Structure of Water.-It is generally accepted that the structure of water around solutes has an important effect on the thermodynamic properties of aqueous solutions. The forces between protein molecules are greatly affected by these structures. NMR spectra were determined for water absorbed in collagen, the main protein of connective tissue. These spectra show that the water molecules diffuse rapidly through the pores, but do not rotate isotropically.

Polymer Characterization.-Physical properties such as viscosity and plasticity are mainly dependent on the distribution of molecular weights in the polymer sample. Techniques for fractionation have now been developed for polyethylene, the commercially most important plastic at present. Several instruments can be used for molecular weight determinations of the fractions, provided they have been calibrated.

Variation of Unit Cell Dimensions.-It has been shown that the orthorhombic unit cell dimensions of a given polyethylene are not unique, but depend upon the physical history of samples crystallized from either the melt or solution. Variations in cell dimensions result from variations in crystallization temperature, annealing temperature, and annealing time as well as from deformation of the crystals. The origins of the effects cannot be assigned with certainty but the observed changes are consistent with a mechanism involving strain from the molecular folds on the crystal surfaces. Corresponding to the changes in dimensions are significant changes in unit cell density and apparent changes in degree of crystallinity. The effects of cell variation no doubt play a role in the variation of many macroscopic physical properties with physical history of commercial polymer samples.

Stacking Faults in Alloys.-One basic parameter that determines the mechanical properties of materials is the stacking fault energy. Since technologically useful structural materials generally consist of combinations of several different phases having different chemical compositions, it is desirable to determine the effects of phase and composition changes on the stacking fault energy. The electron microscope has been applied to a study of cubic and hexagonal phases in several silver-tin alloys in order to establish the effect of phase and composition changes in a simple system. Results indicate a near equality in the stacking fault energy for these two related phases.

Quantitative Microscopy. An optical photographic scanner has been constructed to evaluate photomicrographs taken from a wide range of metallic and nonmetallic materials. Measurements characterizing the internal grain and phase structures are of great

importance in controlling the quality and uniformity of materials. Machine-evaluated photographs are being compared among different laboratories for the purposes of improving capability.

Adherence of Porcelain Enamel on Aluminum.-The Porcelain Enamel Institute is sponsoring a Research Associate Program to investigate the adherence of porcelain enamel to aluminum. The electron microscope and electron probe microanalyzer have been applied to study the interface between the two materials. Some basic differences near the interface have been observed between porcelain enamelaluminum alloy systems showing good adherence with subsequent spalling resistance and those combinations which fail. The electron microscope shows a layer of foreign material present at the interface of those systems which have poor adherence. The electron microprobe also detects an enrichment of magnesium at the interface region on certain alloys that exhibit poor coating adherence. Neither the layer nor magnesium buildup was observed on the systems having good adherence.

Tapered-section through a porcelain enamel-aluminum alloy specimen as seen by the electron microscope shows (arrow) the presence of additional material at the interface. This material affects the degree of adhesion coating.

DATA ON THE PROPERTIES OF MATERIALS

This program area includes making precise measurements of materials properties and the development of new measurement techniques. It also includes fundamental investigations of physical phenomena of importance to science and industry (corrosion, fatigue and fracture, etc.). Establishing the laws relating the physical properties of materials to their characteristics is of prime concern. Theoretical work plays a major role in this area.

Mechanical Properties and Strength of Materials

Elastic Constants of Lower-Symmetry Tetragonal Crystals.-A complete analysis of the elastic constants of CaMoO, were made in order to predict mechanical stability in applications such as a host crystal for laser rods.

This analysis was also of scientific interest. Accurate determinations of elastic constants of single crystals are based on resonance frequency or wave velocity measurements. Wave velocity measurements can be made upon small specimens and as a function of pressure; however, the analysis is complicated for low-symmetry crystals and some of the elastic constants depend upon choosing the appropriate root of an algebraic equation. Fortunately, the elastic constants appear linearly in the expressions associated with the resonance technique; therefore, no ambiguity arises. In this experiment the complete analysis extended the elastic constant determination usually effective only for high-symmetry tetragonal single crystals to lower symmetry tetragonal single crystals.

Properties of Electrodeposited Copper.-An IMR Industrial Research Associate, sponsored jointly by the American Electroplaters' Society, Inc., The International Copper Research Association, and the Copper Development Association, has obtained comprehensive data on the properties of electrodeposited copper. Types of plating baths used in the program included sulfate, cyanide, pyrophosphate, fluoborate, and amine. A total of about 150 sets of operating conditions were used, comprising variations in bath composition, temperature, and current density. Properties measured and correlated include tensile and yield strengths, modulus of elasticity, elongation, internal stress, fatigue limits, hardness, density, electrical resistivity, thermal expansivity, structure, and chemical composition. Effects of cold working and annealing, and effects of cryogenic and elevated temperatures on some of the above properties were also determined.

Ultra Stable Maraging Steel.-Measurements over a two-year period of the temporal dimensional stability of a 12 percent Nickel maraging steel heat treated to meet requirements for a gage block material

indicate that it is one of the most stable materials ever observed in the continuing NBS gage block program. The long time dimensional stability of this important new class of steels has never before been reported. Stability of the two test blocks were -0.03 μin/in/yr and +0.10 μin/in/yr respectively.

Studies of Fiber Reinforced Metals. The problem of identifying the constituents as well as their distribution in an aluminum-silicon alloy containing sapphire fibers was solved, in part, with the use of color photomicrography and the microprobe analyzer. Factors contributing to the strengthening of vapor-deposited copper reinforced with tungsten wires were correlated with ductility, fracture types, and resistivity measurements.

Microplasticity in Metals.-The accuracy of microstrain measurements has been improved by two developments. An IMR designed specimen enables measurements to be made over a well defined gage length, and precision measurements of temperature of the specimen allow for the correction of temperature effects caused by loading and unloading. Neither of these features have been indicated in reports emanating from other laboratories.

Fatigue Cracking in Chromium Coated Iron.-Chromium diffusion coatings on commercial Armco Iron lead to changes in microstructure, stress distribution, and strength in the outer layer of the base material. These factors affect the mechanism of fatigue formation. In coated samples, the crack initiates below the Cr-rich zone, generally in the grain boundaries where precipitated chromium carbides were discovered. Residual tensile stresses introduced below the coated layer by the process are considered an accelerating factor for crack formation.

Fatigue Crack Propagation Under Constant Load Amplitude.—A theoretical-mathematical evaluation of fatigue crack propagation in cyclic bending was completed. It was found that fatigue crack propagation can be expressed as a function of stress intensity factor. The experimental results agree closely with the theoretical model. The influence of localized (heterogenous) structural changes on the proposed model were also investigated. It appears that the length of fatigue crack propagation and the rate of propagation cannot always be expressed as a function of plastic zone size. This was related to the anisotropic conditions of plastic deformation.

Fatigue Due to Reversed Bending and Reversed Torsional Loads.— A recently completed investigation showed that fatigue damage due

to reversed bending loads did not reduce the resistance to fatigue crack initiation under reversed torsional loads. The experiments were conducted by stressing (bending) aluminum alloy specimens until a number of small fatigue cracks had developed, then determining the number of stress cycles required to develop small cracks under reversed torsion. It was found that this number was not significantly different from that required to give equivalent damage in specimens that had not been stressed previously. The reversed sequence was also investigated and it was found that prior stressing in reversed torsion did not affect the resistance to crack initiation in reversed bending. The results indicate that in alloys of this type the damaging effect of fatigue stressing is limited to a relatively small proportion of the crystals which are subjected to potentially dangerous ranges of stress.

Stress Rupture Characteristics of a Titanium Alloy.-In contrast to some theories and some experimental observations on other metals and alloys (e.g., copper and some steels), ductility of Ti-8Al-1Mo-1V increased with increase in test temperature and decrease in strain rate in creep tests with rupture times up to 10,000 hr. Although stressrupture properties of this alloy were greatly affected by notches at low temperature, the presence of notches of varying notch geometries had little or no effect on the behavior at elevated temperatures.

Improved Dental Cements.-Zinc oxide-eugenol (ZOE) cements have found many applications in dentistry. This puttylike material is more compatible with both the hard and soft tissues of the mouth than other dental restorative materials. Previous investigations at IMR have shown that incorporation of o-ethoxybenzoic acid (EBA) and certain additives to ZOE mixtures greatly improved the mechanical properties of these cements.

Recent studies have shown that aluminum oxide acts as a reinforcing agent and further enhances mechanical and physical properties. The high strength and low solubility of the Al2O, reinforced materials, as well as their ease of manipulation, suggested their use for the final cementation of cast crowns and as insulating base. Cements based on the formulations developed at NBS have become commercially available and have been well accepted by the dental profession. Further modification of these products incorporating copolymers of relatively low elastic moduli are showing considerable promise as a rapidly insertable, long-duration temporary restorative. A material of this type is of special interest to the Armed Forces since the prolonged life of this material would permit delay of necessary definitive treatment.