【Article source】
Special Casting & Non-ferrous Alloys, Vol. 44, No. 4, 2024
【Citation Format】
Effect of Single/Dual SiCp on Microstructure and Properties of AZ91D Magnesium Alloy[J]. Special Casting & Nonferrous Alloys,2024,44(4):495-499.
MA Z X，YOU Z Y，JIANG A X，et al. Effects of single/double Size SiCp on microstructure and properties of AZ91D alloy［J］. Special Casting & Nonferrous Alloys，2024，44（4）：495-499.
Guide
Single- and double-size SiC particle-reinforced magnesium matrix composites were prepared by semi-solid mechanical stirring using AZ91D magnesium alloy and SiC particles with average particle sizes of 10 μm and 10 nm as matrix and reinforcing phases, respectively. The results show that the tensile strength of the 10 nm SiCp/AZ91D composites with a volume fraction of 2% SiCp reaches 198 MPa, an increase of 34.7%, the yield strength reaches 113 MPa, an increase of 46.7%, and the elongation reaches 6.4%, which is mainly due to the grain refinement of nano-SiC particles. The fracture mechanism shows that the cracks of SiCp/AZ91D composites mainly propagate along the micron SiCp-AZ91D interface.
【Background】
With the improvement of energy saving, emission reduction and environmental protection requirements, the application of magnesium and its alloys has received extensive attention, but the strength of magnesium alloys is low, which limits its application range. SiC particles have the characteristics of low coefficient of thermal expansion, good wear resistance, high hardness and oxidation resistance, and will not form stable compounds with Mg. SiCp is combined with AZ91D magnesium alloy to ensure that magnesium alloy has the advantage of light weight, while also improving its strength and wear resistance. The composite materials prepared by the two have been used in the preparation of propellers, missile tails and cylinders, etc., and are one of the most widely used magnesium matrix composites. Particle-reinforced metal matrix composites can obtain better mechanical properties, such as increasing particle content, changing particle size, etc.
【Research Highlights】
In this study, AZ91D magnesium alloy was used as the matrix material, and different sizes and double-size SiC particles were used as the reinforcing phase, and the semi-solid mechanical stirring method was used to prepare magnesium matrix composites, and the effects of single/double size SiC particles on the microstructure and mechanical properties of metal matrix composites and the reasons for their strengthening were studied, aiming to provide reference for its application.
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【Methodology】
The chemical composition of AZ91D magnesium alloy is shown in Table 1, according to the binary phase diagram of Mg-Al alloy, the theoretical solidus temperature of AZ91D alloy is 468 °C, the liquidus temperature is about 596 °C, the solidification temperature range is about 128 °C, the tensile strength is 147 MPa, and the yield strength is 77 MPa, which is suitable for semi-solid forming preparation.
The enhanced phase was SiC particles, with average particle sizes of 10 nm and 10 μm, respectively, and the particle morphology is shown in Figure 1. Because the surface of SiC particles is easy to adsorb impurities such as water and oil, which will have a great impact on the properties of the prepared composite materials, it is necessary to pretreat the SiC particles before the test. Firstly, the SiC particles were loaded into a beaker, and the surface impurities were removed with ethanol, and then they were put into a crucible after repeated washing and drying, and preheated at a high temperature of 600 °C for 3 h to improve the dispersion and interfacial bonding of SiC particles, eliminate the agglomeration of SiC particles during mechanical stirring, and make them more evenly dispersed into the magnesium alloy melt.
Fig.1 Surface morphology of SiC particles
In this experiment, three composites were prepared by semi-solid mechanical stirring method: single-size 10 μm/2% SiCp/AZ91D(S1), double-size 10 μm SiC, 10 nm SiC with a ratio of 9∶1, total 2% (S9+M1), and single-size 10 nm/2% SiCp/AZ91D(M1). Wrap SiC into small balls with aluminum foil and put them into a crucible, and preheat them in a resistance furnace at 600 °C for 30 min. The surface of AZ91D magnesium alloy was polished to remove the oxide layer and then put into a crucible, and a mixture of CO2 and SF6 (the volume ratio of CO2 and SF6 was 10:1) was introduced and heated to 720 °C for 30 min. When the temperature dropped to 680 °C, SiC particles preheated to 600 °C were added and kept warm for 10 min. When the temperature continued to drop to 585 °C, semi-solid stirring was carried out for 30 min and 400 r/min, and then the temperature was raised to 720 °C and kept warm for 10 min. The semi-solid blank was obtained by cooling to 595 °C near the liquidus temperature for 30 min, and finally poured into the mold preheated to 300 °C to obtain the semi-solid SiCp/AZ91D composite.
The warp cutting process makes the material into a rectangular standard tensile specimen, the dimensions of which are shown in Figure 2. The tensile test of SiCp/AZ91D composite specimens was carried out by a UTM5105 universal testing machine with a loading speed of 1 mm/min, and the average of the three measurements was taken. The tensile strength, yield strength and elongation of the composite material are determined by the recorded tensile test data. The morphology and distribution of SiC particles of different sizes were observed by ZEM20 scanning electron microscope and Talos F200X TEM, the average grain size of the material was determined, the microstructure was analyzed by EDS spectroscopy, and the composition of the composite phase was determined by Bruker D8 ADVANCE X-ray diffractometer.
Fig.2. Tensile specimen size
[Graphic Analysis]
Figure 3 shows the microstructure of AZ91D and SiCp/AZ91D composites of different sizes. It can be seen that the grains in the SiCp/AZ91D composites are refined after the addition of SiC particles, and most of the SiC particles are segregated along the grain boundaries, mainly because the microstructure of the composites is determined by the melting and solidification stages. At the same time, due to the pushing effect of the liquid/solid interface, the SiC particles are redistributed during the solidification process, and finally the SiC particles tend to gather in the grain boundary region, and the pinning effect of the SiC particles located at the grain boundaries can limit the grain growth of the matrix material and refine the grains of the composites. In addition, nano-SiC particles can provide high-energy interfaces, which are conducive to nucleation of new grains. As can be seen from Fig. 3b and Fig. 3d, the distribution of SiC particles in the 10 μm SiCp/AZ91D composites and the double-size SiCp/AZ91D composites is relatively uniform, and there is no obvious agglomeration of SiC particles. From Fig. 3c, it is found that most of the SiC particles in the 10 nm SiCp/AZ91D composite are relatively uniformly distributed, and some SiC particles are agglomerate, which may be due to the fact that the surface action energy of the nano-SiC particles decreases after agglomeration to a certain size, and it is difficult to disperse uniformly under the action of stirring force. Therefore, it is necessary to further improve the dispersion of nano-SiC particles.
Fig.3 Microstructure of AZ91D and SiCp/AZ91D composites of different sizes
Image Pro-Plus 6.0 software was used to measure the grain size of the corroded composites, as shown in Figure 4, and the statistical results were consistent with the changes in the optical microstructure in Figure 3. Compared with the matrix alloy, the grain in the micro, dual-size and nano-SiCp/AZ91D composites decreased by 28.1%, 39.8% and 49.1%, respectively. XRD analysis of the dual-size SiCp/AZ91D composites is shown in Figure 5. It can be seen that the composition of the two-size SiCp/AZ91D composite is mainly α-Mg and the second phase β-Mg17Al12.
Fig.4. Grain sizes of SiCp/AZ91D composites of different sizes
Fig.5. XRD pattern of dual-size SiCp/AZ91D composites
Figure 6 shows the SEM microstructure and composition analysis of the dual-size SiCp/AZ91 composite. It can be found that the SiC particles are distributed on the grain boundaries of the composite, as shown in Figure 6a. According to the molar ratios of Mg and Al in Figure 6f and Figure 6g, it can be determined that point A is the α-Mg phase, point B is the β-Mg17Al12 phase, and β-Mg17Al12 is distributed irregularly at the grain boundaries in a reticulated pattern. Compared with the AZ91D alloy, the β-Mg17Al12 phase in the SiCp/AZ91D composites becomes discontinuous and refined, changing from massive to sheety, and extends from grain boundaries to the intragranular region and is embedded in the α-Mg matrix, as shown in Fig. 6c and Fig. 6d. Figures 6e and 6h also further confirm that most of the nanoSiC particles are distributed around the micron SiC particles in clusters, which may be caused by adhesion between particles or particle interlocking.
Fig.6. SEM micrographs and composition analysis of dual-size SiCp/AZ91 composites
Fig.7. TEM micrograph of a dual-size SiCp/AZ91 composite
Figure 7 is a TEM image of a dual-size SiCp/AZ91 composite. It can be seen that there is a clean surface between the micron SiC particles and the Mg matrix, and no pores and interfacial reaction products are found at the interface. The nano-SiC particles are distributed around the micro-SiC particles, which is consistent with the SEM observation, and no agglomeration of the nano-SiC particles is observed in the region near the micro-SiC particles, indicating that the nano-SiC particles bind well to the matrix in the composite. As can be seen from Figure 7d, the nano-SiC particles belong to the β-SiCp with a face-centered cubic structure. However, the [011-0] region axis can be observed as a α-SiCp with a hexagonal structure, as shown in Figure 7e.
Fig. 8 shows the effect of AZ91D alloy and SiC particles of different sizes on the mechanical properties of the composites at room temperature. It can be seen that the mechanical properties of the composites are improved to varying degrees compared with the AZ91D matrix due to the addition of SiCp of different sizes, and the reasons include: (1) The grain size of fine-grained reinforcement will significantly affect the yield strength of the composites, as shown in Fig. 3. The yield strength of the composites increases with the decrease of grain size, and the yield strength of 10 nm SiCp/AZ91D composites is 46.7% higher than that of AZ91D matrix alloy. The addition of SiC particles improves the toughness of the composites, and the elongation of the composites is increased to varying degrees compared with the AZ91D matrix alloy, which is mainly attributed to the grain refinement of the SiC particles. (2) Dispersion strengthening, although the formation of β-Mg17Al12 brittle phase at the grain boundary of α-Mg in the composite material will reduce the tensile strength of the alloy, but the addition of SiC plays a role in hindering the dislocation movement, making the β-Mg17Al12 in the composite material discontinuous and fine, see Fig. 5f, which will help to improve the tensile strength. (3) Load transfer, the interface between SiC particles and ɑ-Mg is well combined, and the harder SiC particles can protect the softer ɑ-Mg inside during the tensile process, which plays a role in load transfer, thereby further improving the tensile strength, as shown in Figure 8. (4) Due to the difference in thermal expansion coefficients between micron SiC and α-Mg matrix, the dislocation density increases, and the contribution of the presence of micron-sized SiC particles to the improvement of strength is mainly attributed to the thermal mismatch strengthening and fine-grained strengthening, and the pinning effect of nano-SiC particles on the grain is more significant and easier to distribute inside the grain. Therefore, nano-SiC particles have better Orowan strengthening and fine-grained strengthening than micro-SiC particles, and the synergistic effect of dual-size SiC particles makes the yield strength, tensile strength and elongation of dual-size composites mixed with a small number of nano and micron SiC particles increased by 15.7%, 18.3% and 50.0%, respectively, compared with single-size micron SiCp/AZ91D composites.
Fig.8. Mechanical properties of AZ91D and SiCp/AZ91D composites
The effect of different sizes of SiC particles on the hardness of AZ91D alloy is shown in Figure 9. It can be seen that the hardness of composites with different sizes of SiC particles is greatly improved compared with AZ91D alloy. The microhardness (HV) of the matrix alloy is 68, and when 10 nm SiC particles are added, the microhardness of the composite is 112, which is increased by 64.7%. The reason is that the fine-grained strengthening effect of SiC particles and their fine β-Mg17Al12 form a stress field with the matrix, which can resist the pressure on the surface, so that the atoms on the crystal lattice are not easy to shift when subjected to pressure, and the hardness increases macroscopically. Combined with the XRD analysis in Figure 5, the hard-brittle phase Mg2Si generated further improves the hardness of the composites.
Fig.9. Average hardness of AZ91D and SiCp/AZ91D composites
Figure 10 shows the SEM and TEM microstructures of a two-size composite material after tensile testing. It can be seen that the fracture mainly occurs at the interface between the micron SiC particle enrichment region and the AZ91D matrix, because the SiC particles can withstand higher external loads compared with the AZ91D matrix, and during the tensile process, the matrix near the interface of the SiC-AZ91D is prone to stress concentration, resulting in dislocation activation and accumulation around the micron and nano SiC particles. There is a high-density dislocation between the micron SiC particles and the AZ91D matrix, as shown in Figure 10b. In addition, a large number of deformed twins were observed in the vicinity of the micron SiC particles, see Figure 10c, indicating that high-density dislocations were also present in the twins. There are also high-density dislocations and dislocation accumulation around nano-SiC particles, as shown in Fig. 10d.
Fig.10 SEM and TEM microstructures of double-size composites after tensile testing
Fig. 11 shows the TEM interface morphology of SiC particles and AZ91D matrix in dual-size composites. It can be found that cracks appear around some micron SiC particles, while the interface between the nano-SiCp and AZ91D substrates is well bonded and no cracks appear. Because the resistance of SiC particles to the plastic deformation of AZ91D matrix during the tensile process depends on the particle size, the ability of micron SiC particles to resist plastic deformation is higher than that of nano SiC particles, which leads to the stress concentration at the interface between micron SiC particles and AZ91D matrix, the uneven deformation of SiCp/AZ91 composites during the tensile process, and the edge of some micron particles may produce large stress concentrations, when the stress is greater than the interface bonding strength, Cracks occur at the interface between the AZ91D matrix and the micron SiC particles. As the particle size decreases from microns to nanometers, the surface defects of the particles are reduced, resulting in high interfacial bonding strength. On the other hand, the ability to resist plastic deformation is weakened, so the stress concentration is small, and the interface between nano-SiCp and AZ91D matrix is well combined, and it is not easy to crack.
Fig.11. TEM morphology of SiC particles and AZ91D matrix in dual-size composites
【Main conclusions】
(1) The microscopic morphology of single and double size SiCp/AZ91D prepared by semi-solid mechanical stirring method shows that the addition of microparticle and nanoparticle SiC particles can effectively reduce the grain size, and the β-Mg17Al12 phase in the composite material becomes discontinuous and refined.
(2) The mechanical properties of SiCp/AZ91D composites at room temperature are better than those of AZ91D matrix alloys. Among them, the tensile strength, yield strength, elongation and hardness of SiCp/AZ91D with the highest mechanical properties of 10 nm are increased by 34.7%, 46.7%, 200% and 64.7%, respectively, compared with AZ91D, and the improvement of strength is mainly attributed to the fine-grained strengthening effect of SiC particles.
(3) During the tensile process, the cracks of SiCp/AZ91D composites mainly propagate along the interface of micron SiCp-AZ91D, while no microcracks appear around the nano-SiC particles.