Preparation and mechanical properties of SiC/2024 composite by semisolid casting

A SiC/2024 composite was made by semisolid casting. The wetting between SiC and Al matrix is improved by treating SiC particles at a high temperature, coating K2ZrF6, and adding Mg to the Al melt. An effective way to remove the gas around SiC particles was also found. Microstructures were observed under optical microscope (OM) and scanning electron microscopy (SEM). The results show that SiC particles and Al matrix are well bonded and no gaps or cavities around the particles are observed. SiC particles distribute homogeneously in the Al matrix. The existence of SiC particles results in the increase of wear resistance and strength.

Characterization and evaluation of interface in SiC_p/2024 Al composite

35% SiCp/2024 Al（volume fraction） composite was prepared by powder metallurgy method. The microstructures of Si Cp/Al interfaces and precipitate phase/Al interfaces were characterized by HRTEM, and the interface conditions were evaluated by tensile modules of elasticity and Brinell hardness measurement. The results show that the overall Si Cp/Al interface condition in this experiment is good and three kinds of Si Cp/Al interfaces are present in the composites, which include vast majority of clean planer interfaces, few slight reaction interfaces and tiny amorphous interfaces. The combination mechanism of Si C and Al in the clean planer interface is the formation of a semi-coherent interface by closely matching of atoms and there are no fixed or preferential crystallographic orientation relationships between Si C and Al. MgAl2O4 spinel particles act as an intermediate to form semi-coherent interface with SiC and Al respectively at the slight reaction interfaces. When the composite is aged at 190 °C for 9 h after being solution-treated at 510 °C for 2 h, numerous discoid-shaped and needle-shaped nanosized precipitates dispersively exist in the composite and are semi-coherent of low mismatch with Al matrix. The Brinell hardness of composites arrives peak value at this time.

Microstructures and mechanical properties of 2024Al/Gr/SiC hybrid composites fabricated by vacuum hot pressing

The 2024Al/Gr/SiC hybrid composite plates with 5%-10% SiC particles （volume fraction） and 3%-6% flaky graphite （Gr） （volume fraction） were fabricated by vacuum hot pressing and hot extrusion processing. The effects of SiC and Gr on the microstructures and mechanical properties of the composites aged at 160, 175 and 190℃ were studied by optical microscopy, scanning electron microscopy （SEM）, and hardness and tensile tests. The results indicate that the SiC particles have a more obvious effect on accelerating the aging response as compared with the Gr. Both the tensile strength and elongation are reduced by the Gr and SiC particles added into the matrix, while the Gr has a more negative influence on the elongation than the SiC particles. The tensile strength （ab）, yield stress （as） and elongation （δ） of the 2024Al/3Gr/10SiC composite aged at 165℃ for 8 h are 387 MPa, 280.3 MPa and 5.7%, respectively. The hybrid composites are characterized by ductile fracture, which is associated with the ductile fracture of the matrix and the tearing of the interface between the matrix and the particles.

Multi-scale Modeling and Finite Element Analyses of Thermal Conductivity of 3D C/SiC Composites Fabricating by Flexible-Oriented Woven Process

Thermal conductivity is one of the most significant criterion of three-dimensional carbon fiber-reinforced SiC matrix composites(3D C/SiC).Represent volume element(RVE)models of microscale,void/matrix and mesoscale proposed in this work are used to simulate the thermal conductivity behaviors of the 3D C/SiC composites.An entirely new process is introduced to weave the preform with three-dimensional orthogonal architecture.The 3D steady-state analysis step is created for assessing the thermal conductivity behaviors of the composites by applying periodic temperature boundary conditions.Three RVE models of cuboid,hexagonal and fiber random distribution are respectively developed to comparatively study the influence of fiber package pattern on the thermal conductivities at the microscale.Besides,the effect of void morphology on the thermal conductivity of the matrix is analyzed by the void/matrix models.The prediction results at the mesoscale correspond closely to the experimental values.The effect of the porosities and fiber volume fractions on the thermal conductivities is also taken into consideration.The multi-scale models mentioned in this paper can be used to predict the thermal conductivity behaviors of other composites with complex structures.

Tensile Mechanical Behavior and Failure Mechanism of a Plain-Woven SiCf/SiC Composites at Room and Elevated Temperatures

Ceramic matrix composites (CMCs) are the preferred materials for solving advanced aerospace high-temperature structural components;it has the comprehensive advantages of higher temperature (~1500˚C) and low density. In service environments, CMCs exhibit complex damage mechanisms and failure modes, which are affected by constituent materials, meso-architecture and inhere defects. In this paper, the in-plane tensile mechanical behavior of a plain-woven SiCf/SiC composite at room and elevated temperatures was investigated, and the factors affecting the tensile strength of the material were discussed in depth. The results show that the tensile modulus and strength of SiCf/SiC composites at high temperature are lower, but the fracture strain increases and the toughness of the composites is enhanced;the stitching holes significantly weaken the tensile strength of the material, resulting in the material is easy to break at the cross-section with stitching holes.

STUDIES ON MICROYIELD BEHAVIOR OF A SiCp/2024Al COMPOSITE

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Corrosion Behavior and Protection Efficiency of 2024Al and SiC_p/2024Al Metal Matrix Composite

The corrosion resistance of 2024 Al and SiC particle reinforced 2024 Al metal matrix composite (SiCp?024AI MMC) in 3.5% NaCl solution was investigated with electrochemical method and immersion test, and the corrosion protection of sulfuric acid anodized coatings on both materials was evaluated by electrochemical impedance spectroscopy. The results showed that the SiCp?024AI MMC is more susceptible to corrosion than its matrix alloy in 3.5% NaCI. For 2024AI, the anodized coating provides excellent corrosion resistance to 3.5% NaCI. The anodized coating on the SiCp?024AI provides satisfactory corrosion protection, but it is not as effective as that for 2024AI because the structure of the anodized layer is affected by the SiC particulates.

Aging Behavior of High Volume Fraction SiC Particles Reinforced 2024Al Composite

2024 Al matrix composite reinforced by SiC particles with 45% volume fraction and 1 um diameter was successfully fabricated by squeeze-exhaust casting method. The aging behavior of SiCp/2024AI composite at four temperatures was investigated and compared to 2024 alloy. It was found that the addition of high volume fraction SiC particles does not alter the aging sequence, but it significantly accelerates the kinetics of precipitation in the composite matrices. Therefore, the aging peak of the composite appears earlier than that of 2024AI alloy. This is attributed to the decrease in the activation energy for the precipitate formation and the increase in the precipitate growth rate due to the high density dislocations in the composite with high volume fraction particles. The high density dislocations, as preferential nucleation sites for precipitates, bring about the tiny and dense precipitates in the composite.

Microstructural characterization and mechanical properties of functionally graded Al2024/SiC composites prepared by powder metallurgy techniques

Al2024/SiC functionally graded materials (FGMs) with different numbers of graded layers and different amounts of SiC were fabricated successfully by powder metallurgy method and hot pressing process. The effects of increasing SiC content and number of layers of Al2024/SiC FGMs on the microstructure and mechanical properties of the composite were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) analyses indicated that Al and SiC were dominant components as well as others such as Al4C3, CuAl2, and CuMgAl2

EFFECT OF OXIDATION OF SiC ON AGING CHARACTERISTICS OF SiC PARTICLES AND SHORT MULLITE FIBERS REINFORCED 2024Al COMPOSITE

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EFFECT OF MATRIX STRENGTHENING ON SLIDING WEAR PROPERTIES OF SiCp/2024Al COMPOSITES

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RESISTANCE BRAZING OF SiC/2024AI COMPOSITE

The resistance brazing of 2024Al reinforced with15 volume percent of Sic particulate was simulated by using a Gleeble - 1500 Tharmal/Mechanical sindator. The results show that too high or too low brazing temperature and pressure result in decrease in shear strength of joint,but brazing time influenc- es it slightly. The action between filler metals and the composite was analyzed by SEM and EPAM. The use of simulation method helped in the design and predication of welding technology,which re- duced the rejection rate as well as the cost of production.

Corrosion Behavior of SiC_p/2024 Al Matrix Composites in 3.5 wt pct Sodium Chloride

The influences of volume fraction and particle size of SiC particulate reinforcements on the corrosion characteristics of SiCp/2024 Al metal matrix composites in aerated 3.5 wt pct NaCI aqueous solution were investigated. The electrochemical behavior was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy, the general corrosion behavior of the composites was studied further by immersion tests. The results showed that pitting susceptibility was about the same for the composites and the alloy. The corrosion potentials were also independent of SiC phase. The corrosion resistance for the composites decreased as the volume fraction increased or particle size decreased.

Effect of aging treatment on mechanical properties of (SiCw+SiCp)/2024Al hybrid nanocomposites

2024Al based composites reinforced by a hybrid of SiC whisker and SiC nanoparticle were fabricated by a squeeze casting route. In the (SiCw+SiCp)/Al composites, the volume fraction of SiC whisker is 20% and that of SiC nanoparticle is 2%, 5% and 7%, respectively. The as cast composites were solution treated followed by aging treatment. The experimental results show that the SiC nanoparticles are more effective in improving the hardness and tensile strength of the composites than SiC whiskers. The hardening kinetics of the composites is enhanced by reinforcements addition and the peak aging time is 4-5 h. The hardness of all the hybrid composite decreases at the initial aging stage, suggesting that dislocation recovery softening process coexists with precipitation hardening. DSC study shows that the GP zone formation of the hybrid composites is suppressed.

Microstructure and damping behavior of SiC_p/Gr/2024Al metal matrix composites by squeeze casting technology

SiCp/Gr/2024Al metal matrix composites were processed by squeeze casting technology. The microstructure of composites was observed by SEM and TEM, and the effects of graphite particulates and SiC particulates on the damping behaviors of composites were also investigated. The results show that the microstructure of composites was dense and homogeneous, without any interfacial reactivity among reinforcement/matrix interfaces. Compared with the damping capacity of 2024A1, the damping capacity of composites was enhanced significantly by addition of SiC or graphite particulates. The main damping mechanisms of SiCp/Al composites were ascribed to the dislocation damping, and those of SiCp/Gr/2024Al were attributed to the intrinsic damping and interface damping.

Mixed rare earth metal conversion coatings on 2024 alloy and Al6061/SiC_p metal matrix composites

The processes of mixed rare earth metal (REM) conversion coatings on 2024 alloy and Al6061/SiC p metal matrix composites (MMC) were introduced. The coatings were examined to be honeycomb like feature by scanning electron microscope. X ray diffraction analysis revealed that the coatings are amorphous structure. The results of X ray photoelectron spectroscopy indicated that the mixed REM conversion coatings consist predominantly of Ce and O, the contents of other rare earth elements (such as La, Pr) are relatively low, the coatings are about 2～4 μm thickness with excellent adhesion and wearability. The results of mass loss test showed that the mixed REM conversion coatings produce corrosion resistant surface of 2024 alloy and Al6061/SiC p. [

Preparation and oxidation property of ZrB_2-MoSi_2/SiC coating on carbon/carbon composites

To improve the oxidation resistance of carbon/carbon composites,ZrB2-MoSi2/SiC coating on the carbon/carbon substrate was prepared.The inner coating of SiC was prepared by pack cementation and the outer coating of ZrB2-MoSi2 was prepared by slurry painting.The phase compositions and microstructures of the coating were characterized by XRD and SEM,respectively.The preparation and the high temperature oxidation property of the coated composites were investigated.The results show that the outer coating of carbon/carbon composites is composed of ZrB2,MoSi2 and SiC phases.The mass losses of the ZrB2-MoSi2/SiC coated samples with SiC nano-whiskers after 30 h and 10 h of oxidation at 1 273 K and 1 773 K were,respectively,5.3% and 3.0%.The ZrB2-MoSi2/SiC coated samples exhibit self-sealing performance and good oxidation resistance at high temperature.

C/SiC/MoSi_2-SiC-Si multilayer coating for oxidation protection of carbon/carbon composites

C/SiC/MoSi2-SiC-Si oxidation protective multilayer coating for carbon/carbon （C/C） composites was prepared by pack cementation and slurry method. The microstructure, element distribution and phase composition of the as-received coating were analyzed by scanning electron microscopy （SEM）, energy dispersive X-ray spectroscopy （EDS） and X-ray diffraction （XRD）. The results show that the multilayer coating was composed of MoSi2, SiC and Si. It could effectively protect C/C composites against oxidation for 200 h with the mass loss of 3.25% at 1873 K in static air. The mass loss of the coated C/C composites results from the volatilization of SiO2 and the formation of cracks and bubble holes in the coating.

Double SiC coating on carbon/carbon composites against oxidation by a two-step method

To improve the oxidation resistance of C/C composites, a double SiC protective coating was prepared by a two-step technique. Firstly, the inner SiC layer was prepared by a pack cementation technique, and then an outer uniform and compact SiC coating was obtained by low pressure chemical vapor deposition. The microstructures and phase compositions of the coatings were characterized by SEM, EDS and XRD analyses. Oxidation behaviour of the SiC coated C/C composites was also investigated. It was found that the double SiC coating could protect C/C composites against oxidation at 1773 K in air for 178 h with a mass loss of 1.25%. The coated samples also underwent thermal shocks between 1773 K and room temperature 16 times. The mass loss of the coated C/C composites was only 2.74%. Double SiC layer structures were uniform and dense, and can suppress the generation of thermal stresses, facilitating an excellent anti-oxidation coating.

Thermal analysis for brake disks of SiC/6061 Al alloy co-continuous composite for CRH3 during emergency braking considering airflow cooling

The mass of high-speed trains can be reduced using the brake disk prepared with SiC network ceramic frame reinforced 6061 aluminum alloy composite （SiCn/Al）. The thermal and stress analyses of SiCn/Al brake disk during emergency braking at a speed of 300 km/h considering airflow cooling were investigated using finite element （FE） and computational fluid dynamics （CFD） methods. All three modes of heat transfer （conduction, convection and radiation） were analyzed along with the design features of the brake assembly and their interfaces. The results suggested that the higher convection coefficients achieved with airflow cooling will not only reduce the maximum temperature in the braking but also reduce the thermal gradients, since heat will be removed faster from hotter parts of the disk. Airflow cooling should be effective to reduce the risk of hot spot formation and disc thermal distortion. The highest temperature after emergency braking was 461 °C and 359 °C without and with considering airflow cooling, respectively. The equivalent stress could reach 269 MPa and 164 MPa without and with considering airflow cooling, respectively. However, the maximum surface stress may exceed the material yield strength during an emergency braking, which may cause a plastic damage accumulation in a brake disk without cooling. The simulation results are consistent with the experimental results well.