Grain-boundary de-segregation and intergranular cohesion in Si-Al-O-N ceramics

The influence of heat-treatment on high-temperature creep and sub-critical crack growth in hot-pressed Si-Al-O-N ceramics has been analyzed from microstructural evidence and determination of stress exponents and activation energies. The most significant change is the suppression of cavitation during creep and of the cavity-interlinkage mechanism for slow crack propagation. A creep mechanism of grain-boundary diffusion is characterized by stress exponent n=1 and unusually high activation energy >820 kJ mol^–1. The microstructural origin of the transformation in grain-boundary dominated properties is mainly the removal of triple-junction glassy residues within which cavities are nucleated. This is caused by grain-boundary diffusion of metallic impurities (Mg, Mn, Ca) into a surface silica oxidation layer, and consequent crystallization of the remaining glass components as . There is a continued improvement in grain-boundary cohesion and increased difficulty of grain-boundary diffusion following the stage at which triple-junction glass is removed. The resultant ceramics, in addition to superior mechanical behaviour, have an increased temperature for application due to a marked reduction in susceptibility to dissociation above 1400° C.     

Microstructural evolution of Hi-NicalonTM SiC fibers annealed and crept in various oxygen partial pressure atmospheres

It is expected that in the future SiC fiber-reinforced ceramic-matrix composites (CMCs) will be used in high temperature and hostile environments. In this study, Hi-Nicalon^TM SiC fibers were annealed and crept at 1500 °C for 1 hour in air, an argon flow and an ultra high-purity argon flow in order to investigate the effects of atmospheres and load conditions on the decomposition behavior and microstructural evolution of the fibers. After the fibers were annealed and crept in air, a silica layer with cracks was formed on the fiber surface. Under the creep load, the silica layer became thicker and porous due to the oxidation mechanism change from diffusion of ionic oxygen to transportation of oxygen molecules. An oxygen-enriched amorphous layer was formed at the fiber surface in the case of annealing in an argon flow, whereas SiC crystals were produced by the gas-phase reaction on the fiber surface when the fiber was crept in an argon flow. In an ultra high-purity argon flow, SiC crystals grew on the surface of both annealed and crept fibers. Growth of -SiC grain was enhanced under low oxygen partial pressure atmospheres and creep load.     

Plastic deformation kinetics of fine-grained MgO in tension

The plastic deformation kinetics of 99.4% MgO (d _o = 5.3 m) was investigated at 1500°–1600°C in uniaxial tension. It was determined that the stress exponent n 1 and the activation energy Q = 204 kJ/mole. Neither the present results on MgO, nor data in the literature on MgO and other oxide ceramics, can be fully explained by the models usually proposed for the plastic deformation of fine-grained oxide ceramics. The present results are however in good agreement with the model for grain boundary diffusion plastic flow recently developed by Kim, Estrin and Bush.     

Creep-sintering of polycrystalline ceramic particulate composites

The sintering of particulate composites consisting of a polycrystalline zinc oxide matrix with 10 vol % zirconia inclusions of two different sizes (3 and 14 m) was investigated at a constant heating rate of 4 °C min^–1 under an applied stress of 300 kPa. The presence of the inclusions produced a decrease in both the creep rate and the densification rate but the ratio of the densification to creep rate remained constant during the experiment. The ratio of the densification rate to creep rate for the composites was 1.5 times greater than that of the unreinforced matrix regardless of inclusion size. The creep viscosity of the composites was higher than that of the unreinforced matrix and increased slightly with decreasing inclusion size.     

The creep of sapphire filament with orientations close to the c-axis

Sapphire filament oriented within 2 1/2° of the crystallographic c-axis underwent creep by a mechanism other than slip on the basal planes at temperatures above 1600° C. There was a stress below which creep could not be detected; this decreased from 180 MNm^–2 at 1600° C to 65 MNm^–2 at 1800° C. The total tensile strain obtained never exceeded 5%. Fracture occurred during a linear stage of creep in which the stress exponent of the strain-rate was approximately 6. The creep mechanism appeared to be slip on {20¯2¯1} 01 T2 (morphological unit cell). A filament in which the c-axis lay at 6° to the filament axis deformed by localized basal slip. The accompanying local latice rotations produced fracture at a small overall strain, usually less than 0.5%. The results demonstrate extreme anisotropy of creep in sapphire crystals.     

A study of the Bailey-Orowan equation of creep

A new method was developed to study the Bailey-Orowan equation of creep, _c=r/h, where _c is the creep rate,r is the recovery rate andh is the work-hardening coefficient. The method was to vary the strain rate,, around the creep rate, _c, and to measure the corresponding stress rate,. In a plot of stress rate against strain rate, a straight line was obtained. The slope of the straight line was equal toh, and the intersection of the straight line with the stress axis was equal to –r, as in the equation=–r+h. The creep test under a constant stress is a special case of this equation when the stress rate,, is zero. The above measurement was carried out within a very small stress variation, less than 1% of the total stress, so that the values ofr andh were not disturbed. The creep test was performed on Type 316 stainless steel. The creep rate was shown to be equal to the ratior/h, but the value ofh was approximately equal to Young's modulus at the testing temperature, rather than, as is commonly believed, to the work-hardening coefficient.     

Production of ultrafine alpha alumina powders and fabrication of fine grained strong ceramics

Hydrous alumina powders, pure, seeded with alpha alumina, containing ammonium nitrate and containing both ammonium nitrate and seeds, were prepared by hydroxide precipitation. Their crystallization and sintering behaviour were investigated and mechanical properties of the ceramics were tested. Pure hydrous alumina transformed to alpha alumina crystals, with a size of ca. 200 nm, at 1200°C, after undergoing the usual metastable phase changes during heat-treatment. The powder needed to be sintered at 1600°C to achieve a high density. The ceramic had an average grain size of ca. 9 m. Seeding lowered the transformation temperature to ca. 1120°C and caused the transformation to begin at ca. 600°C. The material could be sintered at 1500°C and had a grain size of 2 m. The nitrate, predominantly present as ammonium nitrate, lowered the transformation temperature to ca. 1150°C and altered the proportion of the intermediate phases. However, the materials still had to be sintered at 1500°C to achieve >97% density. When both seed particles and nitrate ions were present the material almost completely transformed at 950°C to uniform crystals of alpha alumina with a size <60 nm that sintered to >99% theoretical density at 1450°C. The final ceramic had a uniformly grained (<1.0 m) microstructure and exhibited strength up to 800 M Pa.     

Oxidation of Zirconium and Zr + 1% Nb Alloy in Air under Nonstationary Conditions

We present some results of the experimental investigation of the influence of thermal cycling on the kinetics of interaction of zirconium iodide and KTTs-110 (Zr–1% Nb) alloy with air. The tests were carried out both in the stage of heating to 1000°C and in the course of long-term (6 h) holding for the mean temperatures T = 750, 800, 950°C (970°C) and the amplitude of thermal cycles 100°C (±50°C). We measured the specific increment of mass of the metal specimens (per unit area of the surface), the thickness and state of the surface films, and the microhardness of the metal surface. The results obtained in the course of cyclic variation of temperature were compared with similar data accumulated in the process of heating at a constant rate followed by isothermal holding. It is shown that thermal cycling with holding in the -region intensifies the processes of gas saturation and oxide formation promoting the redistribution of the diffusing substance between the oxide film and the diffusion zone in favor of the latter. The process of thermal cycling in the -region results in the catastrophic oxidation of both zirconium iodide and the KTTs alloy and, in the case of rapid heating, decreases the rate of interaction due to the stabilization of the high-temperature phase with enhanced protective properties. It is possible to conclude that the procedure of thermal cycling may intensify the processes accompanying the thermochemical treatment of products made of zirconium alloys.     

The torsional creep of carbon fibre reinforced epoxide resins

The torsional creep of composite specimens containing 60% by volume of unidirectional HT-S carbon fibre, and of unreinforced epoxide resin has been studied. Measurements were made at temperatures of 25, 50, 75 and 90° C, in two environments — air and water. Torsional preloads of up to 40% of the ultimate torsional strength were applied. All the specimens showed primary and secondary creep behaviour during the 170 h test period, and a few resin ones tertiary creep. The effects, on the secondary creep rate, of varying the proportion of hardener in the matrix and the cure schedule were marked. The lowest creep rate for a given set of test conditions was obtained when the stoichiometric amount of hardener was used and the maximum cure given. Using specimens of this optimum type, more detailed studies of creep were performed. In all cases the activation energy for secondary creep lies between 5 and 6.3 kcal (g mol)^–1 indicating that the same basic mechanism occurs in each instance. The shear stress,, was found to be proportional to the logarithm of the secondary creep rate.     

High creep exponents in a nearly-lamellar γ-based titanium aluminide intermetallic

Secondary creep data are reported for an extruded nearly-lamellar Ti-48Al-1.5Cr-alloy tested in a temperature range of 700 to 900°C. Within this temperature regime, this alloy exhibits a two-stage creep deformation behavior, with relatively high (approximately 8–12) creep exponents occurring in the high stress/high temperature regime. The high exponents in this regime are explained by dynamic recrystallization phenomena observed ₂ + in the nearly-lamellar microstructure.     

Creep of chemically vapour deposited SiC fibres

The creep, thermal expansion, and elastic modulus properties for chemically vapour deposited SiC fibres were measured between 1000 and 1500°C. Creep strain was observed to increase logarithmically with time, monotonically with temperature, and linearly with tensile stress up to 800 MPa. The controlling activation energy was 480 ± 20 kJ mol^–1. Thermal pretreatments near 1200 and 145O° C were found to significantly reduce fibre creep. These results coupled with creep recovery observations indicate that below 1400°C fibre creep is anelastic with negligible plastic component. This allowed a simple predictive method to be developed for describing fibre total deformation as a function of time, temperature, and stress. Mechanistic analysis of the property data suggests that fibre creep is the result of -SiC grain boundary sliding, controlled by a small percentage of free silicon in the grain boundaries.     

Estimation of remaining creep life of an aluminium alloy from creep crack density measurements

Long transverse test pieces of fully aged RR58 plate were stressed in tension at 278 and 308 MPa at 120° C for various fractions of their creep lives. The test pieces were subsequently sectioned, mechanically and electrolytically polished and the numbers of cracks per square millimetre were measured by optical microscopy. The crack density, n, increased linearly with creep strain at both stress levels. No accurate assessment of the variation of n with time was possible. Good agreement between the crack densities measured on duplicate microsections was achieved when the crack density was greater than 10 cracks mm^–2. The crack densities in the uniformly strained portions of 11 test pieces from the same plate, fractured at 150° C at stresses within the range 200 to 290 MPa were also measured. The crack density decreased from 45 cracks mm^–2 at 200 MPa to 4 cracks mm^–2 at 290 MPa. A regression equation n/ge=164 – 0.57 (where is the applied stress) was derived assuming linear n versus relationships at 150° C. The 90% confidence limits were derived for the determination of an unknown stress level from a single measurement of n/. Of the creep life prediction methods discussed, only the correlation of creep crack density and creep strain is of sufficient accuracy and this only when the creep stress and creep temperature are low, i.e. only for those conditions which would develop a high crack density at small fractions of the creep life.     

Mechanical behaviour and formulation of stress-strain relation for non-linear viscoelastic cellulose nitrate under cyclic loadings

The cyclic deformations under various repeated stresses are quantitatively investigated using non-linear viscoelastic cellulose nitrate heated to 60° C. The non-elastic strain or creep-plastic strain is remarkably influenced by the repeated stress and the stress rate. The cyclic deformations corresponding to the repeated stress less than a certain stress level attain the saturated state called the shake down after some cycles. The stress-strain relations of the non-linear viscoelastic media in the loading and unloading processes are deduced from the invariant theory using an hypothesis of creep potential. The non-linear viscoelastic observations obtained on the cellulose nitrate at 60° C under cyclic loadings are found to fit the deduced relations for the loading and unloading processes independent of the repeated stress and the stress rate.     

An X-ray diffraction and NMR study into the mechanism of zircon formation from aqueous sols

Solid solutions of the formula Zr₁–xSi _x O₂ (x=0–0.5) were produced by an aqueous inorganic sol-gel route. X-ray diffraction and solid state magic-angle spinning-nuclear magnetic resonance (MAS-NMR) were used to characterize the products at various calcination temperatures. The solubility limit of silica in zirconia at <950°C was found to be x=0.15 and that this amount stabilized the tetragonal zirconia phase to higher temperatures and inhibited the formation of zircon. Withx=0.5, zircon formation was 70% at only 1100°C.     

Effects of heat treatments on the creep properties of a hot pressed silicon nitride ceramic

Effects of heat treatment in an argon atmosphere at high temperatures for varying times on the creep properties of a Y₂O₃-Al₂O₃ (8-2 wt%) doped hot pressed silicon nitride (HPSN) ceramic were investigated. It was observed from the creep measurements that higher temperature, i.e. 1360C, and longer time, i.e. 8 h, heat treatment in an argon atmosphere improved the creep properties, (e.g. secondary creep rate) of this material. Heat treatment at a lower temperature of 1300C and for a shorter time of 4 h did not change the creep behaviour. Improvement of the creep properties was related to the crystallization of an amorphous grain boundary phase by heat treatment. Secondary creep rate parameters of the as-received material: stress exponent, n (2.95–3.08) and activation energy, Q (634–818 kJ mol^S–1), were in the range of values found by other investigators for various hot pressed silicon nitride ceramics.     

Phenomenological rheological model and the criterion of failure of metals under uniaxial stress

The article suggests an energy alternative of the phenomenological rheological equations and criteria of failure of metals for a broad range of stresses including stresses higher than yield stress. The approach is based on the hypothesis that the damage to material is proportional to the linear combination of the work of true stress on creep strain and plastic strain. From the same positions the article describes the nonmonotonic nature of the curve of elastoplastic strain, the nonlinear nature of the curve of creep limit, it identifies the stage of avalanche creep, etc. The model was experimentally checked on the alloy ÉP742 at 650 and 750°C. Agreement between theoretical and experimental data was obtained.Translated from Problemy Prochnosti, No. 11, pp. 13–19, November, 1991.     

Effect of gas-phase aluminum-nitride layer morphology on the thermal resistance of Mo-AlN-Mo cylindric packages

Results of measurements of the thermal resistance of Mo–AlN–Mo cylindric packages are related to the microstructure of the ceramic and intermediate layers. It is shown that only a special morphology of the nitride ceramic layer makes it possible to obtain a high-density contact between the ceramic and metal layers which is not destroyed upon subsequent heating of the package and upon thermocycling. The required socalled composite morphology of nitride ceramics can be obtained only within a narrow range of synthesis temperatures.Institute of Structural Macrokinetics of the Russian Academy of Sciences, Chernogolovka, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 6, pp. 917–922, 1995.     

Development of composite dielectrics with high specific capacitance and stable temperature characteristics

A new alternative of tailoring the dielectric characteristics of a BaTiO₃-based ceramic is established in this study. The ceramic dielectrics were made by either two or three constituents having a composition of (Ba_0.96Ca_0.04)(Ti_1.0–x Zr _x Mn_0.01)O₃ (BCTZ), where x ranges from 0 to 0.22, and sintered with Ni inner electrodes at 1300°C for 4 h in a reducing atmosphere. Both alternative stacking, i.e., layer-by-layer of different compositions, and bulk stacking configurations were prepared by screen-printing, resulting in a composite dielectric of different characteristics. It is obtained that the Curie temperature (T _c) of the BCTZ ceramics decreases with an increase of Zr in the dielectrics, i.e., –8°C per mole of Zr. In addition, the stacking configuration, the proportion and the number of constituents in the composite materials control the dielectric characteristics of the multilayer ceramic capacitors. On the basis of the principles outlined, a multilayer ceramic dielectric having k-value in excess of 8000 with the X7R specification (–55 +125°C, ±15%), which consists of two BCTZ ceramics with Curie temperatures of –20 and 100°C, was successfully developed.     

Creep of austenitic 20% Cr/35% Ni stainless steels at low stresses

The present work comprises measurements of the secondary creep-rate at different stress levels with rates between about 2×10^–5 %/h and 10%/h and the grain-boundary sliding at 700° C in two austenitic 20 wt % Cr/35 wt % Ni stainless steels. One alloy was a pure 20 wt % Cr/35 wt % Ni steel, whereas the other contained about 0.5 wt % Ti and 0.5 wt % Al so that it precipitated during creep at 700° C. Special care was taken to assure equivalent microstructure in the specimens and precise creep conditions so as to obtain accurate and reproducible creep-rates. Both materials exhibited decreasing stress-dependence of the creep-rate at low stresses. Neither the stress-dependence of the creep-rate, nor the absolute creep-rate was consistent with diffusion-creep. The amount of grain-boundary sliding was measured separately by means of scribed grid lines on the creep specimens for the pure material at stresses above the creep yield. The values for the component of the creep-rate due to grain-boundary sliding coincide very well with the extrapolated line of the low-stress branch of the creep-rate/stress curve. All these results taken together suggest that the most likely explanation of the creep yield in 20 wt % Cr/35 wt % Ni steels is the one based upon grain-boundary sliding.     

Synthesis of thermosetting copolymer of polycarbosilane and perhydropolysilazane

A copolymer of polycarbosilane and perhydropolysilazane was obtained by reacting polycarbosilane with titanium n-butoxide and perhydropolysilazane. Titanium n-butoxide and perhydropolysilazane were essential for the polymer to show a thermosetting property. The thermosetting copolymers were converted into silicon carbide-based ceramics by pyrolysis in a stream of nitrogen to 1000 °C with about 80 wt% ceramic yield. The main phase of the pyrolysis product at 1500 °C in nitrogen was small crystallite -SiC. Elemental carbon, based on rule-of-mixtures composition, in the final ceramics could be reduced by varying the ratio of polycarbosilane/perhydropolysilazane. The copolymer was dry spun and pyrolysed to produce ceramic fibre. Pyrolysis in nitrogen to 1500 °C yielded a silicon carbide-based fibre with low oxygen and low elemental carbon content. A tensile strength of 1.8 GPa and an elastic modulus of 220 GPa were obtained for the fibre which ranged from 10–12 m in diameter. Crystallization to -Si₃N₄, -SiC, and -Si₃N₄ proceeded on annealing in nitrogen at 1700 °C for 1 h.