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Al–SiC and hybrid Al–SiC-graphite MMCs with up to 70 vol% SiC and 10 vol% graphite were fabricated using spark plasma sintering. Detailed studies of the microstructure, Vickers hardness, longitudinal elastic constant C11, as well as thermal shock resistance of the MMC samples were carried out. Non-destructive ultrasound phase spectroscopy was used to measure the C11 elastic constant of the MMCs, and the measured values were compared with different micromechanical models for particle reinforced composites. For both two- and three-component MMCs, the Hashin Shtrikman lower bound fitted best to the experimental results in the absence of any porosity. Thermal shock resistance of the MMCs was carried out by heating in air to 500 °C and subsequently quenching in water for ten times. The extent of thermal shock-induced structural alteration was determined via a systematic study of the change in density, hardness, and C11 of the samples. It is observed that the thermal shock resistance of the MMCs depends upon multiple factors such as SiC content, graphite content, and the amount of residual porosity. While the reduction in mechanical properties of the Al–SiC MMCs due to graphite addition is according to the expected line, the present study shows that graphite addition also reduces the thermal shock resistance of the MMC.

Metal/ceramic composites are in high demand in several industries because of their superior thermo-mechanical properties. Among various composite types, the interpenetrating phase composites (IPCs) with percolating metallic and ceramic phases offer manifold benefits, such as a good combination of strength, toughness, and stiffness, very good thermal properties, excellent wear resistance, as well as the flexibility of microstructure and processing route selection, etc. The fabrication of metal/ceramic IPCs typically involves two steps - i) processing of an open porous ceramic body, and ii) infiltration of metallic melt in the pores to fabricate the IPC. Although significant progress has been made in recent years for developing both porous ceramics and melt infiltration methods, to the best of the knowledge of the authors, no review article summarizing all the aspects of processing and properties of IPCs has been published till date. This review article is aimed at filling this gap. Starting with a brief introduction about the current status and applications of IPCs, the various processing routes for fabricating open porous ceramic preforms and melt infiltration techniques have been discussed. Subsequently, the data available for various important physical, mechanical, and thermal properties for IPCs have been critically analyzed to thoroughly understand their dependence on various structural and processing parameters. To compare the properties of IPCs with other relevant materials, seven different Ashby material property maps have been used, and the domains for IPCs have been created in them. For each map, the concept of material indices has been employed to critically discuss how IPCs perform in relation to other material classes for various optimum design conditions. Finally, a detailed future outlook for further research on IPCs has been provided.

Metal matrix composites consisting of Al-alloy as matrix and B4C particles as ceramic reinforcement are widely used for numerous advanced applications requiring high strength, hardness, wear resistance, good thermal properties, neutron radiation shielding characteristics, etc. Among the processing routes for such composites, the liquid metallurgy route possesses the advantages of being cost-effective and capable of manufacturing bulk composites. Numerous efforts were made so far to employ different Al-alloys and a wide range of processing parameters for fabricating such composites via casting route. These studies display a considerable variation in the composites' final mechanical properties and therefore warrants proper selection of the processing parameters to target for the desired applications. Within the scope of this work, the vast literature available on cast Al-B4C composites has been systematically reviewed to critically analyze the influence of the various processing parameters on different relevant mechanical properties. Based on the thorough analysis, the combinations of processing parameters that yield optimum mechanical performance in the case of some selected Al-alloy-based composites have been identified. The inferences drawn in this work will immensely facilitate the selection of process parameters for the development of future particle-reinforced cast Al-B4C composites.

Interpenetrating phase composites (IPCs) are a class of composites consisting of two or more topologically co-continuous phases and have three-dimensionally percolating interconnected structure. The co-continuous and interconnected architecture of the IPCs can provide superior properties compared to the conventional particulate reinforced composites at room and elevated temperatures. IPCs having metal/ceramic phases are finding applications in various fields like (thermal management materials, aerospace, automobile industries, light-weight armor materials, etc.). In the present work, Al-Si alloy/SiC foam IPC was fabricated by infiltrating the alloy melt into SiC foam via the squeeze-casting process. Open porous SiC foams, fabricated via the replication method, and having two different pore sizes (10 and 20 PPI) were used for IPC fabrication. The SiC foams had both macropores between the SiC foam struts and micropores within the strut walls. Results show that the Brinell hardness of the composite fabricated using a small pore size (20 PPI) SiC foam is higher than the composite fabricated with a large pore size SiC foam (10 PPI). Thorough structural characterization via scanning electron microscope and localized study of hardness distribution in different regions of the composite using nanoindentation were carried out.

Elastic properties, compressive stress–strain behaviour and progressive damage evolution of poly-domain metal/ceramic composite samples fabricated by infiltration of Al12Si melt in freeze-cast alumina preforms are studied. Two different preform freezing temperatures were employed to vary the lamellae size while infiltration was carried out using two different techniques – squeeze-casting and die-casting. Due to the faster cooling kinetics at the lower freezing temperature, the lamellae size in the composites based on these preforms are finer and this results into higher compressive strength and stiffness of this composite along the freezing direction. Among the two techniques employed for melt infiltration, the very fast rate of pressure application in die-casting distorts the lamellar structure of the ceramic along the freezing direction. As a result, in die-cast composite samples, the strength and stiffness along the freezing direction are reduced significantly in comparison to the samples infiltrated by squeeze-casting. In-situ scanning electron microscopy under external compression was used to study the progressive damage mechanism in one poly-domain composite sample infiltrated by squeeze-casting. Transverse cracking of the high-angle ceramic lamellae is identified as the predominant damage mechanism.

The mechanism of internal load transfer in a newly developed interpenetrating metal/ceramic composite is studied for the first time in this work using energy dispersive synchrotron X-ray diffraction. The composite was fabricated by infiltration of Al12Si melt in an open porous alumina preform, fabricated using a mixture of two different polymer waxes as pore formers, by gas pressure infiltration. Several diffraction planes of all three crystalline phases of the composite – alumina, silicon and aluminum solid solution are investigated. Lattice strains, both along and transverse to the loading direction, in each phase are calculated from the shifts in the measured diffraction peak positions. Results show that until about 100 MPa applied compressive stress, all three phases undergo elastic deformation. At higher applied stresses aluminum undergoes plastic deformation and correspondingly load is transferred to alumina. The load carrying capability of alumina is however limited by incipient damage in this phase. At applied compressive stresses higher than 187 MPa, load carried by the alumina phase continuously decreases and correspondingly more load is again carried by the aluminum solid solution. Throughout, the fraction of load carried by silicon remains almost constant, suggesting no significant load transfer within the metallic alloy itself.

Thorough analysis of the elastic as well as the thermal expansion behaviour of an interpenetrating Al12Si/alumina composite material is carried out in this work. The open porous preforms used for composite fabrication are also processed in this work using 1:1 mixture of one fine and one coarse polymer wax as pore formers. The ratio of ceramic powder to pore formers is varied to fabricate preforms having different ceramic contents. Study of the elastic properties shows that while the porous preforms display transverse isotropic behaviour, the composites behave as quasi-isotropic material with the composite Young's modulus being closest to the predictions of the Feng model for interpenetrating composites. Study of the thermal expansion behaviour of the composite shows that the composite behaviour is controlled by the unreinforced metallic alloy. In spite of its percolating interpenetrating phase morphology, the evolution of thermal expansion coefficient of the composite with both temperature and ceramic content shows that particle reinforced composites predict the behaviour of the composite more closely. This has been discussed in light of the microstructure of the studied composite material and an outline of the proposed future research is provided.

A systematic methodology for developing hybrid Al–SiC-graphite functionally graded composite materials with the highest feasible SiC-content in the top-most layer, without resulting in residual porosities, has been established. The distribution of ceramic reinforcements in individual layers is very homogeneous, and the interlayer regions are free from any defect. Monolithic composites, having composition identical to the overall average compositions of the FGMs, were also fabricated, and their mechanical properties were compared with the corresponding FGM properties. Due to the locally high SiC-content, the top-layer hardness of the FGM is significantly higher than the corresponding monolithic composite. A systematic study of the flexural stress-strain behavior of individual mono-layers and the overall FGM was carried out at different orientations. For identical mono-layer compositions, the orientation of the FGM has a strong influence on its flexural stress-strain behavior. The failure stress is significantly higher when the layer containing high SiC-content is at the compressively loaded side during the 4-point bend test.

Thermal expansion behavior of an interpenetrating metal/ceramic composite having ceramic contents in the range of 34−60 vol.% is studied in this work. Four thermal cycles were carried out between room temperature and 500 °C at a constant heating/cooling rate of 5 °C/min. Both thermal strain and thermal expansion coefficient decrease with increasing ceramic content in the composite. Evolution of the thermal expansion coefficient with temperature shows anisotropic nature. At all ceramic contents, highest thermal strain and thermal expansion coefficient are obtained along the preform press direction. The extent of thermal expansion anisotropy depends upon both temperature and ceramic content in the composite. The evolution of the observed thermal expansion anisotropy has been attributed to the elastic anisotropy of the preforms and composites, considering as well the elastic-plastic flow behavior of the metallic matrix. The results have been compared with several analytical theoretical models.

Internal load transfer in an interpenetrating metal/ceramic composite has been studied in this work using energy dispersive synchrotron X-ray diffraction. One of the samples was loaded in tension and the other one in compression. In each case, the sample was first loaded into the elastic-plastic regime, unloaded to zero stress, and reloaded beyond the prior maximum stress. Results show that at stress amounts greater than 100 MPa aluminum deforms plastically and the load is transferred to alumina and silicon. Unloading and reloading typically show reverse plastic deformation, Bauschinger effect and strain hardening in aluminum.

The elastic–plastic deformation and internal load transfer in metal/ceramic composites are studied in this work both numerically and experimentally. The composite was fabricated by squeeze-casting AlSi12 melt in an open porous preform made by freeze-casting and drying of alumina suspension. Such composites exhibit a complex microstructure composed of lamellar domains. Single-domain samples were extracted from bulk material. Uniaxial compression tests were carried out parallel to the direction of the alternating metallic alloy and ceramic lamellae in the plane normal to the direction of freeze-casting. This loading mode is selected as highest load transfer occurs when loaded along the ceramic lamellae. Numerical modeling was done using the finite element method using quasi-3D microstructure based on metallographic 2D section and a modified Voigt homogenization technique assuming plastic behavior of the metallic alloy, absence of any damage and ideal interface between the phases. Internal load transfer mechanism was predicted for composites with different ceramic volume fractions. Results show that at any applied stress, as the ceramic content increases, the phase stress in alumina along the loading direction continuously decreases. Experimental validation of the numerical results is carried out by in-situ compression test along with energy dispersive synchrotron X-ray diffraction in one sample with 41 vol% ceramic. Results show that both the numerical techniques yield similar results, which match well with the experimental measurements. The ratio of the phase stress to the applied stress in alumina reaches a highest value between 2 and 2.5 up to a compressive stress of about 300 MPa. At higher applied stresses both the experimentally determined lattice microstrain and the phase stress along the loading direction in alumina decrease due to the initiation of possible damage. This study shows that the applied economic and more flexible homogenization technique is a viable tool for modeling of this composite structure.

The internal load transfer and compressive damage evolution in an interpenetrating Al 2 O 3 /AlSi12 composite have been studied in this work. The composite was fabricated by squeeze-casting eutectic aluminium–silicon alloy melt in a porous alumina preform. The preform was fabricated from a mixture of cellulose fibres and alumina particles via cold pressing and sintering. In an earlier work we reported the internal load transfer in the same composite material under monotonic compression and tension studied using energy dispersive synchrotron X-ray diffraction [20] . The current work is a continuation of this earlier study, aimed at obtaining further understanding about load transfer occurring during load reversal and damage behaviour during external compression. The micromechanical load partitioning between the three phases present in the composite is studied during one load cycle starting in compression followed by unloading and reloading in tension until failure. Average strain and stress value in each phase is calculated from several diffraction planes of each phase and as a result the reported strain and stress are representative of the bulk material behaviour. The load transfer results allow identifying the occurrence of a substantial Bauschinger effect in the Al solid solution phase and progressive damage evolution within the alumina phase. In situ compression test inside a scanning electron microscope showed that failure of the composite occurred by propagation of cracks through the ceramic rich regions, oriented at approximately 45° to the loading direction.

Silicon carbide reinforced aluminum alloy matrix composites offer excellent thermo-mechanical properties and are thus attractive for applications limited by thermal stresses. Open porous silicon carbide preforms are fabricated in this work using polymer wax as pore formers. Two different waxes with different particle size were used to fabricate preforms with different pore structures. Wax content was varied to introduce open porosities up to 64 vol%. Structural characterization was carried out using scanning electron microscopy and micro computed tomography, whereas ultrasound phase spectroscopy was used to determine three longitudinal and three shear elastic constants. The amount of porosity increases with the amount of total wax used. Uniaxial pressing prior to isostatic pressing flattens the pores and as a result the preforms behave transverse isotropically with respect to the press direction. For the same total wax content, while the mixture ratio of two waxes has a minor influence on total porosity, the mixture ratio strongly influences the elastic constants. Optimum elastic constants along all directions are obtained with a mixture of two wax types with a higher content of the larger wax particles.

The complete stiffness matrices of several metal/ceramic composites were analysed using the complementary ultrasonic spectroscopic techniques ultrasound phase spectroscopy (UPS) and resonant ultrasound spectroscopy (RUS). Three different aluminum/alumina composites having complex interpenetrating architectures were studied: a composite based on freeze-cast ceramic preform, a composite based on open porous ceramic preform obtained by pyrolysis of cellulose fibres, and a composite based on discontinuous fibre preform. Six of the nine independent elastic constants describing orthotropic elastic anisotropy were pre-determined by ultrasound phase spectroscopy and used as initial guess input for resonant ultrasound spectroscopy analysis, making the final results of all nine elastic constants more reliable. In all cases, consistent and reproducible results are obtained. Finally the experimental results were compared with effective elastic constants calculated using micromechanical modelling and a good correspondence between both is obtained.

An interpenetrating composite fabricated by squeeze-casting AlSi12 melt in a porous alumina preform is studied in this work. The preform was fabricated by pyrolysis of cellulose fibres used as a pore-forming agent and subsequent sintering of alumina particles. The resulting preform had both micropores within the ceramic walls and macropores between those walls, which were infiltrated by the liquid metal. Lattice strains under externally applied tension and compression in all three phases of the composite were measured using energy dispersive synchrotron X-ray diffraction. Multiple diffraction peaks in each phase were analysed, allowing information about the interplanar anisotropy of each phase to be obtained. Results show that load transfer occurs from aluminium to stiffer and stronger alumina and silicon phases in both elastic and plastic regions. Stresses as well as stress concentration factors along the loading direction in all three phases were calculated from the measured continuum mechanics equivalent lattice strain and the macroscopic elastic constants of each phase. The stress concentration factor in aluminium is less than unity throughout, and it decreases with increasing applied stress owing to load transfer. The maximum stress in alumina along the loading direction is reached in the region of plastic deformation of aluminium. Subsequently, at applied stresses higher than −300 MPa in the sample under compression, stress in alumina along the loading direction decreases owing to damage.

Aluminium/alumina composites based on ceramic preforms prepared via freeze-casting are examined. Domains composed of alternating but also interpenetrating ceramic and metallic lamellae are observed. Single-domain samples were extracted from composites processed under different conditions. In situ scanning electron microscopy analyses were carried out to investigate the damage evolution under compressive load. The composite is strong and brittle when loaded along directions parallel to the freezing direction. When compressed in other directions, the behavior is controlled by the soft metal. The plastic anisotropy is less pronounced than theoretical predictions for laminates, which is explained by the presence of bridges between the ceramic lamellae. Coating the preform with Cu or Cu2O prior to melt infiltration reduces the compressive strengths of the composites. Transmission electron microscopy analysis shows that even in the case of a Cu coating a Cu2O layer is formed during processing, weakening the interface and preventing the dissolution of Cu in the aluminium alloy.

The aim of this study is to analyze the mechanics of a new class of metal/ceramic composites on a mesoscopic length scale. These composites are produced by melt infiltration of porous ceramic preforms produced by freeze casting and subsequent sintering. This production route has a high application potential since it offers a cost-effective way to obtain composites with ceramic content in the 30 to 70 vol.%range. The as-produced material exhibits a hierarchical domain structure with each domain composed of alternating layers of metallic and ceramic lamellae. Residual stresses present in all phases of the composite produced by infiltrating alumina preforms with a eutectic aluminum-silicon alloy have been measured. Integral as well as spatially resolved measurements were carried out on single-domain samples at the high-energy, energy-dispersive diffraction (EDDI) beamline at the synchrotron radiation source BESSY (Berlin, Germany). Results show that strongly fluctuating residual stresses are introduced by the production process, which can be rationalized by taking into account the thermal expansion mismatch of alloy and preform.

Freeze casting offers a new technique to fabricate ceramic preforms for metal/ceramic composites. Internal load transfer under external compressive loading in such composites has been studied for the first time using energy dispersive synchrotron X-ray diffraction. The results show that load transfer takes place from the soft metallic alloy to the hard ceramic which has been explained in the light of generalized Hooke's law and classical laminate theory.

The elastic properties of single domains of lamellar AlSi12/Al2O3 composites produced by metal infiltration of freeze cast preforms have been examined. The anisotropic elastic constants determined from ultrasonic phase spectroscopy (UPS) experiments have been compared to microstructure based FE-models created with the program OOF2, micromechanical models (Mori–Tanaka and inverse Mori–Tanaka) and an analytical model for an ideal laminate. The influences of lamellae orientations and ceramic contents on the elastic constants have been investigated. Along the lamellae directions the microstructure based FE model and the inverse Mori–Tanaka model are in good agreement with experimental results. Perpendicular to the lamellae the experiment shows a stiffer than expected elastic behaviour.

Innovative metal/ceramic composites produced by melt infiltration of ceramic preforms prepared by a freeze-casting technique are examined in this study. These composites exhibit a characteristic hierarchical structure: On a mesoscopic length scale, lamellar domains with sizes up to several millimetres are observed. The aim of the present study was to analyze the anisotropic elastic properties of such a composite on different length scales. Experimental studies were carried out on samples containing about 44% alumina ceramic and 56% aluminium-based alloy (Al–12Si). Ultrasonic wave velocity measurements were carried out on macroscopic poly-domain samples as well as on miniature single-domain samples. The elastic constants derived from these measurements are discussed in light of elasticity models established for lamellar and for fibrous composites.

The aim of this article is to compare the influence of the phase architecture on the mechanical properties of two interpenetrating MMCs with same metallic and ceramic phases and similar phase contents. One of the composites was fabricated by infiltrating freeze-cast alumina preforms, while the other composite was fabricated by infiltrating open porous alumina foam. Tests were carried out to determine the three longitudinal elastic constants, elastic-plastic flow behavior under compression and mechanism of internal load transfer under compression. Results show that phase morphology has a significant influence on the composite mechanical properties. Highest stiffness and compressive strengths are observed along the freezing direction in the freeze-cast MMC and this result from the significantly higher fraction of load carried by the alumina phase in this MMC type. Foam based MMC shows more isotropic behavior and its properties lie between the longitudinal and transverse properties for freeze-cast MMC.

Thorough elastic analysis of porous silicon carbide preforms fabricated using two different polymer waxes as pore formers is carried out. Both the amount and the mixture ratio of the waxes were varied to fabricate preforms with different pore morphologies and porosities in the range of 27–67 vol%. Results show that both the longitudinal and the shear elastic constants decrease with increasing porosity. The rate of decrease in the elastic constants follows a model based on minimum solid area up to an intermediate porosity level. Uniaxial pressure applied prior to cold isostatic pressing and sintering significantly reduces the stiffness along the press direction. For the same initial powder mixture type, the elastic anisotropy of the preforms increases with an increase in the applied uniaxial pressure. The extent of anisotropy is strongly dependent on both the SiC/wax ratio as well as the mixture ratio between the two wax types. At low pore volume fractions a higher volume content of the smaller diameter wax and at high pore volume fractions a higher volume content of larger diameter wax lead to preforms with lowest anisotropy. A map is finally proposed to describe the dependence of the preform elastic properties on the type of initial powder mixture used.

This article provides an overview of recent research carried out about the mechanical properties of an innovative metal/ceramic composite fabricated by squeeze-casting liquid Al-12Si in freeze-cast alumina preforms. The composite consists of domains made of alternating metallic and ceramic lamellae. Elastic and elastic-plastic deformation behavior and mechanism of internal load transfer under external compressive stresses in individual domains with different orientations were studied. Results show that the stiffness is highest along the freezing direction and lowest along the direction perpendicular to it. When compressed along directions parallel to freezing direction, the composite is strong and brittle. When compressed along other directions, the behavior is controlled by the soft metallic alloy. Studies of internal load transfer under external compressive stress along different directions show that the mechanism of internal load transfer differs significantly when loading direction changes from that along the freezing direction to the direction perpendicular to it.

An interpenetrating composite fabricated by squeeze-casting a eutectic aluminium–silicon alloy into a porous alumina preform is studied in this work. The preform was fabricated by pyrolysis of cellulose fibres used as pore forming agent, pressing of the green ceramic body and subsequent sintering of alumina particles. The resulting preform had both micropores within the ceramic walls and macropores between those walls, which were infiltrated by the liquid metal. Composites with alumina contents varied in the range of 18–65 vol.% were studied. Three longitudinal and three shear elastic constants of the composites were determined using ultrasound phase spectroscopy on rectangular parallelepiped samples. Complete stiffness matrix of one sample was determined by modifying the sample geometry by cutting at the corners of the sample and subsequent ultrasonic measurements. All composites exhibit a moderately anisotropic behavior, which can be attributed to a non-random pore orientation distribution caused by uni-axial pressing of the preforms prior to sintering. The experimental results are compared with several theoretical micromechanical models.

Mechanical properties of cellular hollow sphere structures are studied in this work. The material was fabricated by coating the metallic powder slurry on expanded polystyrol (EPS) spheres, drying, forming under compression, debinding, and final sintering of the spheres to each other. Longitudinal elastic wave velocities were measured using ultrasound phase spectroscopy while compression tests were carried out up to a homologous temperature of 0.6. Dependence of the relative Young’s modulus on the relative density is similar to conventional open-cell foams. Compression stress–strain plots show the three stages of elastic deformation, plateau, and densification. With increasing temperature the overall level of the compressive stress–strain plots shifts to lower stresses. The hollow sphere solids show slightly better high temperature strength in comparison to the base metal. However, due to the considerable scatter in the experimental data points, this improvement seems to be insignificant. Structural observations on samples deformed to within the plateau region clearly show the heterogeneous progress of deformation.

Metal-ceramic composites are widely applied in the different brunches of industry. The composites are produced by squeeze-casting of the ceramic preform by molten aluminum alloy. The lamellar microstructure is obtained during freezing of ceramic suspension. The internal structure of the domains can be controlled via freeze-casting parameters. The material has high anisotropy level and its effective properties depend on lamella orientation. The aim of this study is numerical simulation of the inelastic behavior of the material and its verification by experiment. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)