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<jats:p> Precipitation hardening, in which small particles inhibit the movement of dislocations to strengthen a metal, has long been used to improve mechanical strength, especially of aluminum alloys. The small size of precipitates and the many possible variants of the orientation relation have made their structural determination difficult. Small precipitates in commercial aluminum-magnesium-silicon alloys play a crucial role in increasing the mechanical strength of these alloys. The composition and structure of the β” phase in an aluminum-magnesium-silicon alloy, which occur as precipitates (typically 4 nanometers by 4 nanometers by 50 nanometers) and are associated with a particularly strong increase in mechanical strength, were determined. Element analysis indicates that the composition is Mg <jats:sub>5</jats:sub> Si <jats:sub>6</jats:sub> . A rough structure model was obtained from exit waves reconstructed from high-resolution electron microscopy images. The structure was refined with electron nanodiffraction data (overall <jats:italic>R</jats:italic> value of 3.1 percent) with the use of a recently developed least squares refinement procedure in which dynamic diffraction is fully taken into account. </jats:p>

<jats:p>The sophisticated use of self-organizing materials, which include liquid crystals, block copolymers, hydrogen- and π-bonded complexes, and many natural polymers, may hold the key to developing new structures and devices in many advanced technology industries. Synthetic materials are usually designed with only one structure-forming process in mind. However, combination of both complementary and antagonistic interactions in macromolecular systems can create order in materials over many length scales. Here polymer materials that make use of competing molecular interactions are summarized, and the prospects for the further development of such materials through both synthetic and processing pathways are highlighted.</jats:p>

<jats:p>Multilayer films of organic compounds on solid surfaces have been studied for more than 60 years because they allow fabrication of multicomposite molecular assemblies of tailored architecture. However, both the Langmuir-Blodgett technique and chemisorption from solution can be used only with certain classes of molecules. An alternative approach—fabrication of multilayers by consecutive adsorption of polyanions and polycations—is far more general and has been extended to other materials such as proteins or colloids. Because polymers are typically flexible molecules, the resulting superlattice architectures are somewhat fuzzy structures, but the absence of crystallinity in these films is expected to be beneficial for many potential applications.</jats:p>

<jats:p>A systems approach that integrates processing, structure, property, and performance relations has been used in the conceptual design of multilevel-structured materials. For high-performance alloy steels, numerical implementation of materials science principles provides a hierarchy of computational models defining subsystem design parameters that are integrated, through computational thermodynamics, in the comprehensive design of materials as interactive systems. Designed properties combine strength, toughness, and resistance to impurity embrittlement. The methods have also been applied to nonferrous metals, ceramics, and polymers.</jats:p>

<jats:p>Organic molecules can alter inorganic microstructures, offering a very powerful tool for the design of novel materials. In biological systems, this tool is often used to create microstructures in which the organic manipulators are a minority component. Three groups of materials—biomaterials, ceramics, and semiconductors—have been selected to illustrate this concept as used by nature and by synthetic laboratories exploring its potential in materials technology. In some of nature's biomaterials, macromolecules such as proteins, glycoproteins, and polysaccharides are used to control nucleation and growth of mineral phases and thus manipulate microstructure and physical properties. This concept has been used synthetically to generate apatite-based materials that can function as artificial bone in humans. Synthetic polymers and surfactants can also drastically change the morphology of ceramic particles, impart new functional properties, and provide new processing methods for the formation of useful objects. Interesting opportunities also exist in creating semiconducting materials in which molecular manipulators connect quantum dots or template cavities, which change their electronic properties and functionality.</jats:p>

<jats:p>Polymeric materials undergo dramatic changes in orientational order in response to dynamic processes, such as flow. Their rich cascade of dynamics presents opportunities to create and combine distinct alignments of polymeric nanostructures through processing. In situ rheo-optical measurements complemented by ex situ x-ray scattering reveal the physics of three different trajectories to macroscopic alignment of lamellar diblock copolymers during oscillatory shearing. At the highest frequencies, symmetry arguments explain the transient development of a bimodal texture en route to the alignment of layers parallel to the planes of shear. At lower frequencies, larger-scale relaxations introduce rearrangements out of the deformation plane that permit the formation of lamellae perpendicular to the shear plane. These explain the change in the character of the pathway to parallel alignment and the emergence of perpendicular alignment as the frequency decreases.</jats:p>

<jats:p> <jats:bold>A Tale of Two Continents.</jats:bold> A Physicist's Life in a Turbulent World. ABRAHAM PAIS. Princeton University Press, Princeton, NJ, 1997. xvi, 511 pp. + plates. $35 or £25. ISBN 0-691-01243-1. </jats:p>

<jats:p> <jats:bold>Selection.</jats:bold> The Mechanism of Evolution. GRAHAM BELL. Chapman and Hall, New York, 1996. xxvi, 699 pp., illus. $75 or £55. ISBN 0-412-05521-x. </jats:p> <jats:p> <jats:bold>The Basics of Selection.</jats:bold> GRAHAM BELL. Chapman and Hall, New York, 1996. xxii, 378 pp., illus. Paper, $37.50 or £24.99. ISBN 0-412-05531-7. Briefer edition of Selection: The Mechanism of Evolution. </jats:p>