High pressure consolidation of B6O-diamond mixtures

Sintered composites in the B₆O-xdiamond (x= 0–80 vol%) system were prepared under high pressure and high temperature conditions (3–5 GPa, 1400–1800°C) from the mixture of in-laboratory synthesized B₆O powder and commercially available diamond powder with various grain sizes (<0.25, 0.5–3, and 5–10 m). Relationship among the formed phases, microstructures, and mechanical properties of the sintered composites was investigated as a function of sintering conditions, added diamond content, and grain size of diamond. Sintered composites were obtained as the B₆O-diamond mixed phases when using diamond with grain sizes greater than 0.5 m, while the partial formation of the diamond-like carbon was observed when using diamond grain sizes less than 0.25 m. Microhardness of the sintered composite was found to increase with treatment temperature and pressure, and the fracture toughness slightly decreased. A maximum microhardness of H _v57 GPa was measured in the B₆O-60 vol% diamond (grain size < 0.25 m) sintered composite under the sintering conditions of 5 GPa, 1700°C and 20 min.     

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.     

Formation of plate-like lanthanum-β-Aluminate crystal in Ce-TZP matrix

The reaction route and morphology of lanthanum--aluminate (LBA) crystals formed in Ce-TZP matrix were studied by examining the crystal phase changes and the microstructures in relation to the heat-treatment time, heat-treatment temperatures and the particle size of raw Al₂O₃ powders. In the Ce-TZP matrix, the LBA crystal was formed by the reaction between La₂O₃ and Al₂O₃ through the LaAlO₃ phase as the intermediate. La₂Zr₂O₇ forms at 800 °C and remains in the temperature range 800–1500 °C, and LaAlO₃ forms between 1200 and 1400 °C. The LaAlO₃ reacts with Al₂O₃ to form LBA above 1500 °C. The diffusion of La³⁺ through the La₂Zr₂O₇ phase was faster than that of Al³⁺. The morphology of LBA crystals was dependent on the particle size of the starting raw Al₂O₃ particle. When submicrometre size Al₂O₃ (0.4m) particles were used as the starting particles, anisotropic, plate-like LBA crystals, about 10m long, were formed during heat treatments. On the other hand, Al₂O₃ of larger grain sizes (3.6, 10.3m) yield conglomerates of LBA crystals. The size of the conglomerates is similar to that of the raw Al₂O₃ particle. The dependence of the morphology of LBA on the particle size of Al₂O₃ can be attributable to the sintering process of the Ce-TZP matrix, leading to the control of the mechanical properties of Ce-TZP ceramics with LBA crystals.     

Dispersion stability of Y-TZP/Ce-TZP powder system and slip casting

The zeta potential and apparent viscosity measurements of 3Y-TZP, 12Ce-TZP and 3Y-TZP/12Ce-TZP suspensions, has allowed the slip casting conditions for the preparation of multilayer composites have been examined. The influence of heat treatment on the sintered density, microstructure and crystalline phase of multilayer composites was also studied. The isoelectric point of both 3Y-TZP and 12Ce-TZP suspensions was near pH 8 and that of 3Y-TZP/12Ce-TZP was at pH 8.6. The suspensions exhibited pseudoplastic flow, showing a decrease in viscosity with increasing shear stress. A small (0.3 wt%) addition of an organic deflocculant gave 3Y-TZP and 3Y-TZP/12Ce-TZP suspensions with 15 and 20 vol% solid contents an appropriate fluidity for slip casting, but an additional electrolyte was required to reduce viscosity in 12Ce-TZP. Dense (>98% of theoretical) multilayer composites with grain size of 0.3–2.2 m were obtained after sintering at 1500°C.     

A comparison of sensitization kinetics in 304 and 316 stainless steels

The effects of tensile and cold rolling strain (up to 40%) over a range of grain sizes ranging from 300 m to 10 m on sensitization (and desensitization) were observed and compared for 304 and 316 stainless steel having a constant carbon content of 0.05%; at 670°C. Rapid sensitization-desensitization was observed for both materials at the smallest grain size, and plots of degree of sensitization (DOS) data with time, temperature, and tensile strain coupled with chromium diffusivity data for 304 stainless steel allowed activation energies to be calculated from corresponding Arrhenius plots utilizing supplemental data from Beltran, et al. [1] at 625°C and 775°C. Values of 1.9 and 2 kcal/mol were found for unstrained and 20% strained samples for 11 m grain size while corresponding values at 175 m grain size were 55 and 32 kcal/mol respectively. Activation energies for unstrained and 10% strained 316 stainless steel for 135 m grain size were found to be 76 and 64 kcal/mol, respectively. Sensitization was more rapid for cold-rolling versus tensile straining in both stainless steels, and there was no detectable sensitization for the largest grain size regime of the 316 stainless steel up to 10 h aging time at 670°C.     

Magnetic Co2Y ferrite, Ba2Co2Fe12O22 fibres produced by a blow spun process

Gel fibres of Co2Y,Ba2Co2Fe12O22, were blow spun from an aqueous inorganic sol and calcined at temperatures of up to 1200°C. The ceramic fibres were shown by X-ray diffraction to form crystalline Co2Y at 1000°C, and surface area and porosity measurements indicated an unusually high degree of sintering at this temperature. The fibres also demonstrated a small grain size of 1–3 m across the hexagonal plane and 0.1–0.3 m thickness at 1000°C. This only increased to 3 m in diameter and 1 m thickness even at temperature up to 1200°C. The fibrous nature combined with the improved microstructures could be an important factor in improving the magnetic properties of this material.     

Effects of grain size on pyroelectric and dielectric properties of Pb0.9La0.1TiO3 ceramics

Pyroelectric and dielectric properties of Pb0.9La0.1TiO3 ceramics with grain sizes in the range of 1.3–6.0 m were investigated. The temperature stability of the pyroelectric coefficient at room temperature was improved as the grain size became smaller. The pyroelectric coefficient of Pb0.9La0.1TiO3 at room temperature reached the lowest value near the grain size of 2.7 m. Above 2.7 m, the relationship between the pyroelectric coefficient and the grain size could be explained by the increase of 90° domain switching. However, below 2.7 m, the relationship was not clear. The variation of the relative dielectric constant of Pb0.9La0.1TiO3 with the grain size showed a similar relationship with that of the pyroelectric coefficient. The variation of the pyroelectric figure of merit on voltage responsivity as a function of the grain size was similar to that of the pyroelectric coefficient because the pyroelectric coefficient varied more strongly than the relative dielectric constant. © 1998 Chapman & Hall     

On the serrated yielding in Cu-14.1 at%AI polycrystals

Cu-14.1 at %Al polycrystals with three different grain sizes (76,113 and 157 m) have been tested for the Portevin-LeChatlier effect under various conditions of temperature (–196 to 200°C) and strain rate (2.78 × 10^–5 to 5.56 × 10³ sec^–1). In the above range of strain rate, serrated yielding was observed in the temperature range 60 to 160° C. The strain rate dependence of the onset of serrations is most probably due to the diffusion of vacancy-solute atom pairs, as indicated by the low value of the activation energy for migration (0.77 eV). The correlation governing the test variables at the onset of serrations appears to be: = (const)₀ ^{2.2 ± 0.2 –0.87 + 0.03}, where , ₀ and are strain rate, critical strain for the onset of serration and grain size, respectively. The onset of serrations is most probably due to dynamic strain ageing, although the possibility of short range ordering is not ruled out.On leave from Mechanical Engineering Department, Ferdowsi University, Mashhad, Iran.     

Effect of thermomechanical treatments on microstructure and properties of Cu-base leadframe alloy

The effect of thermomechanical treatments (TMT) on the microstructuresand properties of Cu-1.5Ni-0.3Si-0.03P-0.05Mg leadframe alloy wasinvestigated. The Cu-base leadframe alloy was received as hot rolledplates with 8 mm thickness. The hot rolled plates were solutiontreated at 700°C or 800°C for 1 hour, and coldrolled with 40–85% reduction, then followed by aging treatment at450°C. The leadframe alloy solution treated at 800°Cshowed larger grain size of 15 m comparing with the grain size of10 m in leadframe alloy solution treated at 700°C. Theleadframe alloy with smaller grain size of 10 m showed highertensile strength and lower electrical resistivity than that withlarger grain size of 15 m. The dislocation density increased withincreasing reduction ratio of cold rolling from 40% to 85% andresulted in finer Ni₂Si precipitates. Tensile strength increasedand electrical resistivity decreased with increasing reduction ratioof cold rolling due to the formation of finer Ni₂Si precipitates.Two types of thermomechanical treatments were performed to enhance theproperties of leadframe alloy. One type of thermomechanical treatmentis to refine the grain size through the overaging, cold rollingfollowed by recrystallization. The recrystallization process improvedthe tensile strength to 540 MPa and elongation to 15% by reducing thegrain size to 5 m. The other type of thermomechanical treatmentis to refine the precipitate size by two-step aging process. Thetwo-step aging process increased the tensile strength to 640 MPa andreduced the electrical resistivity to1.475 × 10^–8 m by reducing the size of Ni₂Si precipitates to 4 nm.     

Microstructure-property studies in friction-stir welded, Thixomolded magnesium alloy AM60

Thixomolded magnesium alloy AM60 plates joined by friction-stir welding were observed to be as strong or stronger than the unwelded base material. The thixotropic microstructures of the as-molded plates consisted of either 3% or 18% primary solid fraction of -Mg globules (45 m average size) in a eutectic mixture consisting of -Mg grains (10 m) surrounded by Mg₁₇Al₁₂ intermetallic grains in the -Mg grain boundaries (having an average size of 1–2 m). This complex, composite microstructure became a homogeneous (Mg + 6% Al)), recrystallized, equiaxed grain (10–15 m) microstructure in the weld zone.     

Pressureless sintered ATZ and ZTA ceramic composites

Alumina-20 wt% zirconia (ATZ) and zirconia-20 wt% alumina (ZTA) composites were prepared by conventional sintering of commercial powders, with average particle sizes in the range 0.35–0.70 m. Sintering at 1650 °C for 4 h resulted in final densities up to 96%. Bending strength and hardness increased with the final density. The tetragonal volume fraction was strongly dependent on both the final density and tetragonal grain size. The relatively high fracture toughness of 9 MPa m^1/2 was associated with the highly dense microstructure consisting of tetragonal grains of the critical size.     

Study on barium titanate ceramics prepared by various methods

Barium titanate (BaTiO₃) ceramics have been fabricated using powders prepared by sol-gel, coprecipitation and mixed oxide methods. The powders prepared by sol-gel and coprecipitation have average crystallite diameters of 100 nm and 300 nm, respectively while the diameter of the mixed oxide powder is 1–3 m. When sintered at the same temperature of 1320°C, the three BaTiO₃ ceramics have very different grain size, with the one prepared by the mixed oxide method having the largest grain size of 20 m. The dielectric permitivity increases as the grain size of the ceramic becomes smaller. The room temperature (25°C) dielectric permittivity, pyroelectric and piezoelectric properties of these ceramics have been measured as functions of the poling field. The BaTiO₃ ceramic fabricated from nanosized powder derived from the coprecipitation method is found to have the smallest grain size and better properties than prepared from the sol-gel route, and is thus a good candidate for use in devices that required thick (10 to 20 m) ferroelectric films.     

The role of Coble creep and interface control in superplastic Sn-Pb alloys

The creep behaviour of superplastic Sn-2 wt% Pb and Sn-38.1 wt % Pb is investigated at temperatures between 298 and 403 K and for grain sizes between 2.5 and 260m. In Sn-2 wt% Pb with grain sizes larger than 50 m, diffusion-controlled Coble creep is found and it is experimentally shown that this type of creep is inhibited in smallgrained specimens. Measurements covering low stresses ( 0.1 MPa) and strain rates ( 10^–10 sec^–1) rule out any explanation which relies on a threshold stress for plastic deformation. The observations are explained by a model in which, at low stresses or small grain sizes, Coble creep is rate-limited not by diffusion of vacancies but by the rate of emission and absorption at the curved dislocations in the grain boundaries which are the ultimate sources and sinks of vacancies.     

Grain size effects on the dielectric properties of ferroelectric Bi2VO5.5 ceramics

Dielectric properties and microstructural characteristics of ferroelectric bismuth vanadate (Bi₂VO_5.5) ceramics exhibiting grain sizes of 7, 10, 20 and 25 m have been studied. Microstructural studies indicate the presence of ferroelectric 90° domain patterns on the surface as well as in the bulk of the coarse-grained ceramics. The dielectric constant and the loss tangent both at room temperature and in the vicinity of the Curie temperature have been found to increase with increasing grain size. The Curie temperature (725 K) is found to shift slightly (by about 7 K) towards higher temperatures as the grain size increases (7–25 m). The magnitude of the dielectric anomaly around 725 K is found to be higher for coarse-grained ceramics. The dielectric constant and the loss have been found to decrease with increase in frequency (1–100 kHz) for all the ceramics studied. The increase in dielectric constant with increasing grain size is attributed to a decrease in thickness of the relatively more insulating grain boundary layer.     

Mechanical properties of ultra-fine grained zirconia ceramics

The mechanical properties of tetragonal zirconia (TZP) materials doped with Y, Ce or Ti were studied as a function of temperature and grain size. Fine grained Y-TZP (grain size < 0.3 m) shows values for fracture toughness and strength at room temperature, which are comparable with the coarse grained transformation toughened materials, despite lacking transformation toughening. The morphology of the fracture surface points to crack deflection as the most important toughening mechanism. At 800 °C fracture toughness and strength are higher than in coarse grained Y-TZP materials. Doping Y-TZP with Ce or Ti results in a similar trend in mechanical properties, for fine grained material, as for the Y-TZP materials.     

Preparation of Si3N4 whiskers by reaction of wheat husks with NH3

-Si₃N₄ whiskers that are 1–10 mm long and 0.5–1.1 m thick were obtained by the reaction of wheat husks with NH₃ at 1250–1450 °C. A maximum whisker yield of about 30% was obtained at 1450 °C with the addition of an iron impurity. Whiskers with 1.3–2.2 m thickness (average 1.6 m) were obtained by the addition of an H₂S impurity. Thin whiskers with periodic thick and thin diameters were also obtained.     

On crystal alignment in polycrystalline YBCO materials based on TEM observation

Previously, we have observed that parallel lattice fringes show up regularly in MTG samples, indicative of good alignment of unit cells in strips of size 100 nm × a few times 100 nm. We have prepared films ( 80 nm) for TEM observation using YBCO material fabricated by the conventional solid-state reaction method in order to study the crystal texture in a length scale from a few 100 nm down to 3 Å. Parallel lattice fringes are observed to stretch across a cross-section 0.3×0.5 m². Cross lattice fringes intersecting at 90° and 45° are observed as rare events. This result suggests that the degree of alignment in the crystal texture is higher than expected inside a grain of size a few m.     

Dual-phase magnesia-zirconia ceramics with strength retention at elevated temperatures

Two-phase polycrystalline ceramics containing MgO and ZrO₂ were fabricated by pressureless sintering powder compacts in air to near theoretical density. MnO was added as a densification aid in most compositions. For samples fabricated with 20 vol% ZrO₂ and 80 vol% MgO (which actually contained 23 vol% ZrO₂(ss) after sintering because some of the MgO dissolved in zirconia), densities in excess of 98% theoretical were achieved at temperatures as low as about 1250° C. However, most of the samples were typically sintered at 1420±10° C. The grain sizes of the two phases, ZrO₂(ss) and MgO(ss), were of the order of 1.4m. Thermal etching of the specimens showed the presence of very uniform sized domains (approximately 240 nm in size) in zirconia grains. Some samples were also fabricated in which 8 mol% CaO was added in order to stabilize the high-temperature cubic polymorph of zirconia to room temperature. The grain sizes of the two phases in this composition were also of the order of 1.4m. No domains were observed in zirconia grains in CaO-doped samples. Fracture strength was measured as a function of volume fraction of zirconia. Strength values in excess of 500 MPa have been measured on samples fabricated with 40 vol% zirconia (the amount of zirconia (ss) is 43 vol%). Samples of similar composition but with CaO doping exhibited strength of the order of 300 MPa despite an essentially identical grain size and density. Fracture toughness of samples containing CaO was 3.0 MPa m^1/2 while that of the samples without CaO was 5.2 MPam^1/2. No monoclinic phase was observed on either the fracture or the ground surfaces of CaO-doped and undoped samples. Fracture strength and toughness, measured as a function of temperature up to 1000° C, were found to be nearly independent of temperature. The temperature independence of the strength suggests that strengthening and toughening in this material does not occur by transformation toughening.     

Precipitation characteristics of μ-phase in wrought nickel-base alloys and its effect on their properties

Thermal exposures consisting of 1–16000 h at 540, 650, 760, and 870°C were used to study the susceptibility of selected nickel-base alloys to precipitation of -phase and its effect on mechanical strength and corrosion resistance. Analytical electron microscopy and X-ray diffraction were used to characterize the -phase. A -phase of the type Mo₆Ni₇ in nickel-base alloys was found to be stabilized by critical concentrations of iron in an excess of about 3 wt%. Generally, the -phase had a characteristic defect structure consisting of twins and stacking faults, and it exhibited a preferential tendency for precipitation at existing molybdenum-rich carbide particles within the alloy matrix and at grain boundaries. Precipitation of -phase was found to produce a moderate loss of room-temperature tensile ductility; however, it resulted in a considerable degradation of impact toughness and corrosion resistance. In contrast, it had no significant effect on elevated temperature tensile properties. A correlation was found to exist between the Ni/Fe + Co ratio as well as the Mo + W content of the alloy and susceptibility to precipitation of -phase.     

Microstructure and properties of the sintered composite prepared by hot pressing of TiN-coated alumina powder

Alumina powders (average grain size: 50 m) coated with TiN film of thickness 0.5 and 1.2 m were prepared by rotary powder-bed chemical vapour deposition for 15 and 90 min, respectively. These Al₂O₃-TiN composite powders were hot-pressed at 1800 °C and 40 MPa for 30 min. The microstructure of the Al₂O₃-TiN sintered composite was composed of a TiN network homogeneously distributed on the grain boundaries of alumina. The mechanical properties (hardness, bending strength and fractured toughness) and thermal conductivity of the sintered composite were found to depend on the composition and microstructure of the sintered composite, even with a small content (3–7 wt%) of TiN. The resistivity of the sintered composite was 10^–1-10^–3 cm. The relatively high electrical conductivity of the Al₂O₃-TiN composite was caused by the grain boundary conduction of TiN.