Tapered nanopillar structures of different cross-sectional geometries including cone-, pencil-like, and stepwise are prepared from anodized aluminum oxide templates. The shape effect on the adhesion strength is investigated in experiments and simulation. Cone-shaped nanopillars are highly bendable under load and can recover after unloading, thus, warranting high adhesion strength, 34 N cm(-2) . The pencil-like and stepwise nano-pillars are, however, easily fractured and are not recoverable under the same conditions.

Magnesium (Mg) and its alloys hold great potential as an energy-saving structural material for automative, aerospace applications. However, the use of Mg alloys has been limited due to poor ductility and formability. Poor mechanical properties of Mg alloys origin from the insufficient number of slip systems, and deformation twinning plays an important role to accommodate plastic deformation. Here, we report a comprehensive experimental and modeling study to understand crystal size effect on the transformation in deformation modes in twin oriented Mg single crystals. The experiments reveal two regimes of size effects: (1) single twin propagation, where a typical "smaller the stronger" behavior was dominant in pillars ≤ 18 µm in diameter, and (2) twin-twin interaction, which results in anomalous strain hardening in pillars > 18 µm. Molecular dynamics simulations further indicate a transition from twinning to dislocation mediated plasticity for crystal sizes below a few hundred nanometers. Our results provide new understanding of the fundamental deformation modes of twin oriented Mg from nano-scale to bulk, and insights to design Mg alloys with superior mechanical properties through dimensional refinement. This subsequently can materialize into more utilization of Mg alloys as a structural material in technologically relevant applications.

The large fraction of material residing at grain boundaries in nanocrystalline metals and alloys is responsible for their ultrahigh strength, but also undesirable microstructural instability under thermal and mechanical loads. However, the underlying mechanism of stress-driven microstructural evolution is still poorly understood and precludes rational alloy design. Here we combine quantitative in situ electron microscopy with three-dimensional atom-probe tomography to directly link the mechanics and kinetics of grain boundary migration in nanocrystalline Al films with the excess of O atoms at the boundaries. Site-specific nanoindentation leads to grain growth that is retarded by impurities, and enables quantification of the critical stress for the onset of grain boundary migration. Our results show that a critical excess of impurities is required to stabilize interfaces in nanocrystalline materials against mechanical driving forces, providing new insights to guide control of deformation mechanisms and tailoring of mechanical properties apart from grain size alone.

The relationships between microstructure, controlled by alloying elements prone to grain boundary segregation, and electrical resistivity in sputtered nanocrystalline Cu were investigated. We find a non-monotonic dependence of the mean grain size on solute concentration for both Cu-Nb and Cu-Fe dilute alloys, with a concentration regime where the grain size increases over that of pure Cu before refining with further alloying. The electrical resistivity follows the same trend, suggesting a non-equilibrium processing route that remarkably gives rise to dilute nanocrystalline Cu alloys with lower resistivity, thermal stability, and enhanced mechanical properties relative to their pure nanocrystalline counterpart. Nanocrystalline (NC) metals have been the subject of intense research activity, driven largely by technological interests in their high hardness and strength. The results from decades of experiments and simulations point to the governing role of deformation physics unique to its coarse-grained counterparts, including grain boundary (GB) sliding, nucleation of dislocations from GBs and their subsequent isolated propagation, GB rotation, and stress-assisted grain growth [1,2]. Owing to the large volume fraction of material in near-GB regions in nanocrystalline metals, the properties of these materials are governed by interfacial phenomena.

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