Grain growth analysis of multiwalled carbon nanotube reinforced bulk Ni composites
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(2) 174. CARBON. 7 0 ( 2 0 1 4 ) 1 7 3 –1 7 8. a wide span of potential pinning phases such as precipitates, compounds, particles and/or fibres. For example, Rios et al. [9] reported the grain boundary pinning produced by Al6Mn precipitates in an Al alloy during annealing. Humphreys observed the same effect in copper composites reinforced with Al2O3 particles [10]. Grain boundary pinning was identified also by the effect of carbides in high performance alloys during thermal treatments [11]. For CNT-reinforced metal matrix composites, the grain refinement strengthening has been identified in several different matrices such as: spark plasma sintered copper [12], liquid-state processed magnesium [13], hot-rolled aluminium [14] and electrodeposited nickel [15]. It was observed that the increment in the MWCNT relative volume represented an improvement in the mechanical properties (hardness and/or yield strength) related to a microstructural refinement. Recently, Bakshi et al. thoroughly studied the influence of the addition of CNTs in the strengthening of Al-based composites [16]. They point out that the interfacial interaction of the CNTs with the matrix plays a fundamental role in the strengthening, finding a limit at 10 vol.% of CNTs. Nevertheless, they state that this limit might be moved forward by the development of a proper dispersion within the matrix. Despite all the reports that could be found in the literature, and to the best of our knowledge, there is no information available regarding the influence of the amount and distribution of CNTs in the grain growth and the final microstructure. In that sense, we study the influence of the MWCNT concentration in the grain growth evolution and final microstructure of the MWCNT/Ni composite system produced by a pressureless sintering process. The precursor blends were prepared by a colloidal mixing process which has already delivered good dispersion and distribution results [17,18]. The grain growth evolution was investigated by means of high-temperature X-ray diffraction (HT-XRD) and electron backscattered diffraction (EBSD). The main objective of this work is to settle empirical reinforcing limits to this blending process for manufacturing metal/CNT composites by analysing the grain growth behaviour as a function of the CNT concentration.. 2.. and 27 vol.%, respectively). Finally, the blends were dried in a ventilated furnace for 4 h until there was no remaining solvent. Thereafter, cylindrical pellets of 8 mm in diameter and 2–3 mm in height were cold pressed in a hydraulic press at 990 MPa. In order to compare their behaviour and the influence of the CNT content, pure Ni samples were also manufactured with the as-received powder under the same conditions. The microstructure of the samples was analysed in a dual beam focused ion beam/scanning electron microscopy (FIB/ SEM) workstation (FEI Helios NanoLab 600) equipped with an electron backscattered diffraction (EBSD) detector (EDAX TSL). EBSD was performed onto the surface of the samples (500 lm · 500 lm) using a voltage of 20 kV, a current of 22 nA, and a step size of 500 nm. The grains were defined as at least two adjacent points with similar orientation within a range of 5 of misorientation. The raw data was cleaned using confidence index standardization within each grain. Later, points with confident index smaller than 0.1 were discarded. The HT-XRD measurements were carried out in an Anton Paar HTK1200 HT-chamber at 106 mbar and mounted in a PANalyical X’Pert MPD X-ray diffractometer. The diffractograms were obtained using a symmetrical hh geometry configuration and a Cu Ka1 radiation (k = 0.15406 nm). The incident and diffracted optical geometries were parallel and the diffraction angle (2h) was varied from 40 to 130 with a step size of 0.0131 and a 0.5 s/step rate. The maximum irradiated area was 11.9 mm2 with a corresponding penetration depth of approximately 9 lm for the (1 1 1) reflection. The applied voltage and current were 40 kV and 40 mA, respectively. For the HT-XRD measurements, the samples were heated from 50 to 950 C at 10 C/min and the diffractograms were obtained every 25 C after 15 min of thermal stabilization. The temperature distribution in the sample was thus homogenised and errors due to temperature gradients were reduced.. 3.. Results and discussion. 3.1.. HT-XRD analysis. Materials and methods. The MWCNT/Ni bulk composites were manufactured from blends processed with a colloidal mixing process and subsequently sintered at 950 C for 2.5 h in vacuum. The starting materials were Ni dendritic powder [19] (Alfa Aesar, 325 mesh, particle size <44 lm) and MWCNT Baytubes C150P (Bayer Materials Science). The MWCNTs were produced by catalytic chemical vapour deposition, with an outer diameter distribution of 5–20 nm, length between 1 and 10 lm and carbon purity over 95%. The colloidal mixing process consists of three steps: dispersion, blending with Ni powder and drying. The first step consists of the dispersion of the as-received MWCNTs agglomerates in an ultrasound bath with Ethylene Glycol for 10 min. The best dispersion was achieved for a CNT/solvent concentration of 0.2 mg/ml [20]. Then, the Ni powder was added to the CNT suspension and mixed in the ultrasound bath for 5 min. The composite powders were made with 1.0, 2.0, 3.0 and 5.0 wt.% of MWCNT (6.5, 12, 18. The analysis of the grain growth of the Ni matrix of all the samples as well as the phase evolution including different concentrations of MWCNT was performed at a first stage by high temperature XRD. Regarding the phase evolution, an essential fact to be considered is the reaction between metals and C. It is well known that certain metals react with CNTs by the application of temperature, degrading them to form carbides. The most significant examples are Al and Ti [21]. However, in the case of CNT/Ni systems no stable carbide is reported [22]. The formation of Ni3C was only observed in non-equilibrium conditions such as highly energetic ball milling [23] or spark plasma sintering [24]. In this study the following Ni diffraction reflections were detected: 111 (44.51), 200 (51.84), 220 (76.36), 311 (92.94), 222 (98.44) and 400 (121.92) [PDF file 04-0850], observing no other additional peaks. For a clearer observation, Fig. 1a shows only the first three diffraction peaks at different temperatures. All the reflections showed an accentuated peak width contraction.
(3) CARBON. 7 0 (2 0 1 4) 1 7 3–17 8. 175. Fig. 1 – (a) Diffractograms of MWCNT/Ni 1.0 wt.% as a function of the temperature (in C). (b) Evolution of the Ni (1 1 1) peak through the process. A clear contraction of the peak is observed and the shift towards lower angles depicts the thermal expansion of the matrix. (A colour version of this figure can be viewed online.). between 200 and 250 C (Fig. 1b), which corresponds to the starting point of the grain growth in the Ni matrix. This starting point was not significantly influenced by the presence of CNTs. For the quantitative determination of the grain growth, the calculations were based on the work published by Thompson and co-workers [25]. They state that the widths of the diffraction peaks contain information about the grain size and strain effects, being mathematically described by a Voigt profile, which is the convolution of a Lorentzian (grain size related) and a Gaussian (strain related) peak. A deconvolution of each peak was performed and the grain size at each temperature was calculated using C (FWHM) for the Lorentzian peak on Scherrer equation (Eq. (1)). D¼. 0:9 k C cos h. ð1Þ. Another important issue to consider is the existence of a systematic instrumental broadening of the peak, which was extracted from the equipment software and corroborated using LaB6 powder. This feature limits the capability of the technique to determine the grain size, allowing the calculation of the grain size up to few microns. The mean grain size estimated from all the observed Ni reflections as a function of the temperature in a reduced range (up to the aforementioned detection limit) is shown in Fig. 2. It can be observed that the tendency remains similar for all the samples. During the first stage, the grain growth is not yet activated and can be correlated to the initial stage sintering where grain growth is significantly slow due to the pinning action of the porosity [26]. Pores migrate to grain boundaries and act like mobility barriers reducing total grain boundary area and the surface energy. Afterwards, a sharp positive slope is observed starting close to 250 C, which corresponds to the beginning of the grain growth. After this temperature, once the grain growth is activated, it evolves at different rates for each sample. After this point, a competition between the pinning effect and the effect of the thermal conductivity begins. Considering the Ni growth as the reference, the 1.0 and 2.0 wt.% samples grow at a faster rate, whereas the 3.0 and 5.0 wt.% samples show a slightly lower growth rate. The increase in the growth rate has been already reported for CNT-containing composites produced via powder metallurgy [27]. The growth acceleration. Fig. 2 – Thermal evolution of the grain size estimated from all the observed Ni reflections. The grain size estimation is limited by the instrumental peak broadening in the diffraction patterns. (A colour version of this figure can be viewed online.). is a direct consequence of the high thermal conductivity of the CNTs (where the thermal energy is preferentially transported), generating a local temperature increase, thus favouring the grain growth [27]. In the case of this study, the 1.0 and 2.0 wt.% grow faster due to a combination of lower amount of agglomerates and porosity, which results in higher thermal conductivity. As reported in our previous work [18] the 3.0 and 5.0 wt.% samples show significantly more agglomeration and porosity, decreasing the overall thermal conductivity and, as a consequence, the growth rate. Fig. 3 shows the different surfaces of the samples. The increment in the agglomeration related to the CNT concentration is observable, being in certain spots for the 3.0 and 5.0 wt.% in the tens of micrometre range. For the 1.0 and 2.0 wt.% the clustering is always in the micron to submicron range. Yamanaka and co-workers explained the difficulties of the pressureless sintering of CNT-reinforced composites [28]. They state that the densification is severely hindered by the presence of CNTs with concentrations above 10 vol.% (approx. 2.0 wt.%). Reagglomeration into clusters is observed and these are believed to obstruct the densification by interposing.
(4) 176. CARBON. 7 0 ( 2 0 1 4 ) 1 7 3 –1 7 8. Fig. 3 – SEM micrographs of the samples. (a) Pure Ni, (b) MWCNT/Ni 1.0 wt.%, (c) MWCNT/Ni 2.0 wt.%, (d) MWCNT/Ni 3.0 wt.%, and (e) MWCNT/Ni 5.0 wt.%. The increase in the agglomeration activity as well as the void formation is noticeable. The darkest spots from (b) to (e) correspond to the agglomerated CNTs, whereas voids present bright edges in all the images.. between Ni particles. For lower concentrations (up to 10 vol.%), grain growth inhibition is seen and adjudicated to a stagnation of the grain boundary mobility generated by the CNTs. The same limit was observed for MWCNT/Ag bulk composites, densified by pressureless sintering [29]. It is reported that above this concentration, reagglomeration occurs, reducing the hardness and electrical conductivity of the composites.. 3.2.. EBSD analysis. The grain size distribution was determined by means of EBSD after the samples were sintered. Fig. 4 shows grain size maps of (500 · 500) lm2 for pure Ni, and MWCNT/Ni 1.0, 2.0, 3.0 and 5.0 wt.%. The parameters and procedures for data acquisition and processing were the same for all the samples. The white zones within the Ni grains correspond to low-EBSD signal regions. It could be either due to porosity or voids where the CNT fillers are located. As the percentage of CNT increases, the grain size of Ni decreases and the voids fraction increases. These voids generate free surface, which may also modify the grain growth kinetics [30]. Indeed, this effect can be observed on the grain size distributions estimated from the EBSD maps and presented in Fig. 5. The values of the resulting mean grain size measured in the distributions are summarised in Table 1. The representing parameter chosen from the grain size distributions (Fig. 5) was the equivalent diameter which represents the diameter of a circumference having the same area measured of the grain. The diameter distribution for the CNTcontaining composites as well as the pure Ni resembles a LogNormal distribution, in agreement to what is reported in the literature [31]. In the case of pure Ni, a wide diameter distribution can be observed with the highest frequency count at around 50 lm. The final grain size is considerably reduced by increasing the CNT-fraction in the composite. For the 1.0 wt.% sample. the higher amount of counts is ranged between 10 and 20 lm. The large grain sizes observed for pure Ni and 1.0 wt.% at indicate abnormal grain growth, whereas for 2.0, 3.0 and 5.0 wt.% a narrowed and even distribution is observed between 1 and 10 lm. Moreover, no significant change in the final size is observed for the 5.0 wt.% sample compared to the 2.0 and 3.0 wt.%. This fact is directly related to the reagglomeration of the CNTs, which can be observed in Fig. 3. The 5.0 wt.% corresponds to 27 vol.%, which reduces the distance between the MWCNTs and thus increases the probability of their reagglomeration. In Zener pinning, the resulting mean grain size is modelled with the following Eq. (2) [32]. D¼. kr fn. ð2Þ. The term k is a proportional dimensionless constant, f is the volume fraction of the second phase and r is the mean reinforcement radius. The stagnation occurs due to a restriction of the grain boundary motion due to the presence of a dispersion of second phase particles (in our case the CNTs), which act as a frictional force to the moving boundary hindering the growth [5]. As seen in Eq. (2), the refinement effect is strongly dependent on the size of the fillers, being directly proportional to their size. Therefore, by increasing the amount of agglomerates, the real size of the reinforcements is increased and the drag force is reduced proportionally resulting in larger final grain sizes. Moreover, another important requirement for the microstructural refinement is the interparticle distance distribution [7]. The interface energy absorption is inversely proportional to the distance between particles and in the case of higher amount of CNTs; the interparticle distance is statistically lower. However, by increasing the reagglomeration, the expected interparticle distance will be increased, thus reducing the refinement effect on the microstructure. Then, based on the reagglomeration on the.
(5) CARBON. 177. 7 0 (2 0 1 4) 1 7 3–17 8. Fig. 4 – Grain size maps obtained by EBSD for the sintered samples. (a) Pure Ni, (b) MWCNT/Ni 1.0 wt.%, (c) MWCNT/Ni 2.0 wt.%, (d) MWCNT/Ni 3.0 wt.%, and (e) MWCNT/Ni 5.0 wt.%. (A colour version of this figure can be viewed online.) to some extent an empirical limit for the grain size control of the composites beyond which no significant additional influence is noticed. In contrast, this reinforcing limit was also determined by Pham and co-workers [33], for powder metallurgical Cu/CNT composites being at 3.0 wt.%. For aluminium-based composites it was settled for a range between 3.0 and 5.0 vol.% [14]. Finally, based on the presented results, it can be stated that the presence of properly dispersed CNTs in the composite is a powerful tool to tailor the grain growth and thus the physical properties of the composite.. 4.. Fig. 5 – Grain size distributions of the sintered samples and obtained from the EBSD maps. The grain size is significantly reduced for samples with concentrations above 2 wt.%. (A colour version of this figure can be viewed online.). higher concentration samples, one might be allowed to state that between 2.0 and 3.0 wt.% the influence of the CNTs is not directly dependent on the CNT concentration. This fact fixes. Conclusions. The grain growth evolution of MWCNT/Ni composites was studied considering different MWCNT concentrations and their influence on the final microstructure. In this sense, the grain growth starting point was found to be between 240 and 250 C, and it is not significantly affected by the amount of MWCNTs. It was observed that, for lower MWCNT concentrations, the growth is driven mainly by a thermal conductivity-related effect, whereas for the higher concentrations (3.0 and 5.0 wt.%), the growth is governed by Zener grain boundary drag. Finally, an empirical reinforcing limit between 2.0 and 3.0 wt.% was determined for the MWCNT/Ni systems,. Table 1 – Mean grain sizes of the composites after the sintering process. The values were obtained by EBSD. Sample Pure Ni MWCNT/Ni MWCNT/Ni MWCNT/Ni MWCNT/Ni. 1.0 wt.% 2.0 wt.% 3.0 wt.% 5.0 wt.%. MWCNT vol.%. Final mean grain size [lm]. Grain size spread [lm]. –. 47.6 ± 3.1 22.4 ± 1.0 4.9 ± 0.1 4.9 ± 0.1 5.3 ± 0.1. 0.51 ± 0.07 0.65 ± 0.03 0.46 ± 0.02 0.46 ± 0.03 0.43 ± 0.02. 6.5 12 18 27.
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