Lamella (surface anatomy)

About: Lamella (surface anatomy) is a research topic. Over the lifetime, 1855 publications have been published within this topic receiving 29025 citations.

Heterogeneous lamella structure unites ultrafine-grain strength with coarse-grain ductility.

Abstract: Grain refinement can make conventional metals several times stronger, but this comes at dramatic loss of ductility. Here we report a heterogeneous lamella structure in Ti produced by asymmetric rolling and partial recrystallization that can produce an unprecedented property combination: as strong as ultrafine-grained metal and at the same time as ductile as conventional coarse-grained metal. It also has higher strain hardening than coarse-grained Ti, which was hitherto believed impossible. The heterogeneous lamella structure is characterized with soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix. The unusual high strength is obtained with the assistance of high back stress developed from heterogeneous yielding, whereas the high ductility is attributed to back-stress hardening and dislocation hardening. The process discovered here is amenable to large-scale industrial production at low cost, and might be applicable to other metal systems.

Molecular model of drawing polyethylene and polypropylene.

Abstract: Morphological studies of plastic deformation of single crystals, thin layers, and bulk samples together with mechanical, X-ray and infra-red data revealed the existence of three stages in cold drawing of crystalline polymer: the plastic deformation of the original spherulitic structure, the discontinuous transformation of the spherulitic into fibre structure by micronecking, and the plastic deformation of the fibre structure The initial material, which has low strength and high ductility, consist of stacks of parallel lamellae with few interlamella links It deforms plastically by stack rotation, sliding of lamellae, phase change and twinning of crystal lattice, chain slip and tilt until the predeformed lamellae reach the position of maximum compliance for fracture by micronecking The micronecks transform every single lamella into microfibrils of between one and three hundred angstroms in width, consisting of folded chain blocks broken off the lamella primarily by chain slip in the boundary layers between adjacent mosaic blocks The chains bridging the crack are partially unfolded during the micronecking process They connect in axial direction the blocks in the microfibril as intrafibrillar tie molecules The number of microfibrils per cm of crack length increases with molecular weight The draw ratio of the microfibrils and the axial separation in the microfibril of the originally adjacent crystal blocks increase with the average distance between microfibrils and, hence, decrease with increasing molecular weight The concentration of micronecks in every stack of lamellae in a thin destruction zone produces a bundle of microfibrils of rather uniform draw ratio Such a fibril measuring a few thousand angstroms in width includes the interlamella ties of the original sample as interfibrillar tie molecules connecting adjacent microfibrils The concentration of micronecks also provides the conditions for a nearly adiabatic heating of the generated fibril by the transformation work in the destruction zone The local temperature rise imparts so much mobility to the chains in the crystal blocks that during subsequent cooling to ambient temperature, the long period becomes adjusted to this temperature The more or less random distribution of destruction zones in the neck makes the transformation from spherulitic to fibre structure appear to be a gradual process in spite of the discontinuous transformation in the micronecks The plastic deformation of the new fibre structure can proceed only by longitudinal sliding of microfibrils past each other, a process limited by interfibrillar tie molecules Hence, high molecular weight samples with many interlamella links exhibit a smaller draw ratio than lower molecular weight material The three stages are to some extent intermixed in the neck In the initial neck characterised by a low draw ratio and rather gentle constriction, the transformation into the fibre structure is not complete, so that some of the remains of the original microspherulitic structure are still present in the necked portion They are destroyed during subsequent drawing which completes the transformation and also deforms the fibre structure The sharply constricted mature neck, however, yields a high draw ratio which is composed of the draw ratio of microfibrils and of subsequent sliding motion of the microfibrils The technically important natural draw ratio is the maximum draw ratio obtained with the sample under the conditions of the experiment It seems to be higher than the draw ratio of the microfibrils

Bone structure: from angstroms to microns.

Abstract: Bone has a complex hierarchical structure, which despite much investigation, is still not well understood. Here we bring together pieces of this complicated puzzle, albeit from different sources, to present a tentative overview of bone structure. The basic building blocks are the extremely small plate-shaped crystals of carbonate apatite, just hundreds of angstroms long and wide and some 20-30 A thick. They are arranged in parallel layers within the collagenous framework. At the next hierarchical level these mineral-filled collagen fibrils are ordered into arrays in which the fibril axes and the crystal layers are all organized into a 3-dimensional structure that makes up a single layer or lamella of bone a few microns thick. The orientations of the collagen fibrils and the crystal layers in alternating lamellae of rat bone differ such that in the thinner lamellae, the fibrils and the crystal layers are parallel to the lamellar boundaries. In the thicker lamellae the fibrils are parallel to the boundary, but the crystal layers are rotated out of the plane of the boundary. In many bones these alternating lamellae are organized into even larger ordered structures to produce what is truly a remarkably ordered material, all the way from the molecular scale to the macroscopic product.

Phase behavior in thin films of cylinder-forming block copolymers.

Abstract: We have experimentally determined a phase diagram for cylinder-forming polystyrene-block-polybutadien-block-polystyrene triblock copolymer in thin films. The phase behavior can be modeled in great detail by dynamic density functional theory. Deviations from the bulk structure, such as wetting layer, perforated lamella, and lamella, are identified as surface reconstructions. Their stability regions are determined by an interplay between surface fields and confinement effects.

Supramolecular self-assembly of macroscopic tubes.

Abstract: The macroscopic molecular self-assembly of an amphiphilic hyperbranched copolymer in acetone generated multiwalled tubes millimeters in diameter and centimeters in length. The thickness of the tube walls approaches 400 nanometers, and the walls have an inhomogeneous lamella structure that alternates between ordered hydrophilic domains and amorphous, partly irregular hydrophilic domains.