Creep of polycrystalline alumina,pure and doped with transition metal impurities

Grain size effects were used to evaluate the relative contributions of aluminium lattice and oxygen grain boundary diffusion to the high temperature (1350 to 1550° C) steady state creep of polycrystalline alumina, pure and doped with transition metal impurities (Cr, Fe). Divalent iron in solid solution was found to enhance both aluminium lattice and oxygen grain-boundary diffusion. Large concentrations of divalent iron led to viscous Coble creep which was rate-limited entirely by oxygen grain-boundary diffusion. Nabarro-Herring creep which was rate-limited by aluminium lattice diffusion was observed in pure and chromium-doped material. Chromium additions had no effect on diffusional creep rates but significantly depressed non-viscous creep modes of deformation. Creep deformation maps were constructed at various iron dopant concentrations to illustrate the relative contributions of aluminium grain boundary, aluminium lattice, and oxygen grain-boundary diffusion to the diffusional creep of polycrystalline alumina.     

Creep of porcelain-containing silica and alumina

Hard porcelain ceramics find many applications because of their high hardness, high mechanical strength and moderate thermal-shock resistance. The addition of alumina as a filler to porcelain increases its strength at room temperature. In the present investigation, four-point-bend creep tests were carried out for porcelain-containing silica (SP-1) and alumina (AP-3) at 800, 900 and 1000°C. The creep data were analysed using a power-law creep, and the stress exponents were estimated. The activation energy for these two materials was found to be 45 kcal mol^–1. The viscosity of the feldspar glassy phase was also determined from the creep tests. The test samples were analysed by scanning electron microscopy (SEM). The X-ray diffraction results (XRD) show that the amount of crystalline phase in the material increases after creep testing.     

Nonsingular toughening and R-curve behavior in nominally pure alumina

Moire interferometry is employed to study toughening in medium to large grain size nominally pure alumina. The fracture scale length, which is characterized by the grain size of the alumina, is systematically varied from 35 to 102 m. R curves are derived from bulk mode I compliance calculations for the differing grain sizes and from the near tip moire fringes. The level of material toughening that arises from the nonsingular processes of crack bridging and grain boundary friction are found by comparing the bulk and near tip moire R curves.     

Creep of polycrystalline sodium chloride containing a dispersion of alumina

The creep of polycrystalline NaCl contaning a fine dispersion of Al₂O₃ particles is analysed in terms of dependence on stress, temperature, volume fraction and size of dispersion, and grain size of samples. Compressive creep experiments around 0.8 Tm show that the dispersion inhibits diffusive creep. The creep is characterized by a threshold stress above which the creep rate increased linearly with applied stress. The threshold stress decreases with increasing temperature and is proportional to the volume fraction of the dispersion in agreement with a model proposed by Burton. The activation energy corrected for the temperature dependence of the threshold stress falls within a narrow range consistent with grain-boundary diffusion of chlorine in sodium chloride. The grain-size dependence is not consistent with a modified diffusive creep model but it is suggested that it may be controlled by inhibited grain-boundary sliding according to a new model.     

Enhancing damping of pure magnesium using nano-size alumina particulates

In the present study, elemental magnesium was reinforced with nano-size alumina particulates. Synthesis of materials was accomplished using the powder metallurgy route. Energy dissipation in the form of damping capacity was determined using free–free type suspended beam arrangement coupled with circle-fit approach. This technique is based on classical vibration theory, by which the geometry and material properties of the metallic specimen are related to measured resonant frequency and structural damping. Using the fact that the ratio of the vibration response to the applied force fits to a circle in the Argand plane for each resonant frequency of the test specimen, the damping factor and natural frequency is predicted accurately for the test specimen. The results revealed that an increase in the alumina content up to 0.4% volume percentage lead to an increase in the damping capacity up to 34%. Attempt is made to correlate the increase in damping with the various microstructural changes arising due to the presence of the nano-size alumina particulates in the composite sample.     

Formation of relatively pure alumina films by anodic polarization

Barrier-type film formation on aluminium in aqueous sodium and potassium hydroxide electrolytes of pH 11 was monitored using transmission electron microscopy of ultramicrotomed sections of the film attached to the substrate. Film growth to relatively low voltages in sodium hydroxide at a constant current density of 50 A m⁻² and associated high current efficiency develops typical barrier-type films of relatively pure alumina, as assessed by Rutherford backscattering spectroscopy, unlike the case with other forming electrolytes in which characteristic anions are generally incorporated in the outer regions of the films. For film growth in potassium hydroxide at a constant current density of 300 A m⁻² to high voltages, which is associated with an overall relatively low current efficiency of formation, an irregular film morphology develops, reminiscent of the appearance of barrier-type films that have suffered dielectric breakdown. In the present situation, where films were formed to voltages less than the breakdown voltage, it is thought that local current concentration, assisted by local heating, contributes to the morphology revealed; “conducting channels” within the film section provide the means for such non-uniform film growth, but precise reasons for their development are not yet clear.     

Wetting of pure aluminum and selected alloys on polycrystalline alumina and sapphire

Using a modification of the dispensed drop method to measure true contact angles of readily oxidizing metals and alloys, the wettability of polycrystalline alumina and A-plane sapphire by pure aluminum and selected aluminum alloys was investigated. The experiments were performed under high vacuum in a horizontal tube furnace. The experimental setup produces a sessile drop free of its natural surface oxide layer minimizing flight time of the drop, and maintaining a drop impingement on the substrate.The experiments showed that there is no significant difference in the wettability of alumina and sapphire by aluminum as well as Al–11.5Si, Al–1Mg and Al–7Cu. On both substrates, aluminum shows a strong increase in contact angle well into the non-wetting regime just above the melting point. The wetting behavior of Al–7Cu on both substrates is slightly but significantly reduced in comparison to pure aluminum. The contact angles of Al–1Mg and Al–11.5Si remain rather constant between the respective liquidus temperatures of the alloys and 800 °C with θ (Al–1Mg) < θ (Al–11.5Si). Only Al–7Cu above 730 °C achieves the contact angle interval of 70–86° suggested to be most beneficial in terms of aluminum foam stabilization.     

The mechanism of simultaneous sintering and phase transformation in alumina

The sintering behaviour of boehmitic alumina gels during the transformation to the stable phase has been studied using dilatometry, transmission electron microscopy, X-ray analysis and differential thermal analysis. The specimens for transmission electron microscopy were prepared from gel specimens, sintered to various predetermined temperatures, using an ion-beam thinning technique. The transmission electron microscope study and X-ray analysis have revealed a characteristic sintering behaviour which is associated with the to phase transition. The transformation to the phase occurs by a nucleation and growth process. During the growth process considerable redistribution of the fine porosity existing within the transition alumina matrix occurs, in the form of large elongated interconnected pores trapped within the nucleating grains. These pores grow rapidly to a size approximately one hundred times that of the grains. This process results in a rapid fall-off in sintering rate at the end of the transformation. A study of the/ interphase interface by transmission electron microscopy has led to the development of a model that accounts satisfactorily for the redistribution of the porosity.     

On geometric phase from pure projections

Abstract

The relation between three recent experiments aimed at demonstrating a strictly geometric phase using pure projections is clarified in the light of a recent claim to such a measurement. The possibility of observing & ± 2π phase jumps in an optical interference experiment is pointed out.     

Evaluation of continuous alumina fibre reinforced composites based upon pure aluminium

Abstract

Composites of super purity aluminium unidirectionally reinforced with Altex or Nextel 610 continuous alumina basedfibre have been made by liquid metal infiltration. The composites were well consolidated, with fibre volume fractions V_f of 0.4 and 0.6 for the Altex composites and 0.7 for the Nextel composite, the higher values being obtained where preforming involved the use of sized fibre tows. Matrix porosity was very low and there was no evidence of any deleterious reaction product having formed at the fibre/matrix interface. Monotonic longitudinal tensile tests of the composites gave Youngs modulus values between 125 and 250 GPa, in line with rule of mixtures (ROM) predictions and evidence of effective load transfer between fibres. The onset of yielding in longitudinal composites was commensurate with the yield stress of unreinforced super purity aluminium for V_f = 0.4 (～20 MPa), but increased to 225 MPafor V_f =0.7. The tensile strengths of the Altex composites were 760 and 930 MPa, values in accord with ROM predictions based upon equal load sharing of fibres up to the mean filament failure stress. Although the Nextel composite had a higher tensile strength of 1250 MPa, this was significantly lower than the ROM value of 1650 MPa and was better described by fibre ‘bundle’ theory. Predictions of the accumulation of fibre damage, by statistical analysis, indicated that filament breakage commenced at an applied stress of ～50 MPa for the Altex composite and ～ 500 MPa for the Nextel composite. Despite damage at the lower stress, however, the Altex composites were able to tolerate many more ‘double’ fibre breaks than the Nextel composite, the failure of which coincided with the onset of the first double break. Transverse tensile tests of the composites gave Young's modulus values between 80 and 170 GPa, in line with ROM predictions. The yield stress increased with increasing V_f, from 10 to 60 MPa, this behaviour being attributed to plane strain deformation caused by the virtually non-deformable fibres constraining matrix flow. The tensile strengths showed a similar trend, with 84 MPa for V_f =0.4 increasing to 168 MPa for V_f = 0.7.     

High temperature mechanical properties of single phase alumina

The creep properties of a commercial, single phase aIurnina have been determined in the temperature range of 1623 to 1723 K. The stress exponent,n, in the relationship ⁿ was determined to be 1.9 and the true activation energy was found to be 635 kJ mol^–1. Normal primary stage creep transients were observed up to strains of 1%. At low stresses, steady-state conditions were not obtained due to the occurrence of concurrent grain growth. It is shown that the steady state creep results are consistent with the occurrence of an interface controlled diffusionaI creep mechanism.     

Creep rupture studies of two alumina-based ceramic fibres

Creep rupture tests were performed in air on two polycrystalline oxide fibres (Al₂O₃, Al₂O₃-ZrO₂) using both filament bundles and single filaments. Tests were performed at applied stresses ranging from 50–150 MPa over the temperature range 1150–1250 °C. Under these conditions, creep rates for the alumina-zirconia fibre ranged from 4.12 × 10^–8–7.70 × 10^–6s^–1. At a given applied stress, at 1200°C, creep rates for the alumina fibre were 2–10 times greater than those of the alumina-zirconia fibre. Stress exponents for both fibres ranged from 1.2–2.8, while the apparent activation energy for creep of bundles of the alumina-zirconia fibre was determined to be 648 ± 100kJmol^–1. For the alumina-zirconia fibre, the two test methods yielded similar steady-state creep rates, but the rupture times were generally found to be longer for bundles than for single filaments. The steady-state creep behaviour of these alumina-based fibres is consistent with an interface-reaction-controlled diffusion-controlling mechanism.     

Coaxial holographic encoding based on pure phase modulation

We describe a simple technique for coaxial holographic image recording and reconstruction, employing a spatial light modulator (SLM) modified in pure phase mode. In the image encoding system, both the reference beam in the outside part and the signal beam in the inside part are displayed by an SLM based on the twisted nematic LCD. For a binary image, the part with amplitude of "1" is modulated with random phase, while the part with amplitude of "0" is modulated with constant phase. After blocking the dc component of the spatial frequencies, a Fourier transform (FT) hologram is recorded with a uniform intensity distribution. The amplitude image is reconstructed by illuminating the reference beam onto the hologram, which is much simpler than existing phase modulated FT holography techniques. The technique of coaxial holographic image encoding and recovering with pure phase modulation is demonstrated theoretically and experimentally in this paper. As the holograms are recorded without the high-intensity dc component, the storage density with volume medium may be increased with the increase of dynamic range. Such a simple modulation method will have potential applications in areas such as holographic encryption and high-density disk storage systems.