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Simultaneously increasing the high-temperature tensile strength and ductility of nano-sized TiCp reinforced Al-Cu matrix composites
Materials Science & Engineering A 717 (2018) 105–112
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Simultaneously increasing the high-temperature tensile strength and ductility of nano-sized TiCp reinforced Al-Cu matrix composites ⁎⁎
Wei-Si Tiana,b, Qing-Long Zhaoa, , Qing-Quan Zhanga, Feng Qiua,b, Qi-Chuan Jianga,b,
T
⁎
a
Key Laboratory of Automobile Materials, Ministry of Education and Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China b State Key Laboratory of Automotive Simulation and Control, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Al-Cu matrix composites Nano-sized TiCp High-temperature tensile properties Precipitates
Adding various amounts of nano-sized TiCp (0.1–0.7 wt%) made a simultaneous improvement in high-temperature strength and ductility of the Al-Cu matrix alloy. The influences of temperature and strain rate on the high-temperature tensile properties and θ′ precipitate sizes in the Al-Cu matrix alloy and the nano-sized TiCp/AlCu composites were investigated. Theoretical calculations suggest that the yield strength increment of the composites at 453 K was primarily attributed to the strengthening effect of both the refined θ′ precipitates and TiCp, while at 493 K only the strengthening effect of TiCp was predominant due to the significant coarsening of θ′ precipitates at this temperature.
1. Introduction Al-Cu alloys exhibit a combination of low density and high strength, which makes them the particularly attractive materials for aerospace and automotive applications where strength-to-weight is the prime design consideration [1,2]. The major contribution to the high strength of heattreatable Al-Cu alloys is the spatially distributed nano-sized plate-like θ′ precipitates which serve as obstacles to dislocation movement. However, the tensile properties of Al-Cu alloys deteriorate significantly at elevated temperatures, due to the rapid coarsening of θ′ precipitates and thermally activated cross slip and climb that lead to easy dislocation movement, which limits their applications at high temperatures [3–5]. Therefore, there is a need to improve the strength of Al-Cu alloys at elevated temperatures. There are some effective strategies to improve the high-temperature mechanical properties of Al-Cu alloys. One way is to refine the sizes and improve the distribution of θ′ precipitates by inoculation or modification. Bai et al. [6] found that 0.2 wt% Zr-based metallic glass inoculant improved the tensile strength and ductility of Al-Cu alloy at 433 K and 493 K by promoting the precipitation of denser and finer θ′ precipitates. However, the strengthening effect of θ′ precipitates would weaken at high temperature (such as 493 K) due to the significant coarsening of θ′ precipitates [6]. Another method is to introduce particles, which are thermally stable at high temperature, into Al-Cu alloys. Generally, such particles can be divided into two types: one is formed during
high-temperature processing or heat treatment, such as Al3Sc particles in the Al-Cu-Sc alloys [7]; the other type is ceramic reinforcement, such as SiC [8], ZrB2 [9] and TiC [10]. Ceramic reinforcements have attracted much attention due to low cost and ease of fabrication [11]. Compared with the unreinforced Al matrix alloy, micro-sized particles reinforced aluminum matrix composites exhibit an enhanced strength but a reduced ductility. However, when the particle size of the ceramic reinforcement is reduced to nanometer scale, the nano-sized particles reinforced aluminum matrix composites show an improved strength without sacrificing the ductility. Zhang et al. [8] fabricated the ex situ 0.5 vol% nano-SiCp/Al2014 composites and 4 vol% micro-SiCp/ Al2014 composites by semi-solid stir casting combined with hot extrusion and found that the nano-SiCp improved the high-temperature tensile strength and fracture strain of Al2014 more significantly than the micro-SiCp. Among the various reinforcement particles, TiC is an outstanding reinforcement material due to its high hardness, high melting point (3433 K), good wettability with aluminum and good thermodynamic stability [12]. Besides, it is reported that the addition of TiC particles into Al-Cu alloys could refine grains and promote the precipitation of θ′ phases [10]. Compared with the ex situ method, the in situ method has many advantages, such as clear particle-matrix interfaces and uniform distribution of ceramic particles [13]. In our previous work [14–16], we used the carbon nanotube (CNT) as the carbon source to prepare the nano-sized TiCp/Al master alloy by the
⁎ Corresponding author at: Key Laboratory of Automobile Materials, Ministry of Education and Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China. ⁎⁎ Corresponding authors. E-mail addresses: [email protected] (Q.-L. Zhao), [email protected] (Q.-C. Jiang).
https://doi.org/10.1016/j.msea.2018.01.069 Received 5 July 2017; Received in revised form 7 November 2017; Accepted 19 January 2018 Available online 31 January 2018 0921-5093/ © 2018 Elsevier B.V. All rights reserved.
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sufficiently by ball milling at the speed of 50 rpm for 48 h, and then were cold pressed into the cylindrical preforms (40 mm in height and 28 mm in diameter). Secondly, the preforms were heated in a self-made vacuum thermal explosion furnace at 1173 K to induce the SHS reaction of Al-Ti-CNT system to fabricate the in situ nano-sized TiCp/Al master alloys. The nano-sized TiCp were near-spherical with a mean diameter of about 93 nm [16]. Thirdly, the composites with different nominal TiCp contents (0.1, 0.3, 0.5 and 0.7 wt%) were fabricated by adding the master alloys into the Al-Cu alloy melt at 1173 K, followed by the mechanical stirring for 2 min. Finally, after the melt was cooled down to 1073 K, it was poured into a preheated steel mold (200×150×12 mm3) to form the composite ingot. After T6 heat treatment (solution at 811 K for 12 h and aging at 438 K for 10 h), the Al-Cu matrix alloy and composites were machined to the tensile dog-bone shaped specimens with a gauge cross-section of 4.0 × 2.5 mm2 and a gauge length of 10.0 mm. The tensile tests were carried out using a material testing machine (INSTRON 5869, UK) at 453 K and 493 K under the strain rates of 10−4, 10−3 and 10−2 s−1. The microstructures of the matrix alloy and composites were characterized by optical microscope (OM, Axio Imager A2m, Zeiss, Germany) and transmission electron microscope (TEM, JEM-2100F, Japan). The mechanically polished surfaces of as-cast samples were anodized at 18 V in a 5 vol% HBF4-distilled water solution for optical metallography. The average diameters and thicknesses of θ′ precipitates were determined by the measurement of more than two hundred θ′ precipitates. The fracture surfaces of the typical specimens after high-
self-propagating high-temperature synthesis (SHS) of Al-Ti-C system, due to the high chemical activity and the fine sizes of CNT. Then the master alloy was added into the molten Al-Cu alloy to fabricate the nano-sized TiCp/Al-Cu composites. The results showed that the nanosized TiCp/Al-Cu composites exhibited high room-temperature tensile strength and excellent elongation [15], as well as the superior creep resistance at 453–493 K [16]. Therefore, the nano-sized TiCp/Al-Cu composites seem promising to exhibit superior high-temperature tensile strength and ductility. In this paper, the high-temperature tensile tests of the nano-sized TiCp/Al-Cu composites were carried out at 453 K and 493 K under strain rates of 10−4, 10−3 and 10−2 s−1. We found that the addition of nanosized TiCp could simultaneously increase the high-temperature tensile strength and ductility of the Al-Cu alloy under all the test conditions. The strengthening mechanisms of the composites at elevated temperatures were also discussed in detail. This work aims to provide a new approach to improve the high-temperature tensile properties of Al-Cu matrix composites. 2. Experimental procedures The Al-Cu matrix alloy had a nominal wt% composition of Al-5.5 Cu-0.45 Mn-0.3 Ti-0.2 Cd-0.2 V-0.15 Zr-0.04 B. The nano-sized TiCp were in situ synthesized by self-propagating high-temperature synthesis (SHS) of 70Al-Ti-CNT system that was at a ratio corresponding to that of stoichiometric TiC. Firstly, the Al-Ti-CNT powder blends were mixed
Fig. 1. Polarized optical images of the as-cast microstructures of the (a) Al-Cu alloy and nano-sized TiCp/Al-Cu composites with different contents of TiCp: (b) 0.1, (c) 0.3, (d) 0.5, (e) 0.7 wt% nano-sized TiCp.
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Table 2 The yield strength σ0.2 (MPa), ultimate tensile strength σb (MPa) and fracture strain δf (%) of the Al-Cu alloy and the 0.7 wt% nano-sized TiCp/Al-Cu composite at 453 K and 493 K under the strain rates of 10−4, 10−3 and 10−2 s−1. 453 K
493 K
Al-Cu alloy
−4
10 10−3 10−2
Table 1 The tensile test data of the Al-Cu alloy and the nano-sized TiCp/Al-Cu composites with different contents of nano-sized TiCp at 493 K and 10−3 s−1 strain rate. σ0.2 (MPa)
σb (MPa)
δf (%)
0 0.1 0.3 0.5 0.7
210 220 234 242 254
241 254 283 288 297
6.2 17.9 11.9 7.0 6.6
Al-Cu alloy
Composite
σ0.2
σb
δf
σ0.2
σb
δf
σ0.2
σb
δf
σ0.2
σb
δf
214 229 262
232 258 307
3.2 4.0 5.4
261 287 304
347 374 407
7.5 9.9 17.9
183 210 222
194 241 258
2.5 6.2 11.4
210 254 264
232 297 338
4.6 6.6 18.5
Fig. 2 shows the tensile engineering stress-strain curves of the Al-Cu matrix alloy and 0.1–0.7 wt% nano-sized TiCp/Al-Cu composites at 493 K under the strain rate of 10−3 s−1, and Table 1 lists the yield strength (σ0.2), tensile strength (σb) and fracture strain (δf). The strength and ductility of the composites were simultaneously improved compared with the Al-Cu matrix alloy. Besides, with increasing mass fraction of nano-sized TiCp, the yield strength and tensile strength increased and the fracture strain tended to decrease. The yield strength, tensile strength and fracture strain of the Al-Cu matrix alloy at 493 K were 210 MPa, 241 MPa and 6.2%, respectively. By adding 0.1 wt% nanosized TiCp, the yield strength, tensile strength and fracture strain of the Al-Cu matrix composite increased to 220 MPa, 254 MPa and 17.9%, respectively. The 0.7 wt% nano-sized TiCp/Al-Cu composite possessed the highest yield and tensile strength (254 and 297 MPa), which were 44 MPa and 56 MPa higher than those of the Al-Cu matrix alloy. Meanwhile, it exhibited a relatively low fracture strain (6.6%), which was similar to that of the Al-Cu alloy (6.2%). As known, the strength and ductility are sensitively dependent on temperature and strain rate [18]. Effects of temperature and strain rate on tensile properties of the 0.7 wt% nano-sized TiCp/Al-Cu composite were investigated due to its highest high-temperature strength at 493 K and under the strain rate of 10−3 s−1. Fig. 3 shows the tensile engineering stress-strain curves of the Al-Cu matrix alloy and 0.7 wt% nano-sized TiCp/Al-Cu composite at 453 K and 493 K under the strain rate of 10−4, 10−3 and 10−2 s−1, and Table 2 summarizes the results. The yield stress and tensile stress of both the Al-Cu matrix alloy and composite increased with increasing strain rate or decreasing test temperature, and the fracture strain tended to increase with the increase of strain rate. At 453 K and 10−4 s−1 strain rate, the yield strength, tensile strength and fracture strain of the 0.7 wt% nano-sized TiCp/Al-Cu composite were 261 MPa, 347 MPa and 7.5%, which were improved by 22.0%, 49.6% and 134.4%, compared with the Al-Cu
Fig. 2. Engineering stress-strain curves of the Al-Cu alloy and the nano-sized TiCp/Al-Cu composites with different contents of TiCp at 493 K and 10−3 s−1 strain rate.
TiC (wt%)
Composite
temperature tensile testing were observed by scanning electron microscope (SEM, VEGA 3 XMU, TESCAN, Czech).
3. Results and discussion Fig. 1 compares the as-cast microstructures of the Al-Cu matrix alloy and the composites. The grains of Al-Cu matrix alloy were coarse with an average size of about 140 µm (see Fig. 1(a)); while the grains of the composites were smaller and nearly equiaxed (Fig. 1(b)-(e)). Besides, the grain sizes in the composites decreased with the increase of TiCp content. The average sizes of the α-Al grains were about 65, 54, 47 and 33 µm in the 0.1, 0.3, 0.5 and 0.7 wt% nano-sized TiCp reinforced Al-Cu matrix composites, respectively. During the solidification process, TiCp could serve as heterogeneous nucleation sites and refine the grain size of α-Al [17].
Fig. 3. Engineering stress-strain curves of the Al-Cu alloy and the 0.7 wt% nano-sized TiCp/Al-Cu composites under the strain rates of 10−2 s−1, 10−3 s−1and 10−4 s−1 at (a) 453 K and (b) 493 K.
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Fig. 4. SEM micrographs of fracture surfaces after tensile testing at 493 K and 10−3 s−1 strain rate of the (a) Al-Cu alloy and nano-sized TiCp/Al-Cu composites with different contents of TiCp: (b) 0.1, (c) 0.3, (d) 0.5, (e) 0.7 wt % nano-sized TiCp.
Cu matrix alloy ruptured in a brittle manner, which was characterized by intergranular fracture and few shallow dimples. In contrast, with the addition of nano-sized TiCp, the fracture surfaces of the composites were covered with a large number of deep dimples and a small amount of intergranular fracture, indicating the increased ductility compared with the Al-Cu matrix alloy. Meanwhile, with the increase of TiCp content, the fracture surfaces of the composites were composed of more amount of intergranular fracture and less amount of ductile-dimpled fracture, which implied a decrease in ductility, as seen in Fig. 4(b)-(e). It can be also seen that the grain sizes in the composites were much smaller than those in the Al-Cu matrix alloy, which was consistent with the optical images in Fig. 1. Fig. 5 shows SEM fractographs of the 0.7 wt % nano-sized TiCp/Al-Cu composite fractured at different temperatures and strain rates. With increasing strain rate from 10−4 s−1 to 10−2 s−1 (Fig. 5(a)-(c)) at 493 K or with decreasing temperature from 493 K
matrix alloy (214 MPa, 232 MPa and 3.2%), respectively. When the test temperature increased to 493 K, the yield strength and tensile strength of the composite decreased to 210 MPa and 232 MPa, which were 27 MPa, 38 MPa higher than those of the Al-Cu matrix alloy (183 MPa and 194 MPa), respectively, and its fracture strain was improved by 84.0% compared with the Al-Cu matrix alloy (2.5%). As the strain rate increased to 10−2 s−1 at 493 K, the yield strength, tensile strength and fracture strain of the composite increased to 264 MPa, 338 MPa and 18.5%, which were 42 MPa, 80 MPa and 7.1% higher than those of the Al-Cu matrix alloy (222 MPa, 258 MPa and 11.4%), respectively. The strength and ductility of the composite were significantly improved under all the test conditions, compared with the Al-Cu matrix alloy. Fig. 4 shows SEM fractographs of the Al-Cu alloy and nano-sized TiCp/Al-Cu composites with different contents of TiCp fractured at 493 K and 10−3 s−1 strain rate. It can be seen from Fig. 4(a) that the Al108
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Fig. 5. SEM micrographs of fracture surfaces of the 0.7 wt% nano-sized TiCp/Al-Cu composite after tensile tests. The experiment conditions are: (a) 493 K and 10−4 s−1 strain rate (b) 493 K and 10−3 s−1 strain rate, (c) 493 K and 10−2 s−1 strain rate (d) 453 K and 10−3 s−1 strain rate.
(Fig. 5(b)) to 453 K (Fig. 5(d)) at 10−3 strain rate, the amount of intergranular fracture decreased and ductile-dimpled fracture increased, which indicated the increased ductility. The precipitates in Al-Cu alloys are θ′ precipitates, which are formed on the {100}α-Al planes in Al-Cu alloys [19]. The TEM images of θ′ precipitates in the Al-Cu matrix alloy and 0.7 wt% nano-sized TiCp/AlCu composite after tensile tests are demonstrated in Fig. 6. After the tensile test at 453 K and 10−4 s−1 strain rate, the average diameter and thickness of the plate-like θ′ precipitates in the Al-Cu matrix alloy (Fig. 6(a)) were about 60 nm and 3 nm, while the much larger number of finer θ′ precipitates with an average diameter of 38 nm and thickness of 3 nm were observed in the composite (Fig. 6(b)). The coarsening of the precipitates was limited when the test temperature (453 K) was a bit higher than the aging temperature (438 K). However, the θ′ precipitates in both the Al-Cu alloy and composite coarsened considerably (roughly doubled in diameter) when increasing temperature to 493 K. However, after the tensile test at 493 K and 10−4 s−1 strain rate, the θ′ precipitates in the composite (Fig. 6(d)) still had finer sizes (average diameter of 92 nm and thickness of 5 nm), compared with those in the AlCu alloy (average diameter of 111 nm and thickness of 5 nm, as shown in Fig. 6(c)). After the tensile test at 493 K and higher strain rate (10−2 s−1), the sizes of θ′ precipitates in the Al-Cu alloy (average diameter of 86 nm and thickness of 3 nm, as shown in Fig. 6(e)) and in the composite (average diameter of 64 nm and thickness of 3 nm, as shown in Fig. 6(f)) were much smaller than those in the samples at lower strain rate (10−4 s−1). This is because the longer time to rupture at lower strain rate at 493 K could lead to the significant coarsening of θ′ precipitates, based on the modified Lifshit-Slyozov-Wagner equation in which the average θ′ radius increases with increasing aging time [7]. Meanwhile, the stress can also cause the coarsening of θ′ precipitates. In our previous research [16], we found that the coarsening of the θ′ precipitates was enhanced in the nano-sized TiCp/Al-Cu composite
crept at 493 K under 160 MPa for 10 h than that in the composite overaged at 493 K for 10 h, which was consistent with the results from Gariboldi et al. [20]. During deformation under the applied stress, moving dislocation attract solutes both to the associated strain fields and to their cores. When moving dislocations encounter the growing precipitates, the collected solutes diffuse to precipitates, causing Ostwald ripening [21,22]. This stress-enhanced diffusion is much faster at higher temperature, and the solute transport by dislocations is more sufficient during longer time, leading to the coarser θ′ precipitates in the samples tested at higher temperature and lower strain rate. In our previous work, it was found that the nano-sized TiCp predominantly dispersed in the grain interior, and the interface between the particles and the Al matrix was clean before tensile tests [15]. Fig. 7 shows the TEM micrograph of the 0.7 wt% nano-sized TiCp/Al–Cu composite after the tensile test at 453 K and 10−4 s−1. It can be seen that the nano-sized TiCp were thermodynamically stable and distributed in the grain interior. As a result, the nano-sized TiCp could serve as effective barriers to dislocation motion and enhance the hightemperature strength of the composites. The enhancement in ductility of the composites is a result of grain refinement as shown in Fig. 1. Grain refinement results in more uniform deformation. Besides, grain refinement also results in more tortuous grain boundary, which is not conducive to crack propagation [23]. Moreover, grain refinement leads to reduced possible flaw sizes that caused stress concentration, thus suppressing the crack nucleation and propagation [24]. Therefore, the composites could withstand more deformation before fracture than the Al-Cu matrix alloy, as demonstrated in Fig. 4. The ductility of both the matrix alloy and the composite decreased with decreasing the stain rate from 10−2 s−1 to 10−4 s−1. This may be because the grain boundary will weaken at high temperature, and weakening effect is more serious when the exposure time of the sample at high temperature is longer at lower strain rate, 109
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Fig. 6. TEM micrographs of θ′ precipitates in the Al–Cu matrix alloy and 0.7 wt% nano-sized TiCp/Al-Cu composite after tensile tests under different test conditions: the (a) Al-Cu alloy and (b) composite at 453 K and 10−4 s−1 strain rate, the (c) Al-Cu alloy and (d) composite at 493 K and 10−4 s−1 strain rate, as well as the (e) Al-Cu alloy and (f) composite at 493 K and 10−2 s−1 strain rate; (g-l) the corresponding statistical results of the diameters of the θ′ precipitates in (a-f), respectively.
active deformation mechanisms at high temperatures [25]. For dislocation glide, the strengthening effect of precipitates and nano-sized ceramic particles is attributed to the Orowan bypass mechanism, and the decrease in the shear modulus G with increasing temperature results in the decrease in Orowan stress at elevated temperature [25]. At high temperatures (T > 0.5Tm, where Tm is the absolute melting temperature of the alloy), it is well known that dislocation tend to surmount the obstacles by local climb mechanism [26]. According to the local climb mechanism proposed by Brown and Ham [27], the back stress due to local climb (Δσb ) is proportional to the Orowan stress (ΔσOro ) and is lower by a factor of k than the Orowan stress as follows:
Δσb = k ΔσOro
Fig. 7. TEM micrograph of the nano-sized TiCp in the 0.7 wt% nano-sized TiCp/Al–Cu composite after the tensile test at 453 K and 10−4 s−1 strain rate.
(1)
where k is usually in the range of 0.4–0.7. It can be seen that the back stress due to local climb is related to the Orowan stress and the higher Orowan stress indicates the higher back stress due to local climb. As a result, no matter which mechanism plays a dominant role, the increment of high-temperature yield strength is proportional to the Orowan stress. Qin et al. [25] used the Orowan mechanism with modified shear modulus to predict the high-temperature yield strength of the B4C/AlSc composite with large precipitates (more than 8 nm) and found that the predicted yield strengths at elevated temperatures were fairly consistent with the high-temperature experimental data. Therefore, in this paper, we calculated the Orowan stress to compare the stress contributed by the θ′ precipitates and nano-sized TiCp in the Al-Cu
leading to more amount of intergranular fracture and decreased ductility, as shown in Fig. 5. Obviously, the microstructure change induced by the addition of nano-sized TiCp, which includes the grain refinement and the change of the θ′ precipitate sizes, contributes to the enhanced tensile properties of the composites, as well as the strengthening effect of the nano-sized TiCp. It is believed that the refined grain sizes could potentially be helpful in simultaneously enhancing the strength and ductility of materials [24]. However, the strengthening effect of grain refinement is negligible at high temperature [6]. Dislocation glide and climb are 110
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significant coarsening of θ′ precipitates at this temperature.
matrix alloy and 0.7 wt% nano-sized TiCp/Al-Cu composite. In order to reveal whether the strengthening effect of θ′ precipitates or TiCp was predominant in the yield strength increment in the composite, we discussed the θ′ precipitate strengthening and the nano-sized TiCp strengthening separately. The Orowan stress due to the plate-like θ′ precipitates strengthening (Δσθ′) can be calculated according to the following modified Orowan equation [19,28]:
Δσθ ′ =
⎛ MGb ⎜ 2π 1-ν ⎜ 1.123d ⎝
1 0.318π af
πd 1.061d -8- a
⎞ 0.981d ⎟ ln ab ⎟ ⎠
Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51571101 and No. 51601066), the Science and Technology Development Program of Jilin Province, China (grant No. 20160520116JH and 20170101215JC), “Thirteenth five-year plan” Science & Technology Research Foundation of Education Bureau of Jilin Province, China (Grant No. 2015-479), and the Project 985-High Properties Materials of Jilin University.
(2) References
where M is the Taylor factor for Al (approximately equal to 3 [19]), G is the shear modulus (24.2 GPa at 453 K and 23.5 GPa at 493 K for Al, according to literature [29]), ν is the Poisson's ratio (1/3 for Al [19]), b is the Burgers vector (0.286 nm for Al [19]), a is the aspect ratio of the precipitates (a= d/t, d and t are the diameter and thickness of the precipitate, respectively) and f is the volume fraction of θ′ precipitates. The volume fraction (f) of the θ′ precipitates in the T6 peak-aged Al5.5Cu alloy used in this work is estimated to be 6% according to the work by Zhao et al. [30], due to the similar Cu content in these two works. It can be seen from Eq. (2) that the aspect ratio of θ′ precipitates plays an important role in controlling the precipitation strengthening effect. By substituting the values of M, G, ν, b, f and statistical results of d and t of the θ′ precipitates into Eq. (2), the theoretical values of Δσθ′ under the strain rate of 10−4 s−1 in the 0.7 wt% nano-sized TiCp/Al-Cu composite are 33 MPa higher at 453 K and 6 MPa higher at 493 K than those of the Al-Cu matrix alloy, respectively. The precipitation strengthening effect in the composite is more significant than that in the Al-Cu matrix alloy at 453 K, while the difference is minor in the composite and Al-Cu matrix alloy at 493 K. The Orowan stress contributed by the nano-sized TiCp (Δσp) can be calculated by the following equation [31]:
Δσp = 0.81
MAGb πd ln( ) 2πλ 4b
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(3)
where λ=0.4d ( π/fV -2) is the interparticle spacing, d and fV are the diameter and volume fraction of the TiCp, respectively. The value of constant A is estimated to be 1.8 [31]. The theoretical values of Δσp in the 0.7 wt% (0.4 vol%) nano-sized TiCp/Al-Cu composite are 26 MPa at 453 K and 25 MPa at 493 K. It can be seen that there is little change in the value of Δσp with the increase of temperature from 453 K to 493 K, which is different from Δσθ′ (decreasing significantly at 493 K due to the coarsening of θ′ precipitates). According to the calculation results, the improvement of the high-temperature yield strength of the composite at 453 K was primarily attributed to the strengthening effect of TiCp and refined θ′ precipitates. The θ′ precipitates coarsened significantly at 493 K, leading to the weakening of precipitation strengthening, thus only the thermodynamically stable nano-sized TiCp mainly contributed to the strength increment of the composite at 493 K. 4. Conclusions 1. The strength of the Al-Cu matrix alloy and the nano-sized TiCp/AlCu composites increased with the decrease in test temperatures from 493 K to 453 K and the increase in strain rates from 10−4 s−1 to 10−2 s−1. In the same condition, the high-temperature strength and ductility of all the nano-sized TiCp/Al-Cu composites were superior to the Al-Cu matrix alloy. 2. The θ′ precipitates coarsened significantly at high temperature and low strain rate. Theoretical calculations suggest that the high-temperature yield strength increment of the composite at 453 K than the Al-Cu matrix alloy was primarily attributed to the strengthening effect of both TiCp and refined θ′ precipitates, while only the strengthening effect of TiCp was dominant at 493 K due to the 111
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Simultaneously increasing the high-temperature tensile strength and ductility of nano-sized TiCp reinforced Al-Cu matrix composites ⁎⁎
Wei-Si Tiana,b, Qing-Long Zhaoa, , Qing-Quan Zhanga, Feng Qiua,b, Qi-Chuan Jianga,b,
T
⁎
a
Key Laboratory of Automobile Materials, Ministry of Education and Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China b State Key Laboratory of Automotive Simulation and Control, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Al-Cu matrix composites Nano-sized TiCp High-temperature tensile properties Precipitates
Adding various amounts of nano-sized TiCp (0.1–0.7 wt%) made a simultaneous improvement in high-temperature strength and ductility of the Al-Cu matrix alloy. The influences of temperature and strain rate on the high-temperature tensile properties and θ′ precipitate sizes in the Al-Cu matrix alloy and the nano-sized TiCp/AlCu composites were investigated. Theoretical calculations suggest that the yield strength increment of the composites at 453 K was primarily attributed to the strengthening effect of both the refined θ′ precipitates and TiCp, while at 493 K only the strengthening effect of TiCp was predominant due to the significant coarsening of θ′ precipitates at this temperature.
1. Introduction Al-Cu alloys exhibit a combination of low density and high strength, which makes them the particularly attractive materials for aerospace and automotive applications where strength-to-weight is the prime design consideration [1,2]. The major contribution to the high strength of heattreatable Al-Cu alloys is the spatially distributed nano-sized plate-like θ′ precipitates which serve as obstacles to dislocation movement. However, the tensile properties of Al-Cu alloys deteriorate significantly at elevated temperatures, due to the rapid coarsening of θ′ precipitates and thermally activated cross slip and climb that lead to easy dislocation movement, which limits their applications at high temperatures [3–5]. Therefore, there is a need to improve the strength of Al-Cu alloys at elevated temperatures. There are some effective strategies to improve the high-temperature mechanical properties of Al-Cu alloys. One way is to refine the sizes and improve the distribution of θ′ precipitates by inoculation or modification. Bai et al. [6] found that 0.2 wt% Zr-based metallic glass inoculant improved the tensile strength and ductility of Al-Cu alloy at 433 K and 493 K by promoting the precipitation of denser and finer θ′ precipitates. However, the strengthening effect of θ′ precipitates would weaken at high temperature (such as 493 K) due to the significant coarsening of θ′ precipitates [6]. Another method is to introduce particles, which are thermally stable at high temperature, into Al-Cu alloys. Generally, such particles can be divided into two types: one is formed during
high-temperature processing or heat treatment, such as Al3Sc particles in the Al-Cu-Sc alloys [7]; the other type is ceramic reinforcement, such as SiC [8], ZrB2 [9] and TiC [10]. Ceramic reinforcements have attracted much attention due to low cost and ease of fabrication [11]. Compared with the unreinforced Al matrix alloy, micro-sized particles reinforced aluminum matrix composites exhibit an enhanced strength but a reduced ductility. However, when the particle size of the ceramic reinforcement is reduced to nanometer scale, the nano-sized particles reinforced aluminum matrix composites show an improved strength without sacrificing the ductility. Zhang et al. [8] fabricated the ex situ 0.5 vol% nano-SiCp/Al2014 composites and 4 vol% micro-SiCp/ Al2014 composites by semi-solid stir casting combined with hot extrusion and found that the nano-SiCp improved the high-temperature tensile strength and fracture strain of Al2014 more significantly than the micro-SiCp. Among the various reinforcement particles, TiC is an outstanding reinforcement material due to its high hardness, high melting point (3433 K), good wettability with aluminum and good thermodynamic stability [12]. Besides, it is reported that the addition of TiC particles into Al-Cu alloys could refine grains and promote the precipitation of θ′ phases [10]. Compared with the ex situ method, the in situ method has many advantages, such as clear particle-matrix interfaces and uniform distribution of ceramic particles [13]. In our previous work [14–16], we used the carbon nanotube (CNT) as the carbon source to prepare the nano-sized TiCp/Al master alloy by the
⁎ Corresponding author at: Key Laboratory of Automobile Materials, Ministry of Education and Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China. ⁎⁎ Corresponding authors. E-mail addresses: [email protected] (Q.-L. Zhao), [email protected] (Q.-C. Jiang).
https://doi.org/10.1016/j.msea.2018.01.069 Received 5 July 2017; Received in revised form 7 November 2017; Accepted 19 January 2018 Available online 31 January 2018 0921-5093/ © 2018 Elsevier B.V. All rights reserved.
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sufficiently by ball milling at the speed of 50 rpm for 48 h, and then were cold pressed into the cylindrical preforms (40 mm in height and 28 mm in diameter). Secondly, the preforms were heated in a self-made vacuum thermal explosion furnace at 1173 K to induce the SHS reaction of Al-Ti-CNT system to fabricate the in situ nano-sized TiCp/Al master alloys. The nano-sized TiCp were near-spherical with a mean diameter of about 93 nm [16]. Thirdly, the composites with different nominal TiCp contents (0.1, 0.3, 0.5 and 0.7 wt%) were fabricated by adding the master alloys into the Al-Cu alloy melt at 1173 K, followed by the mechanical stirring for 2 min. Finally, after the melt was cooled down to 1073 K, it was poured into a preheated steel mold (200×150×12 mm3) to form the composite ingot. After T6 heat treatment (solution at 811 K for 12 h and aging at 438 K for 10 h), the Al-Cu matrix alloy and composites were machined to the tensile dog-bone shaped specimens with a gauge cross-section of 4.0 × 2.5 mm2 and a gauge length of 10.0 mm. The tensile tests were carried out using a material testing machine (INSTRON 5869, UK) at 453 K and 493 K under the strain rates of 10−4, 10−3 and 10−2 s−1. The microstructures of the matrix alloy and composites were characterized by optical microscope (OM, Axio Imager A2m, Zeiss, Germany) and transmission electron microscope (TEM, JEM-2100F, Japan). The mechanically polished surfaces of as-cast samples were anodized at 18 V in a 5 vol% HBF4-distilled water solution for optical metallography. The average diameters and thicknesses of θ′ precipitates were determined by the measurement of more than two hundred θ′ precipitates. The fracture surfaces of the typical specimens after high-
self-propagating high-temperature synthesis (SHS) of Al-Ti-C system, due to the high chemical activity and the fine sizes of CNT. Then the master alloy was added into the molten Al-Cu alloy to fabricate the nano-sized TiCp/Al-Cu composites. The results showed that the nanosized TiCp/Al-Cu composites exhibited high room-temperature tensile strength and excellent elongation [15], as well as the superior creep resistance at 453–493 K [16]. Therefore, the nano-sized TiCp/Al-Cu composites seem promising to exhibit superior high-temperature tensile strength and ductility. In this paper, the high-temperature tensile tests of the nano-sized TiCp/Al-Cu composites were carried out at 453 K and 493 K under strain rates of 10−4, 10−3 and 10−2 s−1. We found that the addition of nanosized TiCp could simultaneously increase the high-temperature tensile strength and ductility of the Al-Cu alloy under all the test conditions. The strengthening mechanisms of the composites at elevated temperatures were also discussed in detail. This work aims to provide a new approach to improve the high-temperature tensile properties of Al-Cu matrix composites. 2. Experimental procedures The Al-Cu matrix alloy had a nominal wt% composition of Al-5.5 Cu-0.45 Mn-0.3 Ti-0.2 Cd-0.2 V-0.15 Zr-0.04 B. The nano-sized TiCp were in situ synthesized by self-propagating high-temperature synthesis (SHS) of 70Al-Ti-CNT system that was at a ratio corresponding to that of stoichiometric TiC. Firstly, the Al-Ti-CNT powder blends were mixed
Fig. 1. Polarized optical images of the as-cast microstructures of the (a) Al-Cu alloy and nano-sized TiCp/Al-Cu composites with different contents of TiCp: (b) 0.1, (c) 0.3, (d) 0.5, (e) 0.7 wt% nano-sized TiCp.
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Table 2 The yield strength σ0.2 (MPa), ultimate tensile strength σb (MPa) and fracture strain δf (%) of the Al-Cu alloy and the 0.7 wt% nano-sized TiCp/Al-Cu composite at 453 K and 493 K under the strain rates of 10−4, 10−3 and 10−2 s−1. 453 K
493 K
Al-Cu alloy
−4
10 10−3 10−2
Table 1 The tensile test data of the Al-Cu alloy and the nano-sized TiCp/Al-Cu composites with different contents of nano-sized TiCp at 493 K and 10−3 s−1 strain rate. σ0.2 (MPa)
σb (MPa)
δf (%)
0 0.1 0.3 0.5 0.7
210 220 234 242 254
241 254 283 288 297
6.2 17.9 11.9 7.0 6.6
Al-Cu alloy
Composite
σ0.2
σb
δf
σ0.2
σb
δf
σ0.2
σb
δf
σ0.2
σb
δf
214 229 262
232 258 307
3.2 4.0 5.4
261 287 304
347 374 407
7.5 9.9 17.9
183 210 222
194 241 258
2.5 6.2 11.4
210 254 264
232 297 338
4.6 6.6 18.5
Fig. 2 shows the tensile engineering stress-strain curves of the Al-Cu matrix alloy and 0.1–0.7 wt% nano-sized TiCp/Al-Cu composites at 493 K under the strain rate of 10−3 s−1, and Table 1 lists the yield strength (σ0.2), tensile strength (σb) and fracture strain (δf). The strength and ductility of the composites were simultaneously improved compared with the Al-Cu matrix alloy. Besides, with increasing mass fraction of nano-sized TiCp, the yield strength and tensile strength increased and the fracture strain tended to decrease. The yield strength, tensile strength and fracture strain of the Al-Cu matrix alloy at 493 K were 210 MPa, 241 MPa and 6.2%, respectively. By adding 0.1 wt% nanosized TiCp, the yield strength, tensile strength and fracture strain of the Al-Cu matrix composite increased to 220 MPa, 254 MPa and 17.9%, respectively. The 0.7 wt% nano-sized TiCp/Al-Cu composite possessed the highest yield and tensile strength (254 and 297 MPa), which were 44 MPa and 56 MPa higher than those of the Al-Cu matrix alloy. Meanwhile, it exhibited a relatively low fracture strain (6.6%), which was similar to that of the Al-Cu alloy (6.2%). As known, the strength and ductility are sensitively dependent on temperature and strain rate [18]. Effects of temperature and strain rate on tensile properties of the 0.7 wt% nano-sized TiCp/Al-Cu composite were investigated due to its highest high-temperature strength at 493 K and under the strain rate of 10−3 s−1. Fig. 3 shows the tensile engineering stress-strain curves of the Al-Cu matrix alloy and 0.7 wt% nano-sized TiCp/Al-Cu composite at 453 K and 493 K under the strain rate of 10−4, 10−3 and 10−2 s−1, and Table 2 summarizes the results. The yield stress and tensile stress of both the Al-Cu matrix alloy and composite increased with increasing strain rate or decreasing test temperature, and the fracture strain tended to increase with the increase of strain rate. At 453 K and 10−4 s−1 strain rate, the yield strength, tensile strength and fracture strain of the 0.7 wt% nano-sized TiCp/Al-Cu composite were 261 MPa, 347 MPa and 7.5%, which were improved by 22.0%, 49.6% and 134.4%, compared with the Al-Cu
Fig. 2. Engineering stress-strain curves of the Al-Cu alloy and the nano-sized TiCp/Al-Cu composites with different contents of TiCp at 493 K and 10−3 s−1 strain rate.
TiC (wt%)
Composite
temperature tensile testing were observed by scanning electron microscope (SEM, VEGA 3 XMU, TESCAN, Czech).
3. Results and discussion Fig. 1 compares the as-cast microstructures of the Al-Cu matrix alloy and the composites. The grains of Al-Cu matrix alloy were coarse with an average size of about 140 µm (see Fig. 1(a)); while the grains of the composites were smaller and nearly equiaxed (Fig. 1(b)-(e)). Besides, the grain sizes in the composites decreased with the increase of TiCp content. The average sizes of the α-Al grains were about 65, 54, 47 and 33 µm in the 0.1, 0.3, 0.5 and 0.7 wt% nano-sized TiCp reinforced Al-Cu matrix composites, respectively. During the solidification process, TiCp could serve as heterogeneous nucleation sites and refine the grain size of α-Al [17].
Fig. 3. Engineering stress-strain curves of the Al-Cu alloy and the 0.7 wt% nano-sized TiCp/Al-Cu composites under the strain rates of 10−2 s−1, 10−3 s−1and 10−4 s−1 at (a) 453 K and (b) 493 K.
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Fig. 4. SEM micrographs of fracture surfaces after tensile testing at 493 K and 10−3 s−1 strain rate of the (a) Al-Cu alloy and nano-sized TiCp/Al-Cu composites with different contents of TiCp: (b) 0.1, (c) 0.3, (d) 0.5, (e) 0.7 wt % nano-sized TiCp.
Cu matrix alloy ruptured in a brittle manner, which was characterized by intergranular fracture and few shallow dimples. In contrast, with the addition of nano-sized TiCp, the fracture surfaces of the composites were covered with a large number of deep dimples and a small amount of intergranular fracture, indicating the increased ductility compared with the Al-Cu matrix alloy. Meanwhile, with the increase of TiCp content, the fracture surfaces of the composites were composed of more amount of intergranular fracture and less amount of ductile-dimpled fracture, which implied a decrease in ductility, as seen in Fig. 4(b)-(e). It can be also seen that the grain sizes in the composites were much smaller than those in the Al-Cu matrix alloy, which was consistent with the optical images in Fig. 1. Fig. 5 shows SEM fractographs of the 0.7 wt % nano-sized TiCp/Al-Cu composite fractured at different temperatures and strain rates. With increasing strain rate from 10−4 s−1 to 10−2 s−1 (Fig. 5(a)-(c)) at 493 K or with decreasing temperature from 493 K
matrix alloy (214 MPa, 232 MPa and 3.2%), respectively. When the test temperature increased to 493 K, the yield strength and tensile strength of the composite decreased to 210 MPa and 232 MPa, which were 27 MPa, 38 MPa higher than those of the Al-Cu matrix alloy (183 MPa and 194 MPa), respectively, and its fracture strain was improved by 84.0% compared with the Al-Cu matrix alloy (2.5%). As the strain rate increased to 10−2 s−1 at 493 K, the yield strength, tensile strength and fracture strain of the composite increased to 264 MPa, 338 MPa and 18.5%, which were 42 MPa, 80 MPa and 7.1% higher than those of the Al-Cu matrix alloy (222 MPa, 258 MPa and 11.4%), respectively. The strength and ductility of the composite were significantly improved under all the test conditions, compared with the Al-Cu matrix alloy. Fig. 4 shows SEM fractographs of the Al-Cu alloy and nano-sized TiCp/Al-Cu composites with different contents of TiCp fractured at 493 K and 10−3 s−1 strain rate. It can be seen from Fig. 4(a) that the Al108
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Fig. 5. SEM micrographs of fracture surfaces of the 0.7 wt% nano-sized TiCp/Al-Cu composite after tensile tests. The experiment conditions are: (a) 493 K and 10−4 s−1 strain rate (b) 493 K and 10−3 s−1 strain rate, (c) 493 K and 10−2 s−1 strain rate (d) 453 K and 10−3 s−1 strain rate.
(Fig. 5(b)) to 453 K (Fig. 5(d)) at 10−3 strain rate, the amount of intergranular fracture decreased and ductile-dimpled fracture increased, which indicated the increased ductility. The precipitates in Al-Cu alloys are θ′ precipitates, which are formed on the {100}α-Al planes in Al-Cu alloys [19]. The TEM images of θ′ precipitates in the Al-Cu matrix alloy and 0.7 wt% nano-sized TiCp/AlCu composite after tensile tests are demonstrated in Fig. 6. After the tensile test at 453 K and 10−4 s−1 strain rate, the average diameter and thickness of the plate-like θ′ precipitates in the Al-Cu matrix alloy (Fig. 6(a)) were about 60 nm and 3 nm, while the much larger number of finer θ′ precipitates with an average diameter of 38 nm and thickness of 3 nm were observed in the composite (Fig. 6(b)). The coarsening of the precipitates was limited when the test temperature (453 K) was a bit higher than the aging temperature (438 K). However, the θ′ precipitates in both the Al-Cu alloy and composite coarsened considerably (roughly doubled in diameter) when increasing temperature to 493 K. However, after the tensile test at 493 K and 10−4 s−1 strain rate, the θ′ precipitates in the composite (Fig. 6(d)) still had finer sizes (average diameter of 92 nm and thickness of 5 nm), compared with those in the AlCu alloy (average diameter of 111 nm and thickness of 5 nm, as shown in Fig. 6(c)). After the tensile test at 493 K and higher strain rate (10−2 s−1), the sizes of θ′ precipitates in the Al-Cu alloy (average diameter of 86 nm and thickness of 3 nm, as shown in Fig. 6(e)) and in the composite (average diameter of 64 nm and thickness of 3 nm, as shown in Fig. 6(f)) were much smaller than those in the samples at lower strain rate (10−4 s−1). This is because the longer time to rupture at lower strain rate at 493 K could lead to the significant coarsening of θ′ precipitates, based on the modified Lifshit-Slyozov-Wagner equation in which the average θ′ radius increases with increasing aging time [7]. Meanwhile, the stress can also cause the coarsening of θ′ precipitates. In our previous research [16], we found that the coarsening of the θ′ precipitates was enhanced in the nano-sized TiCp/Al-Cu composite
crept at 493 K under 160 MPa for 10 h than that in the composite overaged at 493 K for 10 h, which was consistent with the results from Gariboldi et al. [20]. During deformation under the applied stress, moving dislocation attract solutes both to the associated strain fields and to their cores. When moving dislocations encounter the growing precipitates, the collected solutes diffuse to precipitates, causing Ostwald ripening [21,22]. This stress-enhanced diffusion is much faster at higher temperature, and the solute transport by dislocations is more sufficient during longer time, leading to the coarser θ′ precipitates in the samples tested at higher temperature and lower strain rate. In our previous work, it was found that the nano-sized TiCp predominantly dispersed in the grain interior, and the interface between the particles and the Al matrix was clean before tensile tests [15]. Fig. 7 shows the TEM micrograph of the 0.7 wt% nano-sized TiCp/Al–Cu composite after the tensile test at 453 K and 10−4 s−1. It can be seen that the nano-sized TiCp were thermodynamically stable and distributed in the grain interior. As a result, the nano-sized TiCp could serve as effective barriers to dislocation motion and enhance the hightemperature strength of the composites. The enhancement in ductility of the composites is a result of grain refinement as shown in Fig. 1. Grain refinement results in more uniform deformation. Besides, grain refinement also results in more tortuous grain boundary, which is not conducive to crack propagation [23]. Moreover, grain refinement leads to reduced possible flaw sizes that caused stress concentration, thus suppressing the crack nucleation and propagation [24]. Therefore, the composites could withstand more deformation before fracture than the Al-Cu matrix alloy, as demonstrated in Fig. 4. The ductility of both the matrix alloy and the composite decreased with decreasing the stain rate from 10−2 s−1 to 10−4 s−1. This may be because the grain boundary will weaken at high temperature, and weakening effect is more serious when the exposure time of the sample at high temperature is longer at lower strain rate, 109
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Fig. 6. TEM micrographs of θ′ precipitates in the Al–Cu matrix alloy and 0.7 wt% nano-sized TiCp/Al-Cu composite after tensile tests under different test conditions: the (a) Al-Cu alloy and (b) composite at 453 K and 10−4 s−1 strain rate, the (c) Al-Cu alloy and (d) composite at 493 K and 10−4 s−1 strain rate, as well as the (e) Al-Cu alloy and (f) composite at 493 K and 10−2 s−1 strain rate; (g-l) the corresponding statistical results of the diameters of the θ′ precipitates in (a-f), respectively.
active deformation mechanisms at high temperatures [25]. For dislocation glide, the strengthening effect of precipitates and nano-sized ceramic particles is attributed to the Orowan bypass mechanism, and the decrease in the shear modulus G with increasing temperature results in the decrease in Orowan stress at elevated temperature [25]. At high temperatures (T > 0.5Tm, where Tm is the absolute melting temperature of the alloy), it is well known that dislocation tend to surmount the obstacles by local climb mechanism [26]. According to the local climb mechanism proposed by Brown and Ham [27], the back stress due to local climb (Δσb ) is proportional to the Orowan stress (ΔσOro ) and is lower by a factor of k than the Orowan stress as follows:
Δσb = k ΔσOro
Fig. 7. TEM micrograph of the nano-sized TiCp in the 0.7 wt% nano-sized TiCp/Al–Cu composite after the tensile test at 453 K and 10−4 s−1 strain rate.
(1)
where k is usually in the range of 0.4–0.7. It can be seen that the back stress due to local climb is related to the Orowan stress and the higher Orowan stress indicates the higher back stress due to local climb. As a result, no matter which mechanism plays a dominant role, the increment of high-temperature yield strength is proportional to the Orowan stress. Qin et al. [25] used the Orowan mechanism with modified shear modulus to predict the high-temperature yield strength of the B4C/AlSc composite with large precipitates (more than 8 nm) and found that the predicted yield strengths at elevated temperatures were fairly consistent with the high-temperature experimental data. Therefore, in this paper, we calculated the Orowan stress to compare the stress contributed by the θ′ precipitates and nano-sized TiCp in the Al-Cu
leading to more amount of intergranular fracture and decreased ductility, as shown in Fig. 5. Obviously, the microstructure change induced by the addition of nano-sized TiCp, which includes the grain refinement and the change of the θ′ precipitate sizes, contributes to the enhanced tensile properties of the composites, as well as the strengthening effect of the nano-sized TiCp. It is believed that the refined grain sizes could potentially be helpful in simultaneously enhancing the strength and ductility of materials [24]. However, the strengthening effect of grain refinement is negligible at high temperature [6]. Dislocation glide and climb are 110
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significant coarsening of θ′ precipitates at this temperature.
matrix alloy and 0.7 wt% nano-sized TiCp/Al-Cu composite. In order to reveal whether the strengthening effect of θ′ precipitates or TiCp was predominant in the yield strength increment in the composite, we discussed the θ′ precipitate strengthening and the nano-sized TiCp strengthening separately. The Orowan stress due to the plate-like θ′ precipitates strengthening (Δσθ′) can be calculated according to the following modified Orowan equation [19,28]:
Δσθ ′ =
⎛ MGb ⎜ 2π 1-ν ⎜ 1.123d ⎝
1 0.318π af
πd 1.061d -8- a
⎞ 0.981d ⎟ ln ab ⎟ ⎠
Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51571101 and No. 51601066), the Science and Technology Development Program of Jilin Province, China (grant No. 20160520116JH and 20170101215JC), “Thirteenth five-year plan” Science & Technology Research Foundation of Education Bureau of Jilin Province, China (Grant No. 2015-479), and the Project 985-High Properties Materials of Jilin University.
(2) References
where M is the Taylor factor for Al (approximately equal to 3 [19]), G is the shear modulus (24.2 GPa at 453 K and 23.5 GPa at 493 K for Al, according to literature [29]), ν is the Poisson's ratio (1/3 for Al [19]), b is the Burgers vector (0.286 nm for Al [19]), a is the aspect ratio of the precipitates (a= d/t, d and t are the diameter and thickness of the precipitate, respectively) and f is the volume fraction of θ′ precipitates. The volume fraction (f) of the θ′ precipitates in the T6 peak-aged Al5.5Cu alloy used in this work is estimated to be 6% according to the work by Zhao et al. [30], due to the similar Cu content in these two works. It can be seen from Eq. (2) that the aspect ratio of θ′ precipitates plays an important role in controlling the precipitation strengthening effect. By substituting the values of M, G, ν, b, f and statistical results of d and t of the θ′ precipitates into Eq. (2), the theoretical values of Δσθ′ under the strain rate of 10−4 s−1 in the 0.7 wt% nano-sized TiCp/Al-Cu composite are 33 MPa higher at 453 K and 6 MPa higher at 493 K than those of the Al-Cu matrix alloy, respectively. The precipitation strengthening effect in the composite is more significant than that in the Al-Cu matrix alloy at 453 K, while the difference is minor in the composite and Al-Cu matrix alloy at 493 K. The Orowan stress contributed by the nano-sized TiCp (Δσp) can be calculated by the following equation [31]:
Δσp = 0.81
MAGb πd ln( ) 2πλ 4b
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(3)
where λ=0.4d ( π/fV -2) is the interparticle spacing, d and fV are the diameter and volume fraction of the TiCp, respectively. The value of constant A is estimated to be 1.8 [31]. The theoretical values of Δσp in the 0.7 wt% (0.4 vol%) nano-sized TiCp/Al-Cu composite are 26 MPa at 453 K and 25 MPa at 493 K. It can be seen that there is little change in the value of Δσp with the increase of temperature from 453 K to 493 K, which is different from Δσθ′ (decreasing significantly at 493 K due to the coarsening of θ′ precipitates). According to the calculation results, the improvement of the high-temperature yield strength of the composite at 453 K was primarily attributed to the strengthening effect of TiCp and refined θ′ precipitates. The θ′ precipitates coarsened significantly at 493 K, leading to the weakening of precipitation strengthening, thus only the thermodynamically stable nano-sized TiCp mainly contributed to the strength increment of the composite at 493 K. 4. Conclusions 1. The strength of the Al-Cu matrix alloy and the nano-sized TiCp/AlCu composites increased with the decrease in test temperatures from 493 K to 453 K and the increase in strain rates from 10−4 s−1 to 10−2 s−1. In the same condition, the high-temperature strength and ductility of all the nano-sized TiCp/Al-Cu composites were superior to the Al-Cu matrix alloy. 2. The θ′ precipitates coarsened significantly at high temperature and low strain rate. Theoretical calculations suggest that the high-temperature yield strength increment of the composite at 453 K than the Al-Cu matrix alloy was primarily attributed to the strengthening effect of both TiCp and refined θ′ precipitates, while only the strengthening effect of TiCp was dominant at 493 K due to the 111
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