Dynamics of flow of c-axis sapphire

The deformation dynamics in uniaxial tension of c-axis sapphire were investigated at temperatures from 1600 to 1850° C in constant strain-rate tests from 0.00007 min^–1 to 0.0036 min^–1. The activation parameters are consistent with thermally-activated overcoming of the Peierls barrier as the rate-controlling process for flow. From SEM, TEM, and HVTEM examinations of deformed specimens it is deduced that the active slip plane is of the {¯4223}-type. The dislocation structure suggests that the 0 ¯110 directions are the most likely slip directions.     

Deformation dynamics of pore-free Ti4+-doped and pure c-axis sapphire crystals

Single crystal alumina filaments undoped and Ti⁴⁺-doped, having the c-axis within 2° of the filament axis were plastically strained in constant strain-rate tensile experiments. The results of differential strain-rate and differential temperature tests were used to calculate the stress exponents in the strain-rate equation, the activation volumes, and the activation enthalpies. The relatively high values of the stress-exponents (6 to 7), the variations of the activation enthalpy with stress and temperature, and the magnitudes of the strain-independent activation volumes suggest overcoming the Peierls-Nabarro stress as the rate-limiting mechanism for plastic deformation of the alumina crystals. The microscopic observations of the deformed specimens are consistent with slip on the {10¯11} ¯1101 pyramidal system as reported in previous investigations of similar materials.     

Thermal expansion of interlayer spacing and thermal vibrational displacement of carbon atoms in petroleum coke

On petroleum coke heat-treated at various temperatures in the range from 1600 to 3000°C, thermal expansion of the interlayer spacing and the mean-square displacement along the c-axis due to thermal vibration of carbon atoms were measured from room temperature to 1050°C by means of X-ray diffractometry. The static mean-square displacement due to imperfect crystallization was also estimated for each sample.The c-axis spacing increases linearly with temperature and the slopes had almost the same values (1.85×10^–4 to 1.89×10^–4Å/°C) for all samples. The mean-square thermal vibrational displacements of carbon atoms increased almost linearly from 0.012Ų at room temperature to 0.047Ų at 1000°C for all samples, except for the sample heat-treated at 1640°C.A mean free space between the layer planes, that is the interlayer spacing minus twice the total root-mean-square displacement of carbon atoms along the c-axis direction, was obtained in consideration of the mean-square thermal vibrational displacement and the mean-square static displacement. The mean free space increases abruptly with decrease of c-spacing from 6.86 to 6.85Å (heat-treatment temperatures of 1640 and 2100°C respectively), then decreases slowly and eventually again begins to increase with further decrease of c-axis spacing to 6.73Å.     

Deformation mechanisms in titanium dioxide single crystals

Single crystals of TiO₂ grown by a floating-zone method have been compressed in two directions, [1 0 0] and [0 0 1], at temperatures between 300 and 1300°C in an argon atmosphere. They deformed by {10¯1 1 01 slips, except [1 0 0] specimens tested below 600° C, where twinning preceded the slip deformation. Above 800° C, an oxygen reduction occurred during compression tests and the flow stress data were not reproducible. Below 700° C the data were reproducible and the resolved yield stress increased steeply with decreasing temperature. From the temperature and strain-rate dependences, activation analyses have successfully been performed. Activation volume becomes as small as 2b ³ at high stress, and the deformation below 700° C is concluded to be governed by the Peierls mechanism. The total activation enthalpy of deformation by the Peierls mechanism is 0.32eV. Applying the string model for the kink-pair formation, the results are consistent if the 101 dislocation is dissociated into two 1/2101 partial dislocations. No twinning-anti-twinning asymmetry of slip was observed.     

The influence of pyrolysis condition on the partial melting of solution-spun Y1Ba2Cu3O x superconducting filament

The precursor filaments with diameters of 300 and 110 m were spun through a homogeneous aqueous poly(vinyl alcohol) (PVA) solution containing Y, Ba and Cu acetates. The as-drawn filaments were pyrolysed at a slow heating rate of 30 °Ch^–1 to remove volatile components and partially melted. The effect of pyrolysis condition on the partial melting of the filament was examined to enhance the reproducibility of the highJ _c. By controlling the solidifying condition for the filament pyrolysed at 500 and 450 °C, a highJ _c of more than 10⁴ A cm^–2 at 77 K and 0 T was attained. A window of heating condition to obtain the highJ _c for the thin filament was narrow as compared with the thick filament. The optimum condition for the filament pyrolysed at 450 °C became wider than that for pyrolysing at 500 °C.     

Effect of ion implantation on the strength of sapphire at 300–600°C

Ion implantation with ¹¹B⁺ or ²⁸Si⁺ at 1000°C doubled the ring-on-ring flexure strength of c-plane sapphire disks tested at 300°C but had little effect on strength at 500 or 600°C. Disks were implanted on the tensile surface with 2 × 10¹⁷ B/cm² (half at 40 keV and half at 160 keV) or 1 × 10¹⁷ Si/cm² (80 keV). Sapphire implanted with 1 × 10¹⁸ B/cm² had only half as much flexure strength at 300° or 500°C as sapphire implanted with 2 × 10¹⁷ B/cm². Implantation with B, Si, N, Fe or Cr had no effect on the c-axis compressive strength of sapphire at 600°C. Boron ion implantation (2 × 10¹⁷ B/cm², half at 40 keV and half at 160 keV) induced a compressive surface force per unit length of 1.9 × 10² N/m at 20° and 1.4 × 10² N/m at 600°C. The infrared emittance at 550–800° of B-implanted sapphire at a wavelength of 5 m increased by 10–15% over that of unimplanted sapphire. Infrared transmittance of sapphire implanted with B, Si or N at either 1000°C or 25°C is within 1–3% of that of unimplanted material at 3.3 m. Implantation with Fe or Cr at 25°C decreases the transmittance by 4–8% at 3.3 m, but implantation at 1000°C decreased transmittance by only 2–4% compared to unimplanted material.     

Creep of CoO single crystals

Cobalt monoxide single crystals having a [100] orientation were creep tested in compression over ranges of temperature, stress and oxygen pressure. The creep curves were S-shaped and only the inflection creep rate, ₂, was analysed. In the range of 1000 to 1200° C, 850 to 1700 psi and 10^–3 to 1 atm oxygen, ₂ was given by ₂=A po ₂ ^{0.45 7.1}exp(– Q _c /RT) where Q _c =87±6 kcal/mol at 0.01 atm O ₂ and 100±16 kcal/mol at 1 atm O ₂. Slip occurred on two orthogonal {011} 0¯11 slip systems. The presence of subboundaries was observed by optical and transmission electron microscopy. It is suggested that the creep rate is controlled by oxygen diffusion.     

Dielectric properties of synthetic quartz crystals

The dielectric constant (K) and loss (tan), and hence the conductivity), of high-quality synthetic quartz crystals have been measured with the electric field parallel (or perpendicular) to the optical axis (c-axis). These measurements are carried out in the frequency region 102 to 10⁷ Hz and in the temperature range 30 to 400° C. Values ofK at 30° C areE c 4.58 andE c 4.36, and these are frequency independent; tan), values are quite low,E c having larger values at 10² Hz compared toE c. The temperature variation ofK at different frequencies exhibits two regions: (i) a slow increase up to about 280° C which is frequency independent, and (ii) a fast increase beyond 280° C which is frequency dependent,K having larger values at lower frequencies; similar behaviour is exhibited by tan. Log), against 1/T plots show the usual extrinsic and intrinsic regions. The values for activation energy for conduction in the intrinsic region are calculated to be 0.85 and 1.0 eV, respectively, forE c andE c. An attempt is made to understand these results.     

Tensile testing at high temperatures of ex-PCS Si-C-O and ex-PCSZ Si-C-N single filaments

A microtensile tester consisting mainly of an induction-heated furnace, a 0–2 N load cell, a 0.1/1 m sensitivity straining device and hot grips has been designed and used to test ceramic single ceramic filaments at 25–1600°C under vacuum (0.1 Pa) or in controlled atmospheres. Both failure strength and Young's modulus were measured with an isothermal gauge length of 30 mm. A system compliance correction was applied for each test temperature and material. The apparatus was used to characterize an ex-poly-carbosilane Si-C-O fibre (Nicalon NLM-202) and an ex-polycarbosilazane Si-C-N experimental single filament almost free of oxygen (-ray curing). Both materials exhibit a significant strength loss at 1200–1600°C when tested under vacuum, assigned to a decomposition process with an evolution of gaseous species (SiO/CO or N₂) and the formation of a mechanically weak decomposition surface layer. Conversely, the Si-C-N filament undergoes no strength loss when tested in an atmosphere of nitrogen (P=100 kPa) at 1200°C, the decomposition being impeded by the external nitrogen pressure. In all cases, no significant decrease in Young's modulus was observed.     

Growth,morphology and slip system of α-Si3N4 single crystal

Under the conditions of growth temperatures 1500 to 1700° C and total gas pressure 10 to 50 Torr, -Si₃N₄ single crystals have been grown by chemical vapour deposition from a mixture of NH₃, SiCl₄ and H₂. The crystals were transparent and brownish-red to colourless. The effects of the growth conditions on the crystal morphology, growth habit and growth direction have been investigated. On the basal and prismatic planes, the variation in Knoop hardness with orientation of the indenter long-axis has been measured at temperatures up to 1500° C; maximum hardness values were obtained along the 1 0 ¯1 0 direction for the basal plane and along the [0 0 0 1] directions for the prismatic planes. Hardness anisotropy analysis suggests that the active slip systems of -Si₃ N₄ are {1 0 ¯1 0} [0 0 0 1] from room temperature to 1500° C.     

Deformation of single crystals of Cd3Mg

The deformation behaviour of single crystals of Cd₃Mg and Cd-28.5 at.% Mg has been studied over a range of temperature from –196 to 200° C, and the critical resolved shear stresses associated with both basal and {1¯100} 11¯20 prismatic slip have been measured. The CRSS for prismatic slip has been found to show a marked increase with increasing temperature prior to the onset of appreciable disordering while that for basal slip remains athermal over the same range. Both slip systems, however, showed a CRSS peak within the two-phase, ordered plus disordered, region. The athermal CRSS for basal slip in the stoichiometric alloy is shown to be too low to be consistent with a previous explanation advanced to account for the CRSS for basal slip in Mg₃Cd.     

The orientation dependence of the deformation modes of crystalline mercury at 77°K

Single crystals of mercury have been deformed either uniaxially or in four-point bending at 77‡K and the orientation dependence of the operative deformation modes investigated. The experimental techniques adopted are briefly described and results presented for thirty-five specimens. The predominant deformation mode was crystallographic {11ˉ1} 〈1ˉ10^* slip but ‘{ˉ1ˉ35’ twinning and {1ˉ10} kinking were frequently observed. Slip in the close-packed 〈011〉 direction was wavy and restricted to three specimens. The orientation dependence of these deformation modes is interpreted using Schmid-factor contour-plots of the first- and second-most highly stressed variants. The form and nature of the boundaries between different regions of these plots are discussed in detail. The operative deformation modes in the bent specimens can be satisfactorily interpreted by making use of a new analysis of plane plastic strain developed as a result of this work. The results on deformation kinking have also led to a new general theory of the crystallography of this phenomenon. A critical resolved shear stress of 18±2 g mm⁻² was found for {11ˉ1} 〈1ˉ10〉 slip for specimens with Schmid-factors greater than about 0.3. Much larger values were measured or deduced for other specimens and possible explanations of this surprising result are advanced. Several of the observations are considered to be of general importance to the understanding of the mechanical properties of all crystalline materials but they can only be thoroughly investigated on a material, like mercury, of comparatively low crystal symmetry.     

Transmission microscopy of the corrosion scale formed on a stainless steel exposed to liquid sodium

The scale, formed on 304 stainless steel during exposure to sodium for 85 days, was removed electrolytically, examined in transmission in the Hitachi high voltage microscope and found to consist of hexagonal close packed crystallites, typically 0.3 to 1 m in diameter. Approximately 90% of the crystallites were of basal (0001) orientation and the others either of {4, ¯1, ¯3, 15} or {¯2110} orientations. Lattice parametersa=2.96 Å andc=13.89 Å were determined by analysing electron diffraction patterns derived from the scale. Confirmatory evidence was obtained by examining the scale formed after a shorter exposure of 29 days, in the Philips EM 200 microscope. Previous studies [1, 2] have indicated that the scale may be sodium chromite (with lattice parametersa=2.96 Å andc=15.95 Å). The results of the present study are in apparent conflict with this suggestion unless a contraction parallel to thec-axis 13% is assumed.     

Electrical properties of non c-axis oriented SrBi2(Ta0.95V0.05)2O9 thin films

Pulsed laser deposition technique was used to grow off c-axis oriented SrBi₂(Ta_0.95V_0.05)₂O₉ (SBTV) ferroelectric thin films. X-ray diffraction studies revealed the c-axis suppression in the films grown at lower substrate temperature (350°C) followed by annealing at higher temperatures (650°C). In-plane lattice parameters of the films were decreased with increase in annealing temperature. SBTV films annealed at 750°C exhibited enhanced ferroelectric properties with remanent polarization (2P _r) of 31.5 C/cm² and coercive field (E _c) of 157 kV/cm. The dielectric permmitivity of the films increased with increase in annealing temperature and it was attributed to the grain size dependence. The films annealed at 750°C showed maximum value of dielectric permittivity 172 with a tangential loss of 0.1, at 100 kHz. Higher value of dissipation factor at lower annealing temperature is explained in terms of space charge accumulation at grain boundaries. The leakage current densities of the films follow ohmic behavior at low field regime and space charge limited current dominates at higher fields.     

Fracture toughness of concretes at high temperature

The fracture toughness of ordinary and refractory concretes in the range of 20–1300C was investigated, and the stress intensity factor, K _Ic, on three-point bent specimens (according to ASTM E-399 recommendation) determined. With an increase in testing temperature, the stress intensity factor decreases for both concretes. The values of K _Ic at 20C for both concretes are comparable, being equal to 0.64 MNm^–3/2 for ordinary concrete, and 0.72 MNm^–3/2 for refractory concrete, respectively. At 1100C, K _Ic has a value of 0.043 MNm^–3/2 for ordinary concrete, and for the refractory concrete at 1300C, K _Ic=0.34 MNm^–3/2. The method presented for predicting the behaviour of concrete at high temperature may be used in engineering practice.     

Creep of austenitic 20% Cr/35% Ni stainless steels at low stresses

The present work comprises measurements of the secondary creep-rate at different stress levels with rates between about 2×10^–5 %/h and 10%/h and the grain-boundary sliding at 700° C in two austenitic 20 wt % Cr/35 wt % Ni stainless steels. One alloy was a pure 20 wt % Cr/35 wt % Ni steel, whereas the other contained about 0.5 wt % Ti and 0.5 wt % Al so that it precipitated during creep at 700° C. Special care was taken to assure equivalent microstructure in the specimens and precise creep conditions so as to obtain accurate and reproducible creep-rates. Both materials exhibited decreasing stress-dependence of the creep-rate at low stresses. Neither the stress-dependence of the creep-rate, nor the absolute creep-rate was consistent with diffusion-creep. The amount of grain-boundary sliding was measured separately by means of scribed grid lines on the creep specimens for the pure material at stresses above the creep yield. The values for the component of the creep-rate due to grain-boundary sliding coincide very well with the extrapolated line of the low-stress branch of the creep-rate/stress curve. All these results taken together suggest that the most likely explanation of the creep yield in 20 wt % Cr/35 wt % Ni steels is the one based upon grain-boundary sliding.     

High-temperature creep of yttrium-aluminium garnet single crystals

High-temperature creep in single crystals of Y₃Al₅O₁₂ (YAG) was studied by constant strainrate compression tests. The creep resistance of YAG is very high: a stress of ~ 300 MPa is needed to deform at a strain rate of 10^–6 (s^–1) at a temperature as high as 1900 K (~0.84 T _m, (melting temperature)). YAG deforms using the 111 {1¯10} slip systems following a power law with stress exponent n ~ 3 and activation energy E* ~ 720 kJ mol^–1. However, a small dependence of n on temperature and of E* on stress was observed. This stress-dependence of activation energy combined with the observed dislocation microstructures suggests that the high creep resistance of YAG is due to the difficulty of dislocation glide as opposed to the difficulty of climb. Present dislocation creep data are compared with diffusion creep data and a deformation mechanism map is constructed. Large transition stresses (2–3 GPa for 10 m grain size) are predicted, implying that deformation of most fine-grained YAG will occur by diffusion creep.     

Orientation dependent slip in polycrystalline titanium

The factors determining the active slip systems in cold-rolled polycrystalline titanium sheet were investigated. The texture of such a sheet has an important role in determining the active slip systems. Equi-Schmid factor lines for different slip modes were calculated, and transmission electron microscopy proved that pile ups of dislocations of the predicted systems are formed. The active primary slip system was found to be the prismatic a type slip {1¯100} 11¯20 while the secondary system is either prismatic or pyramidal type I {10¯11}. Basal slip of a dislocations could in certain orientations of load direction be the primary slip systems. Dislocations of the (c + a) type play no significant role in the plastic deformation of polycrystalline titanium sheet.     

Film thickness dependence of critical current density for YBa2Cu3O7 films post-annealed at a low oxygen partial pressure

The variation of critical current density at 77 K as a function of film thickness was studied for YBa₂Cu₃O₇ films on (100) LaAlO₃ substrates. Film thicknesses were in the range 0.2–1.6m. The films were deposited by co-evaporation and post-annealed under conditions which have previously resulted in high-quality films (750°C and an oxygen partial pressure of 29 Pa). The critical current density at 77 K exceeds 1 MA cm^–2 for the thinner films, and decreases with increasing film thickness in excess of about 0.4m. The decrease is in rough agreement with a switch fromc-axis toa-axis growth at about this critical thickness. A good anticorrelation was found between room temperature resistivity and critical current density at 77 K. The results are compared to those obtained before by post-annealing at 850°C in 1 atm of oxygen.     

Deformation of single crystals of the L21 ordered Ag2MgZn

The orientation and temperature dependence of slip geometry and yield stress in single crystals of the L2₁ ordered Ag₂MgZn has been studied in compression in the temperature range 290 to 580 K. The slip direction in Ag₂MgZn is exclusively 1 1 1 in this temperature range, but the slip plane varies with crystal orientations; slip occurs on (¯2 1 1) for orientations near the [0 1 1]-[¯1 1 1] boundary, while for the other orientations in the [0 0 1]-[0 1 1]-[¯1 1 1] unit triangle it occurs on the (¯1 0 1). The critical resolved shear stress (CRSS) for slip on both the (¯1 0 1) [1 1 1] and (¯2 1 1) [1 1 1] systems increases abnormally with increasing temperature and reaches a maximum peak at about 0.92 of the critical temperature T _c, for the L2₁-type order. The peak temperature and the shape of the CRSS versus temperature curve are independent not only of crystal orientation but also of the operative slip system. It is suggested that the strength anomaly in Ag₂MgZn be interpreted in terms of the mechanism based on the transition from unit dislocations to superdislocations.