Triple and double twin interfaces in magnesium-the role of disconnections and facets.

Twin boundaries have been shown to deviate from the twinning planes in hcp metals, and facets have often been observed in twin interfaces. This study presents a twinning disconnection-based model for faceting in single, double and triple twin boundaries in magnesium. Primary twinning disconnections predicted via symmetry arguments are shown to produce commensurate facets in single twin boundaries, which are then transformed into commensurate facets in double twin boundaries via the action of secondary twinning disconnections. In contrast, it is shown that for triple twin boundaries with tension-compression-tension twinning sequence, no commensurate facets can be produced by the action of tertiary twinning disconnections. The effect of facets on the macroscopic orientation of twin interfaces is discussed. Theoretical findings are validated by a transmission electron microscopy study of a hot rolled Mg-1.18wt%Al-1.77wt%Nd alloy. Single and double twins are observed, as well as rare triple twins, and the interface between the matrix and a triple twin is captured for the first time. Facets consistent with theoretical predictions are imaged via high-resolution TEM and macroscopic deviations of the boundaries from the primary twinning planes are measured.

The influence of hydrogen on plasticity in pure iron-theory and experiment.

Tensile stress relaxation is combined with transmission electron microscopy to reveal dramatic changes in dislocation structure and sub structure in pure α-Fe as a result of the effects of dissolved hydrogen. We find that hydrogen charged specimens after plastic deformation display a very characteristic pattern of trailing dipoles and prismatic loops which are absent in uncharged pure metal. We explain these observations by use of a new self consistent kinetic Monte Carlo model, which in fact was initially used to predict the now observed microstructure. The results of this combined theory and experimental study is to shed light on the fundamental mechanism of hydrogen enhanced localised plasticity.

Spatially Controlled Occlusion of Polymer-Stabilized Gold Nanoparticles within ZnO.

In principle, incorporating nanoparticles into growing crystals offers an attractive and highly convenient route for the production of a wide range of novel nanocomposites. Herein we describe an efficient aqueous route that enables the spatially controlled occlusion of gold nanoparticles (AuNPs) within ZnO crystals at up to 20 % by mass. Depending on the precise synthesis protocol, these AuNPs can be (i) solely located within a central region, (ii) uniformly distributed throughout the ZnO host crystal or (iii) confined to a surface layer. Remarkably, such efficient occlusion is mediated by a non-ionic water-soluble polymer, poly(glycerol monomethacrylate)70 (G70 ), which is chemically grafted to the AuNPs; pendent cis-diol side groups on this steric stabilizer bind Zn2+ cations, which promotes nanoparticle interaction with the growing ZnO crystals. Finally, uniform occlusion of G70 -AuNPs within this inorganic host leads to faster UV-induced photodegradation of a model dye.

Segregation mediated heterogeneous structure in a metastable ß titanium alloy with a superior combination of strength and ductility.

In ß titanium alloys, the ß stabilizers segregate easily and considerable effort has been devoted to alleviate/eliminate the segregation. In this work, instead of addressing the segregation problems, the segregation was utilized to develop a novel microstructure consisting of a nanometre-grained duplex (α+ß) structure and micrometre scale ß phase with superior mechanical properties. An as-cast Ti-9Mo-6W alloy exhibited segregation of Mo and W at the tens of micrometre scale. This was subjected to cold rolling and flash annealing at 820 oC for 2 and 5 mins. The solidification segregation of Mo and W leads to a locally different microstructure after cold rolling (i.e., nanostructured ß phase in the regions rich in Mo and W and plate-like martensite and ß phase in regions relatively poor in Mo and W), which play a decisive role in the formation of the heterogeneous microstructure. Tensile tests showed that this alloy exhibited a superior combination of high yield strength (692 MPa), high tensile strength (1115 MPa), high work hardening rate and large uniform elongation (33.5%). More importantly, the new technique proposed in this work could be potentially applicable to other alloy systems with segregation problems.

Three-dimensional structure analysis and percolation properties of a barrier marine coating.

Artificially structured coatings are widely employed to minimize materials deterioration and corrosion, the annual direct cost of which is over 3% of the gross domestic product (GDP) for industrial countries. Manufacturing higher performance anticorrosive coatings is one of the most efficient approaches to reduce this loss. However, three-dimensional (3D) structure of coatings, which determines their performance, has not been investigated in detail. Here we present a quantitative nano-scale analysis of the 3D spatial structure of an anticorrosive aluminium epoxy barrier marine coating obtained by serial block-face scanning electron microscopy (SBFSEM) and ptychographic X-ray computed tomography (PXCT). We then use finite element simulations to demonstrate how percolation through this actual 3D structure impedes ion diffusion in the composite materials. We found the aluminium flakes align within 15° of the coating surface in the material, causing the perpendicular diffusion resistance of the coating to be substantially higher than the pure epoxy.