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Different Fe-Al2O3 and FeAl-Al2O3 composites with metallic contents up to 30 vol% have been fabricated via reaction processing of Al2O3, Fe, and Al mixtures. Low Al contents (

Niobium-alumina (Nb-Al 2 O 3 ) composites have been fabricated via the pressureless sintering of compacts of intensively milled Nb-Al 2 O 3 mixtures. Strength and fracture toughness of the composites increase as the niobium content increases. For a composite that contains 50 vol% niobium, strengths of up to 680 MPa, with a corresponding fracture toughness of 6.3±0.3 MPa.m 1/2 and hardness of 6.4 GPa, have been obtained.

A reactive sintering process has been used to produce almost fully dense composites with interpenetrating networks of NbAl 3 and Al 2 O 3 . The process involves the reaction synthesis of niobium aluminides and Al 2 O 3 from compacts of intensively milled aluminum and Nb 2 O 5 powder mixtures. During carefully controlled heating under an inert atmosphere, the oxide reduction by aluminum to form niobium aluminides and Al 2 O 3 , proceeds at temperatures below the melting point of aluminum. At temperatures of >1000°C, the reaction-formed niobium aluminides and Al 2 O 3 sinter. The present paper discusses processing parameters, such as attrition milling, the heating cycle, and the metal:ceramic ratio in the starting mixture, that control microstructure development and mechanical properties.

Mullitization temperatures and mechanical properties of reaction-bonded mullite composites were investigated using silicon carbide (SiC) of two different particle sizes (180 nm and 2.5 μm) as one of the starting components. The smaller SiC particle size resulted in earlier mullitization, lower final densities, and lower strength of these composites. The sintering shrinkage of these composites was investigated. Low-to-zero shrinkage was rendered possible via volume expansions that were associated with the oxidation of SiC and aluminum in the green material. Green bodies that contained 55 vol% aluminum were compacted to 70% of the theoretical density. The materials showed a linear sintering shrinkage of

Wet milling of Al2O3-aluminide alloy (3A) precursor powders in acetone has been investigated by milling Fe/Al/Al2O3 and Fe2O3/Al/Al2O3 powder mixtures. The influence of the milling process on the physical and chemical properties of the milled powders has been studied. Particle refinement and homogenization were found not to play a dominant role, whereas plastic deformation of the metal particles leads to the formation of dislocations and a highly disarranged polycrystalline structure. Although no chemical reactions among the powder components in Fe2O3/Al/Al2O3 powder mixtures were observed, the formation of a nanocrystalline, ordered intermetallic FeAl phase in Fe/Al/Al2O3 powder mixtures caused by mechanical alloying was detected. Chemical reactions of Fe and Al particle surfaces with the atmosphere and the milling media lead to the formation of highly porous hydroxides on the particle surfaces. Hence the specific surface area of the powders increases, while the powder density decreases during milling. The fraction of Fe oxidized during milling was determined to be 0.13. The fraction of Al oxidized during milling strongly depends on the metal content of the powder mixture. It ranges between 0.4 and 0.8.

The microstructures of niobium-based alumina composites prepared by pressureless sintering of compacts of attrition milled Al2O3, Nb, and Al powder mixtures were studied. The addition of a small amount of Al is assumed to assist in rapid sintering. X-ray diffraction analyses show that Al2O3, Nb, NbO, and the intermetallics AlNb2 and AlNb3 are present in the composites. Electron microscopy studies confirm the existence of these phases and reveal dense, fine-grained (≤500 nm) composites. Al2O3 and Nb grains form the matrix. NbO occurs as grains and additionally as small particles within Al2O3 grains and at Al2O3/Al2O3 grain boundaries. The intermetallic AlNb2 and AlNb3 phases do not exceed 300 nm in size if they occur at grain boundaries, and possess even smaller dimensions when occluded within Al2O3 grains or located at Al2O3 triple junctions. While the niobium intermetallics are expected to form during the heating cycle before reaching the sintering temperature, the NbO is assumed to form during the cooling cycle due to precipitation of oxygen dissolved in the niobium.

The general reaction behavior of the 3A process under the thermal explosion mode of synthesis has been investigated via a continuum model. The continuum model uses mass and energy balances to predict temperature difference (Ts,avg−Tf) curves, as well as profile curves of the reactant conversions and sample temperature. In particular, the effect of the dimensionless parameters associated with the rate of local heat generation (β, the thermicity factor), the activation energy (γ, the Arrhenius number), the rate of heat redistribution (α, the modified thermal diffusivity), the rate of heat transfer by convection (Bi, the Biot number or convective heat transfer parameter), and the rate of heat transfer by radiation (Ω, the radiative heat transfer parameter) were investigated. Conditions to control the reaction process, which should produce high-density final products, were determined. It was found that the overall maximum temperature may be reduced for high γ, low β, high α, and high Bi and Ω. In terms of processing conditions, this may be obtained by reducing the initial reactant concentrations, optimizing the particle size, using small sample sizes and high compaction pressure, and increasing the heat loss by using a high thermal conductivity inert gas.

The reaction sintering of Ti x Al y -Al 2 O 3 composites from TiO 2 /Al starting powder mixtures has been characterized by thermogravimetry and differential thermal analysis (TG/ DTA), in situ temperature measurements, and predictions via a continuum model. In order to model the TiO 2 /Al reaction system, it was necessary to first determine the postmill reactant concentrations and the dominant reaction. The postmill reactant concentrations were obtained from TG/DTA measurements in air, while X-ray diffraction (XRD) was used to gain insight into the reaction mechanisms. A continuum model of the process was fitted to in situ temperature measurements by adjusting two parameters. The model was then used to investigate the effects of various processing conditions on the reaction behavior.

Dense alumina/5 vol% SiC nanocomposites were prepared by sol-gel processing using nanosized (180 nm) precoated SiC powders and a commercial boehmite sol. The SiC powder was precoated with boehmite by a controlled heterogeneous precipitation from an aluminum nitrate solution. The coated SiC powder was then dispersed in a boehmite sol, gelled, calcined, and densified by gas pressure sintering under argon atmosphere at 7-8 MPa pressure. The dependence of the calcination conditions on densification, the effect of seeding on the microstructural development, as well as the mechanical behavior of the sintered specimens, are presented and discussed.

Alumina/zirconia matrix composites with platinum-coated sapphire fibers were fabricated using the technology of reaction bonding of aluminum oxide (RBAO) as the synthesis route. The interfacial behavior of both sintered and annealed specimens was investigated by crack path observations and fiber pushout tests. The results show that precious metals like platinum or related alloys can be used as high-temperature interphases for oxide/oxide composites. Weak interfaces were obtained both between fiber and coating as well as between matrix and coating, which should be suitable to achieve a damage-tolerant behavior of the composite.

Three-layer alumina-based composites with Al 2 O 3 -containing mullite as an outer layer and ZrO 2 -containing alumina as an inner layer were fabricated by die-pressing and reaction-bonding. The strength of samples with Vickers indents made at loads up to 300 N was investigated. When compared to monolithic ceramics, three-layer composites exhibit excellent damage resistance due to compressive stresses in the outer layer. Hot isostatically pressed samples show a four-point bend strength of 800 MPa after indentation at 300 N.

Reaction-bonded aluminum niobate-based (AlNbO{sub 4}-based) composites with tailorable microstructures were fabricated through controlled nucleation and growth of needlelike AlNbO{sub 4} grains. Equiaxed to highly elongated grains were obtained by different heat treatments at 1,340 and 1,400 C in air. Scanning electron microscopy observations revealed that increasing the heating rate promoted the development of large, homogeneously distributed needlelike grains. Because of the elongated nature of the AlNbO{sub 4} grains and the high residual stresses at the interface, enhanced crack wake bridging occurred. This bridging accounted for the observed increase of fracture toughness.

The feasibility of fabricating dense, low-shrinkage, mullite/ ZrO2 composites based on the reaction bonding of alumina (RBAO) process and the reaction sintering of zircon is examined. Compacts pressed from an attrition-milled powder mixture of Al, A12O3 and zircon were heated in air according to a two-step heating cycle. The phase evolution and microstructural development during reaction bonding were traced by X-ray diffraction, nuclear magnetic resonance, and scanning electron microscopy on samples extracted from various points along the heating cycle. It is seen that, as in conventional RBAO, AI oxidizes to γ-Al2O3 which then transforms to α-AI2O3 between 1100° and 1200°C. The zircon dissociation commences at ∼1400°C and is practically complete by 1500°C. Mullite enriched in Al2O3 forms initially, but 3:2 stoichiometry is attained in the final product which consists of mullite, t- and m-ZrO2, and residual α-AI2O3. The flexure strength of the composite is superior to that of pure mullite, and ∼80% of the strength is retained up to 1200°C. Although there was no toughness enhancement relative to mullite, this should be achievable by optimizing the fabrication procedure.

High-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) were used to investigate the structure and chemistry of (0001) {alpha}-Al{sub 2}O{sub 3}/Al interfaces in melt-infiltrated polycrystalline alumina composites. HRTEM revealed an interfacial region different from both Al and {alpha}-Al{sub 2}O{sub 3}, with a structural width of 0.8 {plus_minus} 0.2 nm. AEM of the same interfaces revealed a Ca excess of {Lambda} = 2.5 {plus_minus} 0.5 Ca atoms per nm{sup 2} (Ca/nm{sup 2}). AEM of a basal twin boundary in the {alpha}-Al{sub 2}O{sub 3} also revealed a Ca excess ({Lambda} = 1.0 {plus_minus} 0.5 Ca/nm{sup 2}). Since the metal-ceramic interfaces were the free surfaces of pores before melt infiltration, it can be concluded that Ca segregates to the basal surface of alumina, as well as to basal twin boundaries. Furthermore, the Ca at the free surfaces does not reside on only one cation plane, but is spread over 4 {plus_minus} 1 basal cation layers and forms an interfacial phase with a nominal composition of CaO{center_dot}6Al{sub 2}O{sub 3}.

Based on the RBAO technology, low-shrinkage mullite/SiC/Al[sub 2]O[sub 3]/ZrO[sub 2] composite were fabricated. A powder mixture of 40 vol% Al, 30 vol% Al[sub 2]O[sub 3], and 30 vol% SiC was attrition milled in acetone with TZP balls which introduced a substantial ZrO[sub 2] wear debris into the mixture. The precursor powder was isopressed at 300--900 MPa and heat-treated in air by two different cycles resulting in various phase ratios in the final products. During heating, Al oxidizes to Al[sub 2]O[sub 3] completely, while SiC oxidizes to SiO[sub 2] only on its surface. Fast densification (at > 1300 C) and mullite formation (at 1400 C) prevent further oxidation of the SiC particles. Because of the volume expansion associated with the oxidation of Al (28%), SiC (108%), and the mullitization (4.2%), sintering shrinkage is effectively compensated. The reaction-bonded composites exhibit low linear shrinkages and high strengths: shrinkages of 7.2%, 4.8%, and 3%, and strengths of 610, 580, and 490 MPa, corresponding to compaction pressure of 300, 600, and 900 MPa, respectively, were achieved in samples containing 49--55 vol% mullite. HIPing improved significantly the mechanical properties: a fracture strength of 490 MPa and a toughness of 4.1 MPa[center dot]m[sup 1/2] increased to 890more » MPa and 6 MPa[center dot]m[sup 1/2], respectively.« less

Important parameters controlling the reaction-bonding of Al2O3 (RBAO) process are Al content, particle size, and green density. Successful fabrication of high-strength bodies requires fine and homogeneous powders. Low milling intensity does not lead to the required particle fineness, whereas overmilling causes extensive oxidation and hydrolyzation of Al. Oxidation of Al during milling is enhanced by ZrO2 additions. As much of the Al should be oxidized by solid/gas reaction as possible. Milled RBAO precursor powder contains physically-adsorbed and chemically-bonded (aluminum hydroxide) water. Decomposition of chemically-bonded water promotes cracking in the temperature range 450–550°C. Bloating of RBAO occurs when the surface region becomes dense before complete decomposition of aluminum hydroxide in the interior of the body.

Thermal shock data are presented for two types of Al/Al2O3 composites (35 vol% Al) with different microstructural scale and compared to a monolithic alumina. The fine-scale Al/A12O3 material exhibits high room-temperature strength (765 MPa) and a gradual decrease in retained strength with increasing quenching temperature difference, δT, and improved thermal shock resistance—as compared to monolithic alumina—even at δT= 600°C. The coarse-scale Al/Al2O3 composite shows high long-crack fracture toughness (10.5 MPa·m1/2) and almost no strength degradation up to δT= 500°C. A qualitative picture for the effect of microstructural scale on thermal shock resistance emerges by employing a simple fracture mechanics description contrasting the crack driving force for transient stress fields with R-curve behavior.

Crack velocity curves for Y-TZP and Al2O3-toughened Y-TZP were determined for long cracks in compact tension specimens with an in situ fracture device on the stage of an optical microscope. Indications for a crack velocity threshold were found for both materials. Above this threshold, at 2.6 MPa·m1/2 for Y-TZP and 3.6 MPa·m1/2 for Al2O3-toughened Y-TZP, chemically assisted subcritical crack growth occurs over an extended regime of applied stress intensity factors of width 2.1–2.8 MPa·m1/2. It is recognized that the dependence of the shielding term on the crack-tip stress field renders transformation-toughened materials particularly susceptible to stress-corrosion cracking. This interrelation leads to the definition of a steady-state velocity at constant applied stress intensity factor. This velocity is obtained in the situation where the shielding term is fully defined by the present crack-tip stress field, not depending on prior loading history.

Mullite ceramics were fabricated according to the recently developed reaction-bonded Al2O3 (RBAO) technology. Green compacts consisting of mechanically alloyed Al, SiC, and Al2O3 were heat-treated in two steps. During the first hold at 1200°C, Al and SiC were oxidized to form Al2O3 and SiO2. On further heating, mullite was formed which then sintered during the second hold at 1550°C. All reactions involved in the process were associated with volume expansions that almost compensated for the shrinkage on sintering. Processing details and microstructure development are discussed. Reaction-bonded mullite ceramics exhibit high fracture strength, e.g., 290 MPa at a density of 97% of theoretical density.

SiC- and Al2O3-platelet-reinforced mullite-matrix composites were fabricated by both conventional powder processing and a pressure filtration route with constant filtration rate. Consolidation during the pressure filtration experiment was monitored as a function of time with respect to actual pressing stages. The packing density of green bodies was strongly affected by the different processing methods. Either fracture toughness (3.2 MPa · m1/2) or bending strength (344 MPa) increased, depending on surface conditioning of the SiC platelets. Pressure filtration was shown to decrease processing flaws in the platelet-reinforced bodies leading to substantial improvement of the mechanical properties.

The thermal-stress fracture behavior of duplex ceramics is investigated by quenching in water and in oil. Comparison with the matrix materials shows that the critical quenching temperature difference, {Delta}T{sub c}, is not or is only slightly reduced, even for duplex ceramics of significantly reduced strength. In sintered composites, thermal-stress-induced microcracking within pressure zones and crack initiation at pressure zone--matrix interfacial defects develop before {Delta}T{sub c} is reached. These effects are accompanied by a gradual reduction in strength. At {Delta}T{sub c}, critical crack propagation occurs. The retained strength after thermal shock of duplex ceramics is significantly improved compared with the respective matrix materials. This behavior can be related reasonably well with the K{sup R}-curve behavior.

The fracture toughness behavior during crack growth (KR-curve behavior) of duplex ceramics is investigated. Different types of KR-curves can be distinguished depending on the microstructural designs of these materials which are characterized by the volume fraction and size of the dispersed pressure zones, and by their effective volume expansion. According to their KR-curve behavior, duplex ceramics can be subdivided into two groups consisting of “short-range” and “long-range” toughened materials. The experimental results are discussed regarding the appearance of different toughening mechanisms which are documented by crack path micrographs. An unusual toughening effect, a “crackbranching chain reaction,” is documented by in situ observations. The critical stress to nucleate the observed process zone development is calculated and compared with the internal stress intensity factor Ki which has been previously proposed for these materials and with the material strength.

The directed oxidation of some molten aluminum alloys was studied. Alloys containing Si, Zn, and Mg are readily converted into Al2O3/Al composites with the characteristic directed metal oxidation process (DIMOX) metal-channel structure. No reaction took place when alloys without Mg were used. However, when Mg-free melts containing Si and Zn were in contact with bulk spinel (MgAl2O4), the typical reaction product grew from the metal pool, although without formation of the spinel starter layer which was thought to be a prerequisite for DIMOX growth. Furthermore, the growth front was more planar than when Mg-containing alloys were used.

A fine-grained zirconia, which has been costabilized with yttria and magnesia, has been prepared. Its stability during subeutectoid annealing at 1100deg;C, and its hydrothermal stability during hydrothermal treatment at 180deg;C, have been determined and they are compared to those of Y-TZP and Mg-PSZ materials.