Studies on the fracture and flexural strength of Al/Sip composite

Pure aluminum matrix composite reinforced with a high volume fraction of silicon particles (Al/Si_p) was fabricated by gas-pressured infiltration. The results of four point flexural strength tests show that Al/Si_p has low flexural strength. The analysis of the fractograph reveals the fracture mechanism of Al/Si_p. The fracture of Al/Si_p is primarily dominated by the fracture of brittle silicon particles and the subsequent link up of damage through the matrix. The pre-existent microcracks in silicon particles that were made during the process of compacting will also lower the flexural strength of Al/Si_p composite. The hybrid particle reinforced pure aluminum matrix composite (Al/Si_p+SiC_p) was fabricated in the same way. Results show the flexural strength can be improved by 11.3% compared with Al/Si_p when 6 vol.% silicon particles are replaced by silicon carbide particles with the same volume fraction and size. The reason is that SiC_p with higher fracture stress and higher elastic modulus can prevent the rapid expansion of cracks through the composite and lower the stress in silicon particles.     

Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites

Abstract

SiC_p/Al composites containing high volume fraction SiC particles were fabricated using a pressure infiltration casting process, and their thermophysical properties, such as thermal conductivity and coefficient of thermal expansion (CTE), were characterised. High volume fraction SiC particulate preforms containing 50–70 vol.-%SiC particles were fabricated by ball milling and a pressing process, controlling the size of SiC particles and contents of an inorganic binder. 50–70 vol.-%SiC_p/Al composites were fabricated by high pressure infiltration casting an Al melt into the SiC particulate preforms. Complete infiltration of the Al melt into SiC preform was successfully achieved through the optimisation of process parameters, such as temperature of Al melt, preheat temperature of preform, and infiltration pressure and infiltration time after pouring. Microstructures of 50–70 vol.-%SiC_p/Al composites showed that pores resided preferentially at interfaces between the SiC particles and Al matrix with increasing volume fraction of SiC particles. The measured coefficients of thermal expansion of SiC_p/Al composites were in good agreement with the estimated values based on Turner's model. The measured thermal conductivity of SiC_p/Al composites agreed well with estimated values based on the 'rule of mixture' up to 70 vol.-% of SiC particles, while they were lower than the estimated values above 70 vol.-% of SiC particles, mainly due to the residual pores at SiC/Al interfaces. The high volume fraction SiC_p/Al composite is a good candidate material to substitute for conventional thermal management materials in advanced electronic packages due to their tailorable thermophysical properties.     

Studies on machinability of Al/Si<Subscript>p</Subscript> + SiC<Subscript>p</Subscript> composite materials

This paper presents the experimental results on the machinability of silicon and silicon carbide particles (SiC_p) reinforced aluminium matrix composites (Al/Si_p + SiC_p) during milling process using a carbide tool. The total volume fraction of the reinforcements is 65 vol%. The milling forces, flank wear of the tool and the machined surface quality of composites with different volume fraction of SiC_p were measured during experiments. The machined surfaces of composites were examined through SEM. The results showed that the flexural strength and Vickers hardness are improved when certain volume fraction of silicon particles are replaced by silicon carbide particles with the same volume fraction and particle size and the effect of SiC_p on machinability is optimal when 9 vol% silicon particles in Al/Si_p was replaced by silicon carbide particles with the same volume fraction and the same particle size. Cracks and pits were found on the machined surfaces of composites due to the intrinsic brittleness of silicon particles.     

Microstructure and properties of Sip/Al–Si surface composites prepared by ultrasonic method

Primary Si particles reinforced Al–Si surface composites (Si_p/Al–Si surface composites) were prepared by means of ultrasonic equipment with a special horn crucible. The microstructure and properties of the surface composites were investigated using optical microscope, scanning electron microscopy (SEM), hardness meter and friction and wear tester. The results show that when Al–12%Si alloy was treated by ultrasonic, Si element was easy to move up because of the decrease of the viscosity of the melt, and the alloy composition at the top of the melt became hypereutectic. So, a mass of primary Si particles formed in this place. The thickness of the surface composite layer in the surface composites decreased with increasing the ultrasonic input power. The average size of the primary Si particles in the surface composite layer was larger than that of Al–Si alloy untreated by the ultrasonic and increased with increasing ultrasonic input power. The top layer hardness of Si_p/Al–Si surface composites is higher than that of Al–Si alloy without ultrasonic treatment and increased with increasing ultrasonic input power. The friction coefficients of the top layers of the surface composites are lower than that untreated by ultrasonic. The friction coefficient decreased with increasing ultrasonic input power. With the increase of the applied load, the friction coefficient of the top layer of the surface composites increased. The wear mass loss of Si_p/Al–Si surface composites is lower than that Al–Si alloy without ultrasonic treatment. The wear resistance of the surface composites was improved with increasing ultrasonic input power.     

Fabrication,interface characterization and modeling of oriented graphite flakes/Si/Al composites for thermal management applications

Highly thermally conductive graphite flakes (G_f)/Si/Al composites have been fabricated using G_f, Si powder and an AlSi₇Mg_0.3 alloy by an optimized pressure infiltration process for thermal management applications. In the composites, the layers of G_f were spaced apart by Si particles and oriented perpendicular to the pressing direction, which offered the opportunity to tailor the thermal conductivity (TC) and coefficient of thermal expansion (CTE) of the composites. Microstructural characterization revealed that the formation of a clean and tightly-adhered interface at the nanoscale between the side surface of the G_f and Al matrix, devoid of a detrimental Al₄C₃ phase and a reacted amorphous Al–Si–O–C layer, contributed to excellent thermal performance along the alignment direction. With increasing volume fraction of G_f from 13.7 to 71.1 vol.%, the longitudinal (i.e. parallel to the graphite layers) TC of the composites increased from 179 to 526 W/m K, while the longitudinal CTE decreased from 12.1 to 7.3 ppm/K (matching the values of electronic components). Furthermore, the modified layers-in-parallel model better fitted the longitudinal TC data than the layers-in-parallel model and confirmed that the clean and tightly-adhered interface is favorable for the enhanced longitudinal TC.     

Influence of processing route on electrical and thermal conductivity of Al/SiC composites with bimodal particle distribution

Al/SiC composites with volume fractions of SiC between 0.55 and 0.71 were made from identical tapped and vibrated powder preforms by squeeze casting (SC) and by two different setups for gas pressure infiltration (GPI), one that allows short (1–2 min) liquid metal/ceramic contact time (fast GPI) and the other that operates with rather long contact time, i.e., 10–15 min, (slow GPI). Increased liquid metal–ceramic contact time is shown to be the key parameter for the resulting thermal and electrical conductivity in the Al/SiC composites for a given preform. While for the squeeze cast samples neither dissolution of the SiC nor formation of Al₄C₃ was observed, the gas pressure assisted infiltration led inevitably to a reduced electrical and thermal conductivity of the matrix due to partial decomposition of SiC leading to Si in the matrix. Concomitantly, formation of Al₄C₃ at the interface was observed in both sets of gas pressure infiltrated samples. Longer contact times lead to much higher levels of Si in the matrix and to more Al₄C₃ formation at the interface. The difference in thermal conductivity between the SC samples and the fast GPI samples could be rationalized by the reduced matrix thermal conductivity only. On the other hand, in order to rationalize the thermal conductivity of the slow GPI a reduction in the metal/ceramic interface thermal conductance due to excessive Al₄C₃-formation had to be invoked. The CTE of the composites generally tended to decrease with increasing volume fraction of SiC except for the samples in which a large expansive drift was observed during the CTE measurement by thermal cycles. Such drift was essentially observed in the SC samples with high volume fraction of SiC while it was much smaller for the GPI samples.     

Fabrication of controlled expansion Al-Si composites by pressureless and spark plasma sintering

A combination of low coefficient of thermal expansion (CTE) and decent thermal conductivity (TC) is the reason for the Al-high vol% Si system to become popular for electronic packaging material. In the present work, two process routes, firstly conventional powder metallurgy and then spark plasma sintering (SPS) were utilized for the fabrication of Al-20-60 wt.% Si composites. In addition, effect of small fraction of CNT addition on the CTE of Al-20?wt% Si was studied. Effect of process parameters on the consolidation of the composites in terms of densification, microstructure evolution along with fractographic analysis and strength was studied. CTE and TC of the sintered composites were measured and correlated with the densification, percentage of Si and morphologies of the sintered products. Overall, better densification could be achieved in SPS and the Al-30%Si and Al-40%Si composites SPSed at 550?°C showed average CTE values of 14.52?×?10^?6/K and 13.36?×?10^?6/K, respectively, in the temperature range of 30–200?°C, which were better than some of the existing alloys with higher Si content. Simultaneously, TC values were 114.4?W/mK and 107.12?W/mK, respectively, for the above two SPSed composites.     

Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications

The fabrication process and thermal properties of 50–71 vol% SiC_p/Al metal matrix composites (MMCs) for electronic packaging applications have been investigated. The preforms consisted with 50–71 vol% SiC particles were fabricated by the ball milling and pressing method. The SiC particles were mixed with SiO₂ as an inorganic binder, and cationic starch as a organic binder in distilled water. The mixtures were consolidated in a mold by pressing and dried in two step process, followed by calcination at 1100 °C. The SiC_p/Al composites were fabricated by the infiltration of Al melt into SiC preforms using squeeze casting process. The thermal conductivity ranged 120–177 W/mK and coefficient of thermal expansion ranged 6–10 × 10^–6/K were obtained in 50–71 vol% SiC_p/Al MMCs. The thermal conductivity of SiC_p/Al composite decreased with increasing volume fraction of SiC_p and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiC_p/Al composite decreased with increasing volume fraction of SiC_p, while thermal conductivity was insensitive to the amount of inorganic binder. The experimental values of the coefficient of thermal expansion and thermal conductivity were in good agreement with the calculated coefficient of thermal expansion based on Turner's model and the calculated thermal conductivity based on Maxwell's model.     

Novel MMC microstructures prepared by melt infiltration of reticulated ceramic preforms

Abstract

Alumina/aluminium composites with an interpenetrating microstructure were made by infiltrating an alumina preform which had the structure of a reticulated ceramic foam. Low density preforms (6% relative density)with porous struts were prepared from a polyurethane suspension of alumina powder which was pyrolysed and partially sintered after foaming. Higher density preforms consisted of ceramic foams with open cells and dense struts. All these preforms were infiltrated with 6061 aluminium alloy using a modified squeeze caster fitted with a vacuum system and fine control of speed and pressure. Such MMCs had a microstructure in which two 3D networks were interpenetrating. One was either a ceramic reinforced alloy (50 vol.-%reinforcement) or a dense ceramic phase while the other was the unreinforced alloy. The microstructure of the preform fitted an established relationship between the ratio of window diameter to cell diameter k and void volume fraction V _p. In addition, a short fibre foam was made to compare with the traditional fibre preforms.     

Powder metallurgy of Al-based composites reinforced with Fe-based glassy particles: Effect of microstructural modification

Al metal matrix composites reinforced with high volume fraction of Fe_50.1Co_35.1Nb_7.7B_4.3Si_2.8 glassy particles were fabricated by mechanical milling followed by hot pressing. Elemental Al powders blended with 60 vol.% of glassy particles were mechanically milled for 1, 5, 10, 15, and 20?h, respectively. Selected samples were sintered by uniaxial hot pressing under Ar atmosphere. The changes in the microstructure along with their mechanical properties were investigated. Structural and microstructural characterization followed by microhardness and compression test results of the bulk composite material is reported. The use of high volume fraction of Fe-based glassy particles as reinforcement led to significant hardening of the Al matrix while leading to a remarkable combination of high strength and plasticity.     

Effect of copper content on the thermal conductivity and thermal expansion of Al–Cu/diamond composites

Al–Cu matrix composites reinforced with diamond particles (Al–Cu/diamond composites) have been produced by a squeeze casting method. Cu content added to Al matrix was varied from 0 to 3.0 wt.% to detect the effect on thermal conductivity and thermal expansion behavior of the resultant Al–Cu/diamond composites. The measured thermal conductivity for the Al–Cu/diamond composites increased from 210 to 330 W/m/K with increasing Cu content from 0 to 3.0 wt.%. Accordingly, the coefficient of thermal expansion (CTE) was tailored from 13 × 10⁻⁶ to 6 × 10⁻⁶/K, which is compatible with the CTE of semiconductors in electronic packaging applications. The enhanced thermal conductivity and reduced coefficient of thermal expansion were ascribed to strong interface bonding in the Al–Cu/diamond composites. Cu addition has lowered the melting point and resulted in the formation of Al₂Cu phase in Al matrix. This is the underlying mechanism responsible for the strengthening of Al–Cu/diamond interface. The results show that Cu alloying is an effective approach to promoting interface bonding between Al and diamond.     

On CTE of SPS consolidated SiCp/Al composites with various particle size distributions

Abstract

The coefficient of thermal expansion (CTE) of spark plasma sintering consolidated SiC_p/Al composites with various size distributions was investigated with the combination of experimental measurements and modelling analyses. The CTE of the composites decreased with increasing particle volume fraction, and large particles played a major role in the decline of CTE. The measured CTE lay between the predictions of Kerner model and Schapery lower bound, but the possible formation of percolating particle network and the influence of matrix plasticisation led to the slight deviation of the experimental values from model predictions. A CTE peak appeared for all the composites with increasing temperature to about 250–300°C due to the action of matrix plasticisation filling the microvoids in the composites. The composites with mixed particles of substantially different sizes were prone to concentrate thermal stresses on large particles, which induced an early appearance of matrix plastic deformation that can result in a comparably low CTE peak temperature.     

The thermal expansion and mechanical properties of high reinforcement content SiCp/Al composites fabricated by squeeze casting technology

With mixing different sized SiC particles, high reinforcement content SiCp/Al composites (V_p=50, 60 and 70%) for electronic packaging applications were fabricated by squeeze casting technology. The composites were free of porosity and SiC particles distributed uniformly in the composite. The mean linear coefficients of thermal expansion (20–100 °C) of SiCp/Al composites ranged from 8.3 to 10.8×10⁻⁶/°C and decreased with an increase in volume fraction of SiC content. The experimental coefficients of thermal expansion agreed well with predicted values based on Kerner's model. The Brinell hardness increased from 188.6 to 258.0, and the modulus increased from 148 to 204 GPa for the corresponding composites. The bending strengths were larger than 370 MPa, but no obvious trend between bending strength and SiC content was observed.     

Microstructure and properties of Sip/Al–20 wt% Si composite prepared by hot-pressed sintering technology

In the present work, 50 vol% Sip/Al–20Si composite was prepared by hot-pressed sintering technology. Si particles were uniformly distributed in the Sip/Al–20Si composite, and only the presence of Si and Al phases were detected by XRD analysis. Dislocations, twins, and stacking faults were found in the Si particles. Several Si phases were found to be precipitated between Al matrix and Si particles. Si/Al interface was clean, smooth, and free from interfacial product. HRTEM indicated that the Si/Al interface was well bonded. The average CTE and thermal conductivity (TC) of Sip/Al–20Si composite were 11.7 × 10^?6/°C and 118 W/(m K), respectively. Sip/Al–20Si composite also demonstrated high mechanical properties (bending strength of 386 MPa). Thus, the comprehensive performance (low density and CTE, high TC, and mechanical properties) makes the Sip/Al–20Si composite very attractive for application in electron packaging.     

Effect of C/C preform density on microstructure and mechanical properties of C/C–SiC composites prepared by alloyed reactive melt infiltration

Abstract

Low cost C/C–SiC composites were prepared by alloyed reactive melt infiltration. Effects of the density of C/C preforms on mechanical properties and microstructure of the C/C–SiC composites are reviewed. The results show that with increasing the density of C/C preforms, the flexural strength of the resulting composites increases, while the density of the composites decreases. The flexural strength can reach 341 MPa for the composite produced from the C/C preform of 1·3 g cm^?3. The phases in the composites produced from low density C/C preforms are Si, SiC, ZrSi₂ and carbon, while no Si phase is found in the composites with high density C/C preforms. Furthermore, the mechanism of the microstructure evolution of the C/C–SiC composites is proposed.     

The processing and characterization of sintered metal-reinforced aluminium matrix composites

The current investigation involves the fabrication and characterization of an aluminium metal matrix composite reinforced with sintered metal preforms. Two types of metallic preforms were used (steel and stainless steel) and a variety of squeeze casting conditions were investigated using systematic design-of-experiments techniques to determine the effect of casting conditions on the composite microstructure and mechanical properties. It was observed that a detrimental reaction phase containing iron, aluminium and silicon formed around the metallic preform particles, with a lower volume fraction of reaction phase forming at the lower melt casting temperature. This reaction phase appears to promote premature fracture by facilitating crack initiation and propagation. The stainless steel-reinforced composites had a smaller volume fraction of reaction phase and exhibited superior properties compared to the steel-reinforced composites. This revised version was published online in November 2006 with corrections to the Cover Date.     

反应自生Al2O3-Al3Ti-Al 复合材料的抗弯曲性能

将压力铸造(Squeeze-Casting) 与燃烧合成(Combustion-Synthesis) 相结合, 利用TiO₂与Al 之间的反应, 成功地制备了金属相Al 含量不同的Al₂O₃-Al₃Ti-Al 原位复合材料系列。运用三点弯曲方法测试了复合材料的抗弯曲强度和弹性模量。结果表明: 复合材料具有较高的弯曲强度(410～ 490M Pa) 和弹性模量(156～ 216GPa) , 随着金属相Al 含量的增加, 弯曲强度开始有所升高,当A l 体积百分数超过40% 后便明显下降。而弹性模量始终呈降低趋势, 复合材料的高强度源于反应生成细小的Al₂O₃颗粒及Al₃Ti 相的增强作用。      

Development of light composite materials with low coefficients of thermal expansion

Abstract

Composite materials based on aluminium are used in different fields where weight, thermal expansion, and thermal stability are key requirements. The aim of the present study was to develop a universal method and scientific approach for evaluating the design of lightweight, Al matrix composites with low coefficients of thermal expansion (CTE) and high dimensional stability, and to produce such composites using the vacuum plasma spray (VPS)process. The methodology is general and could be applied to other composite systems. The VPS-produced Al and Al alloy 6061 based composites were reinforced with a variety of ceramic particles including Si₃N₄, B₄C, TiB₂, and 3Al₂O₃.2SiO₂. These composites have low CTE values ((12–13)×10^-6 K^-1), similar to that of steel, and high dimensional stability (capable of keeping dimensions stable with changes in temperature). They have low porosity (98–99%dense) and a uniform distribution of the strengthening particles. Hot rolling of the VPS-formed composites, followed by heat treatment, resulted in a significant improvement in the mechanical properties. Deformed and heat treated 6061 based composites, containing 20 wt-%TiB₂ and 40 wt-%3Al₂O₃?2SiO₂, showed excellent mechanical properties (ultimate tensile strength 210–250 MPa, elongation >4%).     

A theoretical approach for the thermal expansion behavior of the particulate reinforced aluminum matrix composite Part I A thermal expansion model for composites with mono-dispersed spherical particles

Microstructural observation revealed that the increase in the volume fraction of SiC particles lowers the coefficient of thermal expansion (CTE) of the composite, and the CTE of the metal matrix composites is proportional to the size of the Si phase. To analyze the thermal expansion behavior of aluminum matrix composites, a new model for the CTE of the mono-dispersed binary composite on the basis of Ashelby's cutting and welding approach was proposed. In the theoretical model, it was considered that during cooling relaxation of residual stresses could create an elasto-plastic deformation zone around a SiC or Al₂O₃ particle in the matrix. The size of reinforced particles and other metallurgical factors of the matrix alloy and composite were also considered. In this model, the interacting effect between the reinforced hard particle and the soft matrix is considered by introducing the influence of the elasto-plastic deformation zone around a particle, which is distinguished from the previous models. It was revealed that the CTE of the composite are influenced by the particle volume fraction, the elastic modulus and Poisson's ratio as well as the elasto-plastic deformation zone size and the particle size.     

Investigation of thermal properties of Al–Si matrix reinforced fine SiCp composites

Abstract

The characterisation of thermal expansion coefficient and thermal conductivity of Al–Si matrix alloy and Al–Si alloy reinforced with fine SiC_p (5 and 20 wt-%) composites fabricated by stir casting process are investigated. The results show that with increasing temperature up to 350°C, thermal expansion of composites increases and slowly reduces when the temperature reaches to 500°C. The values of both thermal expansion and conductivity of composites are less than those for Al–Si matrix. Microstructure and particles/matrix interface properties play an important role in the thermal properties of composites. Thermal properties of composites are strongly dependent on the weight percentage of SiC_p.