Effect of Processing on Microstructure and Mechanical Behavior of Magnesium Oxide

Polycrystalline MgO specimens were fabricated from two powders by hot-pressing in graphite or alumina dies and annealing in air, in vacuum, or within the graphite die in vacuum. Parameters affected were density, grain size, and visual appearance, which was considered to be dependent on grain boundary structure. The formation of a boundary liquid phase with one type of powder tended to eliminate the effect of the processing variables. The character (microstructure) of the specimens was correlated with their mechanical behavior in compression at a constant strain rate at 1200°C.     

Effect of Nature of Surfaces on Wetting of Sapphire by Liquid Aluminum

Contact angles of aluminum drops on sapphire measured under vacuum conditions from 660° to 1250°C generally fell into three ranges. Large obtuse contact angles indicating interfacial energies greater than either of the two surface energies were obtained up to about 900°C; van der Waals bonding then existed at a compound interface. In the intermediate range, contact angles were 90° or slightly greater indicating a common interface with an energy, _sγ_l, greater than _sγ_g but less than lγ_g. Acute contact angles indicating a _sγ_l less than _sγ_g and greater than _lγ_g occurred above about 950°C because of the formation of a high temperature complex surface structure with _sγ_g > _lγ_g. A hydroxylated sapphire surface has a lower _sγ_g which increases with gradual dehydroxylation and conversion to the high temperature surface structure with a corresponding change in contact angle through the three ranges. Chemical bonding existed in the latter two ranges. Reactions occurred between Al and the sapphire surface to form volatile species at contact angles less than 90°. Molten Al normally has an oxide coating the effect of which appears to be removed at about 870° C.     

A Thickness Gauge for Ceramic Coatings

An instrument has been developed for the purpose of measuring the thickness of protective coatings of ceramic materials applied on nonmagnetic metal bases. This instrument provides a nondestructive method of measurement and can be used on metallic backings having a wide range of electrical conductivities. Also, the thickness measurement refers to a restricted area and is, therefore, applicable to curved surfaces. Although this instrument was developed primarily for the measurement of the thickness of ceramic coatings on turbine blades and other high-temperature parts of aircraft power plants, it should be generally useful in thickness measurements of paint, plastic, and other nonconducting films.     

Kinetics of Iron-Sodium Disilicate Reactions and Wetting

Thermogravimetric and sessile drop measurements were used to study kinetics of redox reactions between sodium disilicate glass and iron. Two redox reaction sequences were identified; both introduced ferrous oxide into the glass at the interface. One consists of formation of ferrous oxide at the interface by reduction of sodium ions in the glass; this is primarily dependent on the a (FeO) in the metal being less than one. The second consists of oxidation of ferrous ions in the glass by the reduction of sodium ions to form ferric ions which subsequently react with the iron to form ferrous oxide. The reaction rates were shown to be sensitive to temperature, time, total ambient pressure, partial pressure of sodium and oxygen in the atmosphere, and the a (FeO) in the iron. Decrease of contact angles and spreading occur with the redox reaction in which the metal plays an active role, i.e. whose a (FeO) is less than one and whose composition undergoes a change.     

Chemical Durability of Fluoride Glasses: I, Reaction of Fluorozirconate Glasses with Water

The chemical durability of five flouarozirconate glass compositions was studied. Measurements of leachant composition and solution chemistry by plasma emission spettroscopy, pH, and fluoride analysis are reported. Changes in surface structure were monitored by scanning electron microscopy, electron-probe X-ray analysis, and X-ray diffraction. The modifer additives with high aqueous solubility (A1F₃, NaF, LiF, PbF₂) exhibited the highest leach rates with ZrF₄, BaFs₂, and LaF₃ exhibiting lower rates. The leaching order of Zr > Ba > La was maintained for all samples. The leaching process consists of the dissolution of the glass matrix, leaving a thick and porous layer containing Zr, Ba, La, and molecular water. Hydroxylation of the Zr species to form a hydroxyfluoride complex in solution dominated a decrease in pH to moderately acidic values. The resulting increase in component solubility (particularly zirconium fluoride) in the acidic solutions accelerated the glass dissolution rate and demonstrated that, if no buffer is present to avert the pH drift, the chemical durability of these glasses in aqueous environments is extremely poor, being roughly equivalent to that of Na₂O 2SiO₂.     

Analysis of Deformation of a Fireclay Refractory

Rate process theory was applied in a study of the creep behavior of a fireclay refractory (firebrick) after different heat treatments. The analysis was based on the assumption that, in the absence of phase changes, equivalent deformation substructures are produced for a given strain at constant stress independent of temperature. An activation energy of about 170 kcal/mole was determined for the flow process in a heat-treated firebrick, which corresponds closely to the value for mullite specimens. Varying but higher values obtained under different test conditions indicated that phase changes were also occurring during the creep tests. The effect of the amount and texture of the components on the creep behavior and strength of the firebrick specimens is discussed. Useful correlations based on this approach are presented, making it possible to predict creep behavior at constant or variable temperatures and also to calculate stress distributions through a wall containing a temperature gradient after a given strain is reached in a given time.     

Mechanical Properties of Magnesia Single Crystals Corn pression

Stress-strain curves of single crystals of magnesia compressed in the [100] direction are reported at temperatures from –196° to 1200°C.; curves are also shown for different rates of loading at room temperature. The crystals show considerable ductility at all temperatures and at room temperature can be deformed plastically about 6% before fracture at stresses which are about one-quarter of reported polycrystalline fracture strengths. The macroscopic yield drops apparently exponentially from an extrapolated value of 50,000 lb. per sq. in. at absolute zero to about 4500 lb. per sq. in. at temperatures of 900°C. and higher. Heat-treatment has an appreciable effect on the yield stress. The resistance of the material to deformation increases with the number of slip systems and bands activated because of the barriers to dislocation movements which occur at slip band intersections. At about 2 to 3% strain, stress concentrations begin to be relieved by small internal cracks which are not easily propagated. This effect is extensive before final macroscopic failure of the crystal occurs. Preliminary creep tests above the macroscopic yield stress and in the temperature range 800° to 1000°C. show large instantaneous plastic deformations followed by slow constant-rate creep.     

MEAT TEXTURE CHARACTERISATION: COMPARISON OF CHEWING PATTERNS, SENSORY AND MECHANICAL MEASURES

The texture of beef, varied by muscle (semimembranosus and semitendinosus), myofibrillar status (ageing time and cold-shortening) and cooking temperature (60C and 80C), was determined by a sensory profile, chewing patterns (determined from surface electromyograms) and mechanical properties of the cooked meats measured at low and high deformation in compression, shear (cutting) and by dynamic (viscoelasticity) tests. The meat factors were discriminated differently by the different measurements showing that they detected differently the various origins of meat texture. Only the maximum modulus before 70% compression and sensory assessments and EMG gave discrimination among all the meat factors. Two main sensory dimensions were found from the profile: juiciness and tenderness with a third related to the determination of elasticity. Juiciness, which varied most between cooking temperatures, was better related to mechanical parameters at low strain rather than high strain and to initial muscular activity rather than later muscular activity. Conversely, tenderness was better related to high strain mechanical properties and to measurements in the middle or later part of the mastication process suggesting that tenderness assessment requires structural disintegration of meat.