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Materials Science & Engineering A 584 (2013) 88–96
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of titanium nitride nanoparticles on grain size stabilization and consolidation of cryomilled titanium Shehreen S. Dheda a, Christopher Melnyk b, Farghalli A. Mohamed a,n a b
Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575, USA California Nanotechnologies, Inc., Cerritos, CA 90703, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2013 Received in revised form 29 April 2013 Accepted 18 June 2013 Available online 6 July 2013
In this work, titanium nitride (TiN) nanoparticles (∼20 nm) were introduced during cryomilling of commercially pure titanium (CP Ti). Consolidation of cryomilled powders was performed using spark plasma sintering (SPS). Samples were analyzed and tested alongside cryomilled, SPS CP Ti not containing TiN nanoparticles. After cryomilling powders containing TiN nanoparticles and powders not containing TiN had a minimum grain size of ∼20 nm. Microstructure analysis after thermal processing of both samples revealed that grain size retention occurred due to the presence of TiN nanoparticles in CP Ti microstructure. In consolidated samples containing 5 vol% TiN nanoparticles, the minimum average grain size was retained to ∼250 nm, while in samples containing 0 vol% TiN nanoparticles, the minimum average grain size obtained was ∼750 nm. Microhardness testing showed an increased hardness of samples containing TiN nanoparticles due to the retention of smaller grains and the presence of TiN nanoparticles. & 2013 Elsevier B.V. All rights reserved.
Keywords: Cryomilling Spark plasma sintering Commercially pure titanium Nanoparticles Microhardness
1. Introduction The production of nanocrystalline (nc) and ultrafine-grained (UFG) materials is gaining popularity amongst researchers because of the enhanced properties, such as strength [1,2] and corrosion resistance [3], that are exhibited by these materials compared to their coarse grained counterparts. Severe plastic deformation (SPD) is a class of techniques that is used to produce nc (d o100 nm) and UFG (d∼250 nm–900 nm) materials [4]. Of the various SPD techniques that exist, high pressure torsion [4,5], hydrostatic extrusion [6,7], and cryomilling [8,9] are capable of producing nc or near-nc materials. Of these techniques, cryomilling is advantageous [8] because cryomilling is a repeatable technique that can be used to produce large quantities of nc materials, and has been used on a variety of metals and alloys, including commercially pure Ti [10]. Cryomilling is a variation of conventional room temperature mechanical milling and involves mechanically milling a metallic powder in a cryogenic liquid medium, usually liquid nitrogen or liquid argon [8]. During high temperature consolidation, grain growth occurs due to heating. It was previously found [10,11] that during cryomilling in liquid nitrogen, nitrogen and oxygen react with the material during milling to form nanoscale nitrides and oxides,
n
Corresponding author. Tel.: +1 949 824 5807; fax: +1 949 824 2541. E-mail address: [email protected] (F.A. Mohamed).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.06.079
which have been shown to lead to grain size stabilization during heating. To enhance this effect, researchers [12–14] are now exploring the addition of nanoparticles during milling of materials. For example, Tang et al. [12] produced nanostructured Al-5083 with 6.5 vol% SiC particles (∼25 nm in size). They found that after degassing at 400 1C for 20 h and HIPping at 400 1C for 2 h, the grain size was maintained as ∼100–200 nm in certain regions where the particle concentration was 8 vol%. In addition, Maung et al. [13] recently produced cryomilled nc-Al with 1 wt% diamantine particles (o 5 nm in size). Grain size stability within the nc range was observed when the powders were heated to different temperatures between 150 1C and 500 1C. For commercially pure (CP) Ti, it has been found that cryomilling of CP Ti in liquid nitrogen results in brittle consolidated samples due to the diffusion of nitrogen atoms into octahedral interstitial sites within the titanium crystal structure (RN (65 pm)/RTi (140 pm) ¼ 0.464) [15]. Liquid argon is another choice for a cryogenic liquid medium and has been shown to be successful for milling of CP Ti. However, the formation of nanoscale oxides and nitrides were not observed in the microstructure of CP Ti after cryomilling in liquid argon [16]. As a result, grain growth stabilization is reduced within the material. In this investigation, TiN nanoparticles were introduced during cryomilling of CP Ti powder in liquid argon in order to replicate the grain size stabilization effect of nanoscale particles during heating that accompanies the consolidation process. It is the purpose of this paper to report and discuss the results of the present investigation.
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Table 1 Parameters used for cryomilling. Cryomilling parameter
Value
Quantity of Ti powder Size of stainless steel balls Ball-to-Powder Ratio (BPR) Cryomilling medium Temperature Impeller rotation speed Time TiN nanoparticles Stearic acid
500 g ∼6 mm in diameter 30:1 Liquid Argon ∼ 0002 1867 10 1C 180 rpm 8h 0 vol% or 5 vol% 0.05 wt% (with TiN)
Table 2 Chemical compositions of as-receive (AR) and cryomilled CP Ti powders. Sample Element
AR CP Ti powder wt%
Cryomilled CP Ti powder wt%
Cryomilled CP Ti powder with 5 vol%TiN wt%
O N H C Fe Residuals Ti
0.229 0.016 0.0222 0.018 0.010 o 0.081 Balance
0.94 0.34 0.06 0.02 0.09 –
2.22 1.96 0.09 0.10 0.09 –
2. Experimental procedure Cryomilling and consolidation work were conducted at California Nanotechnologies, Inc. (Cerritos, Ca). The parameters used for cryomilling of CP Ti are listed in Table 1 and the chemical composition of the as-received CP Ti powder is listed in Table 2. Different concentrations of TiN nanoparticles (∼20 nm in size) were used in separate cryomilling runs: 0 vol% and 5 vol%. During milling runs that included the addition of TiN nanoparticles, stearic acid was added as a process control agent (PCA) to prevent agglomeration of powder particles. This step was taken as a precaution because it was unknown if addition of TiN nanoparticles would affect agglomeration of particles during milling. Stearic acid was not added for the cryomilling run that did not include TiN because it was determined through previous milling runs that agglomeration of particles did not occur during cryomilling of CP Ti in liquid argon and that the presence of stearic acid during cryomilling did not affect the size and morphology of cryomilled powders. It should be mentioned, in their work on CP–Ti, in which a comparison was made between the mechanical behavior of cryomilled Ti using argon and that using nitrogen, Ertorer et al. [16] did not add a PCA during cryomilling runs involving liquid nitrogen as the powders did not agglomerate. After cryomilling, powders were degassed at 500 1C for 10 h at pressures no greater than 10 0002 6 Torr. Since cryomilling is a powder metallurgy technique, consolidation is required to produce bulk samples. Various consolidation techniques exist such as hot isostatic pressing (HIP) [17], quasiisotactic pressing [18], and spark plasma sintering (SPS) [19]. In the present study, SPS is used to consolidate powders after cryomilling. During SPS, the material powder is packed into a die (usually made of graphite). A pulsed DC current is applied through the die and powder, resulting in heating of the die and powder. The temperature is monitored by a thermocouple placed in the center of the die wall. A pressure is simultaneously applied to the powder during heating, resulting in consolidation. SPS differs from other techniques because SPS involves internally heating the material powder by application of a pulsed DC current through
89
the powder [20]. In other techniques, the powder is externally heated using an external heating source requiring longer times at elevated temperatures. Given this advantage in heating source, higher internal temperatures can be reached within the powders much more rapidly reducing the total time at elevated temperatures during SPS. Cryomilled powders containing 5 vol% TiN were then spark plasma sintered using the parameters in Table 3 to produce samples labeled as 5volTiN1, 5volTiN2, and 5volTiN3. Cryomilled powders that did not contain TiN were not degassed and were directly spark plasma sintered using the parameters in Table 3 to produce samples labeled as 0volTiN1, 0volTiN2, 0volTiN3, and 0volTiN4. There are two reasons for not degassing 0volTiN. First, PCA was not added during cryomilling. Second, it was noticed that degassing the cryomilled 0 vol% TiN powder led to significant grain growth. For all samples, a heating rate of 50 1C/min was used and the sintering pressure was first applied when the sintering temperature was reached and held constant during sintering. Spark plasma sintered samples had dimensions approximately 1 in. diameter and 0.25 in. thick. For microstructure analysis, powders were mounted in phenolic resin and mechanically polished. Transmission electron microscopy (TEM) specimens were prepared from these polished samples using the focused ion beam (FIB) in an FEI Quanta 3D FEG SEM. For each sample, slices were removed from the interiors of particles (as shown in Fig. 1) and thinned down using the FIB to form an electron transparent specimen. The specimens were analyzed in a Philips/FEI CM-20 TEM at 200 kV. Grain size measurements of 200 grains were performed for each sample using ImageJ software. Chemical analysis of the cryomilled powders was carried out by Luvak Inc., a professional chemical analysis company located in Boylston, MA. Archimedes method was used to determine the porosity of the spark plasma sintered samples. For grain size analysis, samples Table 3 Parameters used for SPS. Sample
Pressure (MPa) Sintering temperature (1C) Sintering time (min)
0volTiN1 0volTiN2 0volTiN3 0volTiN4 5volTiN1 5volTiN2 5volTiN3
60 85 85 85 85 85 85
822 750 700 600 600 600 750
10 5 5 5 5 10 5
10 μm Fig. 1. SEM image of FIB cut powder particle during TEM sample preparation. A slice of materials was removed and further thinned using FIB to produce an electron transparent sample for examination in the TEM.
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were analyzed using the TEM and/or SEM. For TEM a slice of material was cut from the sample, polished using SiC papers of grit sizes up to 800 grit, and thinned to a thickness of ∼60 μm. 3 mm disc samples were cut and twin jet electropolished with a solution of 15% HCl and 85% methanol at 233 K and 15 V. TEM analyses were performed using a Philips/FEI CM-20 TEM operated at 200 kV. For SEM analysis, a slice of material was cut from the sample, polished using SiC papers of grit sizes up to 800 grit, and final polished with 0.25 μm diamond solution and colloidal silica. Microstructures of the samples were imaged using an FEI Magellan 400 XHR SEM operated at 1 kV or 3 kV and 0.8 nA using the through-the-lens detector (TLD) detector with immersion field lens setting in back-scattered electron (BSE) mode with the sample at a working distance of ∼2 mm. Grain size measurements of 200 grains were performed for each sample using ImageJ software. Electron energy loss spectroscopy (EELS) analysis of sample 5volTiN3 was performed using a Philips/FEI CM 300 equipped with a 2 k Gatan imaging filter (National Center of Electron Microscopy, Lawrence Livermore Berkeley Labs) and operated at 300 kV. For microhardness testing, SPS samples were polished using SiC papers and final polished with colloidal silica suspension. Hardness testing was performed using a Buehler Micromet 5101 hardness tester at room temperature with a square base diamond indenter, well known as a Vickers indenter. 20 measurements were performed for each sample using a load of 200 g with a holding time of 25 s.
3. Results and discussion 3.1. Powder analysis After cryomilling for 8 h, powders containing 0 vol% TiN and 5 vol% TiN were each observed to have a minimum grain size of ∼20 nm. The microstructure of the 5 vol%TiN powder is shown in Fig. 2. Oleszak and Shingu [21] have observed that the crystallite size decreases with milling time leading to a minimum grain size, which is a characteristic of each metal. Eckert et al. [22] have suggested that the minimum average grain size, dmin, is obtained as a result of a balance between the formation of dislocation structure and its recovery by thermal processes. By utilizing the aforementioned findings and suggestions, Mohamed [23] has developed a theoretical dislocation model, which quantitatively describes the dependence of the minimum grain size on several physical parameters in an nc-material. The model may be
100 nm Fig. 2. Bright field TEM images of cryomilled CP Ti containing and 5 vol% TiN nanoparticles.
represented by [23] 2
dmin =b ¼ A3 expð0002βQ =RTÞðDPO Gb =vo kTÞ0:25 ðγ=GbÞ0:5 ðG=HÞ1:25
ð1Þ
where b is the value of Burgers vector, A3 is a dimensionless constant, β is a constant (∼0.04), Q is the self-diffusion activation energy, R is the gas constant, T is the absolute temperature, DPO is the frequency factor for pipe diffusion, G is the shear modulus, νo is the initial dislocation velocity, k is Boltzmann's constant, γ is the stacking fault energy and H is the hardness (¼ 3s, where s is the normal stress). As shown elsewhere [24], the model predicts that the minimum grain size for Ti is about 17 nm, which is very close to the experimental value obtained (20 nm) for both 0 vol% TiN and 5 vol% TiN. Also, the present results show that the presence of 5 vol% TiN has no effect on the minimum grain size in Ti. Chemical compositions of the cryomilled powders are listed in Table 2. Cryomilling has been known to introduce some amounts of O, N, H, C, and Fe impurities into the powders [10]. Increase in O, N and H content could be related to air leaking into the tank during cryomilling and also due to exposure to air during transfer of the cryomilled powder from the tank to the glove box. Increase in Fe and C contents is due to the introduction of these elements from the stainless steel milling apparatus (tank, shaft and balls). Cryomilled powders containing TiN nanoparticles had a higher N content because of the presence of the nanoparticles. As expected, milled powders processed with PCA contained higher levels of O, C, and H. Selected area diffraction (SAD) patterns for the cryomilled powders did not indicate the presence of the titanium oxides or titanium carbides. After degassing, the average grain size of the 5 vol% TiN powder was found to be ∼90 nm, shown in Fig. 3a. The SAD pattern in Fig. 3b has diffraction spots (indicated by arrows) that correspond to TiN. Fig. 3c shows a TiN nanoparticle (labeled P) at a grain boundary. 0 vol% TiN cryomilled powder that was also degassed exhibited a grain sizes ranging from nanocrystalline to as large as ∼1–2 μm. In order to obtain grain sizes less than 1 μm after SPS, 0 vol% TiN cryomilled powders that were not degassed were used to produce bulk 0 vol% TiN samples, while degassed 5 vol% TiN cryomilled powders were used to produced bulk samples containing TiN. 3.2. SPS and microstructure analysis To obtain minimum grain size and minimum porosity in consolidated samples, various parameters were initially evaluated for production of bulk cryomilled, SPS samples of CP Ti containing 0 vol%TiN nanoparticles (labeled as 0volTiN). Table 4 lists the average grain sizes and porosities that resulted for each sample. 0volTiN1 was SPSed at 822 1C at 60 MPa for 10 min. Pressure was applied via uniaxial direct pressing and maintained for the duration of the sintering. 60 MPa was the highest practical pressure that could be applied without fracture of the graphite die surrounding the powder. A temperature of 822 1C was used because it is lower than the α-β phase transition temperature (∼890–950 1C, depending on the impurity content [25]), while a sinter time of 10 min was used in order to ensure maximum possible densification. Grain size analysis of this sample resulted in an average grain size of ∼1.75. This grain size is larger than the ultrafine-grain size range (d∼200 nm–1 μm). The grain growth observed is likely a result of thermal exposure during sintering. The high grain size could be due to the high sintering temperature and time. During SPS, the temperature measured is that of the center of the graphite die wall. It is generally believed that the temperature within the sample is higher than that of the die [26]. Given this, it is possible that during sintering the temperature reached higher than the α-β temperature. In the β phase diffusion processes are favored because the crystal lattice is less packed,
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Table 4 Average grain sizes, porosity fractions, and hardness values of cryomilled and spark plasma sintered samples.
Fig. 3. (a) Bright field TEM image of cryomilled, degassed CP Ti containing 5 vol% TiN nanoparticles. (b) Corresponding SAD pattern indicating diffraction spots for TiN, indicating its presence in the Ti matrix. (c) Bright field TEM image of TiN nanoparticle (P) at grain boundary of Ti grain.
resulting in enhanced grain growth [27]. Also, holding the sample at sintering temperature for 10 min allowed for more grain growth to occur. A porosity of ∼2.3% was also obtained for this sample. Ertorer et al. [16] obtained porosities close to ∼0% during SPS of cryomilled CP Ti. Sample porosity depends on sintering parameters [27]. Higher sintering pressures and higher sintering temperatures can result in lower porosities [16,28]. For the remaining samples a nickel based superalloy die was used so a higher pressure of 85 MPa could to be applied. Samples 0volTiN2, 0volTiN3, and 0volTiN4 were sintered at 750 1C, 700 1C
Sample
d (μm)
P
Hv (GPa)
0volTiN1 0volTiN2 0volTiN3 0volTiN4 5volTiN1 5volTiN2 5volTiN3
1.75 1.09 0.99 0.75 0.25 0.44 0.53
0.023 0.013 0.015 0.020 0.040 0.060 0.000
3.26 7 0.116 3.42 7 0.100 3.46 7 0.140 3.40 7 0.090 5.56 7 0.461 5.30 7 0.421 7.05 7 0.244
and 600 1C, respectively. All were held at for a sintering time of 5 min in order to minimize grain growth. A shorter sintering time could be used because the higher pressure promoted more rapid densification. As seen in Table 4, as sintering temperature was decreased, grain size decreased and porosity increased. A higher sintering temperature resulted in greater densification, but also resulted in grain growth, as observed elsewhere for CP Ti [29]. Therefore, it is necessary to choose an optimum sintering temperature that will allow for minimal grain growth and minimal porosity. Sintering temperatures lower than 600 1C were not attempted because lower temperatures would result in reduced densification of samples. For comparison, powders containing TiN nanoparticles were first sintered at 600 1C for 5 min (sample 5volTiN1 in Table 3) to achieve minimum grain size. However, a high porosity of ∼4% was obtained. In order to improve densification and reduce porosity, a longer sinter time was used to produce sample 5volTiN2. In this case, an even higher porosity of ∼6% was achieved. This increase in porosity can be explained as follows. In SPS, it is assumed that the pulsed high current density that is applied to the powders creates localized plasma fields inside of gaps between powder particles or at contact points between particles. When the pulsed current is on, the plasmas can cause breakdown of oxide layers, absorbates, and other contaminants on the particle surfaces, thereby cleaning the particles and enhancing sintering [19,27]. ‘Cleaning’ of the samples results in evolution of gases, such as oxygen. The evolved gases as well as gases that are already present between the powder particles can become trapped in the spaces between particles if diffusion of the gases does not occur rapidly enough before the full sintering pressure is applied [30]. During SPS of sample 5volTiN2, it was observed that when the powder was heated from ambient temperature to 600 1C, there was significant fluctuation in recorded temperature, vacuum, and current. The frequency of fluctuation of vacuum matched the frequency of the fluctuation of applied current during heating from ambient temperature to sintering temperature confirming the breakdown and expansion of absorbates and other gases. The resultant fluctuations in temperature matched in frequency but had a slight delay in relation to the current and vacuum fluctuations, as expected. Temperature of the powder was monitored indirectly by monitoring the temperature in the die wall. Considering the temperature is produced internally there is an inherent delay and negative delta between the measured temperature and the actual internal temperature. Since the SPS process is normally controlled by a closed loop system, the power output controller relies on this measured temperature to make power output adjustments to keep the ramp rate on target. If the lag becomes large enough, it is possible that the close loop control system overcorrects both over and under temp, causing the significant applied current fluctuations observed. These fluctuations occurring near the target sintering temperatures may, in turn, have caused localized sintering along the die–powder interfaces encasing the powder in the interior of the sample. The densified outer layer would be significantly less permeable then the powder compact initially,
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14
25
12 20
Frequency (%)
Frequency (%)
10 8 6 4
15
10
5
2 0 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 4.8 5.4 6.0 6.6 7.2 7.8 8.4 9.0 9.6
0 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 4.8 5.4 6.0 6.6 7.2 7.8 8.4 9.0 9.6
d (µm)
d (µm) 25
35 30
20
Frequency (%)
Frequency (%)
25 15
10
20 15 10
5
5 0 0.0
0.6
1.2
1.8
2.4
3.0
3.6
4.2
4.8
5.4
0 0.0
6.0
0.6
1.2
1.8
d (µm)
2.4
3.0
3.6
4.2
4.8
5.4
6.0
d (µm)
40
25
35
20
25
Frequency (%)
Frequency (%)
30
20 15
15
10
10
5 5 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
d ( µm )
d (µm) 40 35 30
Frequency (%)
25 20 15 10 5 0 0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
d (µm) Fig. 4. Grain size distributions of samples 0volTiN1 (a), 0volTiN2 (b), 0volTiN3 (c), 0volTiN4 (d), 5volTiN1 (e), 5volTiN2 (f), and 5volTiN3 (g). For (a)–(d), the bin size is 0.2 μm and for (e)–(g) the bin size is 0.1 μm.
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93
thus preventing thorough outgassing during the remaining sintering process and leading to a higher porosity in sample 5volTiN2. It is also possible that the presence of the TiN nanoparticles resulted in a decrease in electrical conductivity, which may have affected the current applied through the powders during SPS [31]. In order to obtain greater densification, a higher sintering temperature of 750 1C was used to produce 5volTiN3. In the case of this sample, a porosity of ∼0% was obtained. A higher sintering temperature increased the conductivity of the powder [31]. In addition, negligible fluctuations were observed as temperature
was increased. Gases evolved during sintering were able to escape due to the longer heating time to reach the sintering temperature (that is, before application of sintering pressure) and higher diffusion rates of the gases due to the higher temperature. Both these factors may have contributed to the greater densification of this sample. The differences in average grain size (listed in Table 4) found for the three 5volTiN samples was consistent with the combination of temperature and time used. The sample sintered at 600 1C for 5 min had the smallest grain size of ∼250 nm, while the sample sintered at 750 1C for 5 min had the largest grain size of ∼530 nm.
Fig. 5. (a) Bright field TEM image of 0volTiN4 sample microstructure having average grain size ∼750 nm. (b) Backscatter electron SEM image of 5volTiN3 sample microstructure having average grain size ∼530 nm. (c) Bright field TEM image of 5volTiN1 sample microstructure having average grain size ∼250 nm. Pores in the TEM images appear as bright spots, while those in the SEM image appear as dark spots.
Fig. 6. (a) Bright field TEM image of 5volTiN1 microstructure showing TiN nanoparticles (indicated by arrows) at grain boundaries. (b) SAD pattern corresponding to (a). Diffraction spots indicated with arrows corresponds to TiN. (c) Bright field TEM image of 5volTiN3 microstructure showing TiN nanoparticles (indicated by arrows) at grain boundaries.
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Grain size distributions for all samples are shown in Fig. 4. In general, samples containing 5 vol% TiN nanoparticles had smaller average grain sizes than those containing 0 vol% TiN nanoparticles, as seen in Fig. 4 and Table 4. This is best illustrated by the differences in grain size obtained for samples 0volTiN4 (Fig. 4d) and 5volTin1 (Fig. 4e) sintered at 600 1C for 5 min and samples 0volTiN2 (Fig. 4b) and 5volTiN3 (Fig. 4g) sintered at 750 1C for 5 min. Grain growth involves the migration of high angle grain boundaries [32]. The presence of second phase particles within a microstructure can hinder grain growth due to grain boundary pinning [33]. Fig. 5 shows the microstructures of samples 0volTiN4, 5volTiN1 and 5volTiN3. TiN nanoparticles in 5volTiN1 are indicated by arrows in Fig. 6a. Fig. 6b is the corresponding SAD pattern. The arrows in Fig. 6b indicate diffraction spots that correspond to d-spacings observed for TiN. The presence of these diffraction spots in the SAD pattern indicates the presence of TiN nanoparticles in the Ti matrix. TiN nanoparticles in 5volTiN3 are indicated by arrows in Fig. 6c. In Fig. 6a and c, grains are overlapping. Some particles that are indicated are at grain boundaries of grains that are below other grains. In both samples, TiN nanoparticles were mostly observed at grain boundaries. Fig. 7 also shows an example of a particle (circled) found at a grain boundary in sample 5volTiN3 using EELS. The cluster of bright pixels circled in the nitrogen map (Fig. 7b) obtained indicated a higher concentration of N in the TiN nanoparticle.
grain size in Fig. 8 in order to determine if a Hall–Petch relationship [34,35] existed. Samples containing 0 vol% TiN (indicated by diamond symbols) followed a Hall–Petch relationship with equation:
3.3. Microhardness testing
H v ¼ H o expð0002bPÞ
Microhardness values (Hv) measured for the samples are also in Table 4 and plotted against the inverse square root of the average
where Ho is hardness of the material at full density, P is porosity as a fraction, and b is a constant, usually in the range of 3–9 for most
00021=2
H v ¼ 3:0 þ 0:385d
ð2Þ
When the line in Fig. 8 described by Eq. (2) is extended to grain sizes exhibited by the samples containing 5 vol% TiN (indicated by squares), it is clear that the hardness values obtained for these samples are much higher than those expected for samples containing 0 vol% TiN if they had the same grain sizes. Higher values of hardness are due to the presence of TiN nanoparticles that act as second phase reinforcement particles and increase the hardness of the samples [14,31]. Second phase reinforcement particles interact with dislocations during deformation, hindering or impeding there motion [36,37,32]. As seen in Fig. 8, hardness values of samples containing TiN did not appear to follow a Hall–Petch relationship. In addition to grain size affecting hardness, another important factor that can affect hardness is porosity. When porosity is present in a material, the hardness value that is measured is the ‘apparent’ hardness due to the filling of pores during indentation [38,39]. Various researchers [40–44] have found that hardness values for ceramic materials decrease as porosity increases according to the exponential relationship: ð3Þ
Fig. 7. (a) Bright field TEM image of particle (circled) located at a grain boundary in sample 5volTiN3. Bright circular spots in the image are pores. (b) Nitrogen map obtained by EELS showing the presence of a TiN nanoparticle indicated by the cluster of bright pixels (circled) similar in shape to the particle cirlced in (a). (c) Nitrogen pre-edge image also indicates the presense of the TiN nanoparticle.
S.S. Dheda et al. / Materials Science & Engineering A 584 (2013) 88–96
8.00 7.00
Hv (GPa)
6.00 5.00 4.00 3.00 2.00 0volTin
1.00
5volTiN
0.00 0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
2.20
Fig. 8. Plot of hardness values versus inverse square root of average grain size for 0volTiN and 5volTiN samples.
14
95
after SPS due to grain boundary pinning and higher hardness values brought about by smaller grain sizes and the presence of the nanoparticles. Hardness values of samples containing 5 vol% TiN were in fact much higher than those expected if samples containing 0 vol% TiN had the same grain sizes, but these values did not follow a Hall-Petch relationship with grain size because the presence of large amounts of porosity had a greater effect on hardness values measured for the samples. Overall, it was shown that the introduction of TiN nanoparticles during cryomilling of CP Ti can result in hindered grain growth due to thermal exposure, including consolidation processes such as SPS and enhanced hardness of samples after consolidation. Further parametric optimization and additional secondary processes (such as extrusion) would potentially result in lower porosities and enhanced mechanical properties. In addition, further investigation of the addition of different concentrations of TiN nanoparticles during cryomilling and the effects of TiN nanoparticle concentration on SPS parameters, final grain size, and final porosity will allow for an optimum concentration of TiN nanoparticles to be determined for obtaining cryomilled, spark plasma sintered CP Ti with nc grain sizes.
12 Acknowledgments
Hv (GPa)
10
This work was supported in part by the National Science Foundation under Grant no. DMR-0702978. The authors would also like to acknowledge the Laboratory for Electron and X-ray Instrumentation (LEXI) at UCI for the use of the SEM and TEM microscope facilities as well as the National Center for Electron Microscopy at Lawrence Berkeley National Lab for the use of the TEM and EELS microscope facilities.
8 6 4 2 0 0.00
References
0.01
0.02
0.03
0.04
0.05
0.06
0.07
P Fig. 9. Plot of hardness values versus porosity for 0volTiN and 5volTiN samples.
ceramics [40]. In addition, it has also been found by He and Ma [45], that porous metallic materials processed by mechanical milling also exhibit such a relationship between hardness and porosity. In their study, the hardness of mechanically milled and consolidated Fe–29Al–2Cr intermetallic decreased with an increase in porosity according to the exponential relationship described by Eq. (3). Fig. 9 shows Hv plotted against P for the 5volTiN samples. Data for these samples were fitted to an exponential relationship such that: H v ¼ 7:0expð00024:9PÞ
ð4Þ
The calculated value of b falls within the range of 3–9 determined previously [40]. It is likely that the 5volTiN samples also follow a Hall–Petch relationship. However, due to the high levels of porosity found in the 5volTiN1 and 5volTiN2, the effect of porosity dominated the trends observed for hardness values obtained for the 5volTiN samples.
4. Conclusion In this study, CP Ti powders were cryomilled with 0 vol% and 5 vol% TiN nanoparticles and were spark plasma sintered to obtain bulk samples with varying average grain sizes and porosities. Addition of TiN nanoparticles resulted in smaller final grain sizes
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Free download or read online True Grit pdf (ePUB) book. The first edition of the novel was published in May 21st 1968, and was written by Charles Portis. The book was published in multiple languages including English, consists of 224 pages and is available in Paperback format. The main characters of this fiction, westerns story are Mattie Ross, Tom Chaney. The book has been awarded with , and many others.

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Author: Charles Portis
Original Title: True Grit
Book Format: Paperback
Number Of Pages: 224 pages
First Published in: May 21st 1968
Latest Edition: December 31st 2002
Language: English
Main Characters: Mattie Ross, Tom Chaney, Rooster Cogburn, Ned Pepper
category: fiction, westerns, historical, historical fiction, classics, adventure
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The translated version of this book is available in Spanish, English, Chinese, Russian, Hindi, Bengali, Arabic, Portuguese, Indonesian / Malaysian, French, Japanese, German and many others for free download.

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