Microstructure Evolution and Fretting Wear Mechanisms of Steels Undergoing Oscillatory Sliding Contact in Dry Atmosphere.

Variations in the microstructure and the dominant fretting wear mechanisms of carbon steel alloy in oscillatory sliding contact against stainless steel in a dry atmosphere were evaluated by various mechanical testing and microanalytical methods. These included scanning electron microscopy and energy dispersive spectrometry with corresponding elemental maps of the wear tracks, in conjunction with cross-sectional transmission electron microscopy of samples prepared by focused ion beam machining to assess subsurface and through-thickness changes in microstructure, all as a function of applied load and sliding time. Heavily dislocated layered microstructures were observed below the wear tracks to vary with both the load and sliding time. During the accumulation of fretting cycles, the subsurface microstructure evolved into stable dislocation cells with cell walls aligned parallel to the surface and the sliding direction. The thickness of the damaged subsurface region increased with the load, consistent with the depth distribution of the maximum shear stress. The primary surface oxide evolved as Fe2O3 and Fe3O4 with increasing sliding time, leading to the formation of a uniform oxide scale at the sliding surface. It is possible that the development of the dislocation cell structure in the subsurface also enhanced oxidation by pipe diffusion along dislocation cores. The results of this study reveal complex phase changes affecting the wear resistance of steels undergoing fretting wear, which involve a synergy between oxidative wear, crack initiation, and crack growth along dislocation cell walls due to the high strains accumulating under high loads and/or prolonged surface sliding.

Mechanisms of soft tissue and protein preservation in Tyrannosaurus rex.

The idea that original soft tissue structures and the native structural proteins comprising them can persist across geological time is controversial, in part because rigorous and testable mechanisms that can occur under natural conditions, resulting in such preservation, have not been well defined. Here, we evaluate two non-enzymatic structural protein crosslinking mechanisms, Fenton chemistry and glycation, for their possible contribution to the preservation of blood vessel structures recovered from the cortical bone of a Tyrannosaurus rex (USNM 555000 [formerly, MOR 555]). We demonstrate the endogeneity of the fossil vessel tissues, as well as the presence of type I collagen in the outermost vessel layers, using imaging, diffraction, spectroscopy, and immunohistochemistry. Then, we use data derived from synchrotron FTIR studies of the T. rex vessels to analyse their crosslink character, with comparison against two non-enzymatic Fenton chemistry- and glycation-treated extant chicken samples. We also provide supporting X-ray microprobe analyses of the chemical state of these fossil tissues to support our conclusion that non-enzymatic crosslinking pathways likely contributed to stabilizing, and thus preserving, these T. rex vessels. Finally, we propose that these stabilizing crosslinks could play a crucial role in the preservation of other microvascular tissues in skeletal elements from the Mesozoic.

Growth behaviour of well-aligned ZnO nanowires on a Si substrate at low temperature and their optical properties.

Well-aligned ZnO nanowires were successfully synthesized on a silicon substrate at the low temperature of 550 degrees C by catalyst-free vapour phase deposition. The ZnO nanowires had diameters in the range of 70-100 nm and lengths over several tens of micrometres. The synthesized ZnO nanowires, which had a single-crystalline wurtzite structure, showed a uniform morphology and faceted planes at the tips of the nanowires. The photoluminescence of the ZnO nanowires showed a strong UV band at 3.28 eV and a broad green band at 2.29 and at 2.53 eV at room temperature. A detailed discussion regarding the growth behaviour and the growth mechanism of the ZnO nanowires on the silicon substrate is presented in this work.

Directed assembly of controlled-misorientation bicrystals.

Grain boundaries play a vital role in determining materials behaviour, and the nature of these intercrystalline interfaces is dictated by chemical composition, processing history, and geometry (misorientation and inclination). The interrelation among these variables and material properties may be systematically studied in bicrystals. Conventional bicrystal fabrication offers control over these variables, but its ability to mimic grain boundaries in polycrystalline materials is ambiguous. Here we describe a novel solid-state process for rapidly generating intercrystalline interfaces with controlled geometry and chemistry, applicable to a broad range of materials. A fine-grained polycrystalline layer, contacted by two appropriately misoriented single-crystal seeds, is consumed by an epitaxial solid-state transformation until the directed growth fronts impinge. The seed misorientations establish the geometry of the resulting intercrystalline boundaries, and the composition of the sacrificial polycrystalline layer establishes the chemistry of the boundaries and their adjacent grains. Results from a challenging model system, titanium-doped sapphire, illustrate the viability of the directed assembly technique for preparing high-quality bicrystals in both twist and tilt configurations.

The electrodeposition of high-density, ordered arrays of Bi1-xSbx nanowires.

Here we report the synthesis of dense arrays of Bi1-xSbx nanowires with >5 x 1010 nanowires/cm2. The individual wires are crystalline, relatively homogeneous, and highly textured in a 110 direction, with diameters of 40 nm and a composition of x = 12-15 atom % Sb. By tuning the solution concentrations and controlling the growth rate by controlling the potential, the composition, crystallinity, and morphology of the nanowires can be varied.