A detailed review of the principles governing the oxidation of metals is presented. T h e initial stage of the oxidation process involves the chemisorption of oxygen, frequently followed by dissociation and at least partial ionization. T h e oxygen is probably incorporated by place-exchange and surface rearrangement. Ordered surface adsorption structures occur on many metals, although amorphous structures are found on some metals. Intermediate sub-oxides or ' pseudo-oxides ' form prior to the formation of a stable oxide. Considerable oxygen solution occurs in some metals with the formation of internal superlattice domains and sub-oxide platelet structures. Oxide nuclei form at the surface of most metals, the density of nuclei depending on the oxidation temperature and oxygen pressure, and nucleation theory is important for understanding this behaviour.T h e growth of continuous oxide films or scales is considered in terms of various rate-limiting processes such as anion or cation diffusion through the bulk oxide, mass or electron transport across one of the interfaces, or electron transfer processes associated with the chemisorption step. A large number of rate laws including linear, parabolic, cubic and quartic as well as logarithmic relations have been derived. Most of these have not yet been adequately tested with the possible exception of the Wagner parabolic law. Most of the rate laws derived seem to be satisfactory only over a very limited oxide thickness range. T h e importance of microstructural features of the oxide is emphasized.

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The dark line which is observed in enamel crystallites represents a planar defect involving a single 100 plane of the hydroxyapatite structure. It may occur in the majority of crystals throughout the enamel, although it is only observed in specific diffraction conditions. Its presence may be related both to the formation and growth of crystallites in the developing tooth and to the manner in which the crystallites dissolve during caries. TEM studies show clearly that the central defect is not a dislocation, stacking fault or lattice twin boundary. They further indicate that it cannot represent a structural twin boundary. The remaining possibilities are that it represents a substitution in the HAP lattice, most likely involving carbonate ion, or a separate compatible calcium phosphate phase.

We observe for the first time the transformation of glassy alloys (Pdioo-x-jU;fSi^, x -j^ -20 at.%) into a metastable icosahedral phase with quasicrystalline order upon annealing. The icosahedral symmetry of the latter phase is confirmed by x-ray and electron diffraction. This phase can be formed only within a very narrow compositional range ( < 2 at.%) of U and Si, suggestive of a strong chemical ordering in it. The icosahedral and glassy phases have similar electrical transport properties. However, magnetic-susceptibility results indicate that the electronic structure of the icosahedral phase is very different from its glassy and crystalline (UPda) counterparts.

The magnetic properties of rapidly quenched FeRM alloys where R=La,Y,Pr,Nd,Gd and M=B,Si,Al,Ga,Ge have been investigated over a wide range of chemical compositions. The samples are generally magnetically soft in the as-quenched state. Magnetic hardening is produced by annealing the samples around 700 °C. The best properties have been obtained in samples containing Pr and Nd together with B and Si. An energy product of 13 MGOe and a coercive field of 15 kOe have been obtained in a Fe76Pr16B5Si3 sample. The higher Fe content samples appear to be more promising with a potential energy product of 49 MGOe. Thermomagnetic data show that a structural transformation takes place upon heating the samples to 700 °C. The Curie temperature of the as-quenched phase is around 160 °C while that of the new phase is around 320 °C. Transmission electron microscope studies show fine precipitates (∼100 Å) dispersed in a matrix of different chemical composition. X-ray and electron diffraction data indicate that the precipitates have the Fe21R3B tetragonal structure. The high anisotropy of this phase together with its fine size and distribution give rise to the observed high coercive fields.