Microstructure and strengthening mechanism of bimodal size particle reinforced magnesium matrix composite

One kind of (submicron + micron) bimodal size SiCp/AZ91 composite was fabricated by the stir casting technology. After hot deformation process, the influence of bimodal size particles on microstructures and mechanical properties of AZ91 matrix was investigated by comparing with monolithic A91 alloy, submicron SiCp/AZ91 and micron SiCp/AZ91 composites. The results show that micron particles can stimulate dynamic recrystallized nucleation, while submicron particles may pin grain boundaries during the hot deformation process, which results in a significant grain refinement of AZ91 matrix. Compared to submicron particles, micron particles are more conducive to grain refinement through stimulating the dynamic recrystallized nucleation. Besides, the yield strength of bimodal size SiCp/AZ91 composite is higher than that of single-size particle reinforced composites. Among the strengthening mechanisms of bimodal size particle reinforced composite, it is found that grain refinement and dislocation strengthening mechanism play a larger role on improving the yield strength.     

Effect of submicron size SiC particles on microstructure and mechanical properties of AZ31B magnesium matrix composites

The magnesium matrix composites reinforced with three volume fractions (3, 5 and 10 vol.%) of submicron-SiC particles (∼0.5 μm) were fabricated by semisolid stirring assisted ultrasonic vibration method. With increasing the volume fraction of the submicron SiC particles (SiCp), the grain size of matrix in the SiCp/AZ31B composites was gradually decreased. Most of the submicron SiC particles exhibited homogeneous distribution in the SiCp/AZ31B composites. The ultimate tensile strength and yield strength of the 10 vol.% SiCp/AZ31B composites were simultaneously improved. The study of interface between the submicron SiCp and the matrix in the SiCp/AZ31B composite suggested that submicron SiCp bonded well with the matrix without interfacial activity.     

Microstructure and mechanical properties of SiC nanoparticles reinforced magnesium matrix composites fabricated by ultrasonic vibration

A particulate reinforced magnesium matrix nanocomposite was fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, grain size of matrix in the SiCp/AZ91 nanocomposite decreased and morphology of phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates. Although there were still some SiC microclusters in the nanocomposite, most of the SiC nanoparticles were dispersed well outside the microclusters. The ultimate tensile strength, yield strength and elongation to fracture of the SiCp/AZ91 nanocomposite were simultaneously enhanced compared with that of the AZ91 alloy. The study of interface between the SiC nanoparticles and the matrix in the nanocomposite suggested that SiC nanoparticles bonded well with the matrix without interfacial activity.     

Effects of volume ratio on the microstructure and mechanical properties of particle reinforced magnesium matrix composite

In the present study, the AZ91 alloy reinforced by (submicron + micron) SiCp with four kind volume ratio was fabricated by the semisolid stirring casting technology. The influence of volume ratio between submicron and micron SiCp on the microstructure and mechanical properties of Mg matrix was investigated. Results show that the submicron SiCp is more conducive to grain refinement as compared with micron SiCp. With the increase of volume ratio, the submicron particle dense regions increase and the average grain size decreases. The yield strength of bimodal size SiCp/AZ91 composite is higher than monolithic micron SiCp/AZ91composite. Both Δσ_Hall–Petch and Δσ_CTE increase as the volume ratio changes from 0:10, 0.5:9.5, 1:9 to 1.5:8.5. Among the composite with different volume ratio, the S-1.5 + 10-8.5 composite has the best mechanical properties. The interface debonding is found at the interface of micron SiCp-Mg. As the increase of volume ratio, the phenomenon of interface debonding weakens and the amount of dimples increases.     

Production and properties of SiCp-reinforced aluminium alloy composites

A vacuum infiltration process was developed to produce aluminium alloy composites containing various volume fractions of ceramic particles. The matrix composites of aluminium with 9.42 wt%Si and 0.36 wt%Mg containing up to 55 vol% SiCp were successfully infiltrated and the effect of infiltration temperature and volume fraction of particle on infiltration behaviour was investigated. In addition to aluminium powder, magnesium was used to improve the wetting of SiC particles by the molten aluminium alloy. The infiltration rate increased with increasing infiltration time, temperature and volume fraction of particle, but full infiltration appeared at the optimum process parameters for the various volumes of fraction composite compacts. In addition, the microstructure, hardness, density, porosity and wear resistance of the composites were also examined. It is observed that the distribution of SiC particles was uniform. The hardness and density of the composite increased with increasing reinforcement volume fraction and porosity decreased with increasing particle content. Moreover, the wear rate of the composite increased with increasing load and decreased with increasing particle content.     

搅拌铸造SiCp/2024复合材料热挤压-轧制变形组织及性能

利用搅拌铸造-热挤压-轧制工艺制备SiCp/2024复合材料薄板。通过金相观察(OM)、扫描电镜(SEM)及力学测试等手段研究了该复合材料在铸态、热挤压态及轧制态下的显微组织及力学性能,分析了材料在塑性变形过程中显微组织及力学性能的演变。结果表明,该复合材料铸坯主要由80～100μm的等轴晶组成,粗大的晶界第二相呈非连续状分布,SiC颗粒较均匀地分布于合金基体中;热挤压变形后,晶粒沿挤压方向被拉长,SiC颗粒及破碎的第二相呈流线分布特征;轧制变形后,基体合金组织进一步细化,晶粒尺寸为30～40μm,SiC颗粒破碎明显,颗粒分布趋于均匀,轧制变形对挤压过程中形成的SiC颗粒层带状不均匀组织有显著的改善作用。数学概率统计指出,塑性变形有利于提高颗粒分布的均匀性。力学测试表明,塑性变形后,复合材料的抗拉强度、屈服强度和延伸率显著提高。SiCp/2024铝基复合材料主要的断裂方式为:合金基体的延性断裂、SiC颗粒断裂及SiC/Al界面脱粘。     

Microstructure and mechanical properties of SiCp/AZ91 composite deformed through a combination of forging and extrusion process

In this paper, 10 vol.% SiCp/AZ91 magnesium matrix composites were fabricated by stir casting technology. The as-cast ingots were forged at 420 °C with 50% reduction, and then extruded at 370 °C with the ratio of 16 at a constant ram speed of 15 mm/s. The results showed that the grains were refined during forging. A much finer grain size (∼2.7 μm) of composite matrix was obtained by subjecting the as-forged composite to hot extrusion. The fine SiC particulates restricted the dynamic recrystallized grain growth during the hot extrusion processing, resulting in a remarkable grain refinement. The yield stress and ultimate tensile stress were increased in the as-extruded composite, with the reasons of eliminated casting flaws, the uniform particle distribution and grains refinement. The grain refinement and uniform particle distribution caused an obvious increase in work hardening rate in the as-extruded composite during tensile deformation at room temperature.     

Effect of the SiC particle size on the dry sliding wear behavior of SiC and SiC–Gr-reinforced Al6061 composites

The effect of size of silicon carbide particles on the dry sliding wear properties of composites with three different sized SiC particles (19, 93, and 146 μm) has been studied. Wear behavior of Al6061/10 vol% SiC and Al6061/10 vol% SiC/5 vol% graphite composites processed by in situ powder metallurgy technique has been investigated using a pin-on-disk wear tester. The debris and wear surfaces of samples were identified using SEM. It was found that the porosity content and hardness of Al/10SiC composites decreased by 5 vol% graphite addition. The increased SiC particle size reduced the porosity, hardness, volume loss, and coefficient of friction of both types of composites. Moreover, the hybrid composites exhibited lower coefficient of friction and wear rates. The wear mechanism changed from mostly adhesive and micro-cutting in the Al/10SiC composite containing fine SiC particles to the prominently abrasive and delamination wear by increasing of SiC particle size. While the main wear mechanism for the unreinforced alloy was adhesive wear, all the hybrid composites were worn mainly by abrasion and delamination mechanisms.     

The Role of Particles in Fatigue Crack Propagation of Aluminum Matrix Composites and Casting Aluminum Alloys

Fatigue crack propagation （FCP） behaviors were studied to understand the role of SiC particles in 10 wt pct SiCp/A2024 composites and Si particles in casting aluminum alloy A356. The results show that a few particles appeared on the fracture surfaces in SiCp/Al composites even at high △K region, which indicates that cracks propagated predominantly within the matrix avoiding SiC particles due to the high strength of the particles and the strong particle/matrix interface. In casting aluminum alloy, Si particle debonding was more prominent.Compared with SiCp/Al composite, the casting aluminum alloy exhibited lower FCP rates, but had a slight steeper slope in the Paris region. Crack deflection and branching were found to be more remarkable in the casting aluminum alloy than that in the SiCp/Al composites, which may be contributed to higher FCP resistance in casting aluminum alloy.     

High shear dispersion technology prior to twin roll casting for high performance magnesium/SiCp metal matrix composite strip fabrication

SiC particulate (SiC_p) reinforced AZ31 magnesium alloy composite strips were produced by a novel process. In the process, a high shear technique was utilised to disperse the reinforcing particles uniformly into the matrix alloy, and AZ31/5 vol%SiC_p slurry was solidified into thin strip by a horizontal twin roll caster. The experimental results showed that the AZ31/5 vol%SiC_p strip obtained with high shear treatment exhibited a significantly refined microstructure and uniform distribution of reinforcing SiC particles. High cooling rate in the TRC process was also considered to contribute to the grain refinement of the matrix alloy, together with the possible heterogeneous nucleation effect of the reinforcing particles. The mechanical properties of the high shear treated composites strips showed enhanced modulus, yield strength and ductility by hardness and tensile tests. The experimental results were discussed in terms of the microstructural features and the macroscopic reliability, where necessary, analytical and statistical analyses were conducted.     

Grain refinement and mechanical properties enhancement of AZ91E alloy by addition of ceramic particles

Improved mechanical properties and structural uniformity of Mg-based alloys can be achieved by use of grain-refining additives prior to casting. Ceramic particles of α-Al₂O₃ and SiC can serve as such additives to refine the microstructure of Mg–Al-based alloys. However, direct introduction of ceramic particles into Mg matrix is limited by the poor wetting of those particles by liquid Mg and their massive agglomeration. Mg/α-Al₂O₃ and Mg/SiC master alloys were prepared using a method based on the insertion of the ceramic particles into a molten Mg bath through a Mg-nitride layer formed on the surface of the molten bath. The mixture of Mg/ceramic particles was cooled to room temperature under a nitrogen atmosphere. Mg-15%Al₂O₃ and AZ91E + 10%SiC master alloys were obtained. These master alloys were used to refine AZ91E alloys by introducing various amounts of ceramic particles to manufacture AZ91E + 1%Al₂O₃, AZ91E + 1%SiC, and AZ91E + 3%SiC alloys. These were cast using high-pressure die casting and gravity die casting. The alloy AZ91E + 1%Al₂O₃ was grain refined to ~20 μm and the alloys AZ91E + SiC were grain refined to ~50 μm as against 110 μm in non-refined counterparts. The mechanical properties of the modified alloys are substantially better than those of a non-refined AZ91E alloy which is the result of a combination of grain refinement and reinforcement of the matrix by ceramic particles. Alloy AZ91E + 1%Al₂O₃ exhibited the best mechanical properties.     

Effects of SiCp size and volume fraction on the high cycle fatigue behavior of AZ91D magnesium alloy composites

High cycle fatigue tests (i.e., stress-controlled, axial) were conducted on monolithic AZ91D and AZ91D magnesinm alloy composites processed via squeeze casting and extrusion to contain either 15 gm or 52 gm size SiC particles, at both the 20% and 25% volume fraction reinforcement level. The effects of changes in SiC particle size and volume fraction on the high cycle fatigue behavior have been determined. In addition, the number of cracked particles on the fatigue fracture surfaces, as well as the level of damage beneath the fatigue fracture surfaces were quantified in order to determine the effects of particle size on the evolution of damage during fatigue and during overload failure. Commercial purity Mg specimens containing a large grain size were also tested in fatigue for comparison with the alloy and composite data.     

Fabrication,microstructures, and tensile properties of magnesium alloy AZ91/SiCp composites produced by powder metallurgy

Abstract

The tensile properties and microstructural evolution of hot extruded AZ91 magnesium alloy with and without reinforcement of SiC particles have been investigated in terms of extrusion parameters, such as extrusion ratio and extrusion temperature. Also, the effect of SiC particles on the grain size of the matrix in the composites was evaluated using the Hall-Petch equation. The AZ91 magnesium alloy powders prepared by wet attrition milling from magnesium machined chips were hot pressed with and without SiC particles, hot extruded, and then solution treated. Microstructural observation revealed that both the composites and the magnesium alloy have fine equiaxed grains due to the dynamic recrystallisation during hot extrusion. The tensile strength of both materials increased with increasing extrusion ratio, and the strengths of the composites were higher than that of the magnesium alloy without reinforcement. It was found that the tensile strength of both the materials decreased after solution treatment, and the decrease in tensile strength of the composites was considerably smaller than that of the magnesium alloy. From analyses of the microstructures and the mechanical properties, combined with examination of the H all–Petch relationship, the refinement of the matrix was primarily responsible for the improvement in the yield strength of the composites. The grain growth of the matrix was inhibited by the introduction of the SiC particles.     

等通道角变形对搅拌铸造SiC_P/AZ91复合材料显微组织与室温性能的影响

采用搅拌铸造法成功制备了SiC_P/Mg(AZ91)复合材料并对铸态复合材料进行了等通道角挤压变形(Equal channel angular pressing,ECAP)。结果表明,搅拌铸造态SiC_P/AZ91复合材料的基体组织致密,颗粒与基体结合良好,没有出现宏观团聚;SiC_P大部分聚集在晶界附近区域并呈"项链状"分布。ECAP变形可以有效地消除铸态SiC_P/AZ91复合材料中的SiC_P"项链状"分布,并且随着ECAP道次的增加,SiC_P分布更加均匀;在ECAP过程中,SiC_P发生了一定断裂但并不明显。SiC_P/AZ91复合材料基体晶粒随着变形道次的提高而逐渐细化。基体晶粒细化以及SiC_P分布均匀化是SiC_P/AZ91复合材料屈服强度和抗拉强度随着道次提升而逐渐增加的主要原因。     

Microstructure and mechanical behavior of die casting AZ91D-Fly ash cenosphere composites

The feasibility of incorporating fly ash cenospheres in die cast magnesium alloy has been demonstrated. The effects of fly ash cenosphere additions on the microstructure and some of the salient physical and mechanical properties of magnesium alloy (AZ91D) metal matrix composites were investigated. The control AZ91D alloy and associated composites, containing 5, 10, and 15 wt.% of fly ash cenospheres (added), were synthesized using a die casting technique. A microstructural comparison showed that microstructural refinement – occurred due to the fly ash additions and became more pronounced with an increase in the percentage of the fly ash added. The metal matrix areas nearer to the fly ash particles exhibited a greater degree of refinement than was observed in the areas further away from these particles. Both filled and unfilled fly ash cenospheres, and porosity were observed in the composite microstructures. The composite specimen densities decreased and the coefficient of thermal expansion did not change significantly as the volume percent of fly ash was increased within the range investigated. The hardness values of the composite specimens exhibited an increase in proportion to the increase in percentage of added fly ash. The tensile strength of the composites also increased as the concentration of fly ash cenospheres was increased. In contrast, the Young’s modulus of these composite samples, as measured by non-destructive pulse-echo method, decreased as the percentage of fly ash in the composite was increased. SEM micrographs of the tensile fracture surfaces showed broken cenospheres on the fracture surface and evidence of ‘pull outs’, where fly ash particles were previously embedded in the matrix. Compression testing results showed that the presence of 5 wt.% cenospheres decreased the compressive strength and compressive yield strength of the composite relative to that of the AZ91D matrix alloy. Surprisingly, a significant change in compression strength was not observed for the composites with 10 and 15 wt.% cenospheres in comparison to the AZ91D matrix alloy. In contrast to the tensile tests, no cenosphere remnants were observed on the compressive test fracture surface of the composites. This observation suggests that the fracture of the composite was initiated within the AZ91D matrix by normal void nucleation and growth, followed by crack propagation through the matrix, avoiding any of the cenospheres, leading to composite fracture of the matrix.     

Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites

Thermal conductivity of SiCp/Cu composites was usually far below the expectation, which is usually attributed to the low real thermal conductivity of matrix. In the present work, highly pure Cu matrix composites reinforced with acid washed SiC particles were prepared by the pressure infiltration method. The interfacial microstructure of SiCp/Cu composites was characterized by layered interfacial products, including un-reacted SiC particles, a Cu–Si layer, a polycrystalline C layer and Cu–Si matrix. However, no Cu₃Si was found in the present work, which is evidence for the hypothesis that the formation of Cu₃Si phase in SiC/Cu system might be related to the alloying elements in Cu matrix and residual Si in SiC particles. The thermal conductivity of SiCp/Cu composites was slightly increased with the particle size from 69.9 to 78.6 W/(m K). Due to high density defects, the real thermal conductivity of Cu matrix calculated by H–J model was only about 70 W/(m K). The significant decrease in thermal conductivity of Cu matrix is an important factor for the low thermal conductivity of SiCp/Cu composites. However, even considered the significant decrease of thermal conductivity of Cu matrix, theoretical values of SiCp/Cu composites calculated by H–J model were still higher than the experimental results. Therefore, an ideal particle was introduced in the present work to evaluate the effect of interfacial thermal resistance. The reverse-deduced effective thermal conductivities of ideal particles according to H–J model was about 80 W/(m K). Therefore, severe interfacial reaction in SiCp/Cu composites also leads to the low thermal conductivity of SiCp/Cu composites.     

等径角挤压道次对SiC_p/Al复合材料显微组织的影响

为改善超高强度SiCp/Al复合材料的塑性,以Bc路径对喷射沉积SiCp/7090Al（SiC颗粒体积分数15%,名义尺寸10μm）复合材料进行等径角挤压变形,研究了复合材料显微组织和力学性能的演变规律.结果表明：经过4个道次变形后,获得等轴晶粒,尺寸大约为400 nm;SiC颗粒在剪切应力作用下被破碎,尺寸约2μm;...     

Mechanical properties of hot pressed SiCp and B4Cp/Alumix 123 composites alloyed with minor Zr

In the present study, effect of Zr addition on the microstructure and wear behavior of aluminum alloy composites (AMCs) reinforced with B4Cp and SiCp particles fabricated via hot pressing were investigated. The samples for the study composed of unreinforced aluminum alloy (Alumix 123) and the composites reinforced with 10% B4Cp and % SiCp were prepared by hot isostatic pressing (HIP) method. Similarly, all the samples alloyed with 0.2% Zr were also produced in order to make a comparison. The produced samples were evaluated for microstructural properties and mechanical tests for hardness, tensile and bending strength were performed. Wear test was carried out at 5 mm/s sliding speed under 3.0 N load for the all kind of hot pressed produced samples. The hot pressed composite microstructures have a more uniform distribution of the reinforcements. After HIP process, the composites were successfully produced with high density (>99%). The addition of Zr increased the yield and tensile strength of the samples. The highest strength value was found for the sample Al 123 matrix alloy with Zr. Evaluation of microstructures showed that copper and zirconium dispersed equally within the matrix microstructure without agglomeration. For the composite samples, Al3Zr, appeared as white precipitate, were inspected around B4C and SiC particles. The composite containing SiC particles and Zr had wear resistance value superior to those of the other counterparts.     

Microstructures and wear property of friction stir welded AZ91 Mg/SiC particle reinforced composite

The microstructures and wear property of friction stir welded AZ91 Mg alloy/SiC particle reinforced composite (AZ91/SiC/10p) were investigated. The initial microstructures of the AZ91/SiC/10p were composed of irregularly distributed β-phases (Al₁₂Mg₁₇) and agglomerated SiC particles, while the friction stir weld zone was characterized by the homogeneous distribution of SiC particles, the recrystallized grain structure and the dissolution of β-phase. Thank to the microstructural modification, an improvement in the hardness and wear property of the weld zone were observed as compared to those of the base metal. The hardness near the weld zone was a higher and more homogeneously distributed and the wear resistance within the weld zone, as evaluated by the specific wear loss, was superior, as compared with the base metal.