The toughening behaviours of Cr3C2 particulate-reinforced Al2O3 composites

Enhancement of the fracture toughness of Al₂O₃ was achieved by the additions of 10–40 vol% Cr₃C₂ particulates through a hot-pressing process. The dependence of Cr₃C₂ particle size (0.5, 1.5 and 7.5 m) on the toughening effect was investigated. The maximum fracture toughness of composites could be improved to 5.9, 7.6 and 8.0 MPa m^1/2 for fine, medium and coarse Cr₃C₂ particle reinforced composites, respectively. Both the quantitative analysis of toughening contributions and experimental observations are extensively discussed in terms of Cr₃C₂ particle size, microcracks, as well as thermal residual stress between Al₂O₃ and Cr₃C₂.     

Microstructure and high-temperature strength of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary melt growth composite

A new Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary MGC (Melt Growth Composite) with a novel microstructure has been fabricated by unidirectional solidification. This ternary MGC has a microstructure consisting of continuous networks of single-crystal Al₂O₃, single-crystal EAG and fine cubic-ZrO₂ phases without grain boundaries. The ternary MGC has also characteristic dimensions of the microstructure of around 2–4 m for EAG phases, around 2–4 m for Al₂O₃ phases reinforced with around 0.4–0.8 m cubic-ZrO₂ phases. No amorphous phases are formed at interfaces between phases in the ternary MGC. The ternary MGCs flexural strength at 1873 K is approximately 700 MPa, more than twice the 330 MPa of the Al₂O₃/EAG binary MGC. The fracture manner of the Al₂O₃/EAG/ZrO₂ ternary MGC at 1873 K shows the same intergranular fracture as the Al₂O₃/EAG binary MGC, but is significantly different from the transgranular fracture of the sintered ceramic.     

Microstructure and mechanical properties of 3Y-TZP/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites fabricated by liquid phase sintering

3Y-TZP/Al₂O₃ composites were pressureless sintered with the addition of TiO₂-MnO₂ and CaO-Al₂O₃-SiO₂ glass. The densification, microstructure and mechanical properties of the composites were investigated. It was found that the composites could be densified at a temperature as low as 1400^C by liquid phase sintering. The majority of the grain sizes for both Al₂O₃ and ZrO₂ were below 1 m because of the lower sintering temperature. A bending strength of 934 ± 28 MPa and fracture toughness of 7.82 ± 0.19 MPam^1/2 were obtained for 3Y-TZP/Al₂O₃ (20 vol%) composite. Transformation toughening is considered the responsible toughening mechanism.     

Microstructure and Mechanical Properties of ZrO2(2Y)-Toughened Al2O3 Ceramics Fabricated by Spark Plasma Sintering

Spark plasma sintering (SPS) has been performed for 5 min at 1500°C and 30 MPa using submicrometer-sized Al₂O₃/ZrO₂(2Y) composite powders in the Al₂O₃-rich region. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics show excellent mechanical strength; the strength of 1620 MPa is achieved in the ZTA with 50 mol% ZrO₂. The grain size of Al₂O₃ in ZTA decreases from 1.5 to 0.6 m with increased ZrO₂ content. Almost all the ZrO₂ grains (0.3 m) are located in the boundaries of the Al₂O₃ grains. Mechanical properties are discussed, with an emphasis on the relation between t-/m-ZrO₂ ratios and microstructures of ZTA.     

Densification and mechanical properties of WC particulate reinforced Cr3C2 matrix composites

The mechanical properties of Cr₃C₂ can be improved by adding 14–25 vol % of WC particulates through a hot-pressing process. The Cr₃C₂-20 vol % WC composite exhibits a fracture strength and fracture toughness of 883 MPa and 6.8 MPa m^1/2, respectively, which is a better than 60% increase over the monolithic Cr₃C₂(_f = 526 MPa, K _IC = 4.1 MPa m^1/2). The possible strengthening and toughening mechanisms are disscussed in terms of microstructures, fracture modes (intergranular or transgranular) and micromechanics. The microstructural evolution and fractography which were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) will be discussed.     

Duplex spinel-ZrO2 ceramics

Duplex spinel-ZrO₂ ceramic composites were produced by an emulsion-hot kerosene drying technique. The sintered duplex spinel-ZrO₂ ceramics which had the composition of 55 wt% Al₂O₃-20 wt% ZrO₂-25 wt% MgO, consisted of a spinel matrix, whose grain size was in the range of 1.5 to 2.0 m, and uniformly dispersed zirconia agglomerates having grain sizes ranging from 1.0 to 2.0 m. Zirconia agglomerates began to appear at a temperature of 1500 °C and the duplex spinel-ZrO₂ structure was formed with the weight ratio of Al₂O₃/MgO being within 1.67 to 2.20 and the amount of ZrO₂ addition being within 5 to 25 wt %. The relative density, fracture toughness, flexural strength, and critical temperature difference of the spinel-ZrO₂ composite were 97.8%, 1.98 MPam^0.5, 390 MPa, and 275 °C, respectively.     

Manufacturing and friction welding properties of particulate reinforced 7005 Al

This study has successfully incorporated Al₂O₃, SiC particulates into the 7005Al alloy matrix by using a drag-push method. The reinforced particulates are uniformly distributed in the matrix. This study also discusses the influence of aging treatment on the friction welding properties of 7005Al/10 wt%, 15 m and 6 m SiC_(p) composites and 7005Al/10 wt%, 15 m Al₂O_3(p) composites joint system. Experimental results show that after peak aging treatment was performed, if SiC particulates were used in the strengthening phase, the heat-affected zone (HAZ) had higher density of strengthening particulate, this resulted an increase in the hardness and stress concentration at the fully plasticized zone (Zpl) of the HAZ region, but a decrease in the width of the Zpl zone and the welding strength. And the welded fracture surface morphology had a low-ductile fracture.     

Effect of grain morphology and grain size on the mechanical properties of Al2O3 ceramics

Bending strength, fracture toughness, fracture energy and crack extension resistance were evaluated for Al₂O₃ ceramics with equi-axed and platelet grains. Bending strength was proportional to grain size^–1/2, but flaws with a size of 10 m controlled the strength when the microstructure was finer than 10 m. Fracture toughness, measured by the single etched precracked beam (SERB) method, was proportional to fracture energy^1/2, and increased with the grain size of Al₂O₃ ceramics with equi-axed and platelet grains. However, the toughness of platelet grain ceramics was higher than the ceramics with equi-axed grains, and increased up to 6.6 MPam^1/2 with grain size. Therefore, it is thought that fracture toughness not only depends on grain size, but also on grain morphology; equations were derived to account for this phenomenon.     

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Two types of Al₂O₃/Mo composites were fabricated by hot-pressing a mixture of - or -Al₂O₃ powder and a fine molybdenum powder. For Al₂O₃/5 vol% Mo composite using -Al₂O₃ as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al₂O₃ grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m^1/2). In the system using -Al₂O₃ as a starting powder, nanometre sized molybdenum particles were dispersed within the Al₂O₃ grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al₂O₃/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al₂O₃ grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m^1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.     

Composites of aluminium alloys: fabrication and wear behaviour

In this paper processes for fabrication of aluminium-alloy composites containing paniculate non-metals, the net shape forming of these composites, their microstructures, their friction and wear behaviours and their mechanical properties are described. Composites of two wrought (2014 and 2024) and one cast (201) aluminium alloys containing 2 to 30 wt% of Al₂O₃ and SiC particles in the size range of 1 to 142m were prepared. The non-metallic particles were added to a partially-solid vigorously-agitated matrix alloy. The particles were then retained in the matrix until interface interaction, for example, the formation of MgAl₂O₄ spinel in the case of Al₂O₃ particles, were faciliated. These composites were solidified and subsequently reheated to above their liquidus temperature and formed under high pressure in a closed-die forging type of apparatus. Composites with particulate additions of size larger than 5m possessed homogeneous structures; particles of size 1m, however, tended to cluster. The wear behaviour of the composites was studied using a pin-on-disc type machine. It was shown that composites containing large amounts of non-metals, 20 wt%, exhibit excellent wear resistance whilst those with small to moderate amounts of non-metals possess tensile properties comparable to the matrix alloy. Increasing the amount of particulate additions results in reduced ductility. Finally, a method was investigated of producing components with high weight-fractions of non-metals near their surface.     

Effects of oxidation of Cr3C2 particulate-reinforced alumina composites on microstructure and mechanical properties

Al₂O₃ can be strengthened and toughened by incorporating Cr₃C₂ particles through hotpressing. For instance, an Al₂O₃-10 vol% Cr₃C₂ composite exhibits fracture strength and toughness of 600 MPa and 5.5 Mpa m^0.5, respectively. An annealing treatment in air from 1000–1200C may further substantially strengthen the same composite to give _f = 800 MPa andK _IC = 9.5 MPam^0.5. Possible oxidation reactions and toughening mechanisms are discussed in terms of oxygen diffusion, the formation of micropores beneath the exposed surface, as well as the fracture mode.     

Mechanochemical effects for some Al2O3 powders of dry grinding

Three kinds of Al₂O₃ powders, i.e. two kinds of low-soda Al₂O₃ with average particle sizes of 3.9 and 0.6 m and an electrofused Al₂O₃ with an average particle size of 21.8 m, were ground for up to 300 h in a dry vibration ball mill. Variations in particle-size distribution, specific surface area, crystallite size, lattice strain, effective temperature factor and lattice constant were examined against milling time. The mechanism of grinding was found to differ between low-soda Al₂O₃ and electrofused Al₂O₃. The mechanochemical effects on these Al₂O₃ powders occurred in the order decrease of crystallite size increase of effective temperature factor increase of lattice strain. The length of the a-axis was clearly increased by a prolonged grinding. The difference in the ground state of three specimens was attributed to differences in the physical state of particles originating from the preparation methods, and also to particle size.     

Effect of metal volume fraction on the mechanical properties of alumina/aluminum composites

The influence of metal volume fraction on the mechanical properties of Al₂O₃/Al composites with constant diameter of metal ligaments was studied. Alumina/aluminum composites with interpenetrating networks and metal content between 12 and 34 vol.% were fabricated by gas-pressure infiltration technique. The fabricated composites exhibited good mechanical properties, e.g. the bending strength of 740 MPa for samples containing 12 vol.% of Al. The bending strength of the composites decreased with increasing volume fraction of metal phase. High strength of the fabricated composites was explained by strong interfacial bonding between alumina and aluminum. The fracture toughness of the composites increased, however, with increasing volume fraction of aluminum. The highest fracture toughness values of 6 MPa m were measured for the composites containing 25 vol.% of Al. Fractographic analysis of fractured surfaces showed deformed metal ligaments which demonstrated that crack bridging by plastic deformation of the metal phase is the main toughening mechanism in Al₂O₃/Al composites.     

Microporosity of Dense Anodic Al2O3 Films

Dense Al₂O₃ films 2.5 and 5 m thick produced by high-voltage and two-step anodization were studied by electron microscopy and small-angle x-ray scattering. The volume microporosity of the films and the mean radius of inertia of pores were evaluated. The results indicate that the densification of porous films in the second step of anodization is a result of two concurrent processes: dense oxide growth and dissolution of the primary porous Al₂O₃ layer.     

Property-microstructure correlation in in situ formed Al2O3, TiB2 and Al3Ti mixture-reinforced aluminium composites

The in situ formed Al₂O₃, TiB₂ and Al₃Ti mixture-reinforced aluminium composites were successfully fabricated by the reaction sintering of the TiO₂-B-Al system in a vacuum. With increasing boron content in the TiO₂-B-Al system, the amount of generated TiB₂ in the composites increased and Al₃Ti content decreased. At the same time the distribution uniformity of the in situ formed Al₂O₃ and TiB₂ particulates was obviously improved, and the size of the Al₃Ti particles was reduced. The in situ Al₂O₃ and TiB₂ particulates had sizes from 0.096–1.88 m. The interface between the in situ formed particulates and the aluminium matrix was clean, and no consistent crystallographic orientation relationship was found. The strength and elastic modulus of the composites was significantly improved by lowering the Al₃Ti content. When the boron content in the TiO₂-B-Al system rose, the morphology of the tensile fracture surface of the composites was changed from large fractured Al₃Ti blocks and fine dimples, to fine dimples and pulled-out particulates. The strengthening and fracture of the composites have been modelled.     

Toughening mechanisms in duplex alumina-zirconia ceramics

A series of Al₂O₃-ZrO₂ ceramics has been fabricated using both conventional sintering and a hot-pressing route, which results in various microstructures including (i) Al₂O₃ with well-dispersed ZrO₂ single crystals; (ii) Al₂O₃ With TZP (tetragonal zirconia polycrystals) agglomerates (20 to 50m); and (iii) Al₂O₃-ZrO₂ duplex structures, in which both well-dispersed ZrO₂ single crystals and TZP agglomerates are dispersed. The fracture strength of the composites has been measured by means of three-point bending and the fracture toughness by means of the micro-indentation technique. The microstructural characterization was carried out using scanning and transmission electron microscopy, and phase analysis of the zirconia by means of X-ray diffraction. The high toughness values of 12 M Pa m^1/2 measured for the duplex structure have been correlated with the toughening mechanisms operative and the fracture strength with the matrix grain size and with larger defects present in the structure. A combined toughening process is proposed to account for the improved properties, including transformation toughening, microcrack toughening and crack deflection, which are discussed in context with the property measurements and the microstructural observations.     

Strengthening of alumina by cerium-zirconate (Ce2Zr2O7)

Composite powders of Al₂O₃ and 0 to 30 vol% Ce₂Zr₂O₇ are prepared by a hybrid sol-gel method using Al₂O₃ powders and a sol formed from Zr-alkoxide and cerium nitrate. All the Zr from the sol goes to form the cerium zirconate phase when the powders are calcined in N₂. Pressureless sintering in air at 1500°C yields composites with high density (98%). Maximum values of fracture toughness and strength, 6.5 MPa and 620 MPa respectively, (e.g. 3.5 MPa and 350 MPa for pure Al₂O₃) are obtained in 10 vol% Ce₂Zr₂O₇ composite sintered in air. The dominant mechanism for enhancement in K _IC is believed to be crack bridging. Crack bridging activity in the 10 vol% composite is found to be maximum and extends upto 190 m from the crack tip.     

Effect of matrix liquid phase on interphase formation in SiC fibre-reinforced Si2N2O-Al2O3-CaO composites

Matrix compositions based on Si₂N₂O, with Al₂O₃ and CaO additions, were used to hot press Nicalon SiC fibre-reinforced composites at 1600 °C. With both CaO and Al₂O₃ additions, eutectic melting formed an appreciable volume of liquid phase during hot pressing, which remained as a stable glassy phase in the cooled composites. This liquid phase fostered formation of 240 nm thick carbon-rich interphases between the fibres and the matrix. These interphases showed relatively low interfacial shear strength and resulted in composites which showed non-catastrophic, notch-independent fracture. Matrices using either Al₂O₃ or CaO did not form adequate liquid phase to form coarse interphases, and fracture was catastrophic in nature. Post-heat treatment of the composites at 1000 °C showed peripheral oxidation (removal of the carbon content of the interphase) indicating limited protection afforded when glassy phase was present in the matrix. Controlled cooling in the hot press did not cause the liquid regions to devitrify.     

Mechanical properties of selected glasscrystal composites

Desirable physical properties not provided by single-phase materials are often conveniently attained by multiphase composites. This paper is concerned with the variation of the mechanical properties of bulk glass through the incorporation of a crystalline oxide phase. Glass-Al₂O₃ and glass-ZrO₂ composites containing 20 and 40 vol % spherical, crystalline inclusions, 125 to 150 rn in diameter, were prepared by hot pressing. The effects of internal stress and elastic properties of the crystalline inclusion on composite mechanical properties were studied. Experimental elastic-property values agreed well with theoretical values calculated from Hashin's equations. Flexural strength tests of all composite compositions revealed that the fracture path rarely traversed the crystalline oxide inclusions. The Al₂O₃ and ZrO₂ additions strengthened the glass considerably, except when internal stresses were of sufficient magnitude to cause cracking of the glassy matrix upon cooling and before flexural testing. Glass-Al₂O₃ composites were consistently stronger than the glass-ZrO₂ counterparts. This strength difference is attributed to the higher elastic moduli of the Al₂O₃ composites. Hypotheses which postulate strengthening as a result of restricted flaw size are apparently not applicable to these materials.This paper is based on part of a thesis submitted by W. J. Frey in partial fulfilment of the requirements for the degree of Master of Science in Materials, Rensselaer Polytechnic Institute, June 1966.     

Temperature dependence of flexural strength and microstructure of Al2O3/Y3Al5O12/ZrO2 ternary melt growth composites

New Al₂O₃/Y₃Al₅O₁₂(YAG)/ZrO₂ ternary Melt Growth Composites (MGCs) with a novel microstructure have been fabricated by unidirectional solidification. These MGCs displayed superior high-temperature strength characteristics. The flexural strength increases progressively in the range 650–800 MPa with a rise in temperature from room temperature up to 1873 K. These excellent high-temperature characteristics are closely linked to such factors as: a microstructure consisting of three-dimensionally continuous and complexly entangled single-crystal Al₂O₃ with a hexagonal structure, single-crystal YAG with a garnet structure and fine ZrO₂ with a cubic structure; characteristic dimensions of the microstructure of Al₂O₃/YAG/ZrO₂ ternary eutectic ceramics of around 2–3 m for YAG, around 2–3 m for Al₂O₃ and around 0.4–0.8 m for ZrO₂; and the fact that no amorphous phase is formed at interfaces between any of the phases.