6061Al composites

Author’s Accepted Manuscript Effect of reinforcement content and aging treatment on microstructure and mechanical behavior of B4Cp/6061Al composites Minqiang Gao, Huijun Kang, Zongning Chen, Enyu Guo, Peng Peng, Tongmin Wang www.elsevier.com/locate/msea

PII: DOI: Reference:

S0921-5093(18)31721-0 https://doi.org/10.1016/j.msea.2018.12.042 MSA37314

To appear in: Materials Science & Engineering A Received date: 24 October 2018 Revised date: 7 December 2018 Accepted date: 11 December 2018 Cite this article as: Minqiang Gao, Huijun Kang, Zongning Chen, Enyu Guo, Peng Peng and Tongmin Wang, Effect of reinforcement content and aging treatment on microstructure and mechanical behavior of B4Cp/6061Al c o m p o s i t e s , Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.12.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of reinforcement content and aging treatment on microstructure and mechanical behavior of B4Cp/6061Al composites

Minqiang Gao, Huijun Kang, Zongning Chen, Enyu Guo*, Peng Peng, Tongmin Wang** Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, PR China

*Enyu Guo: [email protected]; **Tongmin Wang: [email protected];

*

Correspondence information:. The general contact information: (1) Mail address:

School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China; (2) Tel/Fax.: +86-411-8470679

Abstract The effects of B4C particle (B4Cp) content and aging treatment on the microstructure and mechanical behavior of B4Cp/6061Al neutron absorber composites prepared by powder metallurgy were investigated. The results demonstrated that the relative density of the B4Cp/6061Al composites decreased slightly by increasing reinforcement content from 0 to 15 wt%. No obvious pores or reaction products were observed at the B4Cp/matrix interfaces by transmission electron microscopy, suggesting that a good, clean interface was achieved. Mg2Si precipitates and dislocations were detected both near the interfaces and in the matrix. Moreover, the aging kinetics could be significantly accelerated by the presence of B4C particles; this behavior was associated with the enhanced dislocation density. The variation in hardness and electrical conductivity as a function of aging time in composites with various B4Cp contents was examined. For the peak-aged composites with increasing B4Cp content from 0 to 15 wt%, the ultimate tensile strength increased from 342.2 to 451.0 MPa, whereas the failure strain decreased from 18.4% to 7.9%. The grain refinement, dislocation strengthening, and work hardening, which contributed to the improvement in yield strength, were theoretically calculated, and the results agreed well with the experimental results.

Keywords: Al matrix composite; Powder metallurgy; Aging treatment; Tensile deformation; Strengthening mechanism

1

1. Introduction Particle-reinforced aluminum matrix composites (PRAMCs), which are viewed as promising structural materials, have been widely used in the aerospace and automotive industries [1, 2]. This is attributed mainly to their low density, low coefficient of thermal expansion (CTE), high strength and stiffness, and good wear resistance [3–6]. Pure aluminum and aluminum alloys are typically reinforced with Al2O3 [7] and SiC [8, 9] particles; these reinforcements can be replaced by B4C particles, which have the following advantages. First, the density of B4C particles (~2.52 g/cm3) is lower than that of Al2O3 particles (~3.9 g/cm3) and SiC particles (~3.21 g/cm3), suggesting that B4C-particle-reinforced aluminum matrix (B4Cp/Al) composites with higher specific strength and stiffness can be obtained [10]. Second, the thermal stability, thermal expansion coefficient, and chemical inertness of B4C particles are similar to those of SiC or Al2O3 particles [11]. Finally, B4Cp/Al composites have been extensively used as desirable neutron absorber materials owing to the specific ability of the

10

B isotope to

capture neutrons [12]. Owing to their unique properties, B4Cp/Al composites have been used in the nuclear industry for storage and transportation of spent fuels [13, 14]. The age-hardening behavior of heat-treatable aluminum matrix composites reinforced by SiC and Al2O3 particles has been investigated in the past few decades. Previous researchers held differing opinions on the aging kinetics of composites. Most studies showed that the aging kinetics of composites could be accelerated by adding reinforcements [15–17]. Only a few studies noted that the incorporation of reinforcements could suppress or have no effect on aging precipitation [18, 19]. According to these studies, the age-hardening ability of composites is controlled mainly by the microstructural characteristics of the matrix. However, little attention has been paid to the precipitation behavior of heat-treatable B4Cp/Al composites with various B4Cp contents. The factors that strengthen B4Cp/Al composites can be summarized as follows. Addition of particles can not only transfer the load but also indirectly change the 2

microstructure of the matrix. Nardone and Prewo [20] proposed the modified shear lag theory to predict the yield strength of the composites; they noted that when the load was transferred through the interface between the particles and the matrix, the particles would bear greater stress than the matrix. However, microstructural evolution was not considered in their studies. Arsenault et al. [21] suggested that the load transfer, the grain refinement and dislocation strengthening of the matrix, and the work hardening could be added linearly, leading to a higher predicted value. Wu and Lavernia [22] reported that the microstructure of the composites can be divided into three regions, i.e., the reinforcement particles, plastic zone, and elastic area. Therefore, it is necessary to consider the interaction mechanism between the micromechanics and microstructure. In this work, the effects of reinforcement content and aging treatment on the microstructure and mechanical behavior of B4Cp/6061Al composites were investigated. B4Cp/6061Al composites with different B4Cp contents (0, 5, 10, and 15 wt%) were prepared by powder metallurgy followed by hot rolling. The relationship between the microstructure and properties as well as the strengthening mechanisms of the B4Cp/6061Al composites were examined. 2. Experimental procedure 6061Al powder with a nominal composition of Al–1.05Mg–0.59Si–0.16Fe–0.29Cu– 0.08Mn–0.03Zn (wt%) and B4C particles were blended in a ball-miller for 10 h. The rotation speed and ball-to-powder weight ratio were 150 rpm and 10, respectively. The mean sizes of the 6061Al powder and B4C particles were 13 and 7 μm, respectively. The mixed powders with 0, 5, 10, or 15 wt% B4C particles were poured into a heat-resistant steel die with an internal diameter of 45 mm and cold-pressed under an axial pressure of 15 MPa. Next, the cold-pressed billets were hot-pressed in a vacuum chamber (~1 × 10-2 Pa) at 580 °C for 45 min under a maximum axial pressure of 35 MPa. The hot-pressed billets with a diameter of 45 mm and height of 20 mm were hot-rolled at 500 °C. Before each rolling pass, the sample was reheated in a furnace for 5 min, and the total deformation was ~87.5%. Finally, a sheet-like sample with a 3

thickness of ~2.5 mm was obtained. These sheets with different reinforcement contents were solutionized at 520 °C for 1.5 h, quenched in cold water, and then aged at 175 °C for various durations from 0 to 24 h. The density of the as-rolled composites with various reinforcement contents was measured using the Archimedes principle. The morphologies of the raw materials were observed by scanning electron microscopy (SEM, JEOL JSM-5600LV), and the distribution of B4C particles was examined by optical microscopy (OM, Olympus GX51). The interfaces, dislocations, and Mg2Si precipitates in the B4Cp/6061Al composites were detected by transmission electron microscopy (TEM, JEOL JEM-2100F) at an accelerating voltage of 200 kV. X-ray diffraction (XRD) measurements were performed using an EMPYREAN diffractometer equipped with a Cu radiation target. The electrical conductivity of the aged samples was recorded using a D60K digital electrical instrument, and the average value of three measurements was calculated. Vickers hardness tests of the composites were conducted under a load of 200g for a dwell time of 10 s. For each sample, at least 8–10 tests were performed to get good statistics. Tensile tests of the peak-aged samples (having a gauge cross section of 4 × 2.5 mm2 and a gauge length of 10 mm) were performed at ~25 °C under an initial strain rate of 10-3 s-1 using an Instron 5982 testing machine.

3. Results and discussion 3.1. Relative density Table 1 lists the relative density of the as-rolled 6061Al alloy and B4Cp/6061Al composites. The data suggest that the 6061Al alloy and all the B4Cp/6061Al composites with various B4Cp contents were almost entirely compact. As the B4Cp content was increased from 0 to 15 wt%, the relative density of the composites decreased slightly from 99.70% to 99.47%. 3.2. Microstructure Fig. 1 shows SEM images and XRD patterns of the as-received 6061Al powder and 4

B4C particles. The 6061Al powder exhibited spherical or near-spherical morphologies, whereas the B4C particles had a polygonal morphology. No obvious impurities were found in the 6061Al powder, and only a very small quantity of free B impurities was detected in the B4C particles. Table 1 Density of the as-rolled B4Cp/6061Al composites with different reinforcement contents. Sample

Measured density, 3

Theoretical density, 3

Relative density,

g/cm

g/cm

%

0 wt% B4Cp/6061Al

2.698

2.700

99.70

5 wt% B4Cp/6061Al

2.686

2.690

99.65

10 wt% B4Cp/6061Al

2.676

2.681

99.62

15 wt% B4Cp/6061Al

2.657

2.671

99.47

Fig. 2 shows OM and SEM images of the as-rolled B4Cp/6061Al composites with various reinforcement contents. The B4C particles were homogenously distributed in the B4Cp/6061Al composites, as shown in Fig. 2(a)–(c). To provide information about the matrix, SEM micrographs with higher magnification are shown in Fig. 2(d)–(f). No obvious micropores were found within the matrix, indicating that the matrix alloy was well-compacted. Fig. 3 shows TEM images of the interfaces between the B4C particles and the Al matrix in the as-rolled B4Cp/6061Al composites with various reinforcement contents. No obvious cracks or pores were observed at the B4Cp/matrix interfaces at any B4Cp content, suggesting that good interfacial bonding was obtained, as presented in Fig. 3(a)–(c). In addition, almost no reaction products were observed near the interfaces. The upper left and lower right insets in Fig. 3(a) are the selected area electron diffraction (SAED) patterns of the Al matrix and B4C particles, respectively. Note that the B4C crystal structure is indexed as rhombohedral structure [23, 24]. For the composites with 15 wt% B4C particles, additional defect structures, marked by white arrows, appeared within the B4C particle. To provide further insight into the details, a high-resolution TEM (HRTEM) image of the interface is presented in Fig. 3(d). The typical diffraction 5

pattern of a twin was found in the B4C particle. The formation of twins can be attributed to the presence of high shear stress and local high stress intensity around the B4C particles [25]. Further, the good interface between the B4C particle and the Al matrix plays a significant role in the load transfer, as shown in Fig. 3(d). Fig. 3(e) presents a fast Fourier transform (FFT) image of the region within the black rectangle in Fig. 3(d), which shows the B4C particle along the [11̅1] zone axis and the Al matrix along the [011] zone axis. The analysis shows that there is no relationship between the crystallographic orientations of the B4C particle and Al matrix. In addition, a continuous amorphous layer with a thickness of ~10 nm was formed at the B4Cp/matrix interface, as shown in Fig. 3(f). Some studies revealed that these amorphous transition regions, which are composed mainly of Al, Mg, and O, could be attributed to oxide layers on the initial 6061Al powder [26, 27]. When the B4Cp/Al composites were cooled from high temperature to room temperature, dislocations formed owing to the CTE mismatch between the reinforcement and the matrix. The addition of B4C particles results in enhancement of the dislocation density (𝜌), which can be calculated as [28] 𝜌=

6∆𝐶𝑇𝐸×∆𝑇×𝑉𝑝 𝑏×𝑑(1 − 𝑉𝑝 )

(1)

where ∆𝐶𝑇𝐸 is the difference in CTE between the reinforcement and the matrix, ∆𝑇 is the temperature difference which generates dislocations, 𝑏 is the Burger’s vector, 𝑑 is the mean diameter of the particles, and 𝑉𝑝 is the volume fraction of particles. In this case, ∆𝐶𝑇𝐸 is ~18.4 × 10-6/K and ∆𝑇 is ~500 K [29], and the dislocation density increases from 1.47 × 1012 to 4.9 × 1012 m-2 with increasing B4Cp content from 5 to 15 wt%. Fig. 4 shows TEM images of the dislocations and precipitates in the peak-aged 5 wt% B4Cp/6061Al composites. Dislocations could be detected near the B4Cp/matrix interface and in the Al matrix, as illustrated in Fig. 4(a) and (c), respectively. The formation of dislocations near the interfaces was attributed mainly to the significant difference in CTE between the reinforcement and the matrix. The presence of 6

dislocations facilitated nucleation of precipitates, thus strengthening the Al matrix. After the T6 heat treatment, Mg2Si precipitates were detected near the interface and in the Al matrix, as shown in Fig. 4(b) and (d), respectively. The average size of the Mg2Si precipitates was ~160 nm in the 5 wt% B4Cp/6061Al composites. In addition, high-density dislocations around the Mg2Si precipitates in the Al matrix were also found in Fig. 4(d). The SAED pattern in the inset of Fig. 4(b) is indexed as the Mg2Si precipitate along the [001] zone axis. Mg2Si precipitates play a key role in strengthening the Al matrix by impeding dislocation movement, improving the strength of the B4Cp/6061Al composites [30]. 3.3. Mechanical and electrical properties Fig. 5 displays the variation in the hardness and electrical conductivity versus aging time in the composites with different B4Cp contents. The aging hardness curves of the 6061Al alloy and composites [Fig. 5(a)] all show a similar trend as the aging time increased from 0 to 24 h; i.e., the hardness values increased steadily, reached a peak, and then decreased gradually (indicating over-aging). The change in hardness was closely related to the Mg2Si phases during heat treatment, as follows: supersaturated solid solution → cluster → Guinier–Preston zone → β″ phase → β′ phase → β-Mg2Si [30]. For the solutionized samples, the hardness increased from 95.2 to 143.9 HV with increasing B4Cp content from 0 to 15 wt%. The time to achieve peak hardness was shorter in the composites than in the 6061Al alloy, indicating that adding B4C particles could accelerate the age-hardening response. According to previous reports [15], this was due mainly to the increase in dislocation density in the matrix, especially near the interfaces [Fig. 4(a)], which was associated with the CTE mismatch between the reinforcement and the matrix. The variation in dislocation density was theoretically calculated using formula (1). In addition, little difference was noted between the peak hardness values of the 10 and 15 wt% B4Cp/6061Al composites because some strain accommodation (i.e., recovery) occurred in the composites. Fig. 5(b) shows the electrical conductivity of the composites with different B4Cp 7

contents versus the aging time at 175 °C. As the aging time increased from 0 to 24 h, the electrical conductivity of the 6061Al alloy and composites increased continuously. This increase is due mainly to precipitation of Mg2Si phases, which decreased the electron-scattering capability of the matrix [31]. According to a study by Gao et al. [32], the electrical resistivity (𝜌) of metallic materials is expressed as 𝜌 = 𝜌0 + ∆𝜌𝑠 + ∆𝜌𝑃 + ∆𝜌𝐷 + ∆𝜌𝑉 + ∆𝜌𝐵

(2)

where 𝜌0 is the resistivity of an ideal pure metal, and ∆𝜌𝑠 , ∆𝜌𝑃 , ∆𝜌𝐷 , ∆𝜌𝑉 , and ∆𝜌𝐵 are the resistivity due to solute atoms, precipitates, dislocations, vacancies, and grain boundaries, respectively. For the same aging time, a higher B4Cp content would introduce more defects consisting of dislocations, interfaces, and grain boundaries into the matrix, degrading the electrical conductivity. Table 2 Mechanical properties of the peak-aged composites with different B4Cp contents. Absorbed

𝜎0.2

UTS

Failure

Hardness

(MPa)

(MPa)

strain (%)

(HV)

0 wt% B4Cp/6061Al

301.6±5.2

342.4±9.1

18.4±2.8

133.5±2.7

66.41

5 wt% B4Cp/6061Al

339.9±6.2

391.2±5.8

14.2±2.5

161.9±2.2

55.68

10 wt% B4Cp/6061Al

362.3±8.3

424.4±4.4

12.1±1.4

167.6±3.2

48.91

15 wt% B4Cp/6061Al

381.5±6.1

451.0±7.7

7.9±0.6

168.9±2.9

36.12

Sample

energy (MJ/m3)

Fig. 6(a) displays typical engineering stress–strain curves of the peak-aged B4Cp/6061Al composites with different B4Cp contents. As the B4Cp content increased from 0 to 15 wt%, the ultimate tensile strength (𝜎UTS ) and yield strength (𝜎0.2 ) exhibited obvious increases from 342.2 to 451.0 MPa and 301.6 to 381.5 MPa, respectively; the failure strain decreased from 18.4% to 7.9%. Table 2 summarizes the mechanical properties of the peak-aged B4Cp/6061Al composites with various B4Cp contents. The tensile strength of composites generally increases with increasing particle content and 8

decreasing particle size [33]. However, it has been reported that when the B4C particle content was further increased to 30 wt%, the ultimate tensile strength of the composites exhibited a decrease, which was associated with an increase in particle agglomeration regions [27]. In this work, similar results were found; i.e., the strength of the peak-aged B4Cp/6061Al composites increased at the expense of the ductility. Pouraliakbar et al. [34] noted that the toughness of a specimen could be calculated by integrating the area beneath the engineering stress–strain curve. The energy absorbed (toughness) of the peak-aged B4Cp/6061Al composites with various reinforcement contents was calculated using the tensile curves in Fig. 6(a). The results (absorbed mechanical energy per unit volume of material) are listed in Table 2. As the B4Cp content was increased from 0 to 15 wt%, the toughness of the specimens decreased continuously; this behavior was related to the early occurrence of the instability point [35]. Therefore, the ductility has a more significant impact on the toughness of the peak-aged B4Cp/6061Al composites. The peak-aged 15 wt% B4Cp/6061Al composites exhibited a higher strain hardening ability than the 6061Al alloy, as shown in Fig. 6(b). This is mainly because the B4C particles undergo much less deformation than the matrix, resulting in the generation of work hardening [36]. B4C particles could effectively constrain deformation of the Al matrix during the tensile process so that higher strength was achieved. Furthermore, the applied load could be transferred from the Al matrix to the B4C particles during tensile deformation as good interface bonding was achieved [Fig. 3(d) and (f)], which also increased the strength. In addition, a large number of dislocations could be generated near the B4Cp/matrix interfaces owing to the significant difference in CTE between the B4C particles and the Al matrix. The dislocation density increased with increasing particle content, thus enhancing the strength of the B4Cp/6061Al composites. Moreover, the grain boundaries were pinned by the B4C particles, resulting in grain refinement [27]. All of these factors contributed to the increase in the yield strength of the composites, which will be further discussed in detail in the next section. The mechanical properties obtained in this work were compared with those of other 9

PRAMCs, namely, Al2O3p/6061Al composites [37–39], SiCp/6061Al composites [37, 40–42], and B4Cp/6061Al composites [11, 27, 43]. The ultimate tensile strength vs. fracture elongation is plotted in Fig. 7. The B4Cp/6061Al composites fabricated in this work showed a good trade-off between the strength and the ductility, in contrast to the other composites. It has also been noted that the mechanical properties of PRAMCs depend on a variety of factors, such as the particle size, reinforcement particle volume fraction, processing parameters, heat treatment conditions, and particle surface treatment. In Ref. [41], the mechanical properties of the composites were improved by using oxidized and acid-pickled SiC particles. In addition, nanoparticle-reinforced aluminum matrix composites reportedly had an excellent combination of strength and ductility [42]. Fig. 8 shows tensile fractographs of the peak-aged B4Cp/6061Al composites with different B4Cp contents. Dimples and tearing ridges were observed in the fracture surfaces, indicating that typical ductile fracture occurred in the matrix alloy during tensile deformation. Mg2Si phases, which are considered to be the source of the voids, play a fundamental role in ductile fracture of alloys and composites [44]. Void nucleation, growth, and coalescence result in the formation of dimples in the fracture surfaces [45]. Furthermore, as the B4Cp content was increased, the dimples became shallower; this behavior was associated with the decrease in ductility. When microcracks propagated to the interfaces between the B4C particles and the matrix, the B4C particles would fracture owing to stress concentration, suggesting that the load was transferred through the B4Cp/matrix interfaces, as shown in Fig. 8(b)–(d). Note that if the B4Cp/matrix interface bonding is weak, decohesion between the B4C particles and the matrix will occur before particle fracture [46]. In this work, interfacial debonding was occasionally observed. Therefore, for the peak-aged B4Cp/6061Al composites, three fracture modes were responsible for deformation and fracture failure: ductile fracture of the matrix, particle fracture, and decohesion between the B4C particles and the matrix. 10

3.4. Strengthening mechanisms The yield strength of PRAMCs can be expressed as [20] 𝜎𝑦,𝑐 = 𝜎𝑦,𝑚 [𝑉𝑝 (𝑆 + 2)/2 + 𝑉𝑚 ]

(3)

where 𝜎𝑦,𝑐 and 𝜎𝑦,𝑚 are the yield strengths of the composite and matrix, respectively; 𝑉𝑝 and 𝑉𝑚 represent the volume fractions of the particles and matrix, respectively; and 𝑆 is the average aspect ratio of the particles and is approximately 1.2 in this work. Formula (3) is the shear lag model, which indicates that the load can be transferred through the interfaces between the particles and the matrix. Therefore, as the B4Cp content was increased from 5 to 15 wt%, the predicted yield strength of the B4Cp/6061Al composites increased from 309 to 327 MPa. However, the value predicted by the shear lag model was smaller than the experimental value, suggesting that the incorporation of B4Cp should result in microstructural strengthening. The presence of B4C particles can impede grain boundary movement during recrystallization [28]. Consequently, the grains of the composites are refined compared to those of unreinforced alloys. Assuming that each B4C particle provides a nucleation site for grains, the contribution from grain refinement can be estimated using the Hall– Petch equation, as follows [47]: 1

∆𝜎𝐻−𝑃 = 𝐾𝑦 𝑑 −2 [(1 − 𝑉𝑝 )/𝑉𝑝 ]

1 6

−

(4)

where ∆𝜎𝐻−𝑃 is the increment of the yield strength of the composites, 𝐾𝑦 is a constant, and 𝑑 is the mean diameter of the B4C particles. For aluminum alloys, 𝐾𝑦 = 0.1 MN/m3/2 [48]. In addition, the increment of the yield strength (∆𝜎𝑑𝑖𝑠 ) of the matrix can be caused by the increase in dislocation density, which is given by [49] ∆𝜎𝑑𝑖𝑠 = 𝛼𝐺𝑏𝜌1/2

(5)

where 𝛼 is a constant and is equal to 1.25 for aluminum alloys, 𝐺 is the shear modulus of the matrix (𝐺 ≈ 26.2 GPa), and 𝜌 is the dislocation density, which can be calculated using formula (1). When the B4Cp/6061Al composites were loaded, the particles showed much less deformation than the matrix. This led to the formation of geometrically necessary dislocations in the Al matrix, thus generating work hardening. 11

The stress increment (∆𝜎𝑔𝑒𝑜 ) due to the geometrically necessary dislocations can be obtained as follows [50]: 𝑉𝑝 𝜀𝑝𝑙 𝑏

∆𝜎𝑔𝑒𝑜 = 𝛽𝐺 √

𝑑

(6)

where 𝛽 is a geometric factor with a value of ~0.4, and 𝜀𝑝𝑙 is the plastic strain of the matrix and is taken as the yield strain. Therefore, the yield strength of the matrix (𝜎𝑦,𝑚 ) should be modified by the addition of B4C particles owing to microstructure strengthening, as follows: ∗ 𝜎𝑦,𝑚 = 𝜎𝑦,𝑚 + ∆𝜎𝐻−𝑃 + ∆𝜎𝑑𝑖𝑠 + ∆𝜎𝑔𝑒𝑜

(7)

∗ where 𝜎𝑦,𝑚 is the modified yield strength of the matrix. Therefore, the yield strength of

the B4Cp/6061Al composites with different B4Cp contents can be predicted. Fig. 9(a) shows the calculated values of the yield strength increment due to the strengthening mechanisms of grain refinement, dislocation strengthening, and work hardening. A few salient observations can be made. First, the contribution of different strengthening mechanisms to the increase in yield strength can be summarized as follows: ∆𝜎𝐻−𝑃 > ∆𝜎𝑑𝑖𝑠 > ∆𝜎𝑔𝑒𝑜 . In other words, load transfer, grain refinement, and dislocation strengthening were the primary strengthening mechanisms in the B4Cp/6061Al composites in this work. Work hardening contributed very little to the improvement in yield strength. Second, the theoretical values of the yield strength of the B4Cp/6061Al composites are close to the experimental measurements, as shown in Fig. 9(b). This also tells us that the micromechanics and microstructure strengthening mechanisms should be considered when estimating the yield strength of PRAMCs.

4. Conclusions On the basis of the above experimental results obtained in this work, the main conclusions can be summarized as follows: (1) As the B4Cp content was increased from 0 to 15 wt%, the relative density of the B4Cp/6061Al composites decreased from 99.70% to 99.47%, and B4C particles were uniformly distributed in the matrix. 12

(2) No obvious flaws or reaction products were observed at the B4Cp/matrix interfaces. For the peak-aged composites, Mg2Si precipitates and dislocations were detected near the interfaces and within the matrix. (3) The time to achieve the peak hardness was shorter in the composites than in the 6061Al alloy, suggesting that adding B4C particles accelerated the age-hardening response. For the same aging time, the electrical conductivity decreased with increasing B4Cp content. (4) For the peak-aged composites, the yield strength increased from 301.6 to 381.5 MPa with increasing B4Cp content from 0 to 15 wt%, which was in good agreement with the theoretical predictions. Load transfer, grain refinement, and dislocation strengthening were the main strengthening mechanisms.

Acknowledgements The authors gratefully acknowledge the financial support of National Key Research and Development Program of China (No. 2017YFA0403803), the National Natural Science Foundation of China (Nos. 51525401, 51774065, 51601028, 51690163), Dalian Support Plan for Innovation of High-level Talents (Top and Leading Talents, 2015R013), fundamental research funds for the central universities (Nos. DUT18RC(3)042, DUT17RC(3)108) and the Open Project of the State Key Laboratory of Rolling and Automation (No. 2017RALKFKT001).

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Fig. 1. SEM micrographs and XRD patterns of the as-received raw materials: (a) and (b) 6061Al powder; (c) and (d) B4C particles.

Fig. 2. OM (a–c) and SEM (d–f) images of as-rolled B4Cp/6061Al composites with various reinforcement contents: (a) and (d) 5 wt%; (b) and (e) 10 wt%; (c) and (f) 15 wt%.

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Fig. 3. TEM characterization of the interfaces in the as-rolled B4Cp/6061Al composites with different reinforcement contents: (a) 5 wt%; (b) 10 wt%; (c) 15 wt%; (d) HRTEM image of the rectangular area in (c); (e) FFT image of the square area in (d); (f) amorphous interface between the B4C particle and the matrix.

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Fig. 4. TEM images of dislocations and precipitates in the peak-aged 5 wt% B4Cp/6061Al composites: (a) dislocation cells and (b) Mg2Si precipitates near the interface; (c) dislocation lines and (d) Mg2Si precipitates in the Al matrix.

Fig. 5. Variation in the hardness and electrical conductivity as a function of aging time in the composites with different B4Cp contents: (a) hardness; (b) electrical conductivity.

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Fig. 6. (a) Representative engineering stress–strain curves of the peak-aged B4Cp/6061Al composites with various B4Cp contents; (b) strain hardening curves of the B4Cp/6061Al composites with 0 wt% and 15 wt% B4Cp.

Fig. 7. Comparison of the ultimate tensile strength and ductility of 6061Al reinforced by Al2O3, SiC, and B4C particles.

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Fig. 8. Fracture surfaces of the peak-aged B4Cp/6061Al composites with different B4Cp contents: (a) 0 wt%; (b) 5 wt%; (c) 10 wt%; (d) 15 wt%.

Fig. 9. (a) Calculated values of the yield strength increment due to the strengthening mechanisms of grain refinement, dislocation strengthening, and work hardening; (b) comparison of calculated and experimental values of the yield strength.

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