Compressive Creep of Polycrystalline Beryllium Oxide

Compressive creep of polycrystalline beryllium oxide was studied in the temperature range 2500° to 2800°F. The apparent activation energy for creep was 96.0 kcal per mole and the creep rate was linearly dependent on the applied stress. Results were consistent with the Nabarro-Herring creep mechanism. Experimental evidence showed that extrapolation of data to 3200°F was possible.     

High-Temperature Plastic Deformation of Polycrystalline Beryllium Oxide

High-density specimens were plastically deformed under four-point transverse bending. Tests were conducted in vacuum in the region 1400° to 1700°C under stresses of 1000 to 4500 psi. The activation energy for creep was 99.0 kcal/mole. Creep rate was directly proportional to the applied stress and inversely proportional to the square of the grain diameter. The deformation behavior is ascribed to a Nabarro-Herring type mechanism. Results show that creep was the same in tension and compression.     

Self-Diffusion of Beryllium in Polycrystalline Beryllium Oxide

The self-diffusion of beryllium in polycrystalline beryllium oxide was measured from 1330° to 2135°C with the ⁷Be tracer. Results are compared with the findings of other investigators, and it is concluded that the differences reported are due to variations in impurity and porosity contents. Diffusion has been shown to be via a vacancy mechanism, with such impurities as aluminum and silicon being the principal contributors of vacancies. A roll-over in the plot of log D versus 1/ T , where D is the diffusion coefficient and T is absolute temperature, was found in this study as in some others. Above about 1500°C the activation energy is no higher than 62 kcal, whereas it is 92 kcal at lower temperatures. It is suggested that this roll-over is due to aluminum impurity. There appears to be a temperature-independent anisotropy of diffusion, with the rate in the 〈 c 〉 direction being 30% higher than that in the 〈 a 〉 direction. It is concluded that for this and other investigations the diffusion coefficient of beryllium in beryllium oxide above the rollover temperature is governed by impurities and in this region it is given by the formula

where F_p is the fraction of porosity and N_v is the molar fraction of vacancies introduced by impurities. There is sufficient scatter in individual 〈 a 〉 and 〈 c 〉 diffusion values so that there is no value in discriminating between directions in this formula.     

High-Temperature Compressive Creep of Polycrystalline Mullite

Aluminosilicates of three compositions with mullite as the major phase were synthesized by a sol-gel process and characterized with bulk and microchemical analyses and microstructural observation. An apparatus for measuring the compressive creep up to 1900 K with a sensitivity of ±1 μm was constructed and used to measure the creep of singlephase mullite, mullite with second-phase glass, and mullite with second-phase corundum. Measurements in air at stresses of 15 to 100 MPa and temperatures of 1471 to 1724 K determined that samples with second-phase glass crept more rapidly than single-phase mullite or mullite with secondphase corundum. The apparent creep activation energies determined at 100 MPa were 742 kJ/mol for the mullite containing glass, 819 kJ/mol for the single-phase mullite, and 769 kJ/mol for the mullite with second-phase corundum. The stress exponents determined at 1724 K were 1.6 for the mullite plus glass, 1.5 for the single-phase mullite, and 1.2 for the mullite with α-Al₂O₃. The creep behavior of the aluminosilicates containing glass were consistent with rate control by the viscous flow of the glass and the measured creep rates were in good agreement with creep rates calculated from a model by Dryden. The creep behavior of the completely crystalline aluminosilicates was consistent with rate control by diffusional creep.     

Dislocation Etchant for Beryllium Oxide

A hydrochloric acid chemical etchant for BeO was refined and evaluated. The etchant, when used for 20 min at 120°C on chemically polished surfaces, produces distinctive pits at dislocations intersecting the (0001) and (101) surfaces. Correspondence of etch pits and dislocations was inferred by comparing the enlargement of pits with etching time and the constancy of pit density with continued etching. On {1010} surfaces, the etchant is not sensitive to sites of defect emergence and hence leaves a relatively flat, featureless surface. The etch rates and activation energies are given for {0001}, {1010}, and {101} surfaces. The reaction mechanism is discussed.     

High-Temperature Creep and Cavitation of Polycrystalline Aluminum Nitride

Dense, polycrystalline AlN samples of grain size between 1.8 and 19 μm were fabricated by hot-pressing. Bar-shaped samples were subjected to creep in four-point bending under static loads in nitrogen atmosphere. The outer fiber stress was varied between 20 and 140 MPa and the temperature between 1650 and 1940 K. Steady-state creep rate, dɛ/d t was proportional to σ ⁿ d ^{− m} where the stress exponent, n , was between 1.27 and 1.43 and grain-size exponent, m , between ∼ 2.2 and ∼ 2.4. The activation energy for creep ranged between 529 and 625 kJ/mol. Both round (r type) and wedgeshaped (w type) cavities were observed in electron micrographs of the deformed samples. No noticeable change in the dislocation density was observed. Contribution of cavitation to the creep rate was estimated using an unconstrained cavity model. Based on this study it is concluded that the dominant mechanism of creep in polycrystalline AlN is diffusional.     

Comparison of Whisker Growth Sites and Dislocation Etch Pits on Single-Crystal Sapphire

It has been shown previously that alumina (sapphire) whiskers grown on single-crystal alumina grow coherently with and in the directions of screw dislocations in the substrate. A possible explanation is that the whiskers grow by the screw-dislocation mechanism at the site of emergent screw dislocations on the substrate surface. The present study was undertaken to determine if the whiskers grow at the sites of substrate dislocations as revealed by etching techniques. To do this, whisker growth sites were compared with the positions of dislocation etch pits. This was done for the prismatic [{112¯0}, <11¯00>] and basal [(0001), (112¯0)] slip systems on (112¯0), (11¯01), and (0001) crystal surfaces. The whiskers and etch pits did not form at the same places on the crystal surfaces. The reason may be that (1) the etch pits form at pure edge dislocations only, (2) surface nucleation generates screw dislocations at which the whiskers grow, or (3) screw dislocations play no part in the growth of these whiskers.     

Creep of Dense, Polycrystalline Magnesium Oxide

Creep of polycrystalline MgO was studied using four-point transverse bending at 1380° to 1800°K and stresses from 1000 to 5000 psi. The effects of temperature, stress, and grain size on the creep rate were determined for grain sizes from 2 to 20μ. Activation energies for creep decreased sharply with increasing grain size from 96,000 cal/mole at 2μ to 54,100 cal/mole at 5.5μ and then remained constant over the grain-size range 5.5 to 20μ. Creep was attributed in part to a stress-directed diffusional mechanism controlled by extrinsic oxygen ion diffusion in the 5.5 to 20μ grain sizes, although the calculated ionic self-diffusion rates were higher than those predicted by the Nabarro-Herring theory. It is suggested that the discrepancy may be due to a vacancy formation mechanism, which is consistent with the observed formation of dislocation substructure and preferentially distributed porosity during creep, as well as with the observed decrease in creep rate with increasing creep strain.     

Elastic Properties of Polycrystalline Yttrium Oxide, Holmium Oxide, and Erbium Oxide: High-Temperature Measurements

The Young's and shear moduli of polycrystalline yttrium oxide, holmium oxide, and erbium oxide were determined from room temperature to 1000°C using the sonic resonance technique. The bulk modulus and Poisson's ratio were computed as functions of temperature for each oxide. The Young's, shear, and bulk moduli decreased linearly with increasing temperature, whereas Poisson's ratio remained constant. The first and second Grüneisen constants, γ and δ, were calculated from the bulk modulus data and shown to be virtually independent of temperature. The Soga-Anderson equation adequately described the bulk modulus data for each oxide.     

High-Temperature Failure of Polycrystalline Alumina: II, Creep Crack Growth and Blunting

Creep crack growth in fine-grain alumina is measured by using surface cracks. A narrow power-law crack growth regime occurs at both 1300° and 1400°C, wherein the power-law exponent and activation energy are comparable to steady-state creep values. Asymptotic crack velocity behavior is exhibited near both the critical stress intensity factor, K_C , and the crack growth threshold, K_th . The threshold occurs near 0.4 K_1C at both 1300° and 1400°C and is associated with a transition in the size and distribution of damage. Displacement measurements indicate that crack tip damage exerts a strong influence on the displacement field, as predicted by recent theories. Furthermore, use of the stress intensity factor as a loading parameter does not produce adequate correlation with displacement measurements and is, therefore, not strictly suitable for nonlinear creeping ceramic poly crystals.     

High-Temperature Creep Behavior of Polycrystalline SrZrO3

The high-temperature creep behavior of sintered polycrystalline SrZrO₃ containing 1.35 wt% Fe₂O₃ was investigated as a function of temperature, stress, grain size, and strain level over the ranges 1160° to 1275°C, 780 to 3110 psi, 0.45 to 2.04 μm, and 0.0014 to 0.014, respectively. A constant-load 4-point (pure bending) method was used to load the specimens. The creep rate is time-dependent, decreasing exponentially with strain, i.e.     , where the decay constant (β=118, measured at the 1560 psi stress level over the strain range 0.0014 to 0.014) is independent of temperature and grain size. No significant grain growth occurred during creep. The activation energy of 169±10 kcal/mol obtained for creep is relatively independent of temperature, stress, grain size, and strain level over the ranges investigated. The creep rate is directly proportional to the cube of the stress and the reciprocal of the grain size; this result is consistent with nonviscous creep theories based on dislocation generation and climb as the rate-controlling deformation mechanism.     

High-Temperature Creep of Polycrystalline Magnesia: II,Effects of Additives *

The creep of magnesia doped with 0.035 to 2.26 cation % of nine other oxides and three binary mixtures thereof and of three seawater products (about 96, 98, and 99.5y0 MgO) was evaluated in transverse bending at 1200° to 1500°C, with strain rates of about 10−²%/hr, and average grain sizes of 5 to 50p. The results obtained were compared with those for pure magnesia. Most additives accentuated the plastic (diffusion-controlled) nature of the creep process presumably by pinning dislocations and/or slowing grain growth. In most cases the rate-determining diffusing species seemed to be the cation, Mg, but in two cases it was suspected that oxygen boundary diffusion was controlling. Porosities above ˜10% appear to increase the temperature dependence of creep, probably by introducing boundary sliding. The agreement of the creep data with those of other diffusion-controlled processes (electrical conductivity, sintering, and grain growth) is demonstrated.     

High-Temperature Creep of Polycrystalline Magnesia: I,Effect of Simultaneous Grain Growth *

The creep of pure magnesia (99.9 +% MgO) was tested in transverse bending at temperatures from 1200° to 1500°C, strain rates near 10⁻²%/hr, and grain sizes of 4 to 50μ. In most cases, grain growth during the test affected the apparent creep behavior more than all the other variables combined. An analytical graphical method was used to separate the grain growth effect from other effects and to obtain more meaningful creep data. Creep occurred primarily by a viscous mechanism (Nabarro-Herring type, cation-lattice-diffusion controlling) with a minor amount of plastic creep (dislocation climb). The agreement with previous creep data was good.     

Optical Spectra of Amorphous and Polycrystalline Beryllium Oxide over a Wide Energy Range of Fundamental Absorption

The complete sets of spectra of the optical fundamental functions for amorphous and polycrystalline beryllium oxides in the energy range 0–40 eV are determined and compared for the first time. The general features of the spectra and their dependence on the long-range order are revealed. The basic parameters of the optical transitions in the range 0–40 eV for amorphous and polycrystalline BeO are obtained for the first time.