J. Cent. South Univ. (2021) 28: 2295-2306

DOI: https://doi.org/10.1007/s11771-021-4770-2

Synthesis of nano- to micrometer-sized B₄C particle-reinforced aluminum matrix composites via powder metallurgy and subsequent heat treatment

LIU Rui-feng(刘瑞峰)^{1, 2}, WANG Wen-xian(王文先)^{2, 3}, CHEN Hong-sheng(陈洪胜)^{2, 4}

1. College of Aeronautics and Astronautics, Taiyuan University of Technology, Taiyuan 030024, China;

2. Shanxi Key Laboratory of Advanced Magnesium-based Materials, Taiyuan 030024, China;

3. College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China;

4. College of Mechanical Engineering, Taiyuan University of Technology, Taiyuan 030024, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: B₄C/6061Al composites reinforced with nano- to micrometer-sized B₄C particles were fabricated via powder metallurgy route consisting of spark plasma sintering (SPS) and hot extrusion and rolling (HER), followed by T6 treatment. The microstructural evolution and mechanical properties were investigated. Results showed that the status of B₄C particles changed from a network after SPS to a dispersion distribution after HER. The substructured grains reached 66.5% owing to the pinning effect of nano-sized B₄C, and the grain size was refined from 3.12 μm to 1.56 μm after HER. After T6 treatment, dispersed Mg₂Si precipitated phases formed, and the grain size increased to 1.87 μm. Fine recrystallized grains around micro-sized B₄C were smaller than those in the areas with uniform distribution of nano-sized B₄C and Mg₂Si. The stress distributions of as-rolled and heated composites were similar, considering that the T6 heat treatment was only effective in eliminating the first internal stress. The Vickers, microhardness, and tensile strength of as-SPSed composites were greatly improved from HV 55.45, 0.86 GPa, and 180 MPa to HV 77.51, 1.08 GPa, and 310 MPa, respectively. Despite the precipitation strengthening, the corresponding values of as-heated composites decreased to HV 70.82, 0.85 GPa, and 230 MPa owing to grain coarsening.

Key words: nano- to micrometer-size; B₄C/6061 Al composites; T6 heat treatment

Cite this article as: LIU Rui-feng, WANG Wen-xian, CHEN Hong-sheng. Synthesis of nano- to micrometer-sized B₄C particle-reinforced aluminum matrix composites via powder metallurgy and subsequent heat treatment [J]. Journal of Central South University, 2021, 28(8): 2295-2306. DOI: https://doi.org/10.1007/s11771-021-4770-2.

1 Introduction

With excellent comprehensive mechanical properties, particle-reinforced Al matrix composites (PAMCs) have acquired wide attention in the defense and aerospace industries [1-4]. For the selection of the enhancement phase, most research has focused on micro-sized particles, but the strengthening effect of the addition of micro-sized particles is at the expense of plasticity. Nano-sized particles, with few surface defects and good wetting with alloy matrix, have attracted considerable attention from scientific researchers in recent years [5-7]. The agglomeration phenomenon leads to the limited volume fraction of nano-sized particles.

Uniformly dispersed nanoparticles are critical for enhancing the mechanical properties of nanocomposites. Therefore, PAMCs reinforced with nano- to-micrometer-sized particles are a promising path that can enhance the mechanical property and maintain excellent plasticity.Casting has been widely used to fabricate PAMCs [8, 9]. Meanwhile, hot isostatic pressing has also been reported [10]. Powder metallurgy [11] and spark plasma sintering (SPS) [12-14], with the advantages of uniform distribution of enhanced particles, low sintering temperature, and high pressure, have been recognized as perfect techniques in the fabrication of PAMCs.

Hot extrusion and rolling (HER) have been extensively used to improve particle distribution and enhance the interface bonding for PAMCs, especially for nanocomposites. T6 heat treatment (solid solution at 530 °C for 2 h and aging at 175 °C for 6.5 h in this work) is also considered an effective method to strengthen nanocomposites [15-17]. CASATI et al [16] fabricated SiC and oxide nanoparticle-reinforced 2618Al matrix composites and demonstrated that ball milling led to the formation of crystal defects, which in turn extend the solid solution and decrease the driving force for aging. MIRJAVADI et al [17] prepared a graphene nanoplatelet-reinforced Al-10Zn-3.5Mg-2.5Cu nanocomposite by casting and hot extrusion, followed by T6 treatment. The nanocomposite strength was greatly improved after hot extrusion and T6 treatment due to precipitation hardening. The purpose of this study is to prepare aluminum matrix composites with excellent strength and plasticity through the combination of advanced powder metallurgy route and heat treatment.

In this study, the effects of HER and T6 heat treatment on the microstructure and mechanical property of nano- to micrometer-sized B₄C particle-reinforced 6061Al matrix composites were investigated. The distribution of B₄C and precipitated phases, recrystallization behavior, nanoindentation, and tensile properties were examined systematically. The fracture failure mechanism was analyzed, and the theoretical yield strength (YS) was calculated for comparison with the experimental YS.

2 Experimental procedure

The 6061Al alloy powders used in this work had an average size of 13 μm, and the average size of nano-to-micrometer-sized B₄C particles was set to 50 nm. The chemical compositions are shown in Tables 1 and 2. First, 6061Al powders and B₄C particles were subjected to high-energy milling (HEM) at a speed of 1400 r/min for 12 h. Second, the remaining 6061Al powders and mixed powder produced by HEM were subjected to low-energy milling (LEM) for 2 h at a speed of 200 r/min. The protection of argon gas was applied to avoid oxidation in the HEM and LEM processes.

Table 1 Chemical composition of 6061Al alloy (wt%)

Table 2 Chemical composition of B₄C (wt%)

After the ball milling process, a vacuum glove box (O₂ content<1×10^-6) was used when the blended powder was placed into graphite dies. Then, DR.SINTER-type SPS331-Lx equipment with a pulse duration of 40 ms and a vacuum chamber was employed in the sintering process. The vacuum pump was in continuous operation. The vacuum was maintained under ~20 Pa, and all gases were pumped out using the vacuum pump during SPS. The sintering temperature was 570 °C, which was considerably lower than the melting point of 6061Al alloy powders (660 °C) and B₄C particles (2350 °C). The final dimension of as-SPSed billets was 30 mm in height and 30 mm in diameter. For improving the distribution of nano-sized B₄C particles and mechanical property, hot extrusion at 500 °C with an extrusion ratio of 16:1 was applied to the as-SPSed samples. The obtained as-extruded samples were 15 mm in width and 3 mm in thickness. After extrusion, the as-extruded samples were rolled at 500 °C with a speed of 20 mm/s, from 3 mm to 1.5 mm in three passes. Lastly, a solid solution treatment at 530 °C for 2 h and aging at 175 °C for 6.5 h, named T6 heat treatment, were applied to modify the B₄C/6061Al composites under a vacuum atmosphere. Figure 1 shows the process diagram.

Microstructural characterization was achieved using a scanning electron microscope (JSM-6700F, equipped with EDS and EBSD system). All the samples were characterized without any coating. The phases were tested by X-ray diffraction (XRD). The nanoindentation properties were characterized by nanoindentation test (Agilent G200) with a load of 30 mN. All loading-unloading processes were repeated 3 times for an accurate characterization.

Figure 1 Process diagram of whole process

The tensile-testing specimens were cut in the direction parallel to the HER direction, and the tensile test was conducted by employing an Instron Series 5569 test machine at room temperature with a speed of 0.2 mm/min.

3 Results and discussion

3.1 Relative density

Table 3 shows the measured and theoretical density values of the B₄C/6061Al composites. The relative density of B₄C/6061Al composites decreases with increasing B₄C volume fraction, and the values of as-SPSed composites are all above 98.2%. For composites with the same volume fraction, the relative density of B₄C/6061Al composites is higher than that of as-SPSed composites after extrusion and rolling deformation. Almost completely dense composites are obtained. After T6 heat treatment, the relative density of composites does not change remarkably.

Table 3 Measured and theoretical density values of B₄C/6061Al composites with different volume fractions

3.2 Microstructure

Figure 2 presents the raw and mixed powders after the ball milling process. As shown, the maximum size of the micro-sized B₄C particles is approximately 3 μm. The nano-sized B₄C particles are uniformly located on the matrix particle surface (Figure 2(c)) after ball milling, which can be attributed to the selection of a micro-sized Al matrix and nano- to micrometer-sized B₄C particles.Figure 2(d) shows the high magnification of Figure 2(c). The nano-sized B₄C particles are embedded into soft matrix particles, and cold welding exists at the Al/ B₄C interface under the tremendous energy output. Nano-sized B₄C distributes evenly after HEM and LEM, as confirmed in Figure 2(f).

Figure 3 shows the EBSD image, EDS, and inverse pole figure of B₄C/6061Al composites at three statuses. Nano-sized B₄C particles mainly distribute at the Al particle boundary for as-SPSed composites (≈network reinforcement architecture) [18, 19]. For as-rolled composites (Figure 3(b)), the nano-sized B₄C particles are redistributed under substantial plastic deformation. After T6 heat treatment, the distribution of nano-sized B₄C remains the same as that of the as-rolled composites, while some precipitated phases appear. The EDS result shows that the precipitated phase in Figure 3(d) is an intermetallic mainly composed of Al, Si, and Mg, which seems to form during the aging treatment, as shown in Figure 3(e). In sum, the atomic ratio shown in Figure 3(e) can prove that the white-like precipitated phases are Mg₂Si. Mg and Si are the main alloy elements, and Mg₂Si phase will precipitate for 6061Al alloys after T6 heat treatment. This condition has been confirmed by previous works [3, 20]. As shown in Figures 3(f)-(h), the grain orientation with a random distribution for as-SPSed composites changes to the majority of {101} and {111} orientations along with HER and T6 treatment, indicating the appearance of an intense texture orientation.

Figure 2 Morphology of raw powders:

Figure 3 Microstructure and EDS of as-SPSed, as-rolled and as-heated B₄C/6061Al composites:

Figure 4 shows the XRD results of B₄C/6061Al composites at three statuses. Only the Al phase is detected in every status. No B₄C and Mg₂Si are tested due to the low quantity of B₄C particles and limited precipitation. Low-volume B₄C and precipitated phases make them difficult to detect; similar results have been found in our previous work [12].

Figure 5 shows the high-angle grain boundary (HAGB>15°, black), low-angle grain boundary (LAGB<15°, green), and distinguished recrystallized (blue), substructured (yellow), and deformed grains (red) of B₄C/6061Al composites at three statuses. For the as-SPSed composites in Figure 5(a), HAGBs occupy the majority, and the average misorientation angle reaches 29.34°. Considerable plastic deformation will introduce substantial subgrains with an extremely small misorientation, and then LAGBs will transform to HAGBs [21]. After HER, the fraction of LAGBs is calculated to be 30%, and the average misorientation angle decreases to 9.56°. In this condition, the dislocation tangles through the pinning effect of nano-sized B₄C form substructures [22]. As shown in Figure 5(b), the fraction of substructured grains increases to 67.8%.

Figure 4 XRD patterns of as-SPSed, as-rolled and as-heated B₄C/6061Al composites

Figure 5 EBSD results of as-SPSed, as-rolled and as-heated B₄C/6061Al composites:

Corrected curve means that misorientations are calculated from neighboring points, and uncorrected curve implies that misorientations are calculated from random points. Random curve stands for the random distribution of misorientation with no preferential orientation. For the as-rolled composites in Figure 5(b), “Uncorrelated” curve shows misfit between 10°-25° and 35°-50° compared with “Random” curve. For PAMCs, the free energy of the Al matrix will be improved, and the nonuniform nucleation will be stimulated when the area around micro-sized particles is full of dislocations [23, 24], named particle-stimulated nucleation (PSN). The density of dislocation in the area with uniformly distributed nano-sized B₄C particles remains or even increases after solid solution treatment and aging. Two factors cause this condition: 1) the generated dislocation will increase the nucleation rate, which will lead to the formation of a dispersed precipitated phase; 2) the evident pinning effect of small precipitated phases and nano-sized B₄C leads to dislocation entanglement [12, 25]. As shown in Figure 5c, the fraction of substructured grains is kept at 66.5% after T6 treatment given only partial recrystallization around micro-sized B₄C particles. The average grain size increases from 1.56 to 1.87 μm during the process of T6 treatment, even if partial grain refinement occurs around micro-sized B₄C particles. In general, a large number of dislocations are retained after T6 heat treatment, which will increase the nucleation rate of the precipitated phase during the process of heat treatment. Substantial phases are precipitated, as shown in Figure 3. Overall, recrystallization behavior occurs around micro-sized B₄C particles, and recovery behavior is the main softening mechanism in the area with dispersed nano-sized B₄C particles in the process of T6 treatment.

Figures 6(a)-(c) show the measured KAM maps of B₄C/6061Al composites at three statuses. As shown, the as-SPSed composites own the lowest stress compared with the as-rolled and as-heated composites. As HER proceeds, the average angle shows an increasing trend, and the stress increases remarkably, especially for the fine-grain zone. Generally, T6 heat treatment is only effective in eliminating the first internal stress, and the lattice distortion caused by numerous defects in the HER deformation process remains. Therefore, the stress distribution after heat treatment is similar to that of the as-rolled composites. Figures 6(d)-(f) exhibit the local strain distribution of the B₄C/6061Al composites. Heterogeneous distribution of strain and component gradient is evidently shown, and the fine-grain region has the maximum strain distribution at three different stages. Figure 7 shows the schematic of the microstructural evolution of the B₄C/6061Al composites under different statuses.

Figure 6 Measured KAM maps and local strain distribution of B₄C/6061Al composites:

Figure 7 Schematic diagram of microstructure evolution of B₄C/6061Al composites:

3.3 Mechanical properties

Microstructural evolution must lead to the corresponding changes in the mechanical properties of PAMCs. Figure 8 shows the load-displacement curve, microhardness, and elasticity modulus results of B₄C/6061Al composites at three statuses. Under the same load of 30 mN, the indentation depth first decreases and then increases as the hot rolling and T6 heat treatment proceed (Figure 8(a)). That is, the microhardness should first increase and then decrease, which is consistent with the results in Figure 8(d). The reasons for the increase in Vickers hardness and microhardness after HER are grain refinement and dislocation strengthening. For as-heated composites, the release of stress and grain coarsening result in decreases in Vickers hardness and microhardness even with the formation of a dispersed precipitated phase. The slope of the unloading curve represents the elasticity modulus, as shown in Figure 8(e). The unloading curves are becoming steeper as the hot rolling and T6 heat treatment proceed, which means that the elasticity modulus shows an increasing trend (Figure 8(e)).

The deformation zone around micro-sized B₄C particles results from the recrystallization during T6 heat treatment. Figure 8(b) shows the loading-unloading curves located at different distances from micro-sized B₄C particles. The indentation depth decreases with increasing distance from micro-sized B₄C particles. The loading-unloading curve of the deformation zone (1.5 μm) almost coincides with the curve obtained by the matrix. That is, the width of the particle deformation zone ranges between 1.0 and 1.5 μm.

Figure 9 shows the tensile stress-strain curves of B₄C/6061Al composites at three statuses. For the composites fabricated by SPS, the YS (0.2% proof stress) and ultimate tensile strength (UTS) are only 104 and 180 MPa, respectively. Nano-sized B₄C particles gather at the particle boundary of matrix particles, which makes the particle boundary a weak point in the process of tensile test. After extrusion, fine-grain strengthening enhances the tensile strength, YS, and elongation of the composites. In the rolling process, YS and UTS increase to 211 MPa and 310 MPa, respectively, accompanied by a slight decrease in the elongation to fracture from 14% to 12.5%. The following factors cause the change: 1) the distribution of nano-sized B4C changes from the aggregation at the boundary of matrix particles to a uniform distribution at the Al matrix after HER, which is vital to the strengthening of nano-sized particle-reinforced metal matrix composites; 2) HER refines the Al grains effectively (from 3.12 to 1.56 μm), and the contribution of fine-grain strengthening accounts for the majority compared with other strengthening mechanisms, which has been confirmed in our previous study [7]. At the same time, the fine matrix grains promote rapid strain hardening at the beginning of the tensile test. Thus, a high UTS is obtained, as shown in the curve of as-rolled composites in Figure 9(a); 3) a mass of dislocations are accumulated after HER, especially for the condition of the pinning effect of nano-sized B₄C particles.

T6 treatment has a remarkable effect on the strength and plasticity of composites. YS and UTS are decreased to 93 and 198 MPa, respectively, whereas the elongation to fracture increases from 12.5% to 15.3% for as-heated composites (solution treatment). After aging treatment, YS and UTS increase to 170 and 230 MPa, respectively, whereas the elongation to fracture decreases to 12.5% owing to the appearance of Mg₂Si precipitated phases. For the nanocomposites, grain boundary strengthening is critical for enhancing YS. That is, the increase in YS by using a dispersive Mg₂Si phase is inadequate to compensate for the decrease in grain coarsening. A similar phenomenon in ductility increase of an Al-Mg-Si alloy fabricated by ECAP, followed by artificial aging, was also found, and the annihilation of dislocation after heat treatment was the main factor [26]. HU et al [27] fabricated TiN/Al nanocomposites by an ultrasonic-assisted casting method, followed by T6 treatment, and the tensile strength and elongation were 235 MPa and 9.5%, respectively. SHRIVASTAVA et al [28] fabricated Al6061-Al₂O₃ nanocomposites by a powder metallurgy route, followed by a solution heat treatment and aging. The results showed that the maximum tensile strength and elongation were 100 MPa and 5.2% for 2 wt.% nanocomposites, respectively. The hardness results indicated that a solution heat treatment followed by aging of composite samples was highly beneficial to the increase in hardness.

Figure 8 Nanoindentation test results of B₄C/6061Al composites:

Figure 9 Tensile stress-strain curves of as-SPSed, as-extruded, as-rolled and as-heated B₄C/6061Al composites

Figure 10 shows the fracture morphology of B₄C/6061Al composites at three statuses. A slice of dimples exists in the matrix after SPS. Meanwhile, entirely whole Al particles, indicated by the yellow arrows, are exposed. The observation of crack in Figure 10(a) demonstrates that the Al particle boundaries are the main crack propagation path. After HER, the as-rolled composites exhibit obvious dimples in the range of 1-3 μm, with severe tearing of the matrix. In Figure 10(c), the tensile fracture is an obvious ductile morphology, with dimples in the range of 1-4 μm, that is, the T6 treatment effectively maintains the plasticity of the B₄C/6061Al composites, which is consistent with the tensile stress-strain curve. Figure 10(d) shows the highly magnified image of the fracture morphology of as-heated composites. The micro-sized B₄C particles are well bonded to the Al matrix, and the crack propagation can be effectively hindered. All the above mentioned factors contribute to the high ductility of as-heated composites. As shown in Figure 10(d), the particle/matrix interface bonding is well, with matrix fracture preferentially compared with the interface during tensile deformation, which can improve the composite strength.

3.4 Strengthening mechanisms

Several strengthening mechanisms, including load transfer, dislocation, Orowan, and fine-grain strengthening, are working together to improve the mechanical properties of PAMCs.

1) Well interface bonding at the atomic level between nano-sized B₄C and Al matrix can bear the tensile load effectively. The load will be transferred from the matrix to the reinforcement through the interface shear, and the increase in YS (σ_load) is shown by the following equation [29]:

△σ_load=0.5σ_m                                              (1)

2) The remarkable difference in the coefficient of thermal expansion between the metal matrix and ceramic particle will generate large thermal mismatch strain at the interface during cooling, and the release of thermal mismatch strain is achieved by releasing the dislocation ring. Therefore, high-density dislocations can strengthen the Al matrix. The increase in YS (σ_dis) owing to the dislocation strengthening is characterized by the following equation [30]:

                     (2)

3) Two paths can be utilized to overcome the obstacle when dislocations interact with particles, that is, dislocation goes straight through the particles or bypassing the particles and continues to move. As for nano-sized particles and the Mg₂Si precipitated phase, the resistance on the dislocation line makes it difficult to go straight through the particles, which can only be achieved via a bypassing way, which is also called Orowan strengthening. The dislocation ring released by the dislocation line will increase the dislocation density and improve the strength, and the increase in YS (σ_Orowan) can be characterized using the following equation [31]:

                      (3)

                        (4)

4) Recrystallization occurs during hot deformation. The formed HAGBs are pinned by the nano-sized B₄C particles and dispersed Mg₂Si phase, and the growth of recrystallized grains is restrained effectively, thereby obtaining fine grains in the composite. The increase in YS (σ_gb) due to fine-grain strengthening can be calculated using the following equation [32]:

                               (5)

Table 4 lists the parameters for the calculation of the theoretical YS of as-heated composites. In summary, the increase in YS can be attributed to the coupling functions of the four strengthening mechanisms listed above. The quadratic sum of the four strengthening mechanisms is always used to calculate the theoretical YS [33]. On the basis of the calculated results in every single strengthening mechanism, the increase in YS through theoretical calculation and experimental measurement is listed in Table 5. △σ_Total (measured) can be calculated by comparing with the yield stress of the 6061Al matrix. The increment in YS via theoretical calculation is almost consistent with the experimental measurement, and only 13.41 MPa deviation exists.

 (6)

Figure 10 Fracture surface of B₄C/6061Al composites:

Table 4 Values of parameters for calculating theoretical σ_YS of 3% as-heated B₄C/6061Alcomposites

Table 5 Calculated contribution of different strengthening mechanisms to σ_YS of 3% as-heated B₄C/6061Al composites

4 Conclusions

The effects of T6 heat treatment and HER on the microstructure and mechanical properties of nano- to micrometer-sized B₄C PAMCs were investigated. The main conclusions are drawn as follows:

1) Nano-sized B₄C is embedded onto the surface of 6061 matrix particles after ball milling and mainly distributes at the connection boundary of the 6061 matrix particles after SPS. Uniform distribution of nano-sized B₄C particles is achieved after HER. The Mg₂Si phase is precipitated after T6 heat treatment.

2) The microstructure of as-SPSed composites is dominated by recrystallized grains. After HER, substructured grains with LAGBs account for the majority due to the pinning effect of nano-sized B₄C. The substructured grains remain and the average grain size increases from 1.56 to 1.87 μm, although fine recrystallized grains caused by PSN can be observed around the micro-sized B₄C particles after T6 heat treatment.

3) The Vickers, microhardness, and tensile strength of as-rolled composites are increased to HV 77.51, 1.08 GPa, and 310 MPa, respectively, owing to the fine-grain and dislocation strengthening effects. The corresponding mechanical properties of as-heated composites decrease to HV 70.82, 0.85 GPa, and 230 MPa owing to grain coarsening.

4) The calculated YS agrees well with the experimental YS for the nano- to micrometer-sized B₄C PAMCs produced by powder metallurgy, including SPS, HER, and T6 heat treatment.

Contributors

LIU Rui-feng provided the concept and edited the draft of manuscript. WANG Wen-xian conducted the literature review and wrote the first draft of the manuscript. CHEN Hong-sheng edited the draft of manuscript.

Conflict of interest

LIU Rui-feng, WANG Wen-xian and CHEN Hong-sheng declare that they have no conflict of interest.

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(Edited by YANG Hua)

中文导读

微/纳双尺寸B₄C颗粒增强铝基复合材料的粉末冶金合成及后续热处理

摘要：采用放电等离子烧结(SPS)、热挤压+轧制(HER)、粉末冶金工艺及后续T6热处理制备了微/纳双尺寸B₄C颗粒增强铝基复合材料。研究了制备过程中复合材料的显微组织演变和力学性能。结果表明：B₄C颗粒分布由SPS状态的网状分布转变为挤压轧制态的均匀分布，平均晶粒尺寸由烧结态的3.12 μm细化至挤压轧制态的1.56 μm。由于纳米B₄C的钉扎作用，亚结构比例达到66.5%。T6热处理后，材料内部析出弥散分布的Mg₂Si。微米B₄C周围分布明显的动态再结晶晶粒，且晶粒尺寸远小于弥散分布Mg₂Si的基体区域。由于T6热处理只能有效地消除第一内应力，因此轧制态和热处理态复合材料的应力分布相似。烧结态复合材料的维氏硬度、显微硬度和抗拉强度分别从HV 55.45、0.86 GPa和180 MPa提高到HV 77.51、1.08 GPa和310 MPa。T6热处理后，尽管内部存在着析出强化，但由于晶粒粗化，复合材料的析出强度分别为HV 70.82、0.85 GPa和230 MPa。

关键词：微/纳双尺寸；B₄C/6061 Al复合材料；T6热处理

Foundation item: Projects(51775366, 51805358) supported by the National Natural Science Foundation of China; Project(20130321024) supported by the Key Science and Technology Program of Shanxi Province, China

Received date: 2020-10-09; Accepted date: 2020-12-21

Corresponding author: WANG Wen-xian, PhD, Professor; Tel: +86-351-6010076; E-mail: wangwenxian@tyut.edu.cn; ORCID: https://orcid.org/0000-0003-0688-767X