Grain boundary sliding-induced deformation in a 30 wt% zirconia–spinel composite: influence of stress

The contribution of grain boundary sliding to total strain has been investigated in a 21 vol% zirconia–spinel composite crept under stresses of 12 and 90 MPa. To this goal, the surface topography and its changes with strain were investigated on a face parallel to the compression axis by atomic force microscopy in contact mode. Due to the low zirconia content, only sliding on spinel–spinel (S–S) and spinel–zirconia (S–Z) boundaries really contributes to strain and was consequently analysed. Insensitive to stress value, boundary sliding can account for 70–80% of the total strain. However, if the two investigated interfaces behave similarly at 90 MPa, at 12 MPa sliding on S–Z boundaries is larger than on S–S ones. That difference is to relate to a stress–strain rate sensitivity dependent on stress, 1.8 and 4.2 at 90 and 12 MPa, respectively, an increase in the stress exponent able to be induced by the existence of a threshold stress that would concern spinel–spinel boundaries.     

Suitability of mullite for high temperature applications

High temperature mechanical behaviour of mullite has been studied. Our study include tensile, flexural and compressive creep behaviour and fracture up to 1400 °C. The results obtained in creep are analysed and compared with previous work in the literature. Two regions with different behaviour can be distinguished. The creep rates in bending, tension and compression are very similar in the first region at low stresses and temperatures. It is shown that in this region creep takes place by accommodated grain boundary sliding assisted by diffusion. At higher stresses slow crack growth from defects present in the sample occurs. The stress at which this transition in the deformation mechanism happens is dependent on several factors, the loading system during testing, the grain size, the amount and distribution of glassy phase and the environment. It is claimed the existence of a network of mullite–mullite grain boundaries free of glassy phase associated to the low surface energy of [001] planes. The diffusion rate through these boundaries controls the creep rate, and explains the high creep resistance of mullite. The results presented in this work lead to the conclusion that the mechanism controlling high temperature deformation resistance of mullite materials in a wide range of stress–temperature working conditions is independent of the glassy phase content. Slow crack growth limit the use of mullite at high stresses and temperatures.     

Low-angle twist grain boundary in SrTiO3 fabricated by spark plasma sintering techniques

A SrTiO₃ bicrystal with a low-angle twist grain boundary was fabricated using the spark plasma sintering (SPS) instrument. The atomic and electronic structure of the grain-boundary core was characterized using scanning transmission electron microscopy techniques. It was determined that the boundary is comprised of 2 types of defects with distinct electronic structures: screw dislocations and dislocations with an [001] edge component. The dislocations with an [001] edge component dissociated into 2 partial dislocations, separated by a stacking fault consisting of 2 Ti–O layers. The screw dislocations are attributed to the twist component of the grain boundary, while dislocations with an [001] edge component are attributed to surface steps on the original (100) SrTiO₃ surfaces prior to diffusion bonding. The observed repeat distances between the dislocations with edge components along the grain-boundary plane are smaller than those discovered during traditional diffusion bonding experiments. The higher planar defect density observed in this study results partly from higher heating rates, lower processing temperatures, and shorter holding times during SPS processing.     

Influence of grain boundary sliding on diffusion in yttria doped zirconia

Grain boundary sliding during high temperature deformation can lead to stress concentrations and an enhancement of diffusion in mobile boundaries. Experiments were conducted on a fine grained 3 mol% yttria stabilized tetragonal zirconia, under conditions associated with superplastic flow involving grain boundary sliding. Tracer diffusion studies under creep conditions and without load indicate that there is no enhancement in either the lattice or grain boundary diffusivities. The experimental creep data are consistent with an interface controlled diffusion creep mechanism.     

Microstructural effects on the sliding wear of transparent magnesium-aluminate spinel

Grain size effects have been investigated in the lubricated sliding wear of three transparent magnesium aluminate (MgAl₂O₄) spinel materials with different grains sizes identified as: Nano, Fine, and Coarse. Only Fine spinel shows classical wear behavior, which is characterized by initial mild wear followed by a sharp transition to severe fracture-controlled wear. Worn surfaces of Fine spinel show extensive grain pullout, consistent with intergranular mode of fracture found in that spinel. Nano and Coarse spinels both show gradual transition from mild wear to severe wear, and both have significantly lower overall wear rates compared to Fine spinel. Worn surfaces in both Nano and Coarse spinels show transgranular fracture and material removal, which is reminiscent of lateral-crack induced chipping. The transgranular fracture mode in Nano spinel can be attributed to stronger grain boundaries in that spinel, which could be due to the Y₂O₃ sintering additive used for grain refinement. Whilst the large scale of the grains in Coarse spinel could be responsible for the transgranular fracture observed in that spinel.     

Characterization of spinel bubbles and preparation and properties of lightweight ceramics

During the selection of materials for anti-lithium ion corrosion, magnesium aluminate spinel has been found to have good corrosion resistance and low cost. During the preparation of light-weight spinel ceramics, it is considered to be kiln body material. In this work, spinel bubbles were prepared using electrofusion injection process. Bulk density, volume density and pressure resistance of spinel bubbles in different particle sizes were counted. Light-weight spinel bubble ceramics with different densities were prepared by using electrofused spinel bubbles as light aggregates and using ρ-Al₂O₃ and caustic burnt magnesia powder as matrix powders. The microstructure of spinel bubbles with different particle sizes and light spinel ceramics was analyzed using scanning electron microscope. Results show that the spinel bubble has complete structure, good sphericity, perfect grain crystallization, and clear edges and corners. Although there are some fine cracks and cavities, grains are tightly bound. Compressive strength and bending strength of light-weight spinel ceramics, whose density were found to lie within the range of 0.99–1.63 g cm^?3, were found to be 5.19–36.33 MPa and 3.48–12.84 MPa, respectively.     

Strain Softening and Hardening during Superplastic-Like Flow in a Fine-Grained MgAl2O4 Spinel Polycrystal

Superplastic-like flow in a fine-grained MgAl₂O₄ polycrystal exhibits strain softening and hardening, which cannot be ascribed to cavity damage and grain growth during deformation, respectively. The softening and hardening can be related to a change in the internal stress, which depends on a decrease and an increase in the density of the intragranular dislocations, respectively, whose motion contributes to the relaxation of stress concentrations exerted through the predominant deformation mechanism of grain-boundary sliding. In these two regions, the rate of deformation is controlled by the continuous recovery of the dislocations limited by lattice diffusion of the oxygen ions.     

Creep behaviour of Al2O3–SiC nanocomposites

Compared with monolithic fine grained Al₂O₃, Al₂O₃ nanocomposites reinforced with SiC nanoparticles display especially high modulus of rupture as well as reduced creep strain. Taking into account the fracture mode change, the morphology of ground surfaces showing plastic grooving, the low sensitivity to wear and the low dependence of erosion rate with grain size, it can be reasonably assumed that the strength improvement is associated with an increase of the interface cohesion (due to bridging by SiC particles) rather than with a grain size refinement involving substructure formation (as initially suggested by Niihara). In the present work, creep tests have been performed and the results agree with such a reinforcement of the mechanical properties by SiC particle bridging Al₂O₃–Al₂O₃ grain boundaries. Indeed, particles pinning the grain boundaries hinder grain boundary sliding resulting in a large improvement in creep resistance. In addition, SiC particles, while counteracting sliding, give rise to a recoverable viscoelastic contribution to creep. Because of the increased interface strength, the samples undergoing creep support stress levels, greater than the threshold value required to activate dislocation motion. The high stress exponent value as well as the presence of a high dislocation density in the strained materials suggests that a lattice mechanism controls the deformation process. Finally, a model is proposed which fits well with the experimental creep results.     

High-temperature mechanical behavior of polycrystalline yttrium-doped barium cerate perovskite

The high-temperature mechanical properties of the mixed ionic-electronic conductor perovskite BaCe_0.95Y_0.05O_3−δ with average grain size of 0.40 μm have been studied in compression between 1100 and 1300 °C in air at different initial strain rates. The true stress-true strain curves display an initial stress drop, followed by an extended steady-state stage. As the temperature decreases and/or the strain rate increases, there is a transition to a damage-tolerant strain-softening stage and eventually to catastrophic failure. Analysis of mechanical and microstructural data revealed that grain boundary sliding is the primary deformation mechanism. The strength drop has been correlated with the growth of ultrafine grains during deformation, already present at grain boundaries and triple grain junctions in the as-fabricated material.     

Analysis and spectroscopy of rare earth doped magnesium aluminate spinel

Abstract

Rare earth cation segregation in magnesium aluminate spinel has been imaged and analysed in an aberration corrected scanning transmission electron microscope. The SuperSTEM at Daresbury Laboratory provided evidence for monolayer segregation of europium ions along grain boundaries in spinel. EELS spectra confirmed the Eu to be within 0.5 nm of the grain boundary region. Spinel is a candidate material for hard window applications owing to its excellent transparency, intrinsic hardness, fracture toughness and resistance to thermal and chemical erosion. Rare earth cation doped spinel maintains good transparency throughout visible and near infrared wavelengths.     

Structure, energy, and structural transformations of graphene grain boundaries from atomistic simulations

Domain/grain boundaries are often introduced into graphene during chemical vapor deposition growth processes. Here, we performed a series of hybrid molecular dynamics simulations to study the structures, energies, and structural transformations of symmetric tilt grain boundaries of graphene. The grain boundary comprises an array of edge dislocations, with the dislocation density increasing upon increasing the grain boundary misorientation angle. The dislocation in the zigzag-oriented grain boundary contains an edge-sharing pentagon/heptagon defect, whereas the dislocation in the armchair-oriented grain boundary contains two paired pentagon/heptagon defects. In some grain boundaries, out-of-plane buckling exists due to the presence of dislocations. In the transition region (the region between the zigzag- and armchair-oriented grain boundaries), the grain boundary structures feature complex mixtures of both zigzag and armchair grain boundaries. We also discuss the grain boundary transformations and migrations that occur upon adding or removing carbon atoms at the grain boundaries for all of our investigated types of grain boundaries.     

Energy Variations in Diffusive Cavity Growth

The assignment of boundary values for the chemical potential and the calculation of energy-release rates for the growth of creep cavities along grain boundaries by self-diffusion are discussed. For simplicity, it is assumed that the boundaries are flat and that surface and grain-boundary diffusion are the dominant transport mechanisms. As matter diffuses from the void surface into and along the grain boundary, misfit residual stresses are induced to alleviate the high stress concentration ahead of the cavity apex. As a result, the contribution of strain-energy terms to the chemical potential can be neglected in typical cases. Also, contrary to the Griffith crack-extension model, the energy dissipation incurred by diffusive removal of material from the cavity surface and deposition in the grain boundary is a major term in the energy transfers associated with cavity growth. The primary energy "sink" in diffusive cavity growth is shown to arise from the work done by the grain-boundary normal stress when matter is inserted in the near-tip region by diffusion, not from the loss of strain energy of matter that is removed from the cavity at its tip or from the work of bond separation. Thermodynamic restrictions on the angle formed by the void surfaces at their apex, where they join the grain boundary, are considered. Boundary values for the chemical potential are derived in a manner appropriate for arbitrarily large but elastic distortions of material near the cavity tip and, in contrast to most previous work in the area, the effects of surface tension (i.e. of "surface stress," as distinct from surface energy) are included.     

Effect of LaB6 addition on compressive creep behavior with simultaneous oxide scale growth of spark plasma sintered ZrB2-SiC composites

A study has been carried out to examine the effect of LaB₆ addition on the compressive creep behavior of ZrB₂-SiC composites at 1300–1400°C under stresses between 47 and 78 MPa in laboratory air. The ZrB₂-20 vol% SiC composites containing LaB₆ (10% in ZSBCL-10 and 14% in ZSBCL-14) besides 5.6% B₄C and 4.8% C as additives were prepared by spark plasma sintering at 1600°C. Due to cleaner interfaces and superior oxidation resistance, the ZSBCL-14 composite has exhibited a lower steady-state creep rate at 1300°C than the ZSBCL-10. The obtained stress exponent (n ∼ 2 ± 0.1) along with cracking at ZrB₂ grain boundaries and ZrB₂-SiC interfaces are considered evidence of grain boundary sliding during creep of the ZSBCL-10 composite. However, the values of n ∼ 1 and apparent activation energy ∼700 kJ/mol obtained for the ZSBCL-14 composite at 1300–1400°C suggest that ZrB₂ grain boundary diffusion is the rate-limiting mechanism of creep. The thickness of the damaged outer layer containing cracks scales with temperature and applied stress, indicating their role in facilitating the ingress of oxygen causing oxide scale growth. Decreasing oxidation-induced defect density with depth to a limit of ∼280 μm, indicates the predominance of creep-based deformation and damage at the inner core of samples.     

The transition from non-ohmic to ohmic characteristics in La2O3 doped ZnO–MgO–TiO2 linear resistance ceramics

La₂O₃ doped ZnO–MgO–TiO₂ based linear resistance ceramics were prepared by the solid phase sintering method. The doping content of La₂O₃ is from 0.0 wt% to 2.5 wt%. The solubility of La₂O₃ in ZnO is less than 0.06 mol% (0.5 wt%), La_0.66TiO_2.993 phases will be formed at grain boundary and change the distribution of spinel phase when La₂O₃ is excessive. For I–V test, undoped sample exhibits typical non-ohmic characteristics, but La-doped samples show excellent ohmic behaviors under low DC and high pulse current (PC). The complex impedance spectrum and the frequency dependent conductivity furtherly demonstrate that La-doped samples possess linear characteristics because there is no grain boundary effect which can affect the electron transmission at grain boundaries. Besides, the decrease of the grain boundary barrier from 0.2135eV for undoped sample to 0.0031eV for 0.5 wt% La doped samples can account for the elimination or reduction of grain boundary effect. In this work, the transition from non-ohmic to ohmic properties by doping La₂O₃ in ZnO–MgO–TiO₂ multiphase ceramics is realized.     

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.     

Dislocation Structures of Low-Angle and Near-Σ3 Grain Boundaries in Alumina Bicrystals

Alumina bicrystals with low-angle and near-Σ3 <0001> tilt grain boundaries were fabricated using diffusion bonding to study the dislocation structures in alumina grain boundaries. The resulting grain-boundary structures were investigated using high-resolution transmission electron microscopy, and the grain-boundary energies were analyzed using theoretical calculations. It was found that partial dislocations with Burgers vectors of the type return ⅓<10[Onemacr]0> were periodically located in the boundaries and that a stacking fault between pairs of partials was formed in such boundaries. The length of the stacking fault decreased with increased misorientation angles, which was reasonably predicted by the theoretical calculation. In the case of a near-Σ3 grain boundary, an array of displacement shift complete dislocations with the Burgers vector of return ⅓<1[Onemacr]00> was periodically formed along the boundaries. These boundaries did not have stacking faults. The spacing between the dislocations decreased with increased deviation angle from the exact-Σ3 boundary with the tilt angle of 60°.     

Sintering behavior of calcium lanthanum sulfide ceramics in field‐assisted consolidation

In this study, calcium lanthanum sulfide (CaLa₂S₄, CLS) ceramics with the cubic thorium phosphate structure were sintered at different temperatures by field‐assisted sintering technique (FAST). Densification behavior and grain growth kinetics were studied through densification curves and microstructural characterizations. It was determined that the densification in the 850°C‐950°C temperature range was controlled by a mixture of lattice or grain‐boundary diffusion, and grain‐boundary sliding. It was revealed that grain‐boundary diffusion was the main mechanism controlling the grain growth between 950°C and 1100°C. The infrared (IR) transmittance of the FAST‐sintered CLS ceramics was measured and observed to reach a maximum of 48.1% at 9.2 μm in ceramic sintered at 1000°C. In addition, it was observed that the hardness of the CLS ceramics first increased with increasing temperature due to densification, and then decreased due to a decrease in dislocations associated with grain growth.     

Effect of grain boundary microcracking on the light transmittance of sintered transparent MgAl2O4

Optically transparent polycrystalline magnesium aluminate spinel (MgAl₂O₄) has been fabricated by hot-pressing followed by hot isostatic pressing (HIPing). The effect of microstructure on the light transmittance of the sintered MgAl₂O₄ is discussed. The sintered MgAl₂O₄ with a thickness of 2 mm has a maximum light transmittance of ∼60 and ∼70% in the UV–visible and near-IR wavelength regions, respectively, although it contains microcracking along its grain boundaries. Light transmittance losses in the sintered MgAl₂O₄ are explained in terms of light scattering at these microcracked grain boundaries: the light transmittance was determined to decrease with increase in the microcracked grain boundary surface area due to pronounced scattering. The light transmittance is well correlated with microcracked grain boundary surface area per unit volume.     

Damage formation mechanisms of sintered silicon carbide during single-diamond grinding

In this research, a single-diamond grinding test was performed on sintered silicon carbide (SSiC) to explore the damage formation mechanism. A scanning electron microscope and a transmission electron microscope (TEM) were used to examine the surface and subsurface morphologies of the grinding groove, respectively. The characteristics of the ground surface morphologies reveal that the single-diamond grinding process of SSiC can be classified into purely ductile, primarily ductile, primarily brittle, and purely brittle stages. Based on the high-resolution TEM (HRTEM) images and the corresponding Fast Fourier transform images of the near-surface region, results reveal that the high density of dislocations and amorphization of SiC grains are responsible for the plastic deformation of SSiC. Most of the cracks congregate on the top grains of the ground surface due to the distinct obstruction of the grain boundary on the cracks propagation, and the cracks generated at the grain boundaries emit into the top grain interiors and go up toward the exposed surface for the distortedly deformed region with higher strain energy; Furthermore, stress concentration caused by the dislocation pileups at grain boundaries represents the crack initiation mechanisms for SSiC. Finally, based on the dislocations pile-up theory, a critical undeformed chip thickness model for boundary crack system nucleation is established, which considers the cutting-edge radius, grinding wheel speed, material properties, and grain size of ceramics.