Grain Boundary Sliding Microdisplacement Measurements During the Creep of Alumina

Grain boundary sliding (GBS) is thought to be the principal driving force for the nucleation, growth, and coalescence of grain boundary cavities during compressive creep of polycrystalline ceramics. It has been shown theoretically that stochastic GBS gives rise to continuous cavity nucleation and transient cavity growth and coalescence, eventually leading to crack formation and failure. This paper will show through experimental measurements, using stereoimaging techniques, that GBS is in fact stochastic. Also, mode II GBS, in-plane grain rotation, and in-grain shear displacements, strains, and strain rate measurements during creep of Lucalox Al₂O₃ will be presented. These displacements, measured on a machine vision system, will be presented in terms of the surrounding microstructural constraint and their lack of angular relation to the compressive load axis.     

A micromechanics study of competing mechanisms for creep fracture of zirconium diboride polycrystals

A micromechanics model was developed to simulate creep fracture of ceramics at high temperatures and material properties pertinent to zirconium diboride (ZrB₂) were adopted in the simulation. Creep fracture is a process of nucleation, growth, and coalescence of cavities along the grain boundaries in a localized and inhomogeneous manner. Based on the grain boundary cavitation process, creep fracture can be categorized into cavity nucleation-controlled and cavity growth-controlled processes. On the other hand, based on the deformation mechanism, the separation between two adjacent grain boundaries can be categorized into diffusion-controlled and creep-controlled mechanisms. In this study, a parametric study was performed to examine the effects of applied stress, cavity nucleation parameter, grain boundary diffusivity, and applied strain rate on cavity nucleation-controlled versus growth-controlled process as well as diffusion-controlled vs. creep-controlled mechanism during creep fracture of ZrB₂.     

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.     

Cavitation Contributes Substantially to Tensile Creep in Silicon Nitride

During tensile creep of a hot isostatically pressed (HIPed) silicon nitride, the volume fraction of cavities increases linearly with strain; these cavities produce nearly all of the measured strain. In contrast, compressive creep in the same stress and temperature range produces very little cavitation. A stress exponent that increases with stress (ε∞σ ⁿ , 2 < n < 7) characterizes the tensile creep response, while the compressive creep response exhibits a stress dependence of unity. Furthermore, under the same stress and temperature, the material creeps nearly 100 times faster in tension than in compression. Transmission electron microscopy (TEM) indicates that the cavities formed during tensile creep occur in pockets of residual crystalline silicate phase located at silicon nitride multigrain junctions. Small-angle X-ray scattering (SAXS) from crept material quantifies the size distribution of cavities observed in TEM and demonstrates that cavity addition, rather than cavity growth, dominates the cavitation process. These observations are in accord with a model for creep based on the deformation of granular materials in which the microstructure must dilate for individual grains to slide past one another. During tensile creep the silicon nitride grains remain rigid; cavitation in the multigrain junctions allows the silicate to flow from cavities to surrounding silicate pockets, allowing the dilatation of the microstructure and deformation of the material. Silicon nitride grain boundary sliding accommodates this expansion and leads to extension of the specimen. In compression, where cavitation is suppressed, deformation occurs by solution—reprecipitation of silicon nitride.     

Diffusional Creep and Cavitational Strains in High-Purity Alumina under Tension and Subsequent Hydrostatic Compression

Diffusional creep and cavitation in pure alumina prepared with three different fabrication processes are compared under tension and subsequent hydrostatic compression. The deformation rates are separated into a volume-conserving creep rate and cavitational rate by measuring the longitudinal and transverse strains intermittently during deformation. Concurrent grain growth causes the volume-conserving strain rate to decrease in a manner consistent with Nabarro-Herring creep. The creep stress index of n = 1.3 and the average activation energy of Q = 480 kJ/mol are also consistent with Nabarro-Herring creep controlled by aluminum lattice diffusion. Anelastic loading and unloading transients are also identified and separated from the creep strains. High-voltage electron microscopy indicates that cavities nucleate at grain edges early and continuously in the creep process. The surfaces of these cavities tend after some growth to exhibit negligible curvature and various dihedral angles. The activation energy of Q = 450 kJ/mol and stress dependence of the cavitation rate of n = 1.3 are consistent with a grain boundary diffusional growth mechanism. The loading mode is found to have no significant effect on the cavitation rate during tensile creep and the subsequent decavitation rate during hydrostatic compression. The cavitation and decavitation rates are in good agreement with the model proposed by Speight and Beere when the effects of grain growth on cavity accumulation on grain boundaries are included. Exaggerated grain growth in high-density specimens can lead to early cavity coalescence and failure.     

Grain Boundary Phases and Wake Zone Characterization in Monolithic Alumina

The postfracture tensile (PFT) technique has been used to characterize the wake zones of two alumina microstructures over a temperature range of 20° to 1280°C. Differing primarily in purity and grain size distribution, the behaviors of a commercial 99.7% alumina and a nominally pure alumina, Lucalox (General Electric, Cleveland, OH), were evaluated. Previously, the authors have demonstrated the dominant role of thermal expansion anisotropy in control of the toughening processes through 600°C. In this paper we relate microstructural aspects to those toughening mechanisms causing the behavioral change near 800°C, typical of commercial aluminas. This temperature coincides closely with the softening point of a glassy phase reported for some commercial aluminas. Since Lucalox exhibits the same behavior, but contains no added glassy phases, two possibilities remain: (1) trace quantities of a grain boundary phase are sufficient to promote the observed behavior, or (2) changes in toughening mechanisms result from more subtle microstructural transitions. Based upon studies of strain rate and time-dependent behavior, we propose that the topographic changes of the fracture surface near this temperature may explain the increase in toughening behavior at high temperatures.     

Microstructure and Creep Behavior of Silicon Nitride and SiAlONs

Microstructural evolution of silicon nitride (Si₃N₄) and SiAlON materials and its influence on creep resistance is reviewed. Grain size, grain morphology, and the ratio of α- to β-phase grains play a part in resistance to creep. The glassy, intergranular phase typically has the strongest influence on creep. Creep data are usually obtained using uniaxial tensile or compressive tests, where creep in tension is controlled by cavitation and grain boundary sliding controls creep in compression. The impression creep methodology is also reviewed. An additional creep mechanism, dilation of the SiAlON grain structure, was found to be active in impression creep.     

Creep Cavitation in Liquid-Phase-Sintered Alumina

The early stages of cavitation during compressive creep of a liquid-phase-sintered alumina have been characterized using small-angle neutron scattering. Grain-boundary cavities were found to nucleate throughout creep, although at a steadily decreasing rate. The cavities were located on two-grain junctions as well as triple points and were spaced approximately 100 to 200 nm apart. The cavity spacing corresponded to the spacings observed for grain-boundary ledges, suggesting that the ledges were the cavity nucleation sites. Cavity nucleation was also found to be relatively independent of the applied stress. This behavior has been rationalized based on the decreasing ratio of ɛ_gbs/ɛ_t, where ɛ_gbs is the strain due to grain-boundary sliding and ɛ_t is the total strain, at increasing stresses. Cavity growth, on the other hand, was highly stress dependent. Above a certain "threshold" stress cavity growth was observed. In all cases, however, the observed growth was transient; i.e., the cavity growth rate decreased with time. Lowering the stress below the "threshold" resulted in a condition in which cavities nucleated but continued growth of the cavities did not occur. In all cases the cavities nucleated and grew, when growth did occur, with relatively equiaxed shapes.     

Cavity Development During Creep Deformation in Alumina with a Bimodal Grain Size Distribution

An alumina sample, codoped with equimolar proportions of magnesia and zirconia, exhibited a bimodal grain size distribution after hot-pressing. Flexural creep experiments were performed on this material at temperatures of 1673 and 1773 K in air. Inspection of the deformed specimens revealed extensive creep cavitation, with cavities developing preferentially in the coarse-grained regions. The nucleation, growth, and interlinkage of the cavities led to the formation of cracks. Crack growth occurred in the coarse-grained regions by the linkage of cavities with the crack tip. However, several cracks were observed to terminate after extending up to a fine-grained region of a specimen. A model has been developed to rationalize the observation that preferential cavitation occurs in the coarse-grained regions of a specimen undergoing creep deformation.     

Microstructural evolution under load and high temperature deformation mechanisms of a mullite/alumina fibre

A two-phase mullite alumina fibre, the 3M Nextel 720 fibre, has been studied in tension and creep. The fibre shows the highest creep resistance of all current commercial fine oxide fibres up to 1500 °C. The creep mechanisms involve progressive dissolution of mullite and simultaneous reprecipitation of alumina into elongated oriented grains and grain boundary sliding by a thin alumino-silicate liquid phase. The rate of grain growth in creep at a given temperature is dependant on the applied stress. The combination of sub-micron size mullite crystallites and alumina grains gives rise to a high sensitivity to alkaline contamination. Stress enhanced diffusion of the contaminants from the fibre surface results in crack nucleation, dissolution of mullite, formation of a liquid phase and slow crack growth. From 1200 °C, this process is coupled with a fast α-alumina grain growth at the fibre surface.     

Compressive Creep and Creep Failure of 8Y2O3/3Al2O3-Doped Hot-Pressed Silicon Nitride

The compressive creep properties of hot-pressed Si₃N₄8Y₂O₃—3Al₂O₃ (wt%) have been investigated in the temperature range of 1543–1603 K in air. The stress exponent, n , of the power creep law was determined to be 1.5, and the activation energy was determined to be 650 kJ/mol. Transmission electron microscopy observations showed that grain-boundary sliding occurred with cavitation formation in the grain-boundary glassy phase. The quasi-steady-state creep results were consistent with that of the diffusion-controlled solution—diffusion—precipitation creep mechanism, and the distinguished failure mechanism was cavitation creep damage controlled by the viscosity of the boundary glassy phase. The compressive creep failure time, obtained at 1573 K, in the stress range of 175–300 MPa, followed the MonkmanGrant relation, indicating that cavity growth was mainly controlled by the creep response of the material.     

The superplastic deep drawing of a fine-grained alumina–zirconia ceramic composite and its cavitation behavior

The synthesis, superplastic deep drawing and cavitation behavior of a fine-grained alumina–zirconia ceramic composite have been investigated. The results show that dense Al₂O₃/ZrO₂ samples with average grain diameter of 230 nm can be elongated to a dome height of at least 12 mm at the punch rate of 0.6 mm·min⁻¹ at 1400 °C. Deformed microstructure of the material indicates that not only the nucleation and growth of internal cavities are effectively suppressed but also the intragranular dislocation structures and sub-boundaries are observed around the nano-particles in alumina grains, which proves that intragranular dislocation creep is playing an important role in deformation. While the research on the deep drawing of the materials with the initial average grain size of 450 nm conforms that a plasticity controlled cavity growth process takes place during deformation.     

Tensile Creep in an in Situ Reinforced Silicon Nitride

The tensile creep of an in situ reinforced silicon nitride is described in terms of the rheological behavior of the thin intergranular film present in this liquid-phase sintered silicon nitride. The high stress exponents and apparent activation energies (at constant stress) can be explained assuming non-Newtonian flow behavior of the film during grain boundary sliding. Time-to-failure is related to the minimum creep rate, even for samples which fail by slow crack growth. In addition, the primary creep region and the relaxation effects observed on unloading are described in terms of grain boundary sliding modified by the presence of a grain boundary phase with a lower elastic modulus than silicon nitride.     

Atmospheric Effects on Compressive Creep of SiC-Whisker-Reinforced Alumina

A SiC-whisker-reinforced alumina composite was crept in compression at 1200° to 1400°C in an air ambient and in nitrogen. The data were described by a power-law-type constitutive relation. The measured value of the stress exponent was n = 1 at 1200°C and n = 3 at 1300° and 1400°C in both ambients. TEM observations were correlated with the measured creep response to determine active deformation mechanisms. Values of n = 1 were associated with diffusional creep and unaccommodated grain-boundary sliding, while values of n = 3 were associated with increased microstructural damage in the form of cavities. Experiments conducted in circulated air resulted in higher creep rates than comparable experiments in nitrogen. The accelerated creep rates were caused by the thermal oxidation of SiC and the resultant formation of a vitreous phase along composite interfaces. The glassy phase facilitated cavitation, weakened interfaces, and enhanced boundary diffusion.     

Grain Boundary Sliding Measurements during Tensile Creep of a Single-Phase Alumina

The grain boundary sliding (GBS) behavior of a single-phase (relatively coarse-grained) alumina material was studied after tensile creep experiments were performed at 1500°C at stress levels of 20 and 35 MPa. Specimens tested at 35 MPa exhibited a number of modes of GBS, including Mode II (shear) displacements, Mode I (opening) displacements, out-of-plane sliding displacements, and in-plane grain rotation. Strains in the grain boundaries due to Mode II GBS ranged from 940% to 4400%. Average Mode II GBS displacements ranged from 0.08 to 0.29 µm in samples tested for 120 and 480 min, respectively, at 35 MPa. The GBS displacements were shown to fit a Weibull distribution. Tensile creep under a 35 MPa stress yielded a GBS rate of 9.5 10^-6µm/s, while the 20 MPa stress resulted in a GBS rate of 2.2 10^-6µm/s. The average Mode II GBS displacements increased linearly with specimen strain, suggesting that GBS may play an important role in creep cavitation during tensile creep. The data also revealed that compatibility and constraint rules appear to govern GBS behavior during tensile creep. GBS behavior during compressive creep will be compared to the tensile creep GBS measurements presented.     

Creep Deformation in an Alumina–Silicon Carbide Composite Produced via a Directed Metal Oxidation Process

Flexural creep studies were conducted in a commercially available alumina matrix composite reinforced with SiC particulates (SiC_p) and aluminum metal at temperatures from 1200° to 1300°C under selected stress levels in air. The alumina composite (5 to 10 μm alumina grain size) containing 48 vol% SiC particulates and 13 vol% aluminum alloy was fabricated via a directed metal oxidation process (DIMOX(tm))† and had an external 15 μm oxide coating. Creep results indicated that the DIMOX Al₂O₃–SiC_p composite exhibited creep rates that were comparable to alumina composites reinforced with 10 vol% (8 (μm grain size) and 50 vol% (1.5 μm grain size) SiC whiskers under the employed test conditions. The DIMOX Al₂O₃–SiC_p composite exhibited a stress exponent of 2 at 1200°C and a higher exponent value (2.6) at ≥ 1260°C, which is associated with the enhanced creep cavitation. The creep mechanism in the DIMOX alumina composite was attributed to grain boundary sliding accommodated by diffusional processes. Creep damage observed in the DIMOX Al₂O₃-SiC_p composite resulted from the cavitation at alumina two-grain facets and multiple-grain junctions where aluminum alloy was present.     

Micromechanics modeling of creep fracture of zirconium diboride–silicon carbide composites at 1400–1700 °C

The creep deformation of the ultra-high temperature ceramic composite ZrB₂–20%SiC at temperatures from 1400 to 1700 °C was studied by a micromechanical mode in which the real microstructure was adopted in finite element simulations. Based on the experiment results of the change of activation energy with respect to the temperature, a mechanism shift from diffusional creep-control for temperatures below 1500 °C to grain boundary sliding-control for temperatures above 1500 °C was concluded from simulations. Also, the simulation results revealed the accommodation of grain rotation and grain boundary sliding by grain boundary cavitation for creep at temperatures above 1500 °C which was in agreement with experimental observations.     

Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC

The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C.     

Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

The creep behavior of a commercial grade of Si₃N₄ was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ∼18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution–precipitation of Si₃N₄. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution–precipitation creep of Si₃N₄ did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.