Mechanical Behavior of Polycrystalline Magnesium Oxide at High Temperatures

High-temperature tensile deformation of paolycrystalline magnesia prepared by ( a ) single-crystal recrystallization and ( b ) hot-pressing, is described. Recrystallized polycrystalline magnesia goes through a brittle-ductile transition at 1700°C (strain rate 10⁻⁴ sec⁻¹). The brittleness below 1700°C is due to a lack of slip systems and grain boundary sliding. At 1700°C grain boundary migration produces corrugations in the interface which interfere with sliding. Above 1700°C the matrix becomes sufficiently plastic through multiple slip and polygonization to accommodate any distortion. Polycrystalline specimens then neck down for completely ductile fracture. Hot-pressed magnesia starts through a transition at 2200°C, i.e. 500°C higher. The increase is attributed to pores and impurity. Porosity is considered to promote grain boundary sliding by ( a ) providing the source for intergranular sliding, ( b ) decreasing the interfacial contact area, and ( c ) preventing grain boundary migration and corrugation. These observations confirm that high-temperature deformation occurs by dislocation glide and climb and by grain boundary sliding and migration.     

Wear and Wear Transition Mechanism in Silicon Carbide during Sliding

Wear mechanisms in SiC during sliding have been investigated experimentally under paraffin oil lubrication. The wear and friction data indicate that a transition in wear mechanism occurs abruptly after a defined period of sliding. The transition point is reached earlier as load increases. Examination of wear samples reveals that surface material is predominantly removed by a plastic grooving process in the initial stage, and by a grain pull-out process after the transition. It is also observed that severe plastic deformation in the form of dislocations is produced during the sliding. The abrupt occurrence of the grain pull-out after a definite sliding time is discussed in relation to the internal stresses associated with this plastic deformation.     

Size effect on hardness for micro-sized and nano-sized YAG transparent ceramics

Traditional micro-sized and nano-sized YAG transparent ceramics were tested by nanoindentation at different peak loads. The micro-sized YAG transparent ceramics show a marked indentation size effect (ISE). However, for the nano-sized YAG transparent ceramics, the hardness was constant in the whole investigated range without any evidence of ISE. We show that the absence of indentation size effect for nano-sized YAG transparent ceramics can be accurately modeled using the plastic deformation mechanism of grain boundary sliding.     

High temperature deformation of a 5 wt.% zirconia-spinel composite: influence of a threshold stress

Creep experiments performed on a 5 wt.% zirconia- MgAl₂O₄ spinel material, in the stress and temperature ranges 8–200 MPa and 1350–1410°C, have shown the importance of grain boundaries in deformation of this material. Deformation can be analysed as the result of two sequential contributions. At low stress, an increase in the apparent stress exponent and the occurrence of a threshold stress, whose value roughly varies inversely proportional to spinel grain size, were observed. At high stress, grain boundary diffusion is the most likely mechanism that controls the grain boundary sliding. These observations are consistent with previous experiments showing that sliding of spinel/spinel boundaries is more difficult than sliding of spinel/zirconia boundaries in the low stress range. The plastic flow is analysed by means of grain boundary dislocations whose density increases with stress. At low stress, when the density of boundary dislocations is low, creep rates are interface-controlled while at high stress, when the boundary dislocation density is large, rates are limited by the long-range diffusion process.     

Size-induced room temperature softening of nanocrystalline yttria stabilized zirconia

Nanocrystalline oxides exhibit exceptional resistance to plastic deformation, manifesting increased strength and hardness with reduced grain size that qualitatively follows the so-called Hall-Petch relationships. However, below a critical grain size, softening has been observed to occur, in the so-called inverse Hall-Petch regime. The mechanisms underlying these phenomena are still not well understood in oxides. Here we observe, using nanopillar compression, that the yield strength initially increases with decreasing grain size for yttria-stabilized zirconia ceramics produced by high-pressure spark plasma sintering. A hardening-to-softening transition occurs at grain sizes below ≈21 nm. The experiments indicate that this transition depends on strain rate, and the onset of the decrease in yield strength occurs before any shear fracture begins. Nanopillar compression combined with in situ electron diffraction demonstrates the onset of softening coincides with an increase in the amount of crystallographic rotation per unit strain, suggesting a change in deformation mechanism.     

Irradiation-induced large bubble formation and grain growth in super nano-grained ceramic

In this work, 0.5TRPO•0.5Gd₂Zr₂O₇ ceramic with an average grain size of only ∼15 nm was prepared by a high pressure (5 GPa/520 °C) sintering method. Phase evolutions and microstructure changes of the as-fabricated super nano and micron-grained ceramics under a high-dose displacement damage induced by 300 keV Kr²⁺ ions were investigated. The results show that the super nano-grained ceramic has low degree of amorphization, obvious grain growth (2–3 times in grain size) and big Kr bubbles (10–68 nm) formation after irradiation. The micron-grained ceramic was severely amorphized after irradiation and many microcracks were formed parallel to its surface. The formation mechanism of Kr bubbles in the super nano-grained ceramic is on account of grain boundary diffusion and migration induced by the accumulation of the injecting Kr ions and irradiation defects. Nevertheless, microcracks formed in the micron-grained sample are caused by the accumulation of Kr atoms.     

Designing nanomaterials with desired mechanical properties by constraining the evolution of their grain shapes

Grain shapes are acknowledged to impact nanomaterials' overall properties. Research works on this issue include grain-elongation and grain-strain measurements and their impacts on nanomaterials' mechanical properties. This paper proposes a stochastic model for grain strain undergoing severe plastic deformation. Most models deal with equivalent radii assuming that nanomaterials' grains are spherical. These models neglect true grain shapes. This paper also proposes a theoretical approach of extending existing models by considering grain shape distribution during stochastic design and modelling of nanomaterials' constituent structures and mechanical properties. This is achieved by introducing grain 'form'. Example 'forms' for 2-D and 3-D grains are proposed. From the definitions of form, strain and Hall-Petch-Relationship to Reversed-Hall-Petch-Relationship, data obtained for nanomaterials' grain size and conventional materials' properties are sufficient for analysis. Proposed extended models are solved simultaneously and tested with grain growth data. It is shown that the nature of form evolution depends on form choice and dimensional space. Long-run results reveal that grain boundary migration process causes grains to become spherical, grain rotation coalescence makes them deviate away from becoming spherical and they initially deviate away from becoming spherical before converging into spherical ones due to the TOTAL process. Percentage deviations from spherical grains depend on dimensional space and form: 0% minimum and 100% maximum deviations were observed. It is shown that the plots for grain shape functions lie above the spherical (control) value of 1 in 2-D grains for all considered grain growth mechanisms. Some plots lie above the spherical value, and others approach the spherical value before deviating below it when dealing with 3-D grains. The physical interpretations of these variations are explained from elementary principles about the different grain growth mechanisms. It is observed that materials whose grains deviate further away from the spherical ones have more enhanced properties, while materials with spherical grains have lesser properties. It is observed that there exist critical states beyond which Hall-Petch Relationship changes to Reversed Hall-Petch Relationship. It can be concluded that if grain shapes in nanomaterials are constrained in the way they evolve, then nanomaterials with desired properties can be designed.     

Fabrication of nanocrystalline copper by explosive loading and its dynamic mechanics properties

Nanocrystalline (NC) copper is fabricated by the method of severe plastic deformation of coarse-grained copper under explosive dynamic loading at high strain rates. The dynamic mechanical properties of NC copper are studied by the split Hopkinson pressure bar method. The results show that it is feasible to fabricate nanocrystalline copper by explosive dynamic plastic deformation of coarse-grained copper and the grain size of NC copper can be smaller than 100 nm. Twinning and formation of dislocations are the main mechanisms of grain refining. The dynamic yield strength of NC copper increases with decreasing average grain size and increasing strain rate.     

Sintering dense nanocrystalline 3YSZ ceramics without grain growth by plastic deformation as dominating mechanism

It is difficult to obtain nanocrystalline ceramic bulks due to its high surface activity at high temperatures. In the study, in order to achieve both high density and ultrafine morphology, the plastic deformation was induced by an ultrahigh pressure at a deliberately selected temperature, which was much lower than the threshold temperature for rapid grain growth. According to the ultra-high pressure route, nanocrystalline 3YSZ ceramics without grain growth were fully densified at 900?°C under 1.5?GPa. Both direct microstructural observations and calculation results proved that the plastic deformation including high temperature yield and sliding played a dominant role in densification during the sintering process.     

Grain-Size Dependence of Sliding Wear in Tetragonal Zirconia Polycrystals

Using a pin-on-plate tribometer with the reciprocating motion of SiC against yttria-doped tetragonal zirconia polycrystal (Y-TZP) plates, the friction and wear of Y-TZP ceramics were investigated as a function of grain size in dry N₂ at room temperature. The results showed that the overall wear resistance increased as the grain size of Y-TZP ceramics decreased. For grain sizes ≤0.7 μm, the wear results revealed a Hall-Petch type of relationship ( d ^−1/2) between wear resistance and grain size. In this case, the main wear mechanisms were plastic deformation and microcracking. For grain sizes ≥0.9 μm, the wear resistance was proportional to the reciprocal of the grain diameter. In this regime, delamination and accompanying grain pullout were the main mechanisms. In this case, the phase transformation to monoclinic zirconia had a negative effect on the wear resistance of TZP ceramics. The coefficient of friction tended to be higher for fine-grained TZP-SiC couples than for coarse-grained TZP-SiC couples, whereas, for a specific regime of grain size, the coefficient of friction was almost independent of the grain size.     

Effect of Grain Boundaries on Plastic Deformation of Sodium Chloride

Sodium chloride single crystals were joined at a melt zone with a hot-wire technique to produce bicrystals of controlled crystallographic orientation. The plastic deformation properties of the bicrystals were measured in bending while the birefringence was observed with a petrographic microscope. The birefringence at the boundary was interpreted to indicate that significant deformation across the grain boundaries occurred. The stress required for deformation across the boundary was found to increase with increased angle between crystallographic axes. Twist boundaries were found to be more resistant than tilt boundaries. Behavior was consistent with the dislocation model for grain boundaries.     

Brittle and Plastic Behavior of Hot-Pressed BeO

The flow and fracture mechanisms of well characterized hot-pressed polycrystalline BeO were studied. Modulus of rupture tests were made from-196O to 1800°C on specimens of constant density and varying grain size. The fracture surfaces were studied with fractography and X-ray rocking curves. Compression tests were performed both below and above the phase transformation temperature for BeO (2050°C). A model was developed for fracture below 1000°C, involving propagation of existing surface cracks through dissimilar barriers to form a crack front of irregular shape which generates dislocations near the tip. Above 1000°C a model of grain boundary sliding to open cracks at grain boundary junctions was indicated. Permanent deformation under compression was attributed to grain separation and void formation.     

Creep behavior of TiCxN1−x-CoTi cermets synthesized by mechanically induced self-sustaining reaction

The plastic flow of TiC_xN_1−x-CoTi cermets has been investigated by uniaxial compression tests carried out in argon atmosphere at temperatures between 1100 and 1200 °C. Two different cermets, with 5 wt.% W or WC content as sintering additives, have been explored to assess the influence of the sintering additives on creep. The microstructural observations of deformed samples and the mechanical results indicate that the hard phase (ceramic grains) controls the plastic deformation. The stress exponent changes from 1 to 2 with increasing strain rate, suggesting a transition in the deformation mechanism from diffusional creep to grain boundary sliding; both with similar activation energy values of about 400 kJ/mol. This value of activation energy agrees with C diffusion in the carbonitride grains as the strain rate controlling mechanism.     

Mechanical Properties of Polycrystalline TiC

The mechanical properties of fine-grained polycrystalline TiC were studied using both four-point bending and compression tests. The ductile-brittle transition (D-B) temperature in compression was determined to be =800°C and was found to depend on grain size. Yield-point behavior was observed for the first time in fine-grained TiC deformed in compression and was found to depend on grain size and test temperature. The yield stress as a function of grain size can be described by a Hall-Petch type of relation, i.e. yield stress α (grain size)-1/2. The dislocations resulting from deformation in compression at lower temperatures were predominately screw in character, with edge dipoles and dislocation loops being present. As the temperature of deformation was increased, the dipoles and loops were gradually annihilated by climb and the dislocations were observed in the form of hexagonal networks with a much-reduced dislocation density. A plot of log yield stress vs 1/T showed a change in slope, which suggests that two rate-controlling mechanisms are in operation during deformation at different test temperatures. Thermal activation analysis at T = 1050° to 1500°C suggested that the rate controlling mechanism during deformation in this temperature range is associated with cross slip.     

Atomistic origin of brittle-to-ductile transition behavior of polycrystalline 3C–SiC in diamond cutting

The machinability of hard brittle polycrystalline ceramic has a strong correlation with internal microstructures and their accommodated deformation behavior. In the present work, we investigate the mechanisms governing the brittle-to-ductile transition behavior of polycrystalline 3C–SiC in diamond cutting by means of molecular dynamics simulations. Simulation results reveal the co-existence of dislocation slip and amorphization-dominated ductile deformation and cracking along grain boundaries-mediated brittle fracture, as well as the correlation of individual deformation modes with machining force variation and machined surface morphology. In addition, inter-granular fracture, grain boundary sliding and grain pull-up are also operating brittle deformation modes of polycrystalline 3C–SiC. The strong competition between above heterogeneous deformation modes determines the brittle-to-ductile transition behavior in grooving of polycrystalline 3C–SiC. Simulation results also demonstrate that grain size has a strong impact on the brittle-to-ductile transition and material deformation behavior of polycrystalline 3C–SiC under diamond cutting.     

Si segregation and its role in reaching transparent YAG

It is well‐known that doping YAG with Si dramatically affects densification and grain growth, and as a result Si is commonly added to YAG as a sintering additive to achieve full density and transparency. In recent studies, the influence of Si was explored, but segregation of Si to grain boundaries in YAG was not detected. The present article contradicts previous findings by revealing an excess of Si at grain boundaries in YAG. The findings were corroborated using atom probe tomography and energy‐dispersive spectroscopy in a scanning transmission electron microscope. To the best of the author's knowledge, this is the first time Si segregation was discovered in SiO₂‐doped YAG, and the first time a dopant concentration profile at grain boundaries in a ceramic material was characterized by atom probe tomography. Finally, a change is proposed for the method to calculate grain‐boundary mobility in the presence of a solute (Lucke and Detert), showing a good correspondence to the experimental results. These results can change the view on solute distribution in ceramic grain boundaries from theoretical and characterization aspects.     

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.     

Microstructure and phase morphology of wood derived biomorphous SiSiC-ceramics

Biomorphous SiSiC-ceramics were prepared by spontaneous Si-melt infiltration of beech and pine wood derived biocarbon templates (C_B-templates) at 1550 °C for 1 h. The microstructure, phase distribution and interface morphology of the anisotropically structured, microcellular SiSiC-ceramic composite were investigated by X-ray diffraction (XRD) as well as by light, scanning and, in detail, by transmission electron microscopy (LM/SEM/TEM). The biomorphous SiSiC-ceramics consist of three different phases: solidified Si in the cell lumina, few residual carbon islands located in the middle of the former wood cell walls and two different, reaction-formed SiC-morphologies. While the majority of the SiC-phase exhibits a grain diameter of a few microns, a second nano-grained SiC-phase was identified at the interface between the coarse-grained SiC-phase and the residual biocarbon. The occurrence of the nano-grained SiC-phase is related to the density and pore size of the initial C_B-template. While in large pores and narrow carbon struts (low density earlywood) of the C_B-template an excess of Si-melt yields the formation of mainly the coarse-grained SiC-phase, in latewood sections (small pores and thick C-struts) of the biocarbon template the nano-grained SiC-phase forms layers of up to several μm in thickness. Due to the uni-directional pore structure of the wood derived C_B-templates, the morphology of the biomorphous SiSiC-ceramics can be analysed in well-defined pore orientations. A microstructural model for the SiC-formation and phase growth will be proposed and discussed with respect to conventional processed SiSiC-materials.