Alumina Preforms Research Articles

Infiltration and directional solidification of CMSX-4 through a particulate ceramic preform

A potential method for producing selectively reinforced, single-crystal superalloy castings was investigated. Porous particulate alumina preforms were infiltrated with the liquid Ni-based superalloy CMSX-4 under a gas pressure of about 1 atm, and then the alloy was solidified directionally through the preform at different rates. Optical microscopy and Laue X-ray diffraction showed that the orientation of the metallic grains was unaffected by the preform and that no stray grains were nucleated there. In the particular experimental configuration, single grain orientations (i.e., single crystals) were achieved at cooling rates ≤15 °C/min. Furthermore, the microsegregation pattern in the composite region showed that the alloy solidified from the center of the interstices toward the alumina particles, further indicating that nucleation did not occur on the preform. Microsegregation in the composite region is lower than in the unreinforced regions due to geometric confinement of solidification in the narrow spaces between the ceramic particles.

Liquid Phase Sintering of Alumina, II. Penetration of Liquid Phase into Model Microstructures

Alumina preforms containing artificial pores were sintered at 1630°C in air and vacuum. Glass penetration into the alumina preforms was conducted at 1600°C in air. It was found that the trapped gases in alumina preforms sintered in air caused the random and incomplete filling of the smaller and larger artificial pores. In contrast, the pores in the alumina preform sintered in vacuum were completely filled during glass penetration.

Residual stresses and magnetic properties of alumina–nickel nanocomposites

Nanocomposites were produced by infiltration of alumina preforms with a nickel-nitrate solution, followed by reduction and sintering. Multiple infiltration and reduction steps resulted in nanocomposites with varying Ni particle concentrations. The strain in the nickel particles was evaluated from X-ray diffraction, and correlated with the magnetic properties of the nanocomposites.

Internal Strain Measurements and X-ray Imaging in Interpenetrating-Phase Al2O3/Al Composites

ABSTRACTInterpenetrating Al2O3/Al composites were created by liquid-metal infiltration of alumina preforms with three-dimensional periodicity produced by a robotic deposition method. Volume-averaged lattice strains in the alumina phase were measured by synchrotron x-ray diffraction at various uniaxial compression stresses up to 350 MPa. Load transfer, which is experimentally found to occur between the aluminum and the alumina phase, is in agreement with simple rule of mixtures models. Spatially resolved measurements showed variations in load transfer at different positions within the composite for the elastic-, plastic-, and damage-deformation regimes. Using phase-enhanced imaging, the extent of damage within the composites was observed.

Nickel–alumina nanocomposite powders prepared by novel in situchemical reduction

Nanocomposite powders of Ni–Al2O3 (5 and 10 vol% Ni) were prepared from porous alumina preforms with high specific area (100 m2 g?1) impregnated with nickel nitrate. Samples were obtained by reduction under a controlled oxygen partial pressure either directly or through an intermediate step, giving nickel aluminum spinel. In both cases, homogeneous dispersions of nickel particles in the alumina matrix were achieved. The Ni particle size ranged from 10 to 100 nm, depending on the preparation conditions and temperature.

Infiltration and physical characteristics of functionally graded alumina/calcium-hexaluminate composites

A novel route to simple processing of functionally graded materials (FGMs) based on alumina/calcium-hexaluminate (CA 6), and the associated infiltration and physical characteristics are described. The processing involves partial infiltration of a porous alumina preform with hydrolysed calcium acetate to yield a homogeneous layer of alumina and a heterogeneous-graded layer of CA 6/alumina. Two contrasting heat-treatment schedules, namely pre-sintered annealing and post-sintered annealing, were used to investigate their influence on the resultant phase compositions and physical properties. Analysis by X-ray diffraction has revealed the existence of concentration gradients, the CA 6 content decreasing with increasing sample depth. The presence of CA 6 appears to hinder the shrinkage and thus densification during sintering, as well as hindering grain growth of alumina matrix. Scanning electron microscopy has revealed the presence of plate-like morphology of CA 6 grains and the graded microstructure within the FGM.

New technique for pressureless infiltration of Al alloys into Al2O3 preforms

Al alloys were infiltrated into alumina preforms without the aid of pressure in N2 as well as in air at and above 750 °C. It was possible to eliminate termination of infiltration that was seen in open conditions (where N2 was in continuous contact with the melt) by modifying the infiltration geometry. This configuration enables the infiltration to continue for longer periods of time, consequently producing greater thickness of composite. In air, Mg placed at the interface getters the in-coming oxygen until the alloy billet melts and seals off the front from the ingress of the furnace atmosphere thereby eliminating the need for prealloying Al with Mg and N2 atmosphere. In addition, experiments in argon revealed that the infiltration requires some critical amount of N2 in the atmosphere.

Equilibrium Amorphous Silicon–Calcium–Oxygen Films at Interfaces in Copper–Alumina Composites Prepared by Melt Infiltration

The microstructure of copper–alumina (Cu‐Al2O3) composites that have been prepared via the melt infiltration of liquid copper into porous alumina preforms was studied in detail, using various transmission electron microscopy (TEM) techniques. Two different samples—with open pore diameters of 0.2 and 0.8 μm—were investigated. For both specimens, a single crystalline copper network that extended throughout the open porosity of the alumina preform was observed. An amorphous glass phase that contained silicon and calcium was observed at the Al2O3/Cu/Al2O3 triple junctions. The diameters of these amorphous pockets, which were strongly faceted along the Al2O3 grains, were up to 20 and 100 nm for the initial pore sizes of 0.2 and 0.8 μm, respectively. A glass phase that contained silicon and calcium also was present at the Cu/Al2O3 interfaces, whereas the Al2O3 boundaries remained dry. Detailed high‐resolution transmission electron microscopy investigations have shown that the interfacial glass phase at the Cu/Al2O3 interfaces exhibited a uniform equilibrium film thickness along the interface region. However, the interfacial film thickness was dependent on the orientation of the Al2O3 grain, and its value varied from 0.4 nm for Al2O3 rhombohedral‐plane termination ((1¯012)) up to 1 nm for Al2O3 basal‐plane termination ((0001)).

Bi-continuous metal matrix composites

Bi-continuous alumina/aluminium composites were made by infiltrating an alumina preform which had the structure of a reticulated ceramic foam. The low density preforms were prepared from a polyurethane suspension of alumina powder which was pyrolysed and sintered after foaming. Higher density preforms consisted of ceramic foams with open cells. 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. 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). Low k foams were infiltrated fully but on cooling below the solidus, interfacial debonding took place due to differential thermal contraction. This was overcome by modifying the processing conditions. High k foams which had high fractional porosity, retained sound interfacial bonding. The composites possess higher elastic modulus than conventional MMCs with a homogeneous reinforcement distribution at a given volume fraction. The loss of electrical conductivity is negligible in the lower volume fraction range because of the three dimensionally continuous aluminium phase. The experimental results are compared with a number of theoretical predictions.

On some distinctive features of the structure of composite ceramic materials obtained by the method of directed reaction impregnation

The method of directed reaction impregnation (DRI), known as a Lanxide process in foreign countries, provides a wide spectrum of ceramic composite materials with a variable content of metallic phase, which possess high physicomechanical properties. An important advantage of the method is the absence of shrinkage, which diminishes substantially the amount of heat-treatment rejects and the cost of the finishing mechanical treatment. The structural characteristics of composites obtained by impregnation of the pore space of \-SiC and A12O3 ceramic matrices with an aluminum alloy with 5 wt.% Si and 2 wt.% Mg are presented. The former matrix is obtained by free pouring of a granular powder of silicon carbide, and the latter matrix is represented by a densely sintered alumina preform with through cylindrical channels. The process is conducted at 1150 – 1200‡C for 24 h in air. The structure of the composites bears Al2O3 crystals and a mixed Al/Al2O3 phase formed from the metallic melt. The microhardness of the phases that compose the structure of the composite with a silicon carbide matrix is determined (HV = 28.4 – 73.2 GPa for Al2O3,HV = 10.3 – 19.7 GPa for SiC, andHV = 0.5 – 0.8 GPa for A1/A12O3). Directed reaction impregnation through cylindrical channels of the aluminum oxide matrix yields highly porous fibers with a typical dimple structure. The impregnation process in this composite is considered from the standpoint of equilibrium (the Le Chatelier principle). Its occurrence in different stages is considered from the standpoint of thermodynamics.

Novel MMC microstructures prepared by melt infiltration of reticulated ceramic preforms

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 Vp. In addition, a short fibre foam was made to compare with the traditional fibre preforms.

Fracture and R-curves in high volume fraction Al2O3/Al composites

Fracture toughness and fracture mechanisms in Al2O3/Al composites are described. The unique flexibility offered by pressureless infiltration of molten Al alloys into porous alumina preforms was utilized to investigate the effect of microstructural scale and matrix properties on the fracture toughness and the shape of the crack resistance curves (R-curves). The results indicate that the observed increment in toughness is due to crack bridging by intact matrix ligaments behind the crack tip. The deformation behavior of the matrix, which is shown to be dependent on the microstructural constraints, is the key parameter that influences both the steady-state toughness and the shape of the R-curves. Previously proposed models based on crack bridging by intact ductile particles in a ceramic matrix have been modified by the inclusion of an experimentally determined plastic constraint factor (P) that determines the deformation of the ductile phase and are shown to be adequate in predicting the toughness increment in the composites. Micromechanical models to predict the crack tip profile and the bridge lengths (L) correlate well with the observed behavior and indicate that the composites can be classified as (i) short-range toughened and (ii) long-range toughened on the basis of their microstructural characteristics.

Microstructures and properties of Al2O3/Al–AlN composites by pressureless infiltration of Al-alloys

Al alloys were infiltrated into Al2O3 preforms in N2 and N2- 2% H2 gas mixture in the temperature regime of 900-1200°C. The kinetics of nitridation during infiltration were continuously monitored by recording weight gained during infiltration of the preform. The weight gains that are attributed to the formation of AlN in the matrix were observed to increase with processing temperature. In addition, higher weight gains were recorded in N2-H2 as compared to in N2 atmosphere. Analysis of the composites indicates that controlled matrices of AlN/Al can be formed by selection of appropriate process parameters viz. temperature and atmosphere. Based on this study it has been demonstrated that net shaped metal matrix composites (MMCs), with varying amounts of AlN in the matrix can be fabricated by infiltrating alumina preforms at $900-1000^oC$ in commercial nitrogen. By controlling the preform characteristics, it is possible to fabricate composites with significant variation in microstructural scale, and consequently matrix hardness and elastic modulus.

Characteristics of a layered and graded alumina/calcium-hexaluminate composite

A novel route to simple processing of a layered and graded material (LGM) based on alumina/calcium-hexaluminate (CA 6) is described. The processing involves partial infiltration of a porous alumina preform with hydrolysed calcium acetate to yield a homogeneous layer of alumina and a heterogeneous graded layer of CA 6/alumina. Analysis by X-ray diffraction (XRD) has revealed the existence of concentration gradients, the CA 6 content decreasing with increasing sample depth. In-situ high temperature neutron diffraction (HTND) shows that the formation temperatures of CA, CA 2, and CA 6 are respectively 1000°, 1200° and 1350°C. The presence of CA 6 appears to hinder the shrinkage and thus densification during sintering, as well as causing a reduction of hardness and flexural modulus. Scanning electron microscopy (SEM) has revealed the presence of plate-like morphology of CA 6 grains and the graded microstructure within the LGM.

Thermodynamic modelling of directed melt oxidation processing for ceramic matrix composites

Equilibrium thermodynamic modelling is used to predict the likely species that will form in the production of ceramic matrix composites using Li, Mg, Na and Zn as external dopants, with growth into an alumina preform. The modelling indicates strong similarities between all these systems. Interpretation of the results suggests that initiator dopants for the directed melt oxidation of aluminium should be volatile at the reaction temperature and should be capable of forming a mixed oxide phase with aluminium.

Infiltration Processing of Metal-Ceramic FGM's

Graded alumina preforms were fabricated replicating the pore structure of carbon and polyurethane soft foams with porosity gradients. Continuous gradients from 20 to 70% porosity with pore sizes down to a few μm were produced by position dependent anodic dissolution of hard coal. Alternatively, a stepwise porosity gradient was introduced in polyurethane soft foams by stacking foam layers compressed by different factors. In this way gradients from 2.4% to 60% open porosity with ligaments of 40-100 μm size were retained. Infiltrating the porosity graded preforms with alumina slips, burnout of the organic component, sintering and metal infiltration of the resulting alumina preforms yielded dense composites with graded interpenetrating network microstructure.

A Technique of Elaboration of Functionally Graded Cermets

Out first results concerning the elaboration of Functionally Graded Cermets (FGC) are presented. The studied material is an alumina matrix with a SnPb alloy graded fraction. In a first step, an unidirectional gradient of graphite volume fraction in alumina is obtained by sedimentation of a mixing of the alumina and graphite powders. Then, during the sintering stage, the graphite burns and acts as porosity inducing element. Finally, the FGC is obtained by pressure infiltration of a molten SnPb alloy into the graded porosity of the alumina preform. The distribution of the SnPb alloy through the composite is characterised by SEM observations and microhardness tests. From a qualitative point of view, the shape of the gradients in the sample, deduced from the hardness measurements, agree with a simple simulation of the graphite gradient in the alumina deposit, based on Stockes law.

Processing of an in-situ Layered and Graded Alumina/Calcium-Hexaluminate composite: Physical Characteristics

A novel route to simple processing of an in-situ layered and graded alumina/calcium-hexaluminate (CA 6) composite is described. The processing involves partial infiltration of a porous alumina preform with hydrolysed calcium acetate to yield a homogeneous layer of alumina and a heterogeneous graded layer of CA 6/alumina. The homogeneous layer is designed to provide strength, hardness, and wear resistance, while the graded layer is tailored to impart toughness and damage tolerance. The effect of CA 6 on the physical and graded characteristics is discussed.

Synthesis and properties of in situ layered and graded aluminium titanate/alumina composites

A novel route to simple processing of an in situ layered and graded aluminum titanate (AT)/alumina composite is described. This process offers a simple means of tailoring the composition and microstructure of ceramic materials. The processing involves partial infiltration of a porous alumina preform with titanium ethoxide to yield a homogeneous surface layer of alumina and a heterogeneous graded layer of AT/alumina. Analysis of the graded layer by X-ray diffraction shows the existence of concentration gradients, the AT content decreasing with depth. Depth profiling of the graded layer also shows gradual changes in elastic modulus, thermal expansion coefficient, and microhardness. This novel process has the potential to fabricate other material systems and architectures.

Microstructure and mechanical properties of alumina/copper composites fabricated by different infiltration techniques

Al2O3/Cu and Al2O3/Cu–O composites were fabricated using gas pressure-assisted and pressureless infiltration of a pure copper and copper–oxygen alloy into plasma sprayed alumina preforms containing 15 vol.% open porosity. Pressure-assisted infiltration of an alumina preform was obtained at 1300°C under argon pressure of 15 MPa. Pressureless infiltration of an alumina preform was obtained at 1300°C for copper–oxygen alloy containing 3.2 wt.% of oxygen. The fabricated materials exhibited superior fracture toughness, bending strength and hardness when compared with uninfiltrated plasma sprayed alumina. For example, an Al2O3/Cu and Al2O3/Cu–O composites had KIc≈6.7 and 6.5 MPa m, respectively, compared to ∼1.77 MPa m for uninfiltrated alumina. SEM micrographs of fractured surfaces of both composites showed deformed metal ligaments which demonstrated that crack bridging by plastic deformation of metal phase is one of the toughening mechanisms.