A potential order-of-magnitude increase in Hall-Petch (H-P)-based strength level for nanoscale grain-size structures is an important enabler of electronic thin film material design applications. Dislocation pileups of smaller lengths in such thin film materials are blocked in a screw orientation at the through-thickness grain boundaries of relatively larger grains. For fully nanopolycrystalline materials, both strength and strain rate sensitivity measurements exhibit complementary H-P reciprocal square root of grain size dependencies. An additional increase in strength level is predicted for transition from a pileup to a single dislocation loop expanding against the grain boundary obstacle. In opposition, disordered grain boundaries are responsible for a reduced H-P stress intensity, kε. And at the limiting high stresses reached at lower-limiting nanoscale grain sizes, reversed H-P dependences are obtained both for the strength and strain rate sensitivity.

We examine the development of stable bimetal interfaces in nanolayered composites in severe plastic deformation. Copper-niobium multilayers of varying layer thicknesses from several micrometers to 10 nanometers (nm) were fabricated via accumulative roll bonding (ARB). Investigation of their 5-parameter character and atomic scale structure finds that when layer thicknesses refine well below one micrometer, the interfaces self-organize to a few interface orientation relationships. With atomic scale and crystal plasticity modeling, we identify that the two controlling factors that determine whether an interface is stable under high strain rolling are orientation stability of the bicrystal and interface formation energy. A figure-of-merit is introduced that not only predicts the development of the prevailing interfaces but also explains why other interfaces did not develop. Through a suite of nanomechanical and bulk test results, we show that ARB composites containing these stable interfaces are found to have exceptional hardness (∼4.5 GPa) and strength (∼2 GPa).

Nickel-yttrium nanocrystalline alloys with an as-milled grain size of approximately 6.5 nm were synthesized using high-energy cryogenic mechanical alloying. The microstructural changes due to annealing were characterized using x-ray line broadening, microhardness, focused ion beam channeling contrast imaging, and transmission electron microscopy. Experiments demonstrated that increasing yttrium content led to stabilization of the nanocrystalline grain size at elevated homologous annealing temperatures. Additionally, it was found that inadvertent contamination with nitrogen during the milling process caused the formation of yttrium nitride (YN) precipitates, which, in turn, resulted in an additional nonlinear hardening effect beyond the expected hardening due to grain-size reduction. Results reveal that kinetic pinning by YN particles is effective in retaining a nanostructure to relatively high temperatures.

We report high-resolution transmission electron microscopy (HRTEM) observation of a high density of dislocations with edge components (∼1016 m−2) in nanocrystalline (NC) body-centered cubic (bcc) Mo prepared by high-pressure torsion. We also observed for the first time of the ½<111> and <001> pure edge dislocations in NC Mo. Crystallographic analysis and image simulations reveal that the best way using HRTEM to study dislocations with edge components in bcc systems is to take images along <110> zone axis, from which it is possible to identify ½<111> pure edge dislocations, and edge components of ½<111> and <001> mixed dislocations. The <001> pure edge dislocations can only be identified from <100> zone axis. The high density of dislocations with edge components is believed to play a major role in the reduction of strain rate sensitivity in NC bcc metals and alloys.

Deformation mechanism maps are well established in the field of high temperature creep for materials having conventional coarse grain sizes but they are almost unknown within the field of nanostructured materials. This paper summarizes the background to deformation mechanism mapping, presents simplified examples that may be used to easily construct appropriate maps for any selected condition, demonstrates the potential extension of this approach to other areas such as creep fracture, and then considers the potential limitations associated with using the same approach to predict the deformation mechanisms in true nanostructured materials. Two representative deformation mechanism maps are shown for an ultrafine-grained alloy processed either by equal-channel angular pressing or by high-pressure torsion.

We have investigated the microstructure, the quasistatic and high-rate mechanical properties of magnesium (Mg)-based metal-matrix composites (MMCs) reinforced with nanoparticles, also termed as metal-matrix nanocomposites (MMNCs), in this case reinforced with nanoparticles of β-phase silicon carbide (β-SiC) the volume fraction ranging from 5 to 15 vol%. The yield and the ultimate strength increase with reinforcement volume fraction up to 10 vol% nanoparticles. MMCs with micrometer-sized SiC particles have higher yield strength than their MMNC counterparts, whereas the ultimate strength shows the opposing trend, suggesting greater strain hardening in the MMNCs. Transmission electron microscopy shows that the average interparticle distance decreases with increasing SiC vol%. Recrystallization was reported as completed during sintering at 575 °C [R.D. Doherty et al., Mater. Sci. Eng. A, 238, 219 (1997)], but dislocations might be generated due to thermal expansion mismatch of Mg/SiC during cooling. The majority of Mg-grains below 20 nm remain around the nanoparticles. As such a reverse volume fraction effect takes place in 15 vol% nanoparticle-reinforced MMNCs, which off sets the strengthening advantage induced by the nanoparticles.

Retention of a nanostructure in thermoelectric materials through rapid sintering (e.g., field-assisted sintering) is generally associated with leaving certain amounts of porosity due to short sintering times. In this study, the influence of porosity on the thermoelectric transport properties in Bi2Te3-based alloys was studied by changing the sintering pressure during spark plasma sintering. N-type Bi2Te3 and p-type (Bi0.2Sb0.8)2Te3 were sintered at 673 K using pressures from 50 to 300 MPa to obtain different levels of porosity. Electrical resistivity, thermal conductivity, Seebeck coefficient, carrier concentrations, and Hall mobility were measured and characterized. The results show that increasing sintering pressure is effective in reducing porosity, which lowers electrical resistivity and increases the carrier concentrations. The transport properties were fitted to general effective medium equations and demonstrate that in p-type (Bi0.2Sb0.8)2Te3 sintered at high pressures, decreases in electrical resistivity and lattice thermal conductivity exceeded the Seebeck coefficient reduction, improving the thermoelectric figure of merit.

The present work reports a soft-chemical pathway for preparing SnO–TiO2 composite nanocrystallites as photocatalyst through co-hydrolysis of tetrabutyl titanate and tin (II) chloride followed by acidic peptization of the hydrolysate under mild conditions. The procedure is simple and straightforward, from which a well-dispersed semitransparent hydrosol sample is obtained. The freestanding nanocrystallites observed in the as-prepared composite show diameters of 3–5 nm. TiO2 nanoparticles have almost entirely transformed into anatase phase, and the trace amounts of Sn in existence are mainly found in SnO crystals with tetragonal structure. The photocatalytic activity of the SnO–TiO2 composites is confirmed through the photodegradation of methyl blue dye under visible light irradiation (λ > 420 nm). As a p-type semiconductor, the incorporated SnO effectively improves the photocatalytic activity of TiO2 through promoting the separation of photo-generated charge carriers, inhibiting their recombination, and facilitating the reduction of O2 by the photo-generated electrons.

High-performance and continuous zeolite MFI membranes have been successfully fabricated, using in situ hydrothermal synthetic method, on α-Al2O3 hollow ceramic fifibers (HCFs). The CO2 separation properties of the as-prepared MFI membrane are studied by single gas permeation and binary gas permeation of CO2/N2 and CO2/CH4. The separation results show that the membrane exhibits high CO2 selectivity with separation factors of 9.2 and 6.0 for CO2/N2 and CO2/CH4, respectively. A preferred permeance for CO2 in the binary gas mixtures is about 3 × 10−7mol/(m2 s Pa). Furthermore, the supported MFI membrane possesses high mechanical strength, strong thermal stability, and high reproducibility, which are expected to have potential applications in industrial CO2 recycling.