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ABSTRACT: Nanostructured materials are faraway from thermodynamic equilibrium and since they have short transport pathways, fast diffusion and rapid transformation kinetics often lead to coarsening and deterioration of the microstructure and the associated properties. Thus, ensuring the stability of the nanoscale structures, for example by utilizing a composite approach, is a key issue. This approach, however, imposes additional constraints on the stability of phases and phase mixtures, and on phase transformations within the nanoscale structural units due to size confinement and the presence of heterophase interfaces. One important aspect concerns the thermochemical phase equilibrium that is valid at the reduced size of the particles or grains. The melting transformation is considered here as a representative transformation that allows in-depth studies by experiment and analytical theory. It was found, that the atomic-level structure of the interfaces had a dramatic impact on the thermodynamics of the phase transformations and that accounting for the presence of the interfaces modified even the commonly accepted rules for constructing constitutional alloy phase diagrams. These issues are to be considered generally for systems with a large specific interface area.
03/2009: pages 87 - 107; , ISBN: 9783527624041
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ABSTRACT: Al–Pb ribbons containing 1 at.% Pb have been produced by melt-spinning and subsequently investigated by high-resolution transmission electron microscopy. It is shown that the lattice mismatch of about 22% between the nanometre-sized Pb inclusions and the surrounding Al matrix is accommodated by a periodic array of misfit dislocations at the Al–Pb interface. The closing failures of Burgers circuits drawn around misfit dislocations on {111} and {100} facets identify the corresponding Burgers vectors as (a 0/4) 211 and (a 0/2) 110, respectively. The Burgers vector of (a 0/4) 211 corresponds to the projected edge part of a 60° (a 0/2) 110 dislocation. The Pb inclusions themselves appear to be free of defects.
Philosophical Magazine Letters 11/2004; 84(11):673-683. · 1.24 Impact Factor
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ABSTRACT: Defect structures of plastically deformed nanocrystalline Pd investigated by high-resolution transmission electron microscopy are presented. Material with an average grain size of about 15 nm was prepared by inert-gas condensation, and this was plastically deformed by cold rolling up to a true strain of 0.32 at a strain rate of about 0.3 s−1. Abundant deformation twinning on {111} planes was found and Shockley partial dislocations identified. Remarkably, in each grain, twinning occurs only on a single set of parallel planes. This implies that only one out of the five independent slip systems required for the general deformation of a grain is active, a finding which suggests that grain rotation and grain-boundary sliding must be active together with twinning.
Philosophical Magazine Letters 05/2004; 84(5):321-334. · 1.24 Impact Factor
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ABSTRACT: Nanoporous metals prepared by alloy corrosion may assume the form of monolithic, millimeter-sized bodies containing around 1015 nanoscale ligaments per cubic millimeter. Here, we report on the fabrication and mechanical behavior of macroscopic, crack-free nanoporous gold samples which exhibit excellent ductility in compression tests. Their yield stress is significantly lower than that expected based on scaling laws or on previous nanoindentation experiments. Electron backscatter diffraction imaging reveals a polycrystalline microstructure with grains larger than 10 μm which acquire a subdomain structure during plastic flow, but remain otherwise intact. We highlight the action of lattice dislocations which can travel over distances much larger than the ligament size. This results in a collective deformation of the many ligaments in each grain. Remarkably, the dislocation cores are partly located in the pore channels. The results suggest a critical view of the conversion between indentation hardness and yield stress in previous work.
Acta Materialia. 57(9):2665-2672.
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ABSTRACT: With nanostructured matter advancing from laboratory samples to application-relevant or even applied materials, basic and prevalent issues related to the stability of the nanoscale structures against coarsening need to be addressed. The assembly of nano-composites, where isolated nanoscaled particulates or fibrous components are encased in an inert matrix, gives one important processing option that stabilizes the microstructure against coarsening. However, the impact of heterophase boundaries between the nanoscale structures and the matrix or also between different phases in multicomponent nanoscale systems on the product properties yet remain to be understood.
Journal of Alloys and Compounds · 2.29 Impact Factor
