The spatial and temporal variability of nearshore sand bar morphology is quantified using a unique data set spanning 2 years. The data consist of daily time exposure images of incident wave breaking on an open coast sandy beach which may be used to infer bar morphology (Lippmann and Holman, 1989). The morphology in each image is classified into an eight state morphologic scheme in which bars are uniquely defined by four independent criteria. The most frequently observed morphologies are the longshore-periodic (rhythmic) bars, observed in 68% of the data. Linear bars occur under highest wave conditions U s = 1.78 m) and are unstable (mean residence time = 2 days). Shore-attached rhythmic bars are the most stable (mean residence time = 11 days) and generally form 5-16 days following peak wave events. Non-rhythmic, three-dimensional bars are very transient (mean residence time = 3 days). Eighty-seven percent of transitions to lower bar types (defined in text) occurred one state at a time, supporting our selection of the ordering of states, and suggesting the suitability of a sequential morphology model. Transitions to higher states occurred under rising wave energy and were evenly spread among the possible higher states, with more substantial changes in morphology resulting from larger wave increases. This suggests that up-state, erosional transitions (based on offshore bar migration) are better described by an equilibrium model where response is better correlated with incident wave energy than with preceding morphological state. Time exposure images were also digitized to yield quantitative estimates of bar crest location as a function of longshore distance. Principal component analysis was used to decompose bar position into two-dimensional (linear) and three-dimensional (longshOre variable) components. Cross-shore (linear) bar position ranges +50 m about the 2-year mean (27 m standard deviation) and dominates bar variability (74.6%). Three-dimensional bar structure accounts for-14% of the variance (12 m standard deviation). Changes in incident wave height precede cross-shore bar migration by less than 1 day. Changes in longshore variability are inversely correlated to changing wave conditions, with bar morphology becoming linear rapidly during storms (on time scales of less than 1 day). Evolution to significantly threedimensional structure typically occurs over 5-7 days following peak wave events.

A technique is presented to remotely measure the scales and morphology of natural sand bars based on the preferential dissipation of wind waves and swell over the crests of the bar. Photographic or video images are recorded and statistical uncertainties associated with incident wave height modulations removed by averaging (time exposures). Ground truth testing of the technique was carried out as part of the SUPERDUCK experiment in October 1986. The time exposures generally provided a good mapping of underlying morphology, allowing detection of the bar and determination of cross‐shore and longshore length scales. However, during high waves, persistent surface foam obscures the relationship of image intensity to local dissipation (modeled theoretically by dissipation of a random wave field), and an enhancement technique of image differencing must be done to remove the bias. Errors in the estimate of bar crest distance from the shoreline are generally less than 35%, but this value depends on the geometry of the particular bar. Logistic simplicity and quantitative capabilities make this technique very attractive.

The local point symmetry of the short-range order in simple monatomic liquids remains a fundamental open question in condensed-matter science. For more than 40 years it has been conjectured that liquids with centrosymmetric interactions may be composed of icosahedral building blocks. But these proposed mobile, randomly orientated structures have remained experimentally inaccessible owing to the unavoidable averaging involved in scattering experiments, which can therefore determine only the isotropic radial distribution function. Here we overcome this limitation by capturing liquid fragments at a solid-liquid interface, and observing the scattering of totally internally reflected (evanescent) X-rays, which are sensitive only to the liquid structure at the interface. Using this method, we observe five-fold local symmetry in liquid lead adjacent to a silicon wall, and obtain an experimental portrait of the icosahedral fragments that are predicted to occur in all close-packed monatomic liquids. By shedding new light on local bond order in disordered structures such as liquids and glasses, these results should lead to a better microscopic understanding of melting, freezing and supercooling.

a b s t r a c tA b-solidifying TiAl alloy with a nominal composition of Tie43.5Ale4Nbe1Moe0.1B (in at.%), termed TNMÔ alloy, was produced by a powder metallurgical approach. After hot-isostatic pressing the microstructure is comprised of fine equiaxed g-TiAl, a 2 -Ti 3 Al and b o -TiAl grains. By means of two-step heat-treatments different fine-grained nearly lamellar microstructures were adjusted. The evolution of the microstructure after each individual heat-treatment step was examined by light-optical, scanning and transmission electron microscopy as well as by conventional X-ray and in-situ high-energy X-ray diffraction. The experimentally evaluated phase fractions as a function of temperature were compared with the results of a thermodynamical calculation using a commercial TiAl database. Nano-hardness measurements have been conducted on the three constituting phases a 2 , g and b o after hot-isostatic pressing, whereas the hardness modification during heat-treatment was studied by macro-hardness measurements. A nano-hardness for the b o -phase is reported for the first time.

No abstract

The electron-density distribution of the high-pressure polymorph of SiO 2 , stishovite [a = 4.177 (1) , c = 2.6655 (5) A Ê , space group P4 2 amnm, Z = 2], has been redetermined by single-crystal diffractometry using synchrotron radiation of 100.42 and 30.99 keV, respectively, in order to obtain essentially absorptionand extinction-free data. Room-temperature diffraction experiments on two samples of irregular shape were carried out on two different diffractometers installed at HASYLAB/DESY, Hamburg, Germany. The structure re®nement on the high-energy data converged at R(F ) = 0.0047, wR(F ) = 0.0038, GoF = 0.78, for a multipole model with neutral atoms and multipole expansions up to seventh order. For each atom, the radial expansion coef®cients of the multipole orders (l > 0) were constrained to a common value. The absence of extinction was indicated by a re®ned correction parameter equalling zero within error limit. The excellent quality of the data is also illustrated by a high-order (HO) re®nement (s > 0.7 A Ê À1 ) yielding R(F ) = 0.0060, wR(F) = 0.0048, GoF = 0.85. Both static deformation electron-density distribution and structure amplitudes compare well with corresponding results obtained from band-structure calculations using the linearized-augmented-plane-wave (LAPW) method. Ensuing topological analysis of the total model electron density distribution revealed bond critical point properties for the two unique SiÐO bonds, indicating a predominantly closed-shell interaction mixed with a signi®cant shared interaction contribution that decreases with increasing interatomic distance. Calculation of atomic basins yielded charges of +3.39 e and À1.69 e for Si and O, respectively, in good agreement with the theoretically calculated values of +3.30 e and À1.65 e. The volumina of the Si and O basins are 2.32 and 10.48 A Ê 3 , corresponding to spheres with radii of 0.82 and 1.36 A Ê , respectively. The results also conform well with correlations between bond length and bond critical point properties reported in the literature for geometry-optimized hydroxyacid molecules. Estimates of the Si cation electronegativity indicate that the change of Si coordination by oxygen from 4 to 6 is accompanied by an increase of the ionicity of the SiÐO bond of about 7%.

The addition of B effectively supports the generation of fine and homogeneous microstructures in as-cast -solidifying -based titanium aluminide alloys. The microstructural refinement in such alloys can be attributed to the borides acting as nucleation sites for new  grains during the solidstate  transformation (Hecht U, Witusiewicz V, Drevermann A, Zollinger J, Intermetallics 2008; 16: 969-978). In the current work it is shown that the cooling rate plays a crucial role in determining whether borides serve as nucleation sites for grain refinement. Surprisingly, if the cooling rate is too high then grain refinement by borides is hampered. The positive effect of borides can be used to obtain grain refinement in these materials by a simple heat treatment, even if the microstructure has been extensively coarsened through prior heat treatment.

Connections established during last century between bond length, radii, bond strength, bond valence and crystal and molecular chemistry are briefly reviewed followed by a survey of the physical properties of the electron density distributions for a variety of minerals and representative molecules, recently generated with first-principles local energy density quantum mechanical methods. The structures for several minerals, geometry-optimized at zero pressure and at a variety of pressures were found to agree with the experimental structures within a few percent. The experimental Si–O bond lengths and the Si–O–Si angle, the Si–O bond energy and the bond critical point properties for crystal quartz are comparable with those calculated for the H