Effects of SiC volume fraction and aluminum particulate size on interfacial reactions in SiC nanoparticulate reinforced aluminum matrix composites

The SiC nanoparticulate reinforced Al-3.0 wt.% Mg composites were fabricated by combining pressureless infiltration with ball-milling and cold-pressing technology at 700 °C for 2 h. The effects of SiC nanoparticulate volume fractions (6%, 10% and 14%) and Al particulate sizes (38 μm and 74 μm) on interfacial reactions were investigated by SEM, TEM and X-ray diffraction. The results show that the MgO at the interface between SiC nanoparticulate and molten Al can provide a barrier for the diffusion of Si, C and Al. Using Al particulate (74 μm) as raw material, the Al₄C₃ phase was not found in the composites containing 6 vol.% and 10 vol.% SiC, but presented in the composites containing 14 vol.% SiC. When SiC content up to 14 vol.%, the products of MgO around SiC nanoparticulate are not enough to provide effective protection from the reaction between SiC and molten Al, therefore the diffusion of Si, C and Al can take place to produce Al₄C₃ and Si phases. Using 38 μm Al particulate as raw material, the fine Al particulate possesses the high reaction activity and can easily be embedded into the gap among the big Mg particulate segregated at the interface, resulting in the appearance of exposure surface of SiCp to the Al and the forming of diffusion channels for the atomics C, Si and Al. So, the formations of Al₄C₃ and Si phases were occurred.     

Microstructure and mechanical properties of magnesium composites prepared by spark plasma sintering technology

Spark plasma sintering (SPS) technology was used to determine the appropriate conditions for SPS sintering of commercially pure magnesium as well as the magnesium alloy AZ31. It was found that the sintering temperatures of 585 °C and 552 °C were the most suitable sintering temperatures for the magnesium and the AZ31 alloy, respectively. Magnesium matrix and AZ31 alloy matrix composites reinforced with SiC particles were then successfully fabricated by the SPS method at sintering temperatures of 585 °C and 552 °C, respectively. A uniform distribution of SiC particles was observed along the boundary between matrix particles. The mechanical properties, i.e. hardness and tensile strength increased with increasing SiC content up to 10 wt%. However, when the SiC content was larger than 10 wt%, the tensile strength decreased due to the agglomeration of SiC particles. The agglomeration of SiC particles was found to lead to the degradation of the interfacial bonding strength between matrix and reinforcement.     

Mullite-gadolinium silicate environmental barrier coatings for melt infiltrated SiC/SiC composites

Slurry based mullite/gadolinium silicate (Gd₂SiO₅) environmental barrier coatings (EBCs) were developed for melt infiltrated (MI) SiC/SiC composites. The coating chemically adhered well on the substrates. Thermal cycling of uncoated MI-SiC/SiC composites conducted between 1350 °C and 90 °C (one hour hot and 15 min cold) in a 96.5% H₂O-3.5% O₂ environment caused severe oxidation damage after 100 cycles resulting in the formation of dense silica layer of about 25 μm maximum thickness. Mullite/Gd₂SiO₅ EBCs provided excellent protection to MI-SiC/SiC against moisture damage with significantly less oxidation of the substrate; only about a 2 μm thick oxide layer formed even after 400 similar thermal cycles. The hair-line cracks formed at the coating/substrate interface after 400 cycles causing partial coating de-lamination.     

EFFECTS OF HEAT TREATMENT ON THE THERMAL EXPANSION BEHAVIOR OF SiC WHISKER REINFORCED ALUMINUM COMPOSITE

1. IntroductionHigh specific stiffness and strength, good fatigue properties and tribological propertiesmake SiC whisker reinforcement aluminum composite as attractive class of candidates formany applications such as aerospace and electronic industries[1--2]. As structural componellts with temperature variable the materials used must have good performance includingnot only high mechalilcal properties, but also lower and stable coefficients of thermal expansion. Therefore, it is very important…     

Ti-coated SiC particle reinforced sintered Fe-Cu-Sn alloy

Ti-coated SiC particles were developed to improve the wear resistance of Fe-Cu-Sn alloy metal matrices designed for diamond tools. The phase structure of the Ti-coated SiC particles was investigated by X-ray diffraction. Ti coating on SiC was composed of Ti₅Si₃, TiC, and Ti. Excellent interfacial bonding between SiC and the matrix was realized. The SiC/iron alloy composites, prepared by hot pressing at 820 °C, were studied by wear and bending strength tests, and scanning electron microscope. For the composites reinforced by uncoated SiC particles, the wear resistance was improved, but the bending strength decreased. The composites with Ti-coated SiC particles outperformed the composites with uncoated SiC particles in both wear resistance and bending strength tests.     

The effect of sintering temperature on some properties of Cu-SiC composite

Copper matrix composites reinforced with 1 wt.%, 2 wt.%, 3 wt.% and 5 wt.% SiC particles were fabricated by powder metallurgy method. Cu and Cu-SiC powder mixtures were compacted with a compressive force of 280 MPa and sintered in an open atmospheric furnace at 900-950 °C for 2 h. Within the furnace compacted samples were embedding into the graphite powder. The presence of Cu and SiC components in composites was verified by XRD analysis. Optical and SEM studies showed that Cu-SiC composites have a uniform microstructure in which silicon carbide particles are distributed uniformly in the copper matrix. The results of the study on mechanical and electrical conductivity properties of Cu-SiC composites indicated that with increasing SiC content (wt.%), hardness increased, but relative density and electrical conductivity decreased. The highest electrical conductivity of 98.8% IACS and relative density of 98.2% were obtained for the Cu-1 wt.%SiC composite sintered at 900 °C and this temperature was defined as the optimum sintering temperature.     

Effect of thermal cycling on the expansion behavior of Al/SiCp composite

The coefficient of thermal expansion (CTE) and accumulated plastic strain of the pure aluminum matrix composite containing 50% SiC particles (Al/SiC_p) during thermal cycling (within temperature range 298–573 K) were investigated. The composite was produced by infiltrating liquid aluminum into a preform made by SiC particles with an average diameter of 14 μm. Experiment results showed that the relationship between the CTE of Al/SiC_p and temperature is nonlinear; CTE could reach a maximum value at about 530 K. The theoretical accumulated plastic strain of Al/SiC_p composites during thermal cycling has also been calculated and compared with the experimental results.     

弥散质点和SiC颗粒复合强化Al基复合材料 Ⅱ．性能和断裂特征

对机械合金化制备的Ａｌ_４Ｃ_３、Ａｌ_２Ｏ_３弥散质点和ＳｉＣ颗粒复合强化Ａｌ基复合材料进行了拉伸试验和断口分析，并测定了弹性模量和热膨胀系数．研究表明，在ＳｉＣ_ｐ／Ａｌ复合材料中引入弥散的Ａｌ_４Ｃ_３和Ａｌ_２Ｏ_３质点可以明显提高复合材料的室温和高温强度，随加入Ｃ含量的增加或Ａｌ粉氧化时间的加长，复合材料的强度提高．在Ａｌ_４Ｃ_３／Ａｌ复合材料的基础上加入ＳｉＣ颗粒可以提高复合材料的弹性模量并进一步降低其热膨胀系数．复合材料断口为大韧窝加细小韧窝的混合断口，随复合材料基体强度的增加，拉伸断口上断裂的ＳｉＣ颗粒数量增多．     

DISPERSOID AND SiC PARTICULATE STRENGTHENED Al COMPOSITES Ⅱ. PROPERTIES AND FRACTURE BEHAVIOUR

对机械合金化制备的Ａｌ_４Ｃ_３、Ａｌ_２Ｏ_３弥散质点和ＳｉＣ颗粒复合强化Ａｌ基复合材料进行了拉伸试验和断口分析，并测定了弹性模量和热膨胀系数．研究表明，在ＳｉＣ_ｐ／Ａｌ复合材料中引入弥散的Ａｌ_４Ｃ_３和Ａｌ_２Ｏ_３质点可以明显提高复合材料的室温和高温强度，随加入Ｃ含量的增加或Ａｌ粉氧化时间的加长，复合材料的强度提高．在Ａｌ_４Ｃ_３／Ａｌ复合材料的基础上加入ＳｉＣ颗粒可以提高复合材料的弹性模量并进一步降低其热膨胀系数．复合材料断口为大韧窝加细小韧窝的混合断口，随复合材料基体强度的增加，拉伸断口上断裂的ＳｉＣ颗粒数量增多．     

An investigation on corrosion behaviour of pressure infiltrated Al-Mg alloy/SiCp composites

In this study, effect of Mg alloying addition (2-8 wt.%) on corrosion behaviour of Al matrix composites, in 3.5 wt.% NaCl environment, has been investigated. Composites were produced by pressure infiltration technique at 750 °C and had a SiC particle (SiC_p) volume fraction of ∼60%. Results were evaluated by using potentiodynamic polarisation measurements, immersion tests, SEM, EDS and XRD analysis. Compared to the pure Al matrix, mass loss of the composites decreased with increasing Mg content. Experimental results revealed that intermetallics as a result of reaction between Al-Mg alloy and SiC particle has beneficial effect on corrosion resistance of the composites due to interruption of the continuity of the matrix channels within the pressure infiltrated composites.     

The thermal stability in air of hot-pressed diboride matrix composites for uses at ultra-high temperatures

The resistance to oxidation in ambient air at a temperature up to 1600 °C of two hot-pressed diborides matrix composites, both containing 19.5% v/o SiC and 3 v/o HfN (as sintering aid), was investigated. The diboride matrix was based on HfB₂ or a ZrB₂/HfB₂ mixture (volume ratio ≈ 1). Both the materials were subjected to repeated heating-cooling cycles at 1600 °C, and a 20 h exposure at 1450 °C in flowing dry air. Modest weight gains and limited corrosion depths highlighted a rather good thermal stability. In accordance with the thermo-gravimetric test at 1450 °C, the oxidation kinetics for both the composites superbly fit a para-linear law. The introduction of the SiC particles provided tangible benefits for the resistance to oxidation. One of the oxidation products, a borosilicate glass, sealed pores and coated the exposed faces, greatly limiting the inward transport of oxygen towards the internal oxide/diboride interfaces.     

Thermal expansion behaviour of aluminium/SiC composites with bimodal particle distributions

The thermal response and the coefficient of thermal expansion (CTE) of aluminium matrix composites having high volume fractions of SiC particulate have been investigated. The composites were produced by infiltrating liquid aluminium into preforms made either from a single particle size, or by mixing and packing SiC particulate of two largely different average diameters (170 and 16 μm, respectively). The experimental results for composites with a single particle size indicate that the hysteresis in the thermal strain response curves is proportional to the square root of the particle surface area per unit volume of metal matrix, in agreement with current theories. Instead, no simple relationship is found between the hysteresis and any of the system parameters for composites with bimodal particle distributions. On the other hand, the overall CTE is shown to be mainly determined by the composite compactness or total particle volume fraction; neither the particle average size nor the particle size distribution seem to affect the overall CTE. This result is in full agreement with published numerical results obtained from finite element analyses of the effective CTE of aluminum matrix composites. Our results also indicate that the CTE varies with particle volume fraction at a pace higher than predicted by theory.     

Comparison of thermal and ablation behaviors of C/SiC composites and C/ZrB2-SiC composites

A comparison was presented of the thermal and ablation behaviors of two carbon fiber reinforced ceramic-matrix composites (one with a SiC matrix and the other with a ZrB₂-SiC matrix). The C/SiC composite possessed a lower thermal conductivity (TC) and a higher emissivity in comparison to the C/ZrB₂-SiC composite. The two composites exhibited the good ablation-resistive properties with no obvious erosion rate after the arc-heated wind tunnel ablation tests. The surface of the C/SiC composite appeared to be coarse and had many rounded protrusions while a denser and more homogeneous glass oxide scale was formed for the C/ZrB₂-SiC composite. The maximum surface and back side temperatures of the C/ZrB₂-SiC composite were about 50 °C lower than those of the C/SiC composite, respectively, which was mainly attributed to the evaporation of the B₂O₃ as well as its higher TC.     

Thermal expansion behavior of a β-LiAlSiO<Subscript>4</Subscript>/Cu composite

A copper matrix composite reinforced by β-LiAlSiO₄ with negative thermal expansion coefficient was fabricated using vacuum hot-pressed sintering technique. The thermal expansion behavior of the composite was investigated, and the average residual stress in the matrix was analyzed by a simple model. The results indicate that the residual stress in the matrix affects the thermal expansion properties. After heat treatment, the coefficient of thermal expansion (CTE) of the composite decreases greatly. The CTE of the composite after thermal cycling between 50–350°C is the lowest.     

Investigation on the characteristics of micro- and nano-structured W-15 wt.%Cu composites prepared by powder metallurgy route

The properties of W-15 wt.%Cu composites were investigated by preparing two distinct composites of micrometer and nanoscale structures. Micrometer composite was produced by mixing elemental W and Cu powders and nanometer one was synthesized through a mechanochemical reaction between WO₃ and CuO powders. Subsequent compaction and sintering process was performed to ensure maximum possible densification at 1000-1200 °C temperatures. Finally, the behavior of produced samples including relative density, hardness, compressive strength, electrical conductivity, coefficient of thermal expansion (CTE) and room temperature corrosion resistance were examined. Among the composites, nano-structured sample sintered at 1200 °C exhibited better homogeneity, the highest relative density (94%) and mechanical properties. Furthermore, this composite showed superior electrical conductivity (31.58 IACS) and CTE (9.95384 × 10^- 6) in comparison with micrometer type. This appropriate properties may be mainly attributed to liquid phase sintering with particle rearrangement which induced by higher capillary forces of finer structures.     

SiC nanowire-toughened MoSi2-SiC coating to protect carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a SiC nanowire-toughened MoSi₂-SiC coating was prepared on them using a two-step technique of chemical vapor deposition and pack cementation. SiC nanowires obtained by chemical vapor deposition were distributed random-orientedly on C/C substrates and MoSi₂-SiC was filled in the holes of SiC nanowire layer to form a dense coating. After introduction of SiC nanowires, the size of the cracks in MoSi₂-SiC coating decreased from 18 ± 2.3 to 6 ± 1.7 μm, and the weight loss of the coated C/C samples decreased from 4.53% to 1.78% after oxidation in air at 1500 °C for 110 h.     

Thixoforming of AA 2017 aluminum alloy composites

A comparative evaluation has been carried out on the microstructure of aluminum based SiC and Al₂O₃ particle reinforced composites produced by semi-solid direct squeeze forming of composite powder at temperatures of 635-645 °C. The study is focused on the distribution of the reinforcement and the intermetallic phases, the porosity content, the microstructure of the matrix phase, the interfacial state and mechanical properties. The particle size of the reinforcements, the time of the high-energy ball milling procedure for the fabrication of composite powder and the semi-solid forming temperature had a strong influence on the quality of sample in terms of distribution of reinforcement and interfacial interaction. Ball milling improves the interface formation between reinforcement and matrix and influences the remelting behaviour. Increasing ball milling time and decreasing semi-solid forming temperature with isothermal holding time resulted in relatively homogenous microstructures and in a reduced amount of interaction between SiC and metal matrix. Best results were obtained for 5 vol.% SiC_p composites after 3 h ball milling, semi-solid formed at 635 °C and held for 10 min.     

Friction and wear behaviors of reciprocatingly extruded Al–SiC composite

This study focuses on the friction and wear behaviors of reciprocatingly extruded Al–SiC composites. To increase the strength of metal matrix composites and refine the grains of the matrix some deformation processes can be applied, such as reciprocating extrusion (RE). For this reason, RE was carried out on a 6061 Al matrix by a SiC (20 μm) reinforced composite one. The billets were extruded under a pressure of 17.5 MPa at 573 K with a 10:1 extrusion ratio. The reciprocating extrusions were carried out by using up to 15 passes.     

Fabrication and mechanical properties of in situ TiC/Ti metal matrix composites

In situ TiC particle reinforced titanium matrix composites (TMCs) were successfully fabricated by reactive sintering of Ti + Mo₂C and Ti + VC compacts. The results of the tensile tests at ambient and elevated temperatures show that the strength of the composites increases with increasing additive content (Mo₂C and VC), and decreases with increasing temperatures. Comparing the two types of TMCs, the Ti + VC composites have a lower strength than the Ti + Mo₂C composites, but can more effectively retain the strength to elevated temperatures. Microstructural analyses show that the main strengthening mechanisms of the TMCs are solid solution, grain refinement and particulate strengthening. Different dominant strengthening mechanisms in different composites are responsible for the variations of the mechanical properties. At elevated temperatures, the volume fraction of TiC particles is the main factor for increasing the strength.     

Hot rolling behaviour of stir-cast Al 6061 and Al 6082 alloys - SiC fine particulates reinforced composites

Rolling of wrought aluminium matrix composites with hard phase-reinforcements such as SiC, is interesting to produce sheets for engineering constructions due to their light weight combined with good strength and wear resistance. In this work, the hot rolling behaviour is studied for stir-cast composites with matrix of Al 6061 and Al 6082 alloys and fine SiCp particulates with size of 15 μm and 8 μm and volume fraction up to 30%. For composite casting, optimum casting procedures and materials pre-treatment has been applied for successful insertion of particles into the melt, better particles/matrix wetting and particles distribution, minimized SiC/Al reaction. From thermomechanical simulation, step rolling is defined to be suitable at a strain rate of 1 s⁻¹ rate for each step, using intermediate heat treatment at 450 °C for a period of 10 s to 1 h. Generally, the quality of rolled product was improved with improving casting quality. Successive hot rolling resulted in decreasing void and the agglomeration clusters and hence enhanced mechanical properties are achieved. The flow behaviour under rolling of Al-particulate metal matrix composites, PMMCs, is analysed and the product is characterised for its mechanical properties.