Effect of Amount of Boron Doping on Compression Deformation of Fine-Grained Silicon Carbide at Elevated Temperature

The effect of the amount of boron doping in the range of 0 to 1.0 wt% on the high-temperature deformation of fine-grained β-silicon carbide (SiC) was investigated by compression testing. Flow stress at the same grain size increased as the amount of boron doping decreased. The stress exponent increased from 1.3 to 3.4 as the amount of boron doping decreased. The strain rates of undoped SiC were ∼2 orders of magnitude lower than those of 1.0-wt%-boron-doped SiC of the same grain size. The apparent activation energies of SiC doped with 1.0 wt% boron and of undoped SiC were 771 ± 12 and 884 ± 80 kJ/mol, respectively. These results suggest that the actual contribution of grain-boundary diffusion to the accommodation process of grain-boundary sliding decreased as the amount of boron doping decreased. Consequently, the apparent contribution of the dislocation glide increased.     

Superplasticity of Silicon Carbide

Nanocrystalline silicon carbide that was doped with boron and carbon (B,C-SiC) and contained 1 wt% boron additive and 3.5 wt% free carbon was fabricated using hot isostatic pressing under an ultrahigh pressure of 980 MPa and a temperature of 1600°C. The average grain size of the material was 200 nm. The tensile deformation behavior of this material at elevated temperature was investigated. The nanocrystalline B,C-SiC exhibited superplastic elongation of >140% at a temperature of 1800°C. High-resolution transmission electron microscopy observation and electron energy-loss spectroscopy analysis revealed that this nanocrystalline SiC did not have a secondary glassy phase at the grain boundary and the grain boundary had a strong covalent nature, which means that an intergranular glassy phase was not necessary to obtain superplasticity of covalent materials.     

Flexural Creep of an in Situ-Toughened Silicon Carbide

Flexural creep behavior is reported for an in situ -toughened SiC between 1100° and 1500°C in four-point bending. The flexural creep rate of this SiC, sintered with aluminum, boron, and carbon (ABC-SiC), exhibits linear stress dependence, low apparent activation energy, and low incidence of cavitation and dislocation production. Most grain boundaries in this ceramic contain 1–5 nm intergranular films. The creep rate is consistent with a grain-boundary transport mechanism involving diffusion along the grain-boundary film-SiC interfaces. The microstructure and grain boundaries have been examined using transmission electron microscopy to assess possible changes during creep, particularly in relation to the applied stress direction.     

Superplastic Flow of Fine-Grained Yttria-Stabilized Zirconia Polycrystals: Constitutive Equation and Deformation Mechanisms

Available deformation data for superplastic yttria-stabilized zirconia polycrystals with grain size <1 µm have been analyzed at temperatures between 1250° and 1450°C as a function of stress, grain size, and impurity content. The apparent stress exponent n for the higher-purity materials (residual impurity content <0.10 wt%) varies from 2 (region II) to greaterthan equal to3 (region I), and then toward 1 when the stress is decreased. The stress for transition between region II and region I decreases when the temperature and/or grain size is increased. The activation energy Q for flow in region II is 460 kJ/mol, which is approximately that for cation lattice diffusion. The grain-size exponent p decreases continuously and Q increases continuously with decreasing stress in region I. The constitutive equation for superplastic flow in region II is identical to that for metallic systems when lattice diffusion is the rate-controlling mechanism. The experimental results have been correlated with a single deformation process that incorporates a threshold stress, below which grain-boundary sliding does not contribute to strain. The threshold stress may result from yttrium segregation at grain boundaries and its interaction with grain-boundary dislocations. A single deformation regime with n = 2 exists for low-purity materials (impurity content >0.10 wt%) over the entire stress range. The strain-rate enhancement with respect to high-purity materials is related to the grain-boundary amorphous phase present in such materials.     

Microstructure and Grain-Boundary Effect on Electrical Properties of Gadolinium-Doped Ceria

The microstructural evolution and grain-boundary influence on electrical properties of Ce_0.90Gd_0.10O_1.95 were studied. The nanoscale powders synthesized from a semibatch reactor exhibited 50% green density and 92% sintering density at 1200°C (∼200°C lower than previous studies). Impedance spectra as a function of temperature and grain size were analyzed. The Ce_0.90Gd_0.10O_1.95 with finest grain size possessed highest overall grain-boundary resistance; this contribution was eliminated at temperatures >600°C, regardless of grain size. The grain conductivity was independent of grain size and was dependent on temperature with two distinct regimes, indicative of the presence of Gd'_Ce− V _o^∘∘ complexes that dissociated at a critical temperature of ∼580°C. The activation energy for complex dissociation was ∼0.1 eV; the value for the grain-boundary was ∼1.2eV, which was size independent.     

Effect of Oxygen Segregation at Grain Boundaries on Deformation of B, C-Doped Silicon Carbides at Elevated Temperatures

The effect of oxygen segregation at grain boundaries on the deformation of 1 wt% boron (B)- and carbon (C)-doped β-silicon carbide (B, C-doped SiC) was investigated by compression testing at 2073 K. We studied the deformation of sinter-forged B, C-doped SiC (sinter-forged SiC), which contained the minimum amount (0.07 wt%) of oxygen as an impurity, and that of hot isostatically pressed B, C-doped SiC (HIPed SiC), which contained 1 wt% oxygen. Oxygen was detected at grain boundary in HIPed SiC by energy-dispersive X-ray spectroscopy, but it was not detected in sinter-forged SiC. The strain rate of sinter-forged SiC was one order of magnitude lower than that of HIPed SiC at the same grain size. The grain growth rate of sinter-forged SiC was lower than that of HIPed SiC also. These results suggest that the oxygen segregation at grain boundaries, together with boron segregation, promoted the grain-boundary diffusion in B, C-doped SiC. But, the oxygen segregation without boron was less effective in promoting deformation than the boron segregation without oxygen.     

Correlation Between Microstructure and Creep Behavior in Liquid-Phase-Sintered α-Silicon Carbide

The influence of increasing the sintering time from 1 to 7 h on the microstructure evolution and the mechanical properties at high temperature was studied in α-silicon carbide (α-SiC) sintered in argon atmosphere with Y₂O₃–Al₂O₃ (10% weight) as liquid phase (LPS-α-SiC). The density decreased from 98.8% to 94.9% of the theoretical value, the grain size increased from 0.64 to 1.61 μm, and some of the grains became elongated. The compression tests were performed in argon atmosphere, between 1450°C and 1625°C and stresses between 25 and 450 MPa, with the strain rate being between 4.2 × 10⁻⁸ and 1.5 × 10⁻⁶ s⁻¹. The stress exponent n and the activation energy Q were determined, finding values of n between 2.4±0.1 and 4.5±0.2 and Q =680±35 kJ/mol for samples sintered for 1 h, and n between 1.2±0.1 and 2.4±0.1 and Q =710±90 kJ/mol for samples sintered for 7 h. The correlation between these results and the microstructure indicates that grain-boundary sliding and the glide and climb of dislocations, both accommodated by bulk diffusion, may be two independent deformation mechanisms operating. At the temperatures of the tests, the existence of solid-state reactions between SiC and the sintering additives is responsible of the microstructural changes observed. These effects are not a consequence of the process of deformation, but rather they are because of the thermal treatment of the material during the creep.     

Flexural Creep Deformation of ZrB2/SiC Ceramics in Oxidizing Atmosphere

Flexural creep of ZrB₂/0–50 vol% SiC ceramics was characterized in oxidizing atmosphere as a function of temperature (1200°–1500°C), stress (30–180 MPa), and SiC particle size (2 and 10 μm). Creep behavior showed strong dependence on SiC content and particle size, temperature and stress. The rate of creep increased with increasing SiC content, temperature, and stress and with decreasing SiC particle size, especially, at temperatures above 1300°C. The activation energy of creep showed linear dependence on the SiC content increasing from about 130 to 511 kJ/mol for ceramics containing 0 and 50 vol% 2-μm SiC, respectively. The stress exponent was about 2 for ZrB₂ containing 50 vol% SiC regardless of SiC particle size, which is an indication that the leading mechanism of creep for this composition is sliding of grain boundaries. Compared with that, the stress exponent is about 1 for ZrB₂ containing 0–25vol% SiC, which is an indication that diffusional creep has a significant contribution to the mechanism of creep for these compositions. Cracking and grain shifting were observed on the tensile side of the samples containing 25 and 50 vol% SiC. Cracks propagate through the SiC phase confirming the assumption that grain-boundary sliding of the SiC grains is the controlling creep mechanism in the ceramics containing 50 vol% SiC. The presence of stress, both compressive and tensile, in the samples enhanced oxidation.     

Sintering of Ultrafine Silicon Powder

The sintering behavior of compacts of ultrafine silicon powder (0.02 to 0.1 μm particle size) was investigated. Two sintering modes occur: normal sintering associated with densification and subnormal sintering without densification. The micro-structure developed in normal sintering has a fine grain size (0.05 to 0.3 μm) and fine porosity; the grains contain stacking faults and twins. The microstructure developed in subnormal sintering exhibits larger grains (∼1 μm in size) and coarse pores. Green densities >42% of theoretical and temperatures >1100°C are required for densification. Densification follows an exponential time and temperature dependence with an activation energy of 470 kJ/mol, indicating bulk diffusion as the transport mechanism. Grain-boundary diffusion is thought to be inhibited by grain-boundary oxide films. The carbon phase-separates into discrete amorphous regions and is thought to have little effect on sintering behavior.     

Ytterbium Cation Diffusion in Yttrium Aluminum Garnet (YAG)—Implications for Creep Mechanisms

The lattice and grain-boundary diffusion coefficients of ytterbium, which substitutes for yttrium, have been determined in high-purity, stoichiometric yttrium aluminum garnet (YAG) polycrystals in the temperature range 1400°–1550°C, in air. Ytterbium oxide thin films were produced on the YAG surfaces by a dipping method. After diffusion treatments, the penetration profiles were established by secondary ion mass spectroscopy, and the diffusion coefficients were calculated from the thin-film solution of Fick's equation. The difference between the volume and grain-boundary diffusion coefficients is ∼5 orders of magnitude in the temperature range studied. The cation activation energies (∼550 kJ/mol) are much larger than those for oxygen (∼300–350 kJ/mol). The effective diffusion coefficient deduced from high-temperature deformation data reported in the literature for YAG polycrystals, assuming grain-boundary sliding accommodated by volume diffusion, is in excellent agreement, both in magnitude and activation energy, with the cation diffusion data.     

Selectivity of Silicon Carbide/Stainless Steel Solid-State Reactions and Discontinuous Decomposition of Silicon Carbide

Solid-state reactions of SiC with a multicomponent system, stainless steel, have been studied at 1125°C. Four-layered reaction products consisting of modulated carbon precipitation zone, random carbon precipitation zone, four-phase mixture zone, and grain-boundary precipitation zone were formed in the reaction zone. The carbon precipitates were embedded in a matrix of complex metal silicides. In addition, extensive interfacial melting was noted. Carbon atom was found to diffuse faster than Si and selectively reacted with Cr to form Cr carbide(s) along the grain boundaries of stainless steel. No Fe carbides or Ni carbides were ever detected. Among the consituents existing in stainless steel, Ni atoms have the highest affinity for Si. An uphill diffusion of Ni toward the SiC reaction front was observed. While the diffusion of Cr and Fe toward SiC followed a downhill concentration gradient, very small amounts of Cr reached the SiC interface. The selective reactions of Si and C with Ni, Fe, and Cr are discussed on the basis of Gibbs free energy of formation of various compounds. The diffusion kinetics of C and Si atoms in selected metal/SiC reactions are discussed on the basis of their chemical affinities for respective metals. The modulation of carbon precipitation is correlated with previous results from Ni/SiC, Ni₃Al/SiC, Fe/SiC, Co/SiC, and Pt/SiC, reactions. A general model describing discontinuous decomposition of SiC is proposed to explain the origin of carbon modulation.     

Strength-Degrading Mechanisms for Chemically-Vapor-Deposited SCS-6 Silicon Carbide Fibers in an Argon Environment

The room-temperature tensile strengths of chemically-vapor-deposited SCS-6 silicon carbide fibers were measured after 1 to 400 h heat treatments in 0.1 MPa of argon at temperatures up to 2100°C. The fibers heat-treated for 1 h above 1400°C and those heat-treated for 400 h above 1300°C showed strength degradation. Scanning and transmission electron microscopic examination of the degraded fibers showed formation of a recrystallization region within the outer zone of the SiC sheath and the growth of SiC particles in the carbon-rich surface coating. The activation energy for the growth of the recrystallization region was ∼370 kJ/mol. The tensile strength of the fibers was found to vary as an inverse function of the recrystallized zone thickness.     

Heat-Resistant Silicon Carbide with Aluminum Nitride and Erbium Oxide

Fully dense SiC ceramics with high strength at high temperature were obtained by hot-pressing and subsequent annealing under pressure, with AlN and Er₂O₃ as sintering additives. The ceramics had a self-reinforced microstructure consisting of elongated SiC grains and a grain-boundary glassy phase. The strength of these ceramics was ∼550 MPa at 1600°C, and the fracture toughness was ∼6 MPa·m^1/2 at room temperature. The beneficial effect of the new additive composition on high-temperature strength might be attributable to the introduction of aluminum from the liquid composition into the SiC lattice, resulting in a refractive grain-boundary glassy phase.     

Superplastic Deformation in Fine-Grained YBa2Cu3O7−x

The superplastic behavior of YBa₂Cu₃O_{7− x} ceramic superconductors was studied. Large compressive deformation over 100% strain was measured in the temperature range of 775°–875°C, with a strain rate of 1 × 10⁻⁵ to 1 × 10⁻³/s, and a grain size of 0.5–1.4 μm. The nature of the deformation was investigated in terms of three deformation parameters: the stress exponent ( n ), the grain size exponent ( p ), and the activation energy ( Q ). The measured values of these parameters were n = 2 ± 0.3, p = 2.7 ± 0.7, and Q = 745 ± 100 kJ/mol. With the aid of the deformation map, the deformation mechanism was identified as grain boundary sliding accommodated by grain boundary diffusion. The conclusion is consistent with the microstructural observations made by SEM and TEM: the invariance of equiaxed grain shape, the absence of significant dislocation activity, no grain boundary second phases, and no significant texture development.     

Electron Probe Micro Analysis of A-Site Inter-Diffusion Between LaFeO3 and NdFeO3

The A-site cation diffusion in LaFeO₃ has been examined by inter-diffusion experiments between LaFeO₃ and NdFeO₃. Dense, polycrystalline bodies were annealed in contact at temperatures between 1100° and 1300°C in ambient air. The bulk- and grain-boundary inter-diffusion coefficients were calculated from concentration profiles determined by electron probe micro analysis of cross sections. The bulk- and grain-boundary inter-diffusion coefficients showed Arrhenius-type behavior with activation energies 610±30 and 600±100 kJ/mol, respectively. Based on the assumption of 1 nm thick grain boundaries the grain-boundary inter-diffusion coefficient was ∼4 orders of magnitude higher than the bulk inter-diffusion coefficient.     

Relating Grain-Boundary Complexion to Grain-Boundary Kinetics I: Calcia-Doped Alumina

Because of the complex nature of internal interfaces it has been a continual challenge to link the grain growth behavior of alumina (especially the onset of abnormal grain growth) to the internal interface structure and chemistry, and the associated atomic transport rate. The present work considers the problem of normal and abnormal grain growth development in calcia-doped alumina, a system noted for its complex abnormal grain growth behavior, in terms of the new concept of interface complexions. Calcia-doped alumina was shown to exhibit four distinct grain-boundary complexions in the temperature range of 1325°–1870°C. All four complexions may coexist at a single temperature. Each complexion is associated with a characteristic grain-boundary mobility, all of which enhances the grain growth kinetics relative to undoped alumina. It was found that the activation energy for the different complexions (normal and abnormal grain growth) was approximately the same in each case (∼450 kJ/mol). This is discussed in the context of interface- versus diffusion-controlled grain growth, and it is concluded that normal and abnormal grain growth in this system is diffusion controlled.     

Creep Mechanism of Polycrystalline Yttrium Aluminum Garnet

The high-temperature deformation behavior of a fine-grained polycrystalline yttrium aluminum garnet (YAG) was studied in the temperature range of 1400° to 1610°C using constant strain rate compression tests under strain rates ranging from 10⁻⁵/s to 10⁻³/s. The stress exponent of the creep rate, the activation energy in comparison with that for single-crystal YAG, and the grain size dependence suggest that Nabarro–Herring creep rate limited by the bulk diffusion of one of the cations (Y or Al) is the operative mechanism.     

Grain Size Effect on Creep Deformation of Alumina-Silicon Carbide Composites

A study of the flexural creep response of aluminas reinforced with 10 vol% SiC whiskers was conducted at 1200° and 1300°C at stresses from 50 to 230 MPa in air to evaluate the effect of matrix grain size. The average matrix grain size was varied from 1.2 to 8.0 μm by controlling the hot-pressing conditions. At 1200°C, the creep resistance of alumina composites increases with an increase in matrix grain size, and the creep rate (at constant applied stress) exhibits a grain size exponent of approximately 1. The stress exponent of the creep rate at 1200°C is approximately 2, consistent with a grain boundary sliding mechanism. On the other hand, the creep deformation rate of 1300°C was not sensitive to the alumina grain size. This was seen to be a result of enhanced nucleation and coalescence of creep cavities and the development of macroscopic cracks as the grain size increases. Observations also indicated that the prevalent site for nucleation and growth of creep cavities in coarsegrained materials is at two-grain junctions (grain faces), whereas in fine-grained materials cavities nucleate primarily at triple-grain junctions (grain edges). Electron microscopy studies revealed that the content of any amorphous phase present at whisker-alumina interfaces is independent of alumina grain size (and hot-pressing conditions). In addition, the alumina grain boundaries are quite devoid of amorphous phase(s). This variation in amorphous phase content does not appear to be a factor in the present creep results.     

Grain-Boundary Diffusion of 51Cr in MgO and Cr-Doped MgO

The grain-boundary diffusion product, D'δ , of ⁵¹Cr in MgO and Cr-doped MgO as a function of grain-boundary orientation and point-defect concentration was determined at T =1200° to 1450°C. A large degree of anisotropy was found in the grain-boundary diffusion behavior in MgO. The ratio of D'δ_|| parallel to D'δ_‖ perpendicular to the growth direction, D'_||/D'_‖ , is 10² for a 5° (100) tilt boundary, decreased to ∼2 in boundaries with tilt angles > 10°. The decrease in D'_||/D'_‖ is due to a large increase in D'_‖ with increasing tilt angle. The results indicate that grain-boundary diffusion in MgO is connected to the orientation of dislocations and the mechanism is one of dislocation pipe diffusion. The grain-boundary diffusion product D'δ increases with increasing Cr concentration in MgO and is ∼4 times larger for MgO containing 0.56 at. % Cr than for the undoped MgO. For all bicrystals studied, the activation energies are within 180 ± 20 kJ/mol which is 60% of the activation energy for ⁵¹Cr diffusion in undoped MgO.     

Fabrication of Nanograined Silicon Carbide by Ultrahigh-Pressure Hot Isostatic Pressing

Dense nanograined SiC ceramics were obtained by using hot isostatic pressing (HIP). The starting powder was ultrafine β-SiC powder, which had a mean particle size of 30 nm and contained 3.5 wt% free carbon. SiC powders-both boron-doped and undoped-were densified via HIP under an ultrahigh pressure of 980 MPa at a temperature of 1600°C. Both doped and undoped SiC attained the same density (3.12 g/cm³) (relative density of 97.1%). The average grain sizes of boron-doped and undoped SiC were 200 and 30 nm, respectively. The compressive flow stress of undoped SiC was 3 times higher than that of boron-doped SiC at temperatures of 1800° and 1700°C; however, the flow stresses of both materials were almost the same at 1600°C. The HIPed SiC that was doped with boron could be deformed at a stress that was one-third lower than that of hot-pressed boron- and carbon-doped SiC with a grain size of 0.8 µm.