High-energy Boundaries Research Articles

Close-up stereo photographs of abyssal bedforms on the Nova Scotian continental rise

Close-up stereo photographs of the seafloor were taken at five stations on the Nova Scotian rise. Stereo viewing reveals abundant relief with heights in the range 1–50 mm which are not apparent in oblique monoscopic pictures. Objects with dimensions of 0.1 mm can be resolved, and areas up to 1.0 cm 2 can be viewed. Photogrammetric contour mapping at one millimeter intervals of longitudinal ripples gives in-situ dimensions of 12 cm height, 90 cm full width, and 410 cm 2 cross-sectional area. A field of ripples spaced at 5 m with a sediment density of 0.7 g cm −3 contains 5.7 kg dry sediment per m 2. Seafloor roughness on the scale of centimeters in abyssal, high-energy boundary layers reflects both recent sedimentary and biological processes. The spatial density of eight classes of bed relief varies on scales of tens of kilometers downslope and is consistent with downslope trends in the occurrence of larger bedforms, texture of surface sediments, and bottom boundary layer conditions. Most relief is biogenic, directly or indirectly, and most is streamlined.

29th HATFIELD MEMORIAL LECTURE: Ferrite

The morphology of ferrite grains formed from austenite is first considered, then the refinement of ferrite grain size by controlled rolling as exemplified by HSLA steels. The influence of alloying elements is briefly examined, followed by a detailed discussion of the precipitation of carbides during and after the formation of ferrite. It is shown that the γ/α interfaces are preferred sites for the precipitation of alloy carbides (interphase precipitation), which occurs on both planar and curved boundaries. In the former, the precipitation is associated with the movement of small ledges across the γ/α interface, whereas the curved bands of precipitate are formed on high-energy boundaries which move by bowing around the particles. A change in the nature of the γ/α interface often leads to fibrous carbide growth, while more rapid rates of cooling lead to the formation of supersaturated ferrite and subsequent precipitation of carbides on dislocations. The dislocations in the ferrite are shown to play an important role in the subsequent coarsening of the carbide dispersion, whether they form at the interfaces or on the dislocations. Finally, the observations on the formation and growth of carbide dispersions are related to the structure and properties of microalloyed steels.

Structure-dependent intergranular segregation of phosphorus in austenite in a Ni-Cr steel

The segregation of P to grain boundaries during austenitization of a low-alloy NiCr steel doped with P has been revealed by an etching solution which preferentially attacks grain boundaries containing P, coupled with observations by Auger electron spectroscopy. The correlation of grain boundary energy with segregation of P was examined by observing dihedral angles between boundaries which constitute triple points on a polished surface. Grain boundary compositions and the effect of austenitizing temperature on the degree of P segregation in the austenite phase were analyzed by a scanning Auger microprobe. It was found that the degree of P segregation can vary greatly from one boundary to another and along individual stepped or curved boundaries. Coherent twin boundaries apparently contained no P, but incoherent twin boundaries did. It was concluded that the degree of segregation is mainly dependent on the degree of grain boundary coherency, or on the free volume associated with the boundary. High energy boundaries generally absorbed more P than low energy boundaries; however, it is not considered that grain boundary energy, per se, is a separate controlling factor.

Sequences of precipitate nucleation

Solid-solid nucleation theory, as recently extended to faceted nuclei, is applied to several problems in understanding experimentally observed sequences of precipitate nucleation involving transtion phases during isothermal ageing. Transition precipitates form due to a low interfacial energy resulting from a greater structural similarity to the matrix than the equilibrium phase or succeeding transition phases and in spite of a usually smaller driving force, ¦ΔG v ¦, for nucleation than any of these competing phases. Transition phases formed after the first one are predicted to nucleate preferentially at the interphase boundaries of their predecessors, also on the basis of interfacial energy, rather than ΔG v considerations. Most examples of precipitation sequences cited from the literature are in agreement with this prediction; the few exceptions found are noted and discussed. The preference of equilibrium phases for nucleating at high-energy grain boundaries and of transition precipitates for nucleation at dislocations and other lower energy sites is explained on the following basis. The high-energy boundaries reduce the net interfacial free energy needed for nucleation of either phase so much that the higher ¦ΔG v ¦ for the equilibrium phase becomes dominant. At the lower energy sites, the smaller reduction in ΔG *, the free energy of activation for nucleation, provided makes the lower interfacial energy of the transition phases the dominant factor in causing such phases to nucleate more rapidly.

Segregation at selective grain boundaries and its role in temper embrittlement of low alloy steels

The experiments have clearly shown that segregation of Sb and Ni occurs in large amounts to only certain grain boundaries in a low alloy steel embrittled at 480°C for 24 hrs. These observations are explained on the basis of Gibb's adsorption; segregation occurring to lower the energy of high-energy, high-angle boundaries. Phosphorous on the other hand, segregated more uniformly to all boundaries in the same steel. The results are used in explaining the segregation and embrittlement behaviour of low alloy steels in quenching and tempering operations. The inhomogenity of segregation observed in the present experiments can be used to advantage at least in some materials. By generating suitable textures so that the high energy boundaries are oriented in preferred directions, the embrittlement can be made anistropic. Such treatments, if plausible, could give rise to large improvement in properties in preferred directions.

Optical Properties and Collective Excitations in MoS2 and NbSe2 in the 1.7 to 30 eV Range

AbstractOptical functions of two layer type compounds MoS2 and NbSe2 are calculated in the 1.7 to 30 eV range from reflectivity data by means of a Kramers‐Kronig analysis. The positions of the plasmon peaks given by the energy loss function, — Im \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \varepsilon _ \bot ^{ - 1} $\end{document}, are in good agreement with those evaluated directly by electron energy loss experiments. The optical effective number of electrons saturates at the high energy boundary of the experimental interval, for a value corresponding to all the valence electrons.

Theory of Pion-Pion Scattering

Strong-interaction dynamics is formulated on two main assumptions. The first is similar to the one used in the Regge-pole-resonance interference model, and the other is that the potentials arising from the exchange of a few dominant particles and resonances in crossed channels can be accepted physically in the low-energy regions and that the convergent high-energy behavior of the potentials should be guaranteed by the Regge-pole theory. A long-range potential which satisfies both the low-energy and the high-energy boundary conditions stated above is introduced. The dynamical model with only Regge-pole exchange potentials (as in the new form of the strip approximation) does not produce low-energy resonance behavior as a dynamical output. We show that a potential which does produce the main low-energy resonance behavior as output can be constructed by an appropriate superposition of single-particle exchange potentials and Regge-pole exchange potentials according to our second assumption. Also, two other potentials arising from the large-$t$ parts of the Mandelstam double spectral functions are defined under the first assumption.

Asymptotic Renormalizability Conditions on Field Propagators

High-energy boundary conditions upon the basic Green's functions which limit the types of divergence arising in any approximational method of solution of a field theory are derived and shown to be related to the recently defined stability criterion.

Existence of solutions of partial-wave dispersion relations

High-energy boundary conditions and the convergence properties and existence of solutions of low-energy partial-wave calculations are studied. The physical significance of the inverse-amplitude solution for low-energy pion-pion scattering is discussed.