced with sub-micron size SiC particulates (average particle size: 0.4 mm) up to 10.5 vol.% have been produced by pulse electrodeposition. Substantial improvements in mechanical properties including hardness, yield and tensile stress were obtained for the nanocomposite material, as compared with conventional Ni–SiC composites with a matrix grain size in the micrometer range. Tensile strengths up to four times that for conventional polycrystalline Ni and two times that for conventional polycrystalline Ni– SiC of comparable SiC content was measured. The tensile and yield strengths of the nanocomposite material with SiC content less than 2 vol.% were higher than those for pure nanocrystalline Ni of comparable grain size. For these nanocomposites an unexpected increase in tensile ductility was also observed when compared to pure nanocrystalline nickel. At higher SiC content ( \2 vol.%) the strength and ductility were found to decrease to the detriment of the nanocomposite. Particle clustering was considered the main cause of this decrease. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ductility; Microhardness; Nickel silicon carbide nanocomposite; Tensile strength
1. Introduction The mechanical properties of polycrystalline metals, alloys and MMCs are very sensitive to their grain size. In many cases the flow stress, at low temperature, depends on the grain size according to the Hall –Petch relationship: |y =|0 +kd − 1/2
(1)
where |y is the yield stress, |0 is the friction stress, d is the grain size and k is the Hall – Petch coefficient (i.e. the Hall –Petch slope) [1,2]. Many models, based on the dislocation theory, have been presented to interpret this phenomenological equation (e.g. [3 – 7]). Restrictions on dislocation generation and mobility imposed by ultrafine grain size are believed to be the dominant factor in raising strength. Numerous investigators have reported very high strengths associated with Hall – Petch behaviour in nanocrystalline materials (e.g. [8 – 13]). For some materials, an unexpected transition from grain-size strength* Corresponding author. Fax: + 1-416-946-3316. E-mail address: [email protected] (U. Erb).
ening to grain-size softening (i.e. ‘inverse’ Hall –Petch behavior) was observed at a critical grain size [8,9,12 – 20]. For example, Palumbo et al. [13] reported softening effects for a series of as-prepared nanocrystalline Ni –P alloys with grain sizes in the range from 20 down to 3 nm. The data showed an initial increase in hardness followed by a deviation from regular Hall –Petch behaviour leading to softening for the smallest grain size range. The maximum in the hardness curve occurred for a grain size of approximately 8 nm. Gleiter [21] first recognized that the intercrystalline component of a material will form a distinct microstructural constituent when a material is processed to have a nanocrystalline structure. A detailed study by Palumbo et al. [22] using grains modeled in the shape of regular tetrakaidecahedrons and assuming a grain boundary thickness of 1 nm showed that the grain boundary volume fraction increases rapidly from a few percent at a grain size of 100 nm to over 50% at a grain size of 2 nm. For grain sizes below 20 nm, the triple junction (i.e. intersection line of three or more grain boundaries) volume fraction reaches significant values and exhibits a greater grain size dependence than does the grain boundary volume fraction. In fact, it has been
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observed that the onset of the deviation from regular Hall –Petch behaviour often occurs at grain sizes where the triple junction volume fraction in the materials reaches significant values, as discussed elsewhere in more detail [13,23]. Some of the unique mechanical properties of nanocomposite materials including increased strength and hardness [24,25], increased wear resistance and decreased coefficient of friction [24], can also in part be attributed to grain size effects caused by the increased interfacial defect volume fraction, in addition to the strengthening effects by the reinforcing phase. The purpose of the present study was: (1) to prepare bulk samples of a novel nickel/silicon carbide (Ni– SiC) ‘nanocomposite’ as well as pure nanocrystalline nickel (nano-Ni) by pulse electrodeposition; (2) to characterize their microstructure using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis and image analysis; and (3) to determine some mechanical properties of both nanocomposite Ni– SiC and nano-Ni using Vickers hardness and uniaxial tensile tests.
2. Experimental procedures Both nanocomposite Ni– SiC and pure nanocrystalline Ni samples were prepared by pulsed current plating [26]. The nanocomposite samples were deposited from a modified Watts bath described elsewhere [27] containing 100 g/l and 20 g/l high purity SiC of 0.4 90.2 mm average particle diameter. A modified Watts bath without SiC was employed in the plating of the nanocrystalline Ni samples [26]. All plating baths contained 5g/l of saccharin and a plating time of 3–4 h duration was used. In order to obtain free-standing samples that could be machined into suitable specimens for the uniaxial tension tests, a Ti cathode with plating area dimensions 40× 20 mm was used. After removal from the Ti substrate the test coupons were cut to the dimensions (in mm), shown in Fig. 1, using electric discharge machining (EDM). The specimens were then ground and polished using standard metallographic techniques. The final thickness of the tensile specimens ranged from 100 to 300 mm.
Fig. 1. Schematic diagram of the minature plate-type tensile coupon (dimensions in mm).
The grain size was determined using XRD in conjunction with the Scherrer method [28] and also by direct measurement of about 250 grains on TEM darkfield micrographs of some samples. A particle distribution analysis was utilized to calculate the vol.% SiC in the deposits using a Globe Image Analyzer. Samples were mounted and polished to a mirror like finish using standard metallographic techniques including polishing with 6 and 1 mm diamond pastes and finally with a 0.3 mm aluminum oxide slurry. Six readings were carried out and averaged for each sample. Microhardness tests were conducted on a Vicker’s hardness tester using a load of 100 g, applied for 20 s. The tensile tests were carried out on a MTS machine with a low capacity load cell (100 kg maximum) and a clip-on high sensitivity extensometer with a gauge length of 5 mm to measure elongation. The tests were conducted under load control condition at 0.2 MPa s − 1. (i.e. equivalent to a strain rate of − 5× 10 − 5 to 10 − 4 s − 1) to the fracture point or until the maximum load was achieved on the load cell. Both load and elongation signals were digitized by an Omega W-800 A/D card and recorded using an IBM PC. From the load/extension data the yield strength (0.2% off-set), tensile strength and elongation were determined for both the nanocomposite Ni–SiC and the nanocrystalline Ni samples. In order to determine the fracture mode and the microstructural changes resulting from deformation, standard metallographic and fractographic techniques were employed.
3. Results
3.1. Sample characterization The grain-sizes and SiC content (in vol.%) of the nanocomposites produced in this study are summarized in Table 1. The SiC content varied from 0.3 vol.% to a maximum of 10.5 vol.% due to the variation in the plating parameters used [27]. A typical microstructure, consisting of irregular shaped particles and agglomerates of SiC embedded in a nanocrystalline Ni matrix, is revealed by the SEM micrograph of a nanocomposite Ni –SiC electrodeposit shown in Fig. 2. The SiC concentration in this sample was 8.7 vol.%. The grain sizes, determined by XRD, ranged from 10.3 to 15.0 nm as indicated in Table 1. A direct measurement of 250 grain diameters from a TEM darkfield micrograph of the nanocrystallline Ni-matrix between the SiC particles of nanocomposite sample E8 (Fig. 3a), produced a grain size distribution with a mean value of 18.39 6.8 nm, shown in the histogram of Fig. 3b. In comparison, the value for the grain size obtained from XRD line broadening [28] for the same
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Table 1 Grain size, vol.% SiC and mechanical properties of nanocomposite Ni–SiC Sample no. n-NiSiC
|y (MPa)
|UTS (MPa)
mf (%)
Hardness (GPa)
G.S (nm)
Vol. % SiCc
E3 E5 E15 E8 E17 E16 E2 E6a E18 E9 E4 E19 E20 E7a E1 E13 MMCb Ni–SiC1 Ni–SiC2 Ni–SiC3 Ni–SiC4
1050 820 800 730 800 775 920 820 775 820 800 925 730 880 840 850
1413 1350 1150 1256 1246 1115 1431 \1163 1275 1072 1064 1298 1070 \1033 1075 955
0.70 2.20 2.50 3.40 2.00 2.80 2.10 \0.65 1.50 0.50 0.60 1.00 1.00 \0.35 0.50 0.35
6.83 90.15 6.49 9 0.29 5.81 90.25 6.13 9 0.27 6.13 90.28 6.15 9 0.22 5.99 90.29 – 6.54 90.35 6.27 9 0.15 6.39 90.36 6.31 9 0.33 6.31 90.22 6.35 9 0.25 6.59 90.16 6.46 9 0.25
12.4 10.6 13.7 14.2 13.7 12.2 10.3 14.2 11.7 14.0 14.2 11.7 12.8 15.0 13.7 14.5
0.3 0.4 0.6 0.7 1.2 1.4 1.8 3.0 3.2 3.4 3.5 3.9 4.0 5.9 8.7 10.5
569 588 588 539
676 735 833 726
B3 B3 B3 B3
3.15 3.90 4.02 4.41
n.a.d n.a. n.a. n.a.
7.1 11.2 14.6 23.4
a
These samples did not fail during testing. Conventional MMCs produced in a Watts bath [35–38]. c The vol.% SiC data was converted from wt.% values taken from the literature [35–37]. d n.a. means not available. b
sample was 14.2 nm which is in reasonable agreement considering that X-ray diffraction averages the grain size over much larger volume than TEM dark field imaging. The grain sizes for the pure nanocrystalline Ni samples ranged from 9.9 to 40 nm (See Table 2).
3.2. Mechanical testing results The results of mechanical testing, i.e. Vickers hardness, 0.2% off-set yield strength (|y), ultimate tensile strength (|UTS) and elongation (mf) are given for both nanocomposite Ni– SiC and pure nanocrystalline Ni in Table 1 and Table 2, respectively. Comparative mechanical properties for conventional polycrystalline Ni –SiC MMCs and annealed polycrystalline Ni are also displayed. In the following sections the results are presented in form of scatter plots containing points labeled ‘E’ for nanocomposite materials and ‘F’ for nanocrystalline Ni without SiC additions according to Table 1 and Table 2 respectively. In order to separate the effects of SiC content and grain size on the mechanical properties of these materials each scatter plot is followed by a histogram comparing relevant properties. In histograms 4b (hardness) and 5b (yield stress) materials with comparable SiC content and/or grain size are compared. Histogram 6b (tensile strength) also shows comparisons for nanocrystalline materials with varying SiC content
and grain size, while histogram 7b (strain to failure) compares nanocrystalline materials. The microhardness values, for the nanocomposite samples appearing in Table 1 range from a low of 5.819 0.25 GPa to a high of 6.839 0.15 GPa, for the grain size range 10.3–15.0 nm. These hardness values are comparable to those for pure nanocrystalline Ni given in Table 2 which reveal a range of 4.8890.05 to 6.8990.10 GPa for a wider range in grain sizes, from 9.9 to 40 nm. Vickers hardness as a function of vol.% SiC for the nanocomposite specimens is shown in the scatter plot of Fig. 4a. A linear regression analysis gives
Fig. 2. SEM micrograph of a nanocomposite electrodeposit containing 8.7 vol.% SiC.
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Fig. 3. (a) TEM darkfield micrograph of nanocomposite Ni – SiC sample E8. (b) Histogram of Ni grain size distribution for the same sample E8.
a correlation coefficient of r= 0.43 indicating a slight trend of increasing hardness with increasing vol.% SiC. The ‘F ’ points on the Y-axis represent the hardness
values of the pure nanocrystalline Ni samples (i.e. containing no SiC). A comparison of the microhardness of a conventional Ni metal matrix composite containing 7.1 vol.% SiC, a nanocomposite Ni–SiC sample with 8.7 vol.% SiC, polycrystalline annealed Ni and a nanocrystalline Ni sample is given in the histogram of Fig. 4b. The yield strength values for the nanocomposite materials, ranging from a low of 730 MPa to a high of 1050 MPa (Table 1), are plotted as a function of vol.% SiC in Fig. 5a. Linear regression analysis of the data (i.e. a correlation coefficient of r= − 0.06) demonstrates that SiC content has little effect on the yield strength, however, the range of yield strength values for the nanocrystalline nickel samples, 630–851 MPa (Table 2), is somewhat lower than that for nanocomposite Ni–SiC. For many of these specimens the yield strength and tensile strength are identical or show very little difference (e.g. for samples F2, F3, and F9 see Table 2). This indicates that the point of fracture is nearly the same as the yield point which implies that pure nanocrystalline Ni is relatively brittle, possessing little plastic deformation. A comparison of yield strength values for a nanocrystalline Ni sample, a nanocomposite Ni–SiC sample with a SiC content of 8.7 vol.%, polycrystalline annealed Ni, and conventional Ni MMC containing 7.1 vol.% SiC is displayed in the histogram of Fig. 5b. The ultimate tensile strength of the nanocomposite Ni –SiC samples ranges from 955 to 1431 MPa (Table 1). The strength values are plotted as a function of vol.% SiC shown in Fig. 6a. Linear regression analysis of the plotted data (i.e. a correlation coefficient of r= − 0.52) reveals a moderate to fairly strong inverse relationship between tensile strength and vol.% SiC. The ultimate strength values of the nanocomposites are also substantially higher than the values for nanocrystalline Ni samples of comparable grain sizes. The values
Table 2 Grain size and mechanical properties of nanocrystalline nickel Sample no. nano-Ni
|y (MPa)
|UTS (MPa)
mf (%)
Hardness (GPa)
G.S (nm)
F3 F4 F5b F8 F2 F9 F7 F1c Niand
851 830 296 830 630 641 770 700 177
851 1056 296 881 661 641 968 \1370 192
NDa 0.60 ND 0.26 0.22 ND 0.50 2.35 50
6.14 9 0.24 6.22 90.23 6.67 90.07 5.65 90.35 6.66 90.24 6.89 90.10 4.88 9 0.05 5.54 90.12 1.48
9.9 10.9 11.7 13.3 13.7 13.8 15.0 40 100 mm
a
No elongation at the level of sensitivity of the MTS machine. The sample contained surface defects c The sample did not fail. d Conventional polycrystalline nickel annealed at 540 °C for 2 h [45]. b
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tively and conventional Ni MMC with a SiC content of 7.1 vol.%. The elongation to failure, mf, values for many of the nanocomposite Ni–SiC samples are relatively high compared with the values for nanocrystalline Ni. The lower the concentration of SiC in the samples the greater the strain to failure. This inverse relationship between elongation and SiC content is evident in the scatter plot, shown in Fig. 7a. Linear regression analysis (i.e. a correlation coefficient of r= − 0.57) indicates a moderate to fairly strong inverse relationship between mf and vol.% SiC. The histogram, shown in Fig. 7b, compares the mf values of two nanocrystalline Ni samples of grain sizes 40 and 13.7 nm, respectively, and
Fig. 4. (a) Scatter plot of microhardness versus vol.% SiC in the deposits. (b) Histogram comparing microhardness of poly- and nanocrystalline materials. 1Taken from Refs. [35 –38]. 2Taken from Ref. [45].
are compared in the same scatter plot of Fig. 6a. The ultimate tensile strength for pure nanocrystalline Ni, shown on the plot, ranges from 641 to 1056 MPa for grain sizes 515 nm (Table 2). One pure nanocrystalline Ni sample, F1, of a larger grain size (40 nm) demonstrates a strength comparable to the strongest nanocomposite samples (]1370 MPa) indicating a greater plastic flow in the matrix due to a more developed dislocation mechanism at larger grain sizes. The histogram displayed in Fig. 6b, compares the tensile strength of polycrystalline annealed Ni, two nanocrystalline Ni samples, two nanocomposite Ni– SiC samples containing 1.8 vol.% SiC and 8.7 vol.% SiC, respec-
Fig. 5. (a) Scatter plot of yield stress versus vol.% SiC. (b) Histogram comparing yield stress of poly- and nanocrystalline materials. 1Taken from Refs. [35 – 38]. 2Taken from Ref. [45].
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variation in grain size from 15.0 to 13.8 nm, for samples F7 and F9, the microhardness values increased significantly from 4.8890.5 to 6.899 0.10 GPa, respectively. Sample F8, with a similar grain size of 13.3 nm, also revealed a significantly lower hardness value of 5.659 0.35 GPa. These anomalies in grain sizes and microhardness values are likely due to variations in the crystallographic textures exhibited by the electrodeposits. XRD analysis of the nanocrystalline Ni samples indicated a preferred (111) crystallographic orientation for the harder samples, including F9, while a predominate (200) orientation was shown for the ‘softer’ samples, including F7 and F8. This effect of texture on the hardness of nanocrystalline Ni materials has been reported earlier by other researchers [8,30].
Fig. 6. (a) Scatter plot of tensile strength versus vol.% SiC for nanocrystalline materials. (b) Histogram comparing tensile strength of poly- and nanocrystalline materials. 1Taken from Ref. [45]. 2Taken from Refs. [35 – 38].
three nanocomposite Ni– SiC samples containing 8.7, 1.2 and 0.7 vol.% SiC, at relatively constant grain size (13.7 –14.2nm) respectively. Refs [35– 37] state that the elongation to failure was less than 3%.
4. Discussion
4.1. Microhardness Table 2 indicates a substantial variation in microhardness values for some pure nanocrystalline Ni samples with similar grain sizes. For example, for a
Fig. 7. (a) Scatter plot of elongation versus vol.% SiC. (b) Histogram comparing the elongation of nanocrystalline materials.
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Fig. 8. Hardness versus the inverse square root of grain size for nanocrystalline Ni.
The histogram of Fig. 4b illustrates the effects of adding SiC particulate and decreasing grain size on the hardness of polycrystalline Ni. The addition of about 7 vol.% of hard SiC ceramic particulate to polycrystalline nickel results in a large increase (\2 × ) in the hardness value (i.e. from 1.48 to 3.15 GPa). Thus, SiC particles have a major hardening effect on a polycrystalline nickel matrix. When the grain size of pure nickel is decreased to the nanocrystalline range (14 nm) another major increase (\4 ×) in the hardness values is observed (i.e. from 1.48 to 6.66 GPa). Therefore, decreasing the grain size to the nanometer range has a greater hardening effect (\ 2 ×) than adding a reinforcing phase to conventional polycrystalline Ni. However, the addition of a comparable amount SiC (8.7 vol.%) to a nanocrystalline Ni matrix has very little effect on the hardness. The hardness values for nanocrystalline Ni and nanocomposite Ni– SiC are similar. Therefore, it can be concluded that the high hardness values for the nanocomposite are due mainly to the hard nanocrystalline nickel matrix, most likely because a strengthening mechanism based on dislocation– particle interaction is not operative at such small grain sizes. Dislocation models are meaningful only within certain limitations at very small grain sizes. For example, the original dislocation model for the Hall– Petch relation was based on the concept that grain boundaries act as barriers to dislocation motion, thus forming dislocation pile-ups at grain boundaries. Pile-up models be-
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come questionable when the pile-up length is of the order of 10–100 nm (i.e. the nanocrystalline range) since the number of dislocations in a pile-up is rapidly decreasing with decreasing grain size [29]. Similarly, the classical Orowan type mechanism is unlikely to operate in the nanocomposites studied here since the reinforcing particles are at least one order of magnitude larger than the average matrix grain size; this resulting in a structure in which one hard particle is surrounded by many differently oriented grains in the nickel matrix. The high hardness of the nanocomposites produced in the present study is therefore primarily due to the intrinsic hardness of nanocrystalline Ni matrix itself [9] rather than the second phase reinforcement. A Hall –Petch plot (i.e. hardness as a function of d − 1/2) for pure nanocrystalline Ni is shown in Fig. 8. As previously observed in other studies on electrodeposited nanocrystals [8,9,13,30,31], a deviation from Hall–Petch behaviour (i.e. softening effect) is observed for the smallest grain sizes. A number of theories based on different factors have been advanced to explain this inverse Hall–Petch relationship. These include triple junction effects [13], diffusional creep [12], texture effects [8,30], grain boundary source models [32], decrease in interfacial excess volume [33] and bow out of Frank–Read sources [34].
4.2. Yield strength The histogram of Fig. 5b indicates that the yield strength of polycrystalline nickel is greatly improved either by adding SiC reinforcement or by reducing the grain size. Adding SiC to nanocrystalline Ni does not have the same significant effect on increasing the yield strength as compared to adding SiC to conventional polycrystalline Ni. For example, as shown in Fig. 5b, a nanocomposite containing approximately 8.7 vol.% SiC has a yield strength only slightly higher than that of a nanocrystalline Ni sample of comparable grain sizes ( 15 nm): compare 840 and 770 MPa, respectively. Adding approximately 7 vol.% SiC to polycrystalline Ni increases the yield strength more than three times (i.e. from 177 to 569 MPa). This can be expected since the onset of composite yielding for a MMC is governed by the onset of matrix yielding which, for nanocrystalline materials, is not under the control of a dislocation mechanism since the grain size is too small, as discussed in Section 4.1. A Hall–Petch plot for yield strength as a function of 1/d 1/2 indicated that the softening effect is not as apparent as it is in Fig. 8 for hardness versus 1/d 1/2.
4.3. Ultimate tensile strength and elongation to failure An anomaly stands out on the scatter plot shown in Fig. 6a: nanocomposite samples of very low volume
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fractions of SiC reinforcement exhibit relatively high tensile strengths. For example, nanocomposite samples E2, E3, and E5 contain less than 2 vol.% SiC, yet, exhibit some of the highest tensile strengths: 1431, 1413 and 1350 MPa, respectively. Particle agglomeration can play a major role in the tensile properties of composite materials [39– 44]. Agglomerates of SiC particles were quite abundant in the
Fig. 9. (a) A schematic of the microstructure of a conventional Ni–SiC MMC showing SiC particles pinning dislocations. (b) A schematic of the microstructure of Nano-Ni – SiC showing the presence of microvoids in SiC agglomerates.
Fig. 10. SEM micrograph of a fracture surface of nano-NiSiC sample E4.
materials produced in this study, as shown for example in the SEM micrograph of sample E1 with a SiC content of 8.7 vol.% (Fig. 2). Thus localized ductile or ‘soft’ regions can occur in a hard, relatively brittle nanocrystalline Ni matrix, due to enhanced void formation between neighbouring particles [40,44], which can effectively increase the porosity of an otherwise fairly dense material. This can lead to major effects on the mechanical and tensile properties of a nanocomposite. A pair of schematic drawings depicting two different microstructures, shown in Fig. 9(a) and (b) explain this reasoning. Fig. 9(a) depicts mobile dislocations being pinned by SiC particles and SiC agglomerates in a conventional polycrystalline Ni–SiC matrix leading to a form of dispersion strengthening. Dislocation pile-ups at grain boundaries also occur. This leads to a large increase in strength over conventional polycrystalline Ni (see Fig. 6b). Fig. 9b indicates a microstructure consisting of particles and agglomerates of SiC in a matrix of nanocrystalline Ni. The individual submicron size particles are much larger than the nano-sized grains of Ni. The presence of micropores between adhering particles of an agglomerate is also depicted. As the concentration of SiC in the nano-Ni matrix increases damage accumulates and increases the porosity, thus weakening the nanocomposite. Therefore, small volume fractions of SiC are more effective at increasing the strength of nanocomposite SiC than larger ones, which are actually detrimental to composite strength. The scatter plot of Fig. 6a as well as the histogram of Fig. 6b also indicate that the tensile strength values of the nanocomposites for low SiC content (B 2 vol.%) are higher than those for pure nanocrystalline Ni of comparable grain size. The same histogram also shows that the strength of the nanocomposites are approximately two times greater than the strength of conventional metal matrix composites with similar SiC content. This increase in strength is again mainly due to the decrease in grain size of the Ni matrix (i.e. from the microregime to the nano-regime). The scatter plot depicting ductility or strain to failure (mf) as a function of SiC content is similar to the plot of tensile strength as a function of SiC content; compare Fig. 6a and Fig. 7a. Both graphs indicate inverse relationships between elongation or strength and SiC content and reveal similar correlation coefficients (−0.57 and − 0.52, respectively). Fig. 7a is also similar to previously published ‘ductility versus vol. fraction ’ plots of conventional MMCs exhibiting ductile fracture [43,44]. A ductile fracture mechanism operative in the nanocomposite Ni–SiC samples with low SiC concentrations is also supported by examining the fracture surfaces of nanocomposite samples (e.g. for sample E4, shown in the SEM micrograph of Fig. 10). A typical fibrous cup and cone morphology containing SiC particles is revealed.
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Fig. 11. SEM micrograph of fracture surface of nano-Ni sample F2.
Many of the pure nanocrystalline Ni samples, listed in Table 2, have very low strains to failure. In some cases there is negligible elongation (e.g. samples F3 and F9), indicating that many of these samples fail by a brittle fracture mechanism as observed in the fracture surfaces [e.g. for nanocrystalline Ni sample F2 (mf = 0.22%), shown in Fig. 11]. Here, the micrograph reveals cleavage-type facets (e.g. parallel plateau and ledge morphology, flat river-patterns, etc.) typical of a brittle fracture mechanism. In the histogram of elongation for nanocrystalline materials, shown in Fig. 7b, pure nanocrystalline Ni (grain size of 14 nm) has the lowest mf value (0.22%), close to that for a nanocomposite sample containing one the highest SiC contents (i.e. 8.7 vol.%). The nanocrystalline sample with the largest grain size (i.e. 40 nm for sample F1) revealed a substantially higher mf value (i.e. 2.35%), comparable to values for nano-Ni– SiC samples of lower SiC content. At 40 nm grain size, nanocrystalline Ni has a relatively higher value of elongation, compared to nano-Ni of smaller grain sizes (e.g. 13.7 nm), since a dislocation mechanism may be operative at the larger grain size. This also explains the increase in tensile strength of the larger grain size nano-Ni over the smaller grain size one. However, the mf value is still relatively low compared to the ductility of conventional polycrystalline nickel, i.e. 50% elongation for annealed Ni [45] (Table 2).
5. Summary Nanocomposite materials consisting of submicron size SiC particulate embedded in a nanocrystalline nickel matrix with grain sizes as small as 10 nm and SiC contents up to about 10.5 vol.% were produced from a modified Watts bath. Microhardness values of the nanocomposite were more than twice those for conventional polycrystalline
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MMCs with similar SiC content and greater than four times the hardness of conventional polycrystalline Ni. The hardness values were similar to the values for pure nanocrystalline Ni of comparable grain size. Nanocomposite Ni–SiC demonstrated a tensile strength greater than four times that of conventional polycrystalline nickel and about two times that for conventional polycrystalline MMCs of comparable SiC content. The tensile strengths for the nanocomposite samples with low SiC content (B 2%) were also higher than for nanocrystalline Ni of comparable grain size. Yield strength values greater than five times that for annealed Ni and about 1.5 times that for conventional polycrystalline MMC of comparable SiC content were measured for nanocomposite Ni–SiC. Most pure nanocrystalline Ni samples were brittle, exhibiting a yield points close to their fracture points. Nanocomposite Ni–SiC revealed an unexpected increase in ductility over that of pure nanocrystalline Ni of comparable grain size, especially at lower SiC contents (B 2 vol.%). The ductility of nanocomposites of lower SiC contents was comparable to the ductility of conventional MMCs. Fractography results revealed a ductile fracture mechanism possibly via a void coalescence process operative in nanocomposite-Ni–SiC and a brittle cleavage-type mechanism in pure nanocrystalline Ni.
Acknowledgements The authors wish to thank Dr N. Wang, Z. Wang and V. Krstic for their contributions to this work. Financial support for this work was provided by the Natural Sciences and Engineering Research Council of Canada and Ontario Power Generation. One of the authors (A.F.Z.) would like to acknowledge financial support by the School of Graduate Studies and Research at Queen’s University.
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[11] G.W. Nieman, J.R. Weertman, R.W. Siegel, Scripta Metall. Mater. 24 (1990) 145. [12] A.H. Chokshi, A. Rosen, J. Karch, H. Gleiter, Scripta Metall. 23 (1989) 1679. [13] G. Palumbo, U. Erb, K.T. Aust, Scripta Metall. Mater. 24 (1990) 2347. [14] K. Lu, M.L. Sui, Scripta Metall. Mater. 28 (1993) 1465. [15] R.O. Scattergood, C.C. Koch, Scripta Metall. Mater. 27 (1992) 1195. [16] N. Wang, Z. Wang, K.T. Aust, U. Erb, Mat. Sci. Eng. A237 (1997) 150. [17] T.G. Nieh, J. Wadsworth, Scripta Metall. Mater. 25 (1991) 955. [18] K. Lu, W.D. Wei, J.T. Wang, Scripta Metall. Mater. 24 (1990) 2319. [19] T. Christman, M. Jain, Scripta Metall. Mater. 25 (1991) 767. [20] V.B. Rabukhin, Phys. Met. Metalloved. 43 (1995) 3027. [21] H. Gleiter, in: N. Hansen, et al. (Eds.), Second Riso Int. Symp. Met. and Mater. Sci., Riso Nat. Lab. Denmark, 1981, p. 15. [22] G. Palumbo, S.J. Thorpe, K.T. Aust, Scripta Met. Mater. 24 (1990) 1347. [23] K.T. Aust, U. Erb, G. Palumbo, Interfacial Effects in Nanocrystalline Materials, in: C. Suryanarayana, J. Singh, F.H. Froes (Eds.), Processing and Properties of Nanocrystalline Materials, The Minerals, Metals and Materials Society, Warrendale, PA, 1996. [24] Y. Ding, D.O. Northwood, A.T. Alpas, Surf. and Coat. Tech. 62 (1993) 448. [25] Nieman G.W., Weertman J.R., Siegel R. Win, in: David C., Van Aken, and Gary S. Was (Eds.), Microcomposites and Nanophase Materials, The Mineral, Metals and Materials Society, Warrendale, PA, 1991, 15.
[26] El-Sherik A.M., Erb U., Production of Nanocrystalline Metals, US Patent 5,433,797 (1995). [27] Zimmerman A.F., Clark D.G., Aust K.T., Erb U., Mat. Letters, submitted for publication. [28] B.D. Cullity, Elements of X-ray Diffraction, 2nd, Addison Wesley Publishing Co. Inc., London, 1978. [29] Y.T. Chou, J. Appl. Physics 38 (1967) 2080. [30] C. Cheung, F. Djuanda, U. Erb, G. Palumbo, Nanstr. Mat. 5 (1995) 513. [31] C. Cheung, G. Palumbo, U. Erb, Scripta Metall. Mater. 31 (1994) 735. [32] S. Li, L. Sun, Z. Wang, Nanostr. Mat. 2 (1993) 653. [33] K. Lu, M.L. Sui, Scripta Metall. Mater. 28 (1993) 1465. [34] J. Lian, B. Baudelet, Nanostr. Mat. 2 (1993) 653. [35] M. Viswanathan, K.S.G. Das, Metal Finishing Feb (1972) 83. [36] M.N. Joshi, M. Totlani, J. Electrochem. Soc. India 28 (1) (1979) 35. [37] E. Broszeit, G. Heinke, H. Wiegand, Metall. 25 (1971) 470. [38] W.H. Safranek, The Properties of Electrodeposited Metals and Alloys, 2, AEFS, Florida, 1986, p. 365. [39] K.E. Puttick, Phil. Mag. 4 (1959) 964. [40] J.R. Fisher, J. Gurland, Metal Sci. 15 (1981) 185. [41] G.L. Roy, J.D. Embury, G. Edwards, M.F. Ashby, Acta Metall. 29 (1981) 1509. [42] A.S. Argon, Im J. Safoglu, Metall. Trans. 6A (1975) 825. [43] A.R. Rosenfield, Met. Rev. 121 (1968) 29. [44] T.W. Clyne, P.J. Withers, An Introduction to Metal Matrix Composites, Cambridge University Press, Cambridge, 1993, pp. 256 – 263. [45] ASM Metals Handbook vol. 2, ASM International, Metals Park, OH, 1993.
Mechanical properties of nickel silicon carbide nanocomposites A.F. Zimmerman a, G. Palumbo b, K.T. Aust a, U. Erb a,* a
Department of Materials Science and Engineering, Uni6ersity of Toronto, Toronto, ON, Canada, MS5 3E4 b Integran Technologies, 1 Meridian Rd., Toronto, ON, Canada, M9W 4Z6 Received 13 March 2001; received in revised form 11 June 2001
Abstract Nanocomposite materials consisting of a nanocrystalline Ni matrix (grain size 10 – 15 nm) reinforced with sub-micron size SiC particulates (average particle size: 0.4 mm) up to 10.5 vol.% have been produced by pulse electrodeposition. Substantial improvements in mechanical properties including hardness, yield and tensile stress were obtained for the nanocomposite material, as compared with conventional Ni–SiC composites with a matrix grain size in the micrometer range. Tensile strengths up to four times that for conventional polycrystalline Ni and two times that for conventional polycrystalline Ni– SiC of comparable SiC content was measured. The tensile and yield strengths of the nanocomposite material with SiC content less than 2 vol.% were higher than those for pure nanocrystalline Ni of comparable grain size. For these nanocomposites an unexpected increase in tensile ductility was also observed when compared to pure nanocrystalline nickel. At higher SiC content ( \2 vol.%) the strength and ductility were found to decrease to the detriment of the nanocomposite. Particle clustering was considered the main cause of this decrease. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ductility; Microhardness; Nickel silicon carbide nanocomposite; Tensile strength
1. Introduction The mechanical properties of polycrystalline metals, alloys and MMCs are very sensitive to their grain size. In many cases the flow stress, at low temperature, depends on the grain size according to the Hall –Petch relationship: |y =|0 +kd − 1/2
(1)
where |y is the yield stress, |0 is the friction stress, d is the grain size and k is the Hall – Petch coefficient (i.e. the Hall –Petch slope) [1,2]. Many models, based on the dislocation theory, have been presented to interpret this phenomenological equation (e.g. [3 – 7]). Restrictions on dislocation generation and mobility imposed by ultrafine grain size are believed to be the dominant factor in raising strength. Numerous investigators have reported very high strengths associated with Hall – Petch behaviour in nanocrystalline materials (e.g. [8 – 13]). For some materials, an unexpected transition from grain-size strength* Corresponding author. Fax: + 1-416-946-3316. E-mail address: [email protected] (U. Erb).
ening to grain-size softening (i.e. ‘inverse’ Hall –Petch behavior) was observed at a critical grain size [8,9,12 – 20]. For example, Palumbo et al. [13] reported softening effects for a series of as-prepared nanocrystalline Ni –P alloys with grain sizes in the range from 20 down to 3 nm. The data showed an initial increase in hardness followed by a deviation from regular Hall –Petch behaviour leading to softening for the smallest grain size range. The maximum in the hardness curve occurred for a grain size of approximately 8 nm. Gleiter [21] first recognized that the intercrystalline component of a material will form a distinct microstructural constituent when a material is processed to have a nanocrystalline structure. A detailed study by Palumbo et al. [22] using grains modeled in the shape of regular tetrakaidecahedrons and assuming a grain boundary thickness of 1 nm showed that the grain boundary volume fraction increases rapidly from a few percent at a grain size of 100 nm to over 50% at a grain size of 2 nm. For grain sizes below 20 nm, the triple junction (i.e. intersection line of three or more grain boundaries) volume fraction reaches significant values and exhibits a greater grain size dependence than does the grain boundary volume fraction. In fact, it has been
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0921-5093(01)01692-6
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observed that the onset of the deviation from regular Hall –Petch behaviour often occurs at grain sizes where the triple junction volume fraction in the materials reaches significant values, as discussed elsewhere in more detail [13,23]. Some of the unique mechanical properties of nanocomposite materials including increased strength and hardness [24,25], increased wear resistance and decreased coefficient of friction [24], can also in part be attributed to grain size effects caused by the increased interfacial defect volume fraction, in addition to the strengthening effects by the reinforcing phase. The purpose of the present study was: (1) to prepare bulk samples of a novel nickel/silicon carbide (Ni– SiC) ‘nanocomposite’ as well as pure nanocrystalline nickel (nano-Ni) by pulse electrodeposition; (2) to characterize their microstructure using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis and image analysis; and (3) to determine some mechanical properties of both nanocomposite Ni– SiC and nano-Ni using Vickers hardness and uniaxial tensile tests.
2. Experimental procedures Both nanocomposite Ni– SiC and pure nanocrystalline Ni samples were prepared by pulsed current plating [26]. The nanocomposite samples were deposited from a modified Watts bath described elsewhere [27] containing 100 g/l and 20 g/l high purity SiC of 0.4 90.2 mm average particle diameter. A modified Watts bath without SiC was employed in the plating of the nanocrystalline Ni samples [26]. All plating baths contained 5g/l of saccharin and a plating time of 3–4 h duration was used. In order to obtain free-standing samples that could be machined into suitable specimens for the uniaxial tension tests, a Ti cathode with plating area dimensions 40× 20 mm was used. After removal from the Ti substrate the test coupons were cut to the dimensions (in mm), shown in Fig. 1, using electric discharge machining (EDM). The specimens were then ground and polished using standard metallographic techniques. The final thickness of the tensile specimens ranged from 100 to 300 mm.
Fig. 1. Schematic diagram of the minature plate-type tensile coupon (dimensions in mm).
The grain size was determined using XRD in conjunction with the Scherrer method [28] and also by direct measurement of about 250 grains on TEM darkfield micrographs of some samples. A particle distribution analysis was utilized to calculate the vol.% SiC in the deposits using a Globe Image Analyzer. Samples were mounted and polished to a mirror like finish using standard metallographic techniques including polishing with 6 and 1 mm diamond pastes and finally with a 0.3 mm aluminum oxide slurry. Six readings were carried out and averaged for each sample. Microhardness tests were conducted on a Vicker’s hardness tester using a load of 100 g, applied for 20 s. The tensile tests were carried out on a MTS machine with a low capacity load cell (100 kg maximum) and a clip-on high sensitivity extensometer with a gauge length of 5 mm to measure elongation. The tests were conducted under load control condition at 0.2 MPa s − 1. (i.e. equivalent to a strain rate of − 5× 10 − 5 to 10 − 4 s − 1) to the fracture point or until the maximum load was achieved on the load cell. Both load and elongation signals were digitized by an Omega W-800 A/D card and recorded using an IBM PC. From the load/extension data the yield strength (0.2% off-set), tensile strength and elongation were determined for both the nanocomposite Ni–SiC and the nanocrystalline Ni samples. In order to determine the fracture mode and the microstructural changes resulting from deformation, standard metallographic and fractographic techniques were employed.
3. Results
3.1. Sample characterization The grain-sizes and SiC content (in vol.%) of the nanocomposites produced in this study are summarized in Table 1. The SiC content varied from 0.3 vol.% to a maximum of 10.5 vol.% due to the variation in the plating parameters used [27]. A typical microstructure, consisting of irregular shaped particles and agglomerates of SiC embedded in a nanocrystalline Ni matrix, is revealed by the SEM micrograph of a nanocomposite Ni –SiC electrodeposit shown in Fig. 2. The SiC concentration in this sample was 8.7 vol.%. The grain sizes, determined by XRD, ranged from 10.3 to 15.0 nm as indicated in Table 1. A direct measurement of 250 grain diameters from a TEM darkfield micrograph of the nanocrystallline Ni-matrix between the SiC particles of nanocomposite sample E8 (Fig. 3a), produced a grain size distribution with a mean value of 18.39 6.8 nm, shown in the histogram of Fig. 3b. In comparison, the value for the grain size obtained from XRD line broadening [28] for the same
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Table 1 Grain size, vol.% SiC and mechanical properties of nanocomposite Ni–SiC Sample no. n-NiSiC
|y (MPa)
|UTS (MPa)
mf (%)
Hardness (GPa)
G.S (nm)
Vol. % SiCc
E3 E5 E15 E8 E17 E16 E2 E6a E18 E9 E4 E19 E20 E7a E1 E13 MMCb Ni–SiC1 Ni–SiC2 Ni–SiC3 Ni–SiC4
1050 820 800 730 800 775 920 820 775 820 800 925 730 880 840 850
1413 1350 1150 1256 1246 1115 1431 \1163 1275 1072 1064 1298 1070 \1033 1075 955
0.70 2.20 2.50 3.40 2.00 2.80 2.10 \0.65 1.50 0.50 0.60 1.00 1.00 \0.35 0.50 0.35
6.83 90.15 6.49 9 0.29 5.81 90.25 6.13 9 0.27 6.13 90.28 6.15 9 0.22 5.99 90.29 – 6.54 90.35 6.27 9 0.15 6.39 90.36 6.31 9 0.33 6.31 90.22 6.35 9 0.25 6.59 90.16 6.46 9 0.25
12.4 10.6 13.7 14.2 13.7 12.2 10.3 14.2 11.7 14.0 14.2 11.7 12.8 15.0 13.7 14.5
0.3 0.4 0.6 0.7 1.2 1.4 1.8 3.0 3.2 3.4 3.5 3.9 4.0 5.9 8.7 10.5
569 588 588 539
676 735 833 726
B3 B3 B3 B3
3.15 3.90 4.02 4.41
n.a.d n.a. n.a. n.a.
7.1 11.2 14.6 23.4
a
These samples did not fail during testing. Conventional MMCs produced in a Watts bath [35–38]. c The vol.% SiC data was converted from wt.% values taken from the literature [35–37]. d n.a. means not available. b
sample was 14.2 nm which is in reasonable agreement considering that X-ray diffraction averages the grain size over much larger volume than TEM dark field imaging. The grain sizes for the pure nanocrystalline Ni samples ranged from 9.9 to 40 nm (See Table 2).
3.2. Mechanical testing results The results of mechanical testing, i.e. Vickers hardness, 0.2% off-set yield strength (|y), ultimate tensile strength (|UTS) and elongation (mf) are given for both nanocomposite Ni– SiC and pure nanocrystalline Ni in Table 1 and Table 2, respectively. Comparative mechanical properties for conventional polycrystalline Ni –SiC MMCs and annealed polycrystalline Ni are also displayed. In the following sections the results are presented in form of scatter plots containing points labeled ‘E’ for nanocomposite materials and ‘F’ for nanocrystalline Ni without SiC additions according to Table 1 and Table 2 respectively. In order to separate the effects of SiC content and grain size on the mechanical properties of these materials each scatter plot is followed by a histogram comparing relevant properties. In histograms 4b (hardness) and 5b (yield stress) materials with comparable SiC content and/or grain size are compared. Histogram 6b (tensile strength) also shows comparisons for nanocrystalline materials with varying SiC content
and grain size, while histogram 7b (strain to failure) compares nanocrystalline materials. The microhardness values, for the nanocomposite samples appearing in Table 1 range from a low of 5.819 0.25 GPa to a high of 6.839 0.15 GPa, for the grain size range 10.3–15.0 nm. These hardness values are comparable to those for pure nanocrystalline Ni given in Table 2 which reveal a range of 4.8890.05 to 6.8990.10 GPa for a wider range in grain sizes, from 9.9 to 40 nm. Vickers hardness as a function of vol.% SiC for the nanocomposite specimens is shown in the scatter plot of Fig. 4a. A linear regression analysis gives
Fig. 2. SEM micrograph of a nanocomposite electrodeposit containing 8.7 vol.% SiC.
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Fig. 3. (a) TEM darkfield micrograph of nanocomposite Ni – SiC sample E8. (b) Histogram of Ni grain size distribution for the same sample E8.
a correlation coefficient of r= 0.43 indicating a slight trend of increasing hardness with increasing vol.% SiC. The ‘F ’ points on the Y-axis represent the hardness
values of the pure nanocrystalline Ni samples (i.e. containing no SiC). A comparison of the microhardness of a conventional Ni metal matrix composite containing 7.1 vol.% SiC, a nanocomposite Ni–SiC sample with 8.7 vol.% SiC, polycrystalline annealed Ni and a nanocrystalline Ni sample is given in the histogram of Fig. 4b. The yield strength values for the nanocomposite materials, ranging from a low of 730 MPa to a high of 1050 MPa (Table 1), are plotted as a function of vol.% SiC in Fig. 5a. Linear regression analysis of the data (i.e. a correlation coefficient of r= − 0.06) demonstrates that SiC content has little effect on the yield strength, however, the range of yield strength values for the nanocrystalline nickel samples, 630–851 MPa (Table 2), is somewhat lower than that for nanocomposite Ni–SiC. For many of these specimens the yield strength and tensile strength are identical or show very little difference (e.g. for samples F2, F3, and F9 see Table 2). This indicates that the point of fracture is nearly the same as the yield point which implies that pure nanocrystalline Ni is relatively brittle, possessing little plastic deformation. A comparison of yield strength values for a nanocrystalline Ni sample, a nanocomposite Ni–SiC sample with a SiC content of 8.7 vol.%, polycrystalline annealed Ni, and conventional Ni MMC containing 7.1 vol.% SiC is displayed in the histogram of Fig. 5b. The ultimate tensile strength of the nanocomposite Ni –SiC samples ranges from 955 to 1431 MPa (Table 1). The strength values are plotted as a function of vol.% SiC shown in Fig. 6a. Linear regression analysis of the plotted data (i.e. a correlation coefficient of r= − 0.52) reveals a moderate to fairly strong inverse relationship between tensile strength and vol.% SiC. The ultimate strength values of the nanocomposites are also substantially higher than the values for nanocrystalline Ni samples of comparable grain sizes. The values
Table 2 Grain size and mechanical properties of nanocrystalline nickel Sample no. nano-Ni
|y (MPa)
|UTS (MPa)
mf (%)
Hardness (GPa)
G.S (nm)
F3 F4 F5b F8 F2 F9 F7 F1c Niand
851 830 296 830 630 641 770 700 177
851 1056 296 881 661 641 968 \1370 192
NDa 0.60 ND 0.26 0.22 ND 0.50 2.35 50
6.14 9 0.24 6.22 90.23 6.67 90.07 5.65 90.35 6.66 90.24 6.89 90.10 4.88 9 0.05 5.54 90.12 1.48
9.9 10.9 11.7 13.3 13.7 13.8 15.0 40 100 mm
a
No elongation at the level of sensitivity of the MTS machine. The sample contained surface defects c The sample did not fail. d Conventional polycrystalline nickel annealed at 540 °C for 2 h [45]. b
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tively and conventional Ni MMC with a SiC content of 7.1 vol.%. The elongation to failure, mf, values for many of the nanocomposite Ni–SiC samples are relatively high compared with the values for nanocrystalline Ni. The lower the concentration of SiC in the samples the greater the strain to failure. This inverse relationship between elongation and SiC content is evident in the scatter plot, shown in Fig. 7a. Linear regression analysis (i.e. a correlation coefficient of r= − 0.57) indicates a moderate to fairly strong inverse relationship between mf and vol.% SiC. The histogram, shown in Fig. 7b, compares the mf values of two nanocrystalline Ni samples of grain sizes 40 and 13.7 nm, respectively, and
Fig. 4. (a) Scatter plot of microhardness versus vol.% SiC in the deposits. (b) Histogram comparing microhardness of poly- and nanocrystalline materials. 1Taken from Refs. [35 –38]. 2Taken from Ref. [45].
are compared in the same scatter plot of Fig. 6a. The ultimate tensile strength for pure nanocrystalline Ni, shown on the plot, ranges from 641 to 1056 MPa for grain sizes 515 nm (Table 2). One pure nanocrystalline Ni sample, F1, of a larger grain size (40 nm) demonstrates a strength comparable to the strongest nanocomposite samples (]1370 MPa) indicating a greater plastic flow in the matrix due to a more developed dislocation mechanism at larger grain sizes. The histogram displayed in Fig. 6b, compares the tensile strength of polycrystalline annealed Ni, two nanocrystalline Ni samples, two nanocomposite Ni– SiC samples containing 1.8 vol.% SiC and 8.7 vol.% SiC, respec-
Fig. 5. (a) Scatter plot of yield stress versus vol.% SiC. (b) Histogram comparing yield stress of poly- and nanocrystalline materials. 1Taken from Refs. [35 – 38]. 2Taken from Ref. [45].
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variation in grain size from 15.0 to 13.8 nm, for samples F7 and F9, the microhardness values increased significantly from 4.8890.5 to 6.899 0.10 GPa, respectively. Sample F8, with a similar grain size of 13.3 nm, also revealed a significantly lower hardness value of 5.659 0.35 GPa. These anomalies in grain sizes and microhardness values are likely due to variations in the crystallographic textures exhibited by the electrodeposits. XRD analysis of the nanocrystalline Ni samples indicated a preferred (111) crystallographic orientation for the harder samples, including F9, while a predominate (200) orientation was shown for the ‘softer’ samples, including F7 and F8. This effect of texture on the hardness of nanocrystalline Ni materials has been reported earlier by other researchers [8,30].
Fig. 6. (a) Scatter plot of tensile strength versus vol.% SiC for nanocrystalline materials. (b) Histogram comparing tensile strength of poly- and nanocrystalline materials. 1Taken from Ref. [45]. 2Taken from Refs. [35 – 38].
three nanocomposite Ni– SiC samples containing 8.7, 1.2 and 0.7 vol.% SiC, at relatively constant grain size (13.7 –14.2nm) respectively. Refs [35– 37] state that the elongation to failure was less than 3%.
4. Discussion
4.1. Microhardness Table 2 indicates a substantial variation in microhardness values for some pure nanocrystalline Ni samples with similar grain sizes. For example, for a
Fig. 7. (a) Scatter plot of elongation versus vol.% SiC. (b) Histogram comparing the elongation of nanocrystalline materials.
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Fig. 8. Hardness versus the inverse square root of grain size for nanocrystalline Ni.
The histogram of Fig. 4b illustrates the effects of adding SiC particulate and decreasing grain size on the hardness of polycrystalline Ni. The addition of about 7 vol.% of hard SiC ceramic particulate to polycrystalline nickel results in a large increase (\2 × ) in the hardness value (i.e. from 1.48 to 3.15 GPa). Thus, SiC particles have a major hardening effect on a polycrystalline nickel matrix. When the grain size of pure nickel is decreased to the nanocrystalline range (14 nm) another major increase (\4 ×) in the hardness values is observed (i.e. from 1.48 to 6.66 GPa). Therefore, decreasing the grain size to the nanometer range has a greater hardening effect (\ 2 ×) than adding a reinforcing phase to conventional polycrystalline Ni. However, the addition of a comparable amount SiC (8.7 vol.%) to a nanocrystalline Ni matrix has very little effect on the hardness. The hardness values for nanocrystalline Ni and nanocomposite Ni– SiC are similar. Therefore, it can be concluded that the high hardness values for the nanocomposite are due mainly to the hard nanocrystalline nickel matrix, most likely because a strengthening mechanism based on dislocation– particle interaction is not operative at such small grain sizes. Dislocation models are meaningful only within certain limitations at very small grain sizes. For example, the original dislocation model for the Hall– Petch relation was based on the concept that grain boundaries act as barriers to dislocation motion, thus forming dislocation pile-ups at grain boundaries. Pile-up models be-
143
come questionable when the pile-up length is of the order of 10–100 nm (i.e. the nanocrystalline range) since the number of dislocations in a pile-up is rapidly decreasing with decreasing grain size [29]. Similarly, the classical Orowan type mechanism is unlikely to operate in the nanocomposites studied here since the reinforcing particles are at least one order of magnitude larger than the average matrix grain size; this resulting in a structure in which one hard particle is surrounded by many differently oriented grains in the nickel matrix. The high hardness of the nanocomposites produced in the present study is therefore primarily due to the intrinsic hardness of nanocrystalline Ni matrix itself [9] rather than the second phase reinforcement. A Hall –Petch plot (i.e. hardness as a function of d − 1/2) for pure nanocrystalline Ni is shown in Fig. 8. As previously observed in other studies on electrodeposited nanocrystals [8,9,13,30,31], a deviation from Hall–Petch behaviour (i.e. softening effect) is observed for the smallest grain sizes. A number of theories based on different factors have been advanced to explain this inverse Hall–Petch relationship. These include triple junction effects [13], diffusional creep [12], texture effects [8,30], grain boundary source models [32], decrease in interfacial excess volume [33] and bow out of Frank–Read sources [34].
4.2. Yield strength The histogram of Fig. 5b indicates that the yield strength of polycrystalline nickel is greatly improved either by adding SiC reinforcement or by reducing the grain size. Adding SiC to nanocrystalline Ni does not have the same significant effect on increasing the yield strength as compared to adding SiC to conventional polycrystalline Ni. For example, as shown in Fig. 5b, a nanocomposite containing approximately 8.7 vol.% SiC has a yield strength only slightly higher than that of a nanocrystalline Ni sample of comparable grain sizes ( 15 nm): compare 840 and 770 MPa, respectively. Adding approximately 7 vol.% SiC to polycrystalline Ni increases the yield strength more than three times (i.e. from 177 to 569 MPa). This can be expected since the onset of composite yielding for a MMC is governed by the onset of matrix yielding which, for nanocrystalline materials, is not under the control of a dislocation mechanism since the grain size is too small, as discussed in Section 4.1. A Hall–Petch plot for yield strength as a function of 1/d 1/2 indicated that the softening effect is not as apparent as it is in Fig. 8 for hardness versus 1/d 1/2.
4.3. Ultimate tensile strength and elongation to failure An anomaly stands out on the scatter plot shown in Fig. 6a: nanocomposite samples of very low volume
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fractions of SiC reinforcement exhibit relatively high tensile strengths. For example, nanocomposite samples E2, E3, and E5 contain less than 2 vol.% SiC, yet, exhibit some of the highest tensile strengths: 1431, 1413 and 1350 MPa, respectively. Particle agglomeration can play a major role in the tensile properties of composite materials [39– 44]. Agglomerates of SiC particles were quite abundant in the
Fig. 9. (a) A schematic of the microstructure of a conventional Ni–SiC MMC showing SiC particles pinning dislocations. (b) A schematic of the microstructure of Nano-Ni – SiC showing the presence of microvoids in SiC agglomerates.
Fig. 10. SEM micrograph of a fracture surface of nano-NiSiC sample E4.
materials produced in this study, as shown for example in the SEM micrograph of sample E1 with a SiC content of 8.7 vol.% (Fig. 2). Thus localized ductile or ‘soft’ regions can occur in a hard, relatively brittle nanocrystalline Ni matrix, due to enhanced void formation between neighbouring particles [40,44], which can effectively increase the porosity of an otherwise fairly dense material. This can lead to major effects on the mechanical and tensile properties of a nanocomposite. A pair of schematic drawings depicting two different microstructures, shown in Fig. 9(a) and (b) explain this reasoning. Fig. 9(a) depicts mobile dislocations being pinned by SiC particles and SiC agglomerates in a conventional polycrystalline Ni–SiC matrix leading to a form of dispersion strengthening. Dislocation pile-ups at grain boundaries also occur. This leads to a large increase in strength over conventional polycrystalline Ni (see Fig. 6b). Fig. 9b indicates a microstructure consisting of particles and agglomerates of SiC in a matrix of nanocrystalline Ni. The individual submicron size particles are much larger than the nano-sized grains of Ni. The presence of micropores between adhering particles of an agglomerate is also depicted. As the concentration of SiC in the nano-Ni matrix increases damage accumulates and increases the porosity, thus weakening the nanocomposite. Therefore, small volume fractions of SiC are more effective at increasing the strength of nanocomposite SiC than larger ones, which are actually detrimental to composite strength. The scatter plot of Fig. 6a as well as the histogram of Fig. 6b also indicate that the tensile strength values of the nanocomposites for low SiC content (B 2 vol.%) are higher than those for pure nanocrystalline Ni of comparable grain size. The same histogram also shows that the strength of the nanocomposites are approximately two times greater than the strength of conventional metal matrix composites with similar SiC content. This increase in strength is again mainly due to the decrease in grain size of the Ni matrix (i.e. from the microregime to the nano-regime). The scatter plot depicting ductility or strain to failure (mf) as a function of SiC content is similar to the plot of tensile strength as a function of SiC content; compare Fig. 6a and Fig. 7a. Both graphs indicate inverse relationships between elongation or strength and SiC content and reveal similar correlation coefficients (−0.57 and − 0.52, respectively). Fig. 7a is also similar to previously published ‘ductility versus vol. fraction ’ plots of conventional MMCs exhibiting ductile fracture [43,44]. A ductile fracture mechanism operative in the nanocomposite Ni–SiC samples with low SiC concentrations is also supported by examining the fracture surfaces of nanocomposite samples (e.g. for sample E4, shown in the SEM micrograph of Fig. 10). A typical fibrous cup and cone morphology containing SiC particles is revealed.
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Fig. 11. SEM micrograph of fracture surface of nano-Ni sample F2.
Many of the pure nanocrystalline Ni samples, listed in Table 2, have very low strains to failure. In some cases there is negligible elongation (e.g. samples F3 and F9), indicating that many of these samples fail by a brittle fracture mechanism as observed in the fracture surfaces [e.g. for nanocrystalline Ni sample F2 (mf = 0.22%), shown in Fig. 11]. Here, the micrograph reveals cleavage-type facets (e.g. parallel plateau and ledge morphology, flat river-patterns, etc.) typical of a brittle fracture mechanism. In the histogram of elongation for nanocrystalline materials, shown in Fig. 7b, pure nanocrystalline Ni (grain size of 14 nm) has the lowest mf value (0.22%), close to that for a nanocomposite sample containing one the highest SiC contents (i.e. 8.7 vol.%). The nanocrystalline sample with the largest grain size (i.e. 40 nm for sample F1) revealed a substantially higher mf value (i.e. 2.35%), comparable to values for nano-Ni– SiC samples of lower SiC content. At 40 nm grain size, nanocrystalline Ni has a relatively higher value of elongation, compared to nano-Ni of smaller grain sizes (e.g. 13.7 nm), since a dislocation mechanism may be operative at the larger grain size. This also explains the increase in tensile strength of the larger grain size nano-Ni over the smaller grain size one. However, the mf value is still relatively low compared to the ductility of conventional polycrystalline nickel, i.e. 50% elongation for annealed Ni [45] (Table 2).
5. Summary Nanocomposite materials consisting of submicron size SiC particulate embedded in a nanocrystalline nickel matrix with grain sizes as small as 10 nm and SiC contents up to about 10.5 vol.% were produced from a modified Watts bath. Microhardness values of the nanocomposite were more than twice those for conventional polycrystalline
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MMCs with similar SiC content and greater than four times the hardness of conventional polycrystalline Ni. The hardness values were similar to the values for pure nanocrystalline Ni of comparable grain size. Nanocomposite Ni–SiC demonstrated a tensile strength greater than four times that of conventional polycrystalline nickel and about two times that for conventional polycrystalline MMCs of comparable SiC content. The tensile strengths for the nanocomposite samples with low SiC content (B 2%) were also higher than for nanocrystalline Ni of comparable grain size. Yield strength values greater than five times that for annealed Ni and about 1.5 times that for conventional polycrystalline MMC of comparable SiC content were measured for nanocomposite Ni–SiC. Most pure nanocrystalline Ni samples were brittle, exhibiting a yield points close to their fracture points. Nanocomposite Ni–SiC revealed an unexpected increase in ductility over that of pure nanocrystalline Ni of comparable grain size, especially at lower SiC contents (B 2 vol.%). The ductility of nanocomposites of lower SiC contents was comparable to the ductility of conventional MMCs. Fractography results revealed a ductile fracture mechanism possibly via a void coalescence process operative in nanocomposite-Ni–SiC and a brittle cleavage-type mechanism in pure nanocrystalline Ni.
Acknowledgements The authors wish to thank Dr N. Wang, Z. Wang and V. Krstic for their contributions to this work. Financial support for this work was provided by the Natural Sciences and Engineering Research Council of Canada and Ontario Power Generation. One of the authors (A.F.Z.) would like to acknowledge financial support by the School of Graduate Studies and Research at Queen’s University.
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