Deformation twinning in a nanocrystalline hcp Mg alloy

Nanocrystalline (nc) hexagonal close-packed (hcp) metals are rarely observed to deform by twinning, which is contrary to face-centered cubic metals. Here we report that, after alloying Mg with 10 at% Ti, deformation twins are observed in an nc Mg Ti alloy processed by mechanical attrition. The formation of deformation twins is attributed to the alloying effect, which may change the energy path for twinning. These results point to a promising approach to design nc hcp alloys for superior mechanical properties. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Dislocation-twin interactions in nanocrystalline fcc metals

Dislocation interaction with and accumulation at twin boundaries have been reported to significantly improve the strength and ductility of nanostructured face-centered cubic (fcc) metals and alloys. Here we systematically describe plausible dislocation interactions at twin boundaries. Depending on the characteristics of the dislocations and the driving stress, possible dislocation reactions at twin boundaries include cross-slip into the twinning plane to cause twin growth or de-twinning, formation of a sessile stair-rod dislocation at the twin boundary, and transmission across the twin boundary. The energy barriers for these dislocation reactions are described and compared. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Microstructure and mechanical properties at different length scales and strain rates of nanocrystalline tantalum produced by high-pressure torsion

Fully dense, nanocrystalline tantalum (average grain size as small as similar to 40 nm) has been processed for the first time by high-pressure torsion. High-resolution transmission electron microscopy reveals non-equilibrium grain boundaries and grains decorated with high-density dislocations. Microhardness measurements and instrumented nanoindentation experiments indicate that the mechanical property is quite uniform except for the central area of the disks. Nanoindentation experiments at different strain rates suggest that the strain rate sensitivity of nanocrystalline tantalum is increased compared to the coarse- and ultrafine-grained counterparts and is accompanied by an activation energy of the order of a few similar to b(3) (b is the magnitude of the dislocation Burgers vector), implying a shift in the plastic deformation mechanism from the screw dislocation dominated regime. We thus infer the plastic deformation mechanisms of nanocrystalline body-centered cubic (bcc) and face-centered cubic metals converge. To examine the stress strain behavior, we have used microcompression to measure the compressive stress strain curves on microscale pillars fabricated by focused ion beam technique. Yield strength as high as 1.6 GPa has been observed. High-strain rate behavior has been investigated using a miniature Kolsky bar system. We have found that at high-strain rates the nanocrystalline tantalum specimens exhibit adiabatic shear banding, a dynamic plastic deformation mode common to many ultrafine-grained and all nanocrystalline bcc metals. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Cross-boundary Substance Uses Among Hong Kong Chinese Young Adults

The study documents the characteristics of Hong Kong residents who used substances in mainland China. Characteristics of such users were compared with those using substances elsewhere. A cross-sectional survey was used. Data analyses were stratified by gender, and multivariate analyses were performed. Respondents, Hong Kong adults aged 18–30 (n = 6,528), were interviewed using a mixed interviewer and computerized mobile phone method, at the Hong Kong-mainland China boundary. Cross-border substance users and other types of substance users had characteristics that were quite different (e.g., education level, perceived harmfulness of substance use in general, having friends who frequently used substances in mainland China). Cross-boundary substance users also found it easier to obtain substances in mainland China than in Hong Kong (males: 60.5%, females: 39.6%) and perceived no chance of being arrested in mainland China because of substance use (men: 28.8%, women: 24.5%). They tended to use substances in discos/rave parties together with friends (men: 77%, women: 81%), and substances were often obtained for free (men: 63.2%, women: 78%). Ecstasy and ketamine were often used (men: 86.5% & 79.4%, women: 81% & 70.2%), and multiple uses of different substances were common (men: 81.3%, women: 68.6%). Lower price, peer influence, and better availability were reasons behind cross-boundary substance use behaviors. Cross-boundary substance users have specific characteristics. Prevention policies should take these characteristics into account. Regional collaborations are highly warranted