Effect of TiN nanoparticles on microstructure and properties of Al2024-TiN nanocomposite by high energy milling and spark plasma sintering

Journal of Alloys and Compounds 726 (2017) 638e650

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Effect of TiN nanoparticles on microstructure and properties of Al2024-TiN nanocomposite by high energy milling and spark plasma sintering Bing Li, Fei Sun*, Qizhou Cai, Jingfan Cheng, Bingyi Zhao State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 May 2017 Received in revised form 2 August 2017 Accepted 3 August 2017 Available online 5 August 2017

Ultra-fine grained (UFG) Al2024-TiN nanocomposite was fabricated by high energy milling (HEM) and spark plasma sintering (SPS). The effect of TiN content (1, 2, 3 and 4 wt.%) on the morphology and microstructure of as-milled powders was investigated, as well as on the microstructure and properties of as-SPSed samples. Results show that TiN can accelerate the milling process, solution of Cu atoms and deformation of Al2Cu during milling. Besides, Al grain size of as-milled powders was refined from ~87 nm to 30e50 nm after TiN was added, staying steady when TiN exceeded 2 wt.%. Moreover, Al2024-2TiN nanocomposite behaved superior comprehensive properties, showing yield and ultimate strength of ~730 MPa and ~871 MPa, respectively, which were 185% and 35% higher than those without TiN, and still possessing ~10% engineering strain. The Al grain sizes of as-SPSed Al2024 and Al2024-2TiN samples were 576.5 nm and 145.4 nm, respectively, exhibiting 6.6 times and 4 times those of their corresponding milled powders. It is suggested that TiN nanoparticles are helpful to refine Al grains during milling and restrain grain growth during sintering. The high strength of Al2024-TiN nanocomposite was mainly attributed to ultra-fine grains, Orowan strengthening and dislocation strengthening due to TiN nanoparticles. © 2017 Elsevier B.V. All rights reserved.

Keywords: Ultra-fine grain Al2024 matrix nanocomposite TiN nanoparticle Spark plasma sintering Mechanical property Microstructure

1. Introduction In recent years, nanoparticle reinforced aluminum matrix composites (AMCs) with nanostructured/ultra-fine grained (NS/ UFG) aluminum grains have been widely investigated to obtain ideal lightweight materials with high strength [1e3], due to their integrated advantages of particle strengthening [4e6] and grain refinement strengthening [7e10]. Among the fabrication methods of NS/UFG AMCs, powder metallurgy (PM) is an attractive and costeffective way. It generally involves obtaining NS/UFG powders by milling and then consolidating the powders by sintering. Since the late 1980s, high energy milling (HEM) has been used to produce nanostructured powders for its low-cost and capability of largescale production with simple equipment [11]. Besides, it is also an effective way to disperse the nanoparticles throughout the matrix [12]. As for powder consolidation method, spark plasma sintering (SPS) is widely utilized due to its advantages of short-time

* Corresponding author. E-mail address: [email protected] (F. Sun). http://dx.doi.org/10.1016/j.jallcom.2017.08.021 0925-8388/© 2017 Elsevier B.V. All rights reserved.

sintering, low-temperature sintering, good densification effect, self-purification of particle's surface and effective bonding with particles [13], which are effective to obtain dense bulk materials. Since NS/UFG grains tend to be highly unstable under the thermal exposure of consolidation, as predicted by the GibbseThomson equation [14], consolidation process is detrimental to obtain ultra-fine matrix grains. Usually, grain growth would occur during the procedure. For example, the average grain size of cryomilled Al5083 powder was ~30 nm, while it increased to ~192 nm after SPS [15]. Eldesouky et al. also found that the grain size of milled AA2124 powders (~20 nm) increased to ~200 nm after SPS [16]. However, some investigations showed that ceramic nanoparticles could improve the thermal stability of NS/UFG powders when introduced into the metal matrix. Researchers found that cyromilled NS powders presented notably thermal stability due to the nanosized and well-dispersed nitrides and oxides formed during cryomilling [17e19]. Maung et al. studied the thermal stability of nanoscale grains in milled aluminum with 1 wt.% ex-situ diamantane nanoparticles [20]. Results showed that the average grain size of the cryomilled NS Al powders containing diamantane was consistently about half that of the powders without

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diamantane after the same temperature treatment. Specifically, the grain size remained less than 100 nm for the NS Al powders with diamantane, even after being annealed for 10 h at the temperature of 773 K. Nanoparticles not only promote the formation of fine grains in fabricating bulk NS/UFG aluminum, but also enhance the materials' mechanical properties significantly. Nanostructured Al5083/n-TiB2 metal matrix composite fabricated by cryomilling and SPS showed an average size of ~74 nm, a compressive strength of 817 MPa and engineering strain of 6.0% [1]. In Tabandeh-Khorshid's study [21], the grain size of bulk Alþ5 wt.% Al2O3 nanocomposite was 50 nm, while that of Al without Al2O3 was 69.7 nm. The Rockwell F Hardness of the former (101) was higher than the latter (92.48). However, many of the studies on NS/UFG AMCs fabricated by PM have been mainly concentrated on the microstructure and mechanical properties of the bulk nanocomposites. The effect of ceramic nanoparticles on the microstructure evolution during milling and sintering procedure has not been investigated systematically. In this study, Al2024 alloy powders and TiN nanoparticles were chosen as the raw materials. High energy milling followed by SPS was adopted to fabricate the bulk materials. The effect of TiN on the morphology, grain size and microstructure of as-milled powders was investigated, as well as on the grain size, microstructure and mechanical properties of as-SPSed samples. The strengthening mechanisms of Al2024-TiN nanocomposite were analyzed. 2. Experimental procedure 2.1. Material preparation N2-atomized Al2024 alloy powders (Al-4.4Cu-1.5Mg-0.3Mn wt.%, average size of 50 mm) and TiN nanoparticles (average size of 50 nm), as shown in Fig. 1, were used as the raw materials. The inset in Fig. 1a shows the cross-sectional microstructure of Al2024 powders. As-milled Al2024-xTiN (x ¼ 1, 2, 3, 4 wt.%) powders were prepared by high energy milling, which was carried out at room temperature using a QM-3SP4 planetary grinder in stainless steel vessel with the protection of 99.99% Ar. The steel balls were composed of two different sizes: 10 mm and 6 mm in diameter, with the weight ratio of 1:3. The ball-to-powder weight ratio was 20:1, with 1 wt.% stearic acid added as the process control agent. The milling was operated at a speed of 400 rpm. A 10 min stop was set after 1 h running. 24 g milled powders were loaded into a graphite mould (Mersen company) with an inner diameter of 30 mm, and then sintered in a

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Fig. 2. The diagram of temperature and pressure variation during sintering.

LABOX-1575 SPS device at 500  C for 10 min, with a heating rate of 50  C/min. Fig. 2 shows the diagram of temperature and pressure variation during sintering. The initial loading pressure was 5 MPa. When the temperature reached 500  C, the pressure was increased to 50 MPa gradually in 3 min. As soon as the holding procedure terminated, the pressure was reduced to 5 MPa again in 15 s. The sintered sample was cooled in the furnace to 100  C. Temperature was detected by a K-type thermocouple inserted into the drilled hole, which was at half height of the graphite mould from outside. The chamber pressure was kept at a level of 5e20 Pa to avoid oxidation during the whole process. 2.2. Material characterization X-ray diffractometer (Cu Ka, l ¼ 1.5406 Å, Shimadzu XRD-7000S, Japan) was used to assess the grain size, lattice strain and the phase composition, with a scanning rate of 2 /min and a scanning range of 10e90 operating at 30 mA and 40 kV. The standard WilliamsonHall method [22] was used to estimate the grain size and lattice strain, which is defined as:

bhkl cosqhkl ¼

Kl þ 4εsinqhkl t

(1)

where K ¼ 0.9 is the shape factor, l is the X-ray wavelength

Fig. 1. Morphology of the raw materials (a) Al2024 powders and (b) TiN nanoparticles.

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(1.5406 Å), qhkl is the Bragg angle, t is the effective grain size perpendicular to the reflecting plane and ε is the lattice strain. After correcting the instrumental broadening according to standard Silicon sample, the line profile breadth bhkl as a full width at halfmaximum (FWHM) was calculated by XRD analyzer. Five Al reflecting planes (111), (200), (220), (311) and (222) were used to construct a linear plot of bhklcosqhkl against sinqhkl. Then, grain size t was obtained from the intercept c (i.e. t ¼ Kl/c) and lattice strain ε from the slope m (i.e. ε ¼ m/4) [23]. The powders were mounted in conductive resin to observe their cross-sectional microstructure. As-polished specimens were etched with Keller's regent (2% HF, 3% HCl, 5% HNO3 and 90% H2O, vol.). Microstructure of the specimens was observed by optical microscope (DMM-480C) and field emission scanning electron microscope (Nova NanoSEM 450) equipped with Oxford X-Max 50

energy dispersive spectroscopy (EDS). 15 kV was chosen as the accelerating voltage of SEM/EDS e-beam. Detailed microstructural characterization on as-SPSed samples was performed using TEM (Tecnai G2 F30, FEI) equipped with an X-ray energy dispersive spectrometer. Vickers hardness was measured with loading force of 200 g for 15 s, and the obtained hardness values were the average of 5 measurements. Archimedes method was used to measure the relative density of bulk materials with a SHIMADZU AUY220 analytical balance. Compressive cylinders (6 mm in diameter and 9 mm in height) were sectioned from the as-SPSed materials by wire electric discharge machining. Quasi-static compressive testing was performed with a rate of 0.2 mm/min at room temperature using a Shimadzu AG-100kN machine. The compressive load direction was parallel to the SPS loading direction. Yield strength was

Fig. 3. SEM images of Al2024 powders milled for (a) 2 h, (c) 20 h and (e) 40 h; Al2024-2TiN powders milled for (b) 2 h, (d) 20 h and (f) 40 h.

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determined by the 0.2% offset method. The average compressive strength was obtained from three tests.

3. Results and discussion 3.1. Characterization of as-milled powders Fig. 3 displays the SEM images of as-milled Al2024 and Al20242TiN powders under different milling time. After 2 h milling, both of the powders were deformed and flattened, and Al2024-2TiN powders showed slightly larger deformation (Fig. 3a & b). When milled for 20 h, Al2024 powders were flaky, while Al2024-2TiN powders were near-spherical with wide particle size distribution (Fig. 3c & d). Further increasing milling time to 40 h, Al2024 powders became near-spherical and Al2024-2TiN powders obtained better sphericity, narrow particle size distribution and obviously smaller particle size than Al2024 powders (Fig. 3e & f). The addition of TiN nanoparticles had significant effect on the size and morphology of as-milled powders. As is known, high energy milling is a process during which the milling balls deform the ductile powders and cause working-

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hardening, and subsequently the working-hardened powders come to a fracture and re-welding stage [24]. Each particle experiences a unique processing history during high energy milling [25]. According to Fogagnolo's study [26], milling process is generally divided into six stages: (1) starting powders; (2) particle flattening; (3) welding predominance; (4) equiaxed particle formation; (5) random welding orientation; (6) steady state. The effect of TiN nanoparticles on milling process can be analyzed from the role as hard milling media. At the initial stage of milling, TiN can cause localized stress and thus accelerate the deformation of Al2024 powders. When the deformed powders were working-hardened to a critical level, the localized stress induced by TiN nanoparticles will promote the fracture of powders. As a result, the milling process was accelerated by TiN and the powders' morphology with TiN addition was different from that without TiN after the same milling time. Fig. 4 shows the SEM images of as-milled Al2024 and Al2024xTiN powders after 40 h milling. It can be seen that the content of TiN nanoparticles affected the size and morphology of as-milled powders under the given 40 h milling. As mentioned above, more TiN induced higher localized stress, thus accelerating the milling

Fig. 4. SEM images of (a) as-milled Al2024 powders and Al2024-xTiN powders (b) x ¼ 1, (c) x ¼ 2, (d) x ¼ 3 and (e) x ¼ 4 after 40 h milling.

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Fig. 5. XRD analysis results of as-milled Al2024 powders with different TiN contents: (a) the whole patterns and (b) localized peak patterns.

process. For Al2024 powders without TiN, most of the particles showed near-spherical shape and some flattened particles can also be found, as indicated by red arrows (Fig. 4a). When 1 wt.% TiN was added, the average particle size became smaller in general, showing irregular shape with a few flattened particles still existing (Fig. 4b). It is suggested that 1 wt.% TiN addition facilitated the deformation and fracture of particles and made most of the them exceed the particle flattening stage. When increasing the content of TiN to 2 wt.%, the particle size was further decreased and nearly all of the particles changed to near-spherical shape (Fig. 4c). As the content of TiN exceeded 2 wt.%, the variation of the particle size was minor compared to as-milled Al2024-2TiN powders, while the sphericity of the particles increased, as marked by yellow arrows (Fig. 4d & f). It is indicated that Al2024 powders with more than 2 wt.% TiN reached the steady state after 40 h milling, showing almost invariant particle size and morphology. As-milled powders with 40 h milling were selected for further characterization and sintering, since flaky powders are bouffant and not readily to be loaded into the mould before sintering [26]. Fig. 5 shows the XRD analysis results of as-milled Al2024 powders with different TiN contents. As shown in Fig. 5a, Al2Cu peaks were still observed in as-milled Al2024 powders, while they disappeared in as-milled Al2024 powders with TiN. Fig. 5b displays the localized patterns between 37.5 and 40 , indicating obvious broadening of Al peaks in powders after milling. Table 1 lists the grain size and lattice strain of Al matrix with different TiN contents, which were calculated according to Williamson-Hall method [22]. As-milled Al2024 powders had a grain size of ~87.0 nm and lattice strain of ~0.233%. When 1 wt.% TiN was added, the grain size decreased to ~46.8 nm and lattice strain increased to ~0.340%. Increasing the content of TiN to 2 wt.%, the grain size continued to decrease and lattice strain increased, showing results of ~35.7 nm and 0.319%, respectively. As the content of TiN was more than 2 wt.%, the grain size and lattice strain reached a steady state. Moreover, it can be also seen that the diffraction peak position of Al shifted to lower diffraction angles after milling and the peak

Table 1 Grain size and lattice strain of Al matrix with different TiN contents. Grain size (nm) as-milled as-milled as-milled as-milled as-milled

Al2024 powders Al2024-1TiN powders Al2024-2TiN powders Al2024-3TiN powders Al2024-4TiN powders

87.0 46.8 35.7 33.5 34.5

± ± ± ± ±

8.0 1.8 1.1 1.0 1.2

Lattice strain (%) 0.233 0.340 0.319 0.315 0.318

± ± ± ± ±

0.0085 0.0067 0.0073 0.0081 0.0091

shifting extent became greater after TiN addition. The angle shifting suggested the expansion of Al lattice, which may be caused by the solid solution of Cu atoms into Al lattice [27]. Further, greater extent indicated larger Al lattice expansion due to more solution of Cu atoms, resulting in less Al2Cu phase. This could be one reason of Al2Cu's peak disappearance in XRD pattern of Al2024 powders with TiN. It can be concluded that TiN nanoparticles can promote grain refinement and lattice strain generation of Al matrix and facilitate the solid dissolution of Cu atoms during high energy milling. Generally, powders experiencing high energy milling will be deformed severely, thus forming crystal defects such as dislocations, point defects and so on. The generation of defects increases the system energy, including lattice strain. In order to reduce the system energy, new grains with lower energy are produced, resulting in grain refinement [28,29]. When TiN was added, the deformation rate was accelerated due to the localized stress caused by TiN. With increasing TiN content, higher localized stress was induced, and therefore the effect of grain refinement and lattice strain generation was enhanced. However, the effect was no longer strengthened when TiN exceeded 2 wt.%. It is suggested that the refinement of grains and lattice strain had a critical point, which may be due to that the system energy achieved equilibrium. As to the greater extent of Cu atoms' dissolution in Al with TiN addition, it is due to the increased lattice strain. With higher lattice strain, the activation energy of penetration was reduced and therefore the penetration of Cu atoms into Al was increased [30]. Since the difference of lattice strain among powders containing 1e4 wt.% TiN was minor, the shifting extents of diffraction angle were almost similar. Fig. 6 shows the cross-sectional SEM images of raw Al2024, asmilled Al2024 and as-milled Al2024-2TiN powders. Before milling, Al2024 powder showed dendrite microstructure with micrometer sized grains, which was the typical characteristic of atomized powders (Fig. 6a). After milling, secondary phases were broken into small particles, as marked by arrows in Fig. 6b & c, which were distributed along the grain boundaries in the raw Al2024 powders. The phases were probably to be Al2Cu by EDS analysis, as listed in Table 2. The secondary phases in as-milled Al2024-2TiN powders displayed smaller size and amount than those of as-milled Al2024 powders. It is suggested that TiN helped to deform the secondary phases during milling. The enhanced deformation of Al2Cu in Al2024-2TiN powders can be ascribed to the extra localized stress induced by hard TiN nanoparticles. Obviously, Al2Cu in Al20242TiN powders were broken and embedded in the Al as very small

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Fig. 6. The cross-sectional SEM images of (a) raw Al2024, (b) as-milled Al2024 and (c) as-milled Al2024-2TiN powders.

Table 2 EDS analysis data in Fig. 5. Point

1 2 3

at.% Al

Cu

Mg

Ti

73.12 76.56 76.12

25.15 21.81 20.62

1.73 1.63 1.48

/ / 1.78

isolated particles, which were virtually undetectable by XRD [11]. This could be another reason of their peak disappearance in XRD patterns. Moreover, the shrinkage porosities between Al grains and secondary phases in raw Al2024 powders disappeared. After milling, randomly dispersed micro pores appeared due to the fragmentation and re-welding of Al matrix, which was identical with the results of Xun's study [24]. 3.2. Mechanical properties of as-SPSed samples Table 3 summarizes the relative density and Vickers hardness of Al2024 alloy and Al2024-xTiN nanocomposite. It is shown that all of the samples reached a relative high relative density, ascribed to the positive effect of plasma on the mobility and diffusion of atoms

by vaporizing and melting the powder particle surface during SPS [31,32]. The addition of TiN had adverse effect on the relative density, which can be attributed to two aspects. One is that TiN could reduce the contacting area of ductile aluminum particles and thus impede their diffusion; the other is that TiN strengthened the Al powders and decreased their deformation ability [33,34]. Both are disadvantageous factors of densification. With increasing TiN content, the Vickers hardness increased accordingly. The hardness of Al2024-4TiN nanocomposite reached ~274HV, achieving 138% increment compared to that without TiN. Fig. 7 shows compressive properties of Al2024 alloy and Al2024xTiN nanocomposite. Notably, the compressive strength of Al2024xTiN nanocomposite was enhanced, which can be seen intuitively from the typical compressive stress-strain curves in Fig. 7a. Before increasing TiN content up to 3 wt.%, the yield strength increased continuously; when TiN addition was 4 wt.%, the nanocomposite fractured before it could undergo plastic deformation and thus showed no yield behavior. The ultimate strength was increased with increasing TiN until 3 wt.% and then felt down with 4 wt.% TiN addition. This decreasing can be ascribed to the more porosities generated in the samples with excessive TiN particles [35]. On the whole, Al2024-2TiN nanocomposite behaved superior comprehensive properties, with yield and ultimate strength of ~730 MPa

Table 3 Relative density and Vickers hardness of Al2024 alloy and Al2024-xTiN nanocomposite.

Relative density (%) Vickers hardness

Al2024

Al2024-1TiN

Al2024-2TiN

Al2024-3TiN

Al2024-4TiN

99.2 ± 0.11 115 ± 5

98.8 ± 0.10 157 ± 4

98.2 ± 0.40 228 ± 7

97.5 ± 0.24 260 ± 5

96.5 ± 0.50 274 ± 6

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Fig. 7. Compressive properties of Al2024 alloy and Al2024-xTiN nanocomposite: (a) typical compressive stress-strain curves and (b) values of strength and strain.

and ~871 MPa, respectively, and ~10% engineering strain. Fig. 8 shows the compressive fractograph of Al2024-2TiN and Al2024-4TiN nanocomposites. Both of the nanocomposites showed brittle fracture. Al2024-2TiN nanocomposite displayed some necking regions and small voids, indicating some plastic deformation before fracture. While necking regions could hardly be seen in Al2024-4TiN nanocomposite, some big pores were observed obviously. The pores could be the sources of cracks when samples were compressed, and thus the Al2024-4TiN nanocomposite showed no obvious deformation and no yield behavior before fracture. 3.3. Microstructure of as-SPSed samples Fig. 9 shows the XRD patterns of as-SPSed samples with

different TiN contents. In addition to Al or/and TiN phases, intermetallic compounds Al2Cu and AlCu were also detected whether adding TiN or not. The presence of Al2Cu was logical because Al2Cu phases were just broken into small particles during milling. As for the appearance of AlCu, it could be ascribed to the exsolution of Cu atoms, since XRD results of as-milled powders (Fig. 5b) suggested that Cu atoms had dissolved into Al during the milling process. As a result, AlCu formed due to the thermal effect during sintering. Fig. 10 shows the optical microstructure of as-SPSed samples with different TiN contents. It can be seen that all the samples showed a microstructure of some light-colored parts and some dark-colored parts. According to the article by Witkin [15], the light-colored parts represented the well-bonded region where sintering was more readily. In Al2024 alloy (Fig. 10a), the

Fig. 8. Compressive fractograph of (a, b) Al2024-2TiN and (c, d) Al2024-4TiN nanocomposites.

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Fig. 9. XRD patterns of as-SPSed samples with different TiN contents.

microstructure was composed of mostly light-colored parts with some dark-colored particles distributing evenly. As increasing TiN content, the dark-colored parts increased gradually. When TiN exceeded 2 wt.% (Fig. 10d & e), a structure of dark-colored parts surrounded by light-colored halos was observed obviously. As is

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known, sintering is a densification process which is accomplished by creep, plastic deformation and mass diffusion under certain temperature and pressure, and thus the loose-packed powders transform to the well metallic-bonding bulk material. The halostructure in this study could be attributed to two main aspects. On one hand, the presence of localized high current density, temperature, and stress at the particle-particle contact regions can fasten the sintering from the simulation of SPS process through FEM (finite element method) by Xiong et al. [13], so the regions between particles were easier to be sintered. On the other hand, the ceramic phase impeded the diffusion of atoms and decreased the sintering rate [36] during sintering. Given the characteristic of SPS and the blocking effect of TiN, the samples showed a declining sintering extent and clearer halo-structure with increasing TiN contents. In order to study the effect of TiN on the as-SPSed samples in detail, Al2024-2TiN nanocomposite, which behaved superior comprehensive properties, was chosen to be characterized further. The SEM images of as-SPSed Al2024 and Al2024-2TiN samples are shown in Fig. 11. It can be seen that both samples displayed a uniform structure with some micro-sized particles dispersed in the matrix, as shown in Fig. 11a & c. Combined with the EDS (Table 4) and XRD (Fig. 9) results, they were most likely to be Al2Cu. The particles in Al2024 sample showed a size of about 1e3 mm, as shown in Fig. 11b. However, they were made up of some big-sized ones (~2 mm) and some small-sized ones (~0.5 mm) in Al2024-2TiN sample, as shown in Fig. 11d. Based on the dispersed particles'

Fig. 10. Optical microstructure of as-SPSed samples (a) Al2024 alloy, (b) Al2024-1TiN, (c) Al2024-2TiN, (d) Al2024-3TiN and (e) Al2024-4TiN nanocomposites.

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morphology and composition in as-milled powders, it can be deduced that most Al2Cu in as-SPSed samples were directly inherited from their powders'. Further, some micro pores were also present in both samples, which became smaller obviously than that in milled powders due to the densification effect of high temperature and pressure during sintering. Fig. 12 shows the TEM images of as-SPSed Al2024 and Al20242TiN samples, as well as Al grain size distribution histograms of them. From Fig. 12a & b, it is revealed that both the samples had a nanoscale grain structure. The strongly diffracting grains were marked by red dotted lines. The grain distribution histograms were achieved by making statistics on the size of the marked grains using Image Plus 6.0 software, as shown in Fig. 12c & d. Each of the histograms was based on over 200 grains' measurement from dozens of TEM images, indicating average diameters of 576.5 nm for Al2024 alloy and 145.4 nm for Al2024-2TiN nanocomposite, respectively. Compared to the powders' grain size listed in Table 1, obvious grain growth was found in both as-SPSed samples. As is known to us, SPS is a sintering process with high temperature, during which powders experience a thermal exposure. Milled aluminum powders containing nano grains and high strain tend to evolve to a more stable state by recovery and recrystallization, and thus grains would grow up. The same result was also observed in the research of milled powders' heat treatment by F. Zhou et al. [37]. It is worth noting that the grain size of as-SPSed sample was 6.6 times its corresponding powders for Al2024 alloy and 4 times for Al2024-2TiN nanocomposite, suggesting that TiN nanoparticles can effectively restrain grain growth during sintering. This is mainly attributed to the Zener pinning of TiN nanoparticles on grain boundaries [38]. Since the addition of TiN increased the fraction of reinforcement and decreased the inter-particle spacing, a stronger pining force was formed. Consequently, Al2024-2TiN nanocomposite showed limited grain growth after sintering. Fig. 13 shows the TEM images of Al2024-2TiN nanocomposite. The distribution of TiN nanoparticles is shown in Fig. 13a. According

Table 4 EDS analysis data in Fig. 11. Point

1 2

at.% Al

Cu

Mg

Ti

75.92 80.46

22.72 17.17

1.36 0.93

e 1.44

to the size and morphology, the dispersed TiN nanoparticles marked by arrows were uniformly distributed throughout the aluminum matrix. Some agglomerates were also observed, as marked by circles. Totally speaking, TiN showed uniform distribution. Fig. 13b displays the high magnification image of zone A in Fig. 13a. In order to ascertain TiN nanoparticle more accurately, high resolution image was taken of the area marked by rectangle in Fig. 13b, as shown in Fig. 13c. From the measurement of interplanar spacing and FFT image, it can be deduced that the particle was TiN. Besides, some nanometer intermetallic compounds appeared in both samples, as shown in Fig. 14. Identified by EDS (Table 5), the nano compounds were likely to be Al2Cu or Al2CuMg phases. Moreover, the average size in Al2024-2TiN sample (~35 nm) was smaller than that in Al2024 sample (~154 nm), which was obtained from the statistical results using Image Plus 6.0 software. 3.4. Strengthening mechanisms of UFG Al2024-2TiN nanocomposite The strengthening mechanisms of Al2024-2TiN nanocomposite involved in this study are mainly due to two aspects: strengthening from the metal matrix and strengthening from the nanoparticles. To be specific, there includes grain boundary strengthening by HallPetch relationship, solid solution strengthening, Orowan strengthening from the secondary phase and nanoparticles, and dislocation strengthening. Hereafter, the overall yield strength increment of Al2024-2TiN nanocomposite will be analyzed mainly

Fig. 11. SEM images of as-SPSed samples (a, b) Al2024 alloy and (c, d) Al2024-2TiN nanocomposite.

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Fig. 12. TEM images of as-SPSed (a) Al2024 alloy and (b) Al2024-2TiN nanocomposite; Al grain size distribution histograms of (c) Al2024 alloy and (d) Al2024-2TiN nanocomposite.

Fig. 13. TEM images of Al2024-2TiN nanocomposite (a) TiN nanoparticle (marked by arrows) distribution in Al matrix; (b) high magnification image of zone A in (a); (c) HRTEM image of a TiN nanoparticle in (b) and corresponding FFT image.

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Fig. 14. Nanometer intermetallic compounds of (a) Al2024 alloy and (b) Al2024-2TiN nanocomposite.

strengthening contribution of solid solution Dsss can approximately be treated to be 0 MPa.

Table 5 EDS analysis data in Fig. 14. Point

at.%

1 2

Al

Cu

Mg

92.15 94.21

5.12 3.42

2.73 2.34

from DsGB (the grain boundary strengthening), DsSS (the solid solution strengthening), DsOr (the Orowan strengthening) and DsDS (the dislocation strengthening). i.) Grain boundary strengthening As the grain size decreases, i.e. the volume of grain boundary increases, the impedance to dislocation motion is enhanced and thus the stress required to deform the material is increased. The grain boundary strengthening is described by the Hall-Petch relationship [39]:

DsGB ¼ kHP

.pffiffiffi d

(2)

where kHP is a constant of Hall-Petch slope, and d is the mean grain size [40]. Recent experimental work has indicated that the kHP value is about 0.09 MPa/m1/2 for bulk Al consolidated from cryomilled powder [17], whilst the kHP is about 0.13 MPa/m1/2 for a nanostructured Al-Cu alloy [40]. Herein, the latter value is adopted. Based on the grain sizes of Al2024 alloy (~576.5 nm) and Al20242TiN nanocomposite (~145.4 nm), the yield strength increment DsGB are 171 MPa and 341 MPa, respectively. ii.) Solid solution strengthening The concentration of solute atoms in the Al matrix decides the solid solution strengthening effect. The strengthening effect of Cu and Mg atoms was estimated to be 10.5 MPa/at%Cu and 5 MPa/at% Mg [41e43], respectively. However, in the study of Chen et al. [44], they found that the solid solution strengthening contribution of Cu and Mg in ultrafine-grained ternary Al-Cu-Mg alloy was negligible, since they formed nanometer-sized solute clusters at selected boundaries/junctions. Quantities of secondary particles containing Cu and Mg atoms in the bulk samples were observed according to the obtained results (Figs. 11 and 14) in this study. As a result, the

iii.) Orowan strengthening The dispersed particles, including secondary particles and reinforcements, act as the barrier to dislocation motion. Their strengthening effect can be predicted by the Orowan equation [40]:

DsOr

pffiffiffi f 1 ¼ MGb ¼ MGb r L

(3)

where M ¼ 3.06 is the mean orientation factor for face-centered cubic Al, G ¼ 27 GPa is the shear modulus, b ¼ 0.286 nm is the Burgers vector, and L is inter-particle spacing. The inter-particle spacing is given by equation [45]:

L¼

qffiffiffiffiffiffiffiffiffiffiffi  pffiffiffiffiffiffiffiffi 2=3Dp p=4f  1

(4)

where f is volume fraction of particles, and Dp the average diameter of particles. For Al2024-2TiN nanocomposite (where f ¼ 0.01047), the DsOr caused by TiN is estimated to be 56.2 MPa. Unfortunately, in this study, enough information has not been obtained to work out the value of secondary particles' volume fraction and inter-particle spacing, so the strengthening contribution of secondary phases cannot be calculated. However, it can be clearly seen that the secondary phases' inter-particle spacing of Al2024-2TiN nanocomposite was smaller than that of Al2024 alloy, as shown in Figs. 11 and 14. Assuming the total weight of secondary particles is a constant for both samples, it is reasonable to deduce that the secondary particles' strengthening effect in Al2024-2TiN nanocomposite is stronger according to equation (3). iv.) Dislocation strengthening Dislocations are common in milled powders because of severe plastic deformation [37]. It is well known that dislocations are obstacles of motion of other dislocations. More dislocations in the material will increase the yield strength. For the MMCs with nanoparticle reinforcements, geometrically necessary dislocations (GNDs) have been studied [46e48]. GNDs are often present to accommodate the non-uniform deformation due to the mismatch

B. Li et al. / Journal of Alloys and Compounds 726 (2017) 638e650

of elastic modulus and different volume variation caused by CTE mismatch. Gao et al. [47] reported that the critical length scale of reinforcement to consider the influence of GNDs in continuum plasticity was below 1 mm. The size of TiN in the current work is ~50 nm. Therefore, it is necessary to consider the strengthening effect of GNDs. Here, we assume that the particles are incoherent with the matrix. For the moment, it is still a challenge to tell the GNDs from the other dislocations experimentally [47]. However, the strengthening contribution from GNDs can be estimated by equation [48]:

pffiffiffi

pffiffiffiffiffiffiffiffi

pffiffiffi

pffiffiffiffiffiffiffiffiffi

DsGND ¼ 3hGb rEM þ 3bGb rCTE

(5)

where h ¼ 0.5 and b ¼ 0.7 are geometric constants; G and b are the

DaDTÞ rε same as mentioned above; rEM ¼ 6f and rCTE ¼ 12fr ðbd represent bd r

r

the GND densities due to mismatch of elastic modulus (EM) and CTE, respectively. fr ¼ 0.01047 is the volume fraction of the reinforcement particles; dr ¼ 50 nm is particle size; Da is the CTE mismatch; ε is the misfit strain due to elastic modulus mismatch (assumed to be the same as the strain due to CTE mismatch DaDT ¼ 0.00648, where DT ¼ 475 K is the difference between the processing temperature and room temperature). This strength increment is estimated to be 106 MPa for Al2024-2TiN nanocomposite. From the analysis above, the strengthening contribution of finer aluminum grains, Orowan strengthening and dislocation strengthening due to TiN nanoparticles has been discussed in detail. Some other strengthening factors, such as dislocations caused by milling and secondary particles, haven't been analyzed concretely, which are also supposed to make some contribution. 4. Conclusions In this study, bulk ultra-fine grained Al2024-TiN nanocomposite was successfully fabricated by high energy milling and spark plasma sintering. The effect of TiN nanoparticles on as-milled powders and as-SPS samples was researched. The main conclusions are summarized as follows: (1) TiN nanoparticles can accelerate the milling process, solution of Cu atoms and deformation of Al2Cu during milling. Besides, they are helpful to refine grains. Al grain size of asmilled powders with 40 h milling was refined from ~87 nm to 30e50 nm after TiN was added, staying steady when TiN exceeded 2 wt.%. (2) Al2024-2TiN nanocomposite behaved superior comprehensive properties, showing yield and ultimate strength of ~730 MPa and ~871 MPa, respectively, which were 185% and 35% higher than those without TiN, and still possessing ~10% engineering strain. (3) TiN nanoparticles can restrain grain growth during sintering. The matrix grain sizes of Al2024 alloy and Al2024-2TiN nanocomposite were 576.5 nm and 145.4 nm, respectively, exhibiting 6.6 times and 4 times those of their corresponding milled powders. (4) The high strength of Al2024-TiN nanocomposite was mainly attributed to ultra-fine grains, Orowan strengthening and dislocation strengthening due to TiN nanoparticles. Acknowledgments This work was financially supported by the Important National Science and Technology Specific Project of China (No. 2012ZX04010-081). We acknowledge the support from State Key

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