Fracture across a grain in an alumina ceramic caused by thermal shock after heating at 800 °C. The fracture surfaces are parallel to the rhombohedral planes. Fracture of this type is a frequent cause of failure in alumina. Studies are being conducted at NBS to better understand brittle fractures and thus to expand the usefulness of ceramic materials.

Low-Temperature Mechanical Properties.-The mechanical properties of selected copper alloys have been measured and an extensive literature search and critical evaluation was completed. The final report will be published as an NBS Monograph.

New Method for Studying Dental Materials.-Scientists in the NBS Dental Research Laboratory are investigating the use of ultrasonic waves to study properties of dental materials. It is well known that the elastic properties of materials can be determined from the velocities of ultrasonic waves within them, but this is the first time that the technique has been tried on dental amalgam.

Tarnishing Behavior Identifies Fatigue Fractures.-IMR researchers have observed an interesting phenomenon that provides a definite distinction between a metal fracture resulting from a single application of load and one that results from the propagation of a fatigue crack. In the course of an investigation on magnesium alloys, it was observed that fracture surfaces, which were bright when first broken,

became discolored after several months storage, but the tarnishing occurred only on the fatigue portion of the fracture. So far as can be determined, this is the first case observed in which the type of fracture has affected the chemical character of the fracture surface. Such information will be of substantial value in the examination of fractured metal parts (such as on an airplane or a large production line machine) to determine the cause of failure.

Reactivity and Corrosion

Protective Oxide Films on Iron.-Efforts are being made to determine methods of forming protective oxide films on iron in various corroding environments. A study has been completed to characterize some of these protective oxide films. By using tritium (radioactive hydrogen) as a tracer, hydrogen was found to be incorporated in those thin (2-5 nm) films which gave protection from corrosion. The hydrogen was found in highest concentrations in the y-Fe2O3 outer portion of the film while lesser amounts were found in the Fe3O4 portion. In a second study, the effects of ultrahigh vacuum annealing of thin oxide films on iron was determined and a model to explain the results was proposed. The study found that after vacuum annealing at 400° C or above, the previously protective oxide films grew when re-exposed to oxygen at 25° C.

The Stress Corrosion of Titanium.-An investigation of the stress corrosion cracking of titanium and titanium 8Al-IV-1Mo in methanol has shown that failure will occur in the methanol vapor phase with times-to-failure being at least an order of magnitude shorter than previously reported. Those effects which significantly altered timeto-failure were system volume, metal surface conditions, and environment composition. The increasing use of titanium in the chemical industry, and in aircraft and space exploration has made such problems and their solution more important.

Protection of Steel Piles in Seawater Environments.-In a joint research effort with the Department of the Army-Coastal Engineering Research Center, over 100 carbon steel and low alloy steel piles (35 feet long, 48 lb/ft) were driven 19 feet into the Atlantic Ocean bottom off the coast of Dam Neck, Va. The results of this study, which will take about 15 years to complete, will demonstrate the best means of protecting steel pilings in seawater. Many types of protective methods are included in the investigation, consisting of coating systems (coaltar epoxy, galvanized, aluminum and zinc flame spray, zinc-rich paints, etc.) and also cathodic protection by zinc and aluminum sacraficial anodes.

Thermodynamics and Kinetic Data

Polystyrene Heat Capacity Values.-Over a dozen publications exist which report different heat capacity values for various samples of the polymer polystyrene. This situation has been clarified by determining values precise to better than 0.1 percent over the temperature range from 10 to 360 °K for a narrow molecular weight distribution atactic polystyrene sample. At temperatures above 100 °K these data agree within the combined limits of experimental uncertainty with most previously published values. At temperatures below about 100 °K, however, significant heat capacity differences appear, especially between the atactic and the isotactic isomers, and even between atactic samples of different molecular weight distributions and of different thermal histories.

Volume Change Accompanying Collagen Aggregation.-The natural polymeric substance collagen is a most important constituent of the human body, making up 30 percent of its protein. Industrially this polymer (the basic constituent of hide) is converted by various tanning operations to leather. In either biological or industrial application the collagen in its useful form is an aggregate of so-called tropocollagen units. By dilatometric measurement the volume of a

dispersed collagen system has been shown to increase by an extremely small amount as aggregates of tropocollagen units are permitted to form. This volume increase is attributed to a change in the ordering of the water molecules immediately surrounding non-polar surfaces of the tropocollagen units as these surfaces are superimposed. The detection of this small but real volume increase provides strong evidence that hydrophobic bonding is the driving force in the aggregation process.

Monte Carlo Studies of Polymer Chain Motion.-Existing theories of the motion of random-coil polymer chains usually neglect the mutual interference of different parts of the chain with each other (the socalled excluded volume effect). In an attempt to learn more about the effects of this interference on chain motion, a high-speed computer was used to simulate the motion of a model polymer chain. The model employed was drastically over-simplified, but contained the essential features of excluded volume interactions. The computer model may be studied either with or without excluded volume effects. Without excluded volume, its behavior has been found to be remarkably similar to the predictions of a theoretical treatment based on a picture of chain motion very different from the computer model.

Physical Adsorption.-Studies are underway to measure the adsorption isotherms necessary for the proper design of cryogenic adsorbers and to develop techniques for correlating and predicting these data. Considerable progress has been made in both of these areas. The adsorption isotherms of nitrogen, methane, hydrogen and their mixtures have been measured at 76 °K over a wide range of pressures. Techniques for correlating and predicting data of this type have also been developed and published.

Properties of Solid Hydrogen.-The dielectric constant of solid hydrogen in equilibrium with liquid has been determined, and refined measurements of the pressure-temperature coordinates of the melting line near the triple point have been made. From the latter, the density of the melting solid was calculated.

Characteristics of Slush Hydrogen.-An investigation was continued of mixtures of liquid and solid (slush) hydrogen. Particle behavior due to aging has been studied in a carefully controlled thermal environment. Flow correlations are being derived as a function of solid fraction. Instrumentation for the determination of slush density (solid fraction) has been extended to include nuclear radiation attenuation. A density reference system is also being developed for calibration of field-type instruments and transfer standards.

Kinetics of Solute-Enhanced Diffusion in Dilute fcc Alloys.-The addition of impurity atoms to a dilute alloy can appreciably change the diffusion coefficient of the matrix solvent atoms, sometimes by more than an order of magnitude. To help understand this property of alloys, theoretical calculations have been made relating the solvent diffusion coefficient to the atom jump frequencies in the vicinity of an impurity. On the basis of these calculations, it is possible to infer detailed information about atomic parameters (vacancy jump frequencies, impurity-vacancy binding energies) from the results of macroscopic diffusion measurements.

Kinetic Theory of Whisker Growth.-Numerical solutions for whisker length as a function of time have been obtained for the surface diffusion model of whisker growth. In this model atoms from the vapor strike the sides of the whisker, are adsorbed, and then surface diffuse along the whisker to the whisker tip where they are incorporated into the solid whisker. Of particular interest is a comparison of the exact numerical solution with the steady state approximation, since, if this approximation is valid, only the product, DE, of the surface diffusion constant D and the mean adatom stay time Σ can be determined. Detectable deviations from the steady state approximation are necessary to determine both D and Σ separately. The results indicate that only extremely accurate measurements on whiskers of very small radii can possibly yield values of both D and Σ.

High-Temperature Phase Relations of Niobium and Tungsten Oxides. The phase diagram was determined of the complex system niobium pentoxide-tungsten trioxide. The phase relations were established using a combination of quenching techniques, microscopic examination and x-ray methods. One region of solid solubility, twelve stable compounds, one metastable compound, and no non-stoichiometric compounds were found.

Properties of Aqueous Salt Mixtures.-A program of water desalinization and purification necessitates a knowledge of the thermodynamic properties of mixed salt solutions and, in particular, how these mixed salt solutions differ from solutions of the separate components. A productive study was undertaken of the properties of mixed salt solutions utilizing accurate isopiestic vapor-pressure measurements. In the two and one-half years during which the project was in effect, seven research papers dealing with the properties of aqueous solutions of the salts present in sea water, singly and in mixtures, were published. Particularly outstanding was the development of a thermodynamic method for studying the behavior of one salt in a mixture containing two or more saline materials.