Grain boundary-mediated plasticity in nanocrystalline nickel

The plastic behavior of crystalline materials is mainly controlled by the nucleation and motion of lattice dislocations. We report in situ dynamic transmission electron microscope observations of nanocrystalline nickel films with an average grain size of about 10 nanometers, which show that grain boundary-mediated processes have become a prominent deformation mode. Additionally, trapped lattice dislocations are observed in individual grains following deformation. This change in the deformation mode arises from the grain size-dependent competition between the deformation controlled by nucleation and motion of dislocations and the deformation controlled by diffusion-assisted grain boundary processes.     

Atomic-level observation of disclination dipoles in mechanically milled, nanocrystalline Fe

Plastic deformation of materials occurs by the motion of defects known as dislocations and disclinations. High-resolution transmission electron microscopy was used to directly reveal the individual dislocations that constitute partial disclination dipoles in nanocrystalline, body-centered cubic iron that had undergone severe plastic deformation by mechanical milling. The mechanisms by which the formation and migration of such partial disclination dipoles during deformation allow crystalline solids to fragment and rotate at the nanometer level are described. Such rearrangements are important basic phenomena that occur during material deformation, and hence, they may be critical in the formation of nanocrystalline metals by mechanical milling and other deformation processes.     

Three-dimensional orientation mapping in the transmission electron microscope

Over the past decade, efforts have been made to develop nondestructive techniques for three-dimensional (3D) grain-orientation mapping in crystalline materials. 3D x-ray diffraction microscopy and differential-aperture x-ray microscopy can now be used to generate 3D orientation maps with a spatial resolution of 200 nanometers (nm). We describe here a nondestructive technique that enables 3D orientation mapping in the transmission electron microscope of mono- and multiphase nanocrystalline materials with a spatial resolution reaching 1 nm. We demonstrate the technique by an experimental study of a nanocrystalline aluminum sample and use simulations to validate the principles involved.     

Plastic deformation recovery in freestanding nanocrystalline aluminum and gold thin films

In nanocrystalline metals, lack of intragranular dislocation sources leads to plastic deformation mechanisms that substantially differ from those in coarse-grained metals. However, irrespective of grain size, plastic deformation is considered irrecoverable. We show experimentally that plastically deformed nanocrystalline aluminum and gold films with grain sizes of 65 nanometers and 50 nanometers, respectively, recovered a substantial fraction (50 to 100%) of plastic strain after unloading. This recovery was time dependent and was expedited at higher temperatures. Furthermore, the stress-strain characteristics during the next loading remained almost unchanged when strain recovery was complete. These observations in two dissimilar face-centered cubic metals suggest that strain recovery might be characteristic of other metals with similar grain sizes and crystalline packing.     

Near-perfect elastoplasticity in pure nanocrystalline copper

Ductile metals and alloys undergo plastic yielding at room temperature, during which they exhibit work-hardening and the generation of surface instabilities that lead to necking and failure. We show that pure nanocrystalline copper behaves differently, displaying near-perfect elastoplastic behavior characterized by Newtonian flow and the absence of both work-hardening and neck formation. We observed this behavior in tensile tests on fully dense large-scale bulk nanocrystalline samples. The experimental results further our understanding of the unique mechanical properties of nanocrystalline materials and also provide a basis for commercial technologies for the plastic (and superplastic) formation of such materials.     

Plastic deformation with reversible peak broadening in nanocrystalline nickel

Plastic deformation in coarse-grained metals is governed by dislocation-mediated processes. These processes lead to the accumulation of a residual dislocation network, producing inhomogeneous strain and an irreversible broadening of the Bragg peaks in x-ray diffraction. We show that during plastic deformation of electrodeposited nanocrystalline nickel, the peak broadening is reversible upon unloading; hence, the deformation process does not build up a residual dislocation network. The results were obtained during in situ peak profile analysis using the Swiss Light Source. This in situ technique, based on well-known peak profile analysis methods, can be used to address the relationship between microstructure and mechanical properties in nanostructured materials.     

A crossover in the mechanical response of nanocrystalline ceramics

Multimillion-atom molecular dynamics simulation of indentation of nanocrystalline silicon carbide reveals unusual deformation mechanisms in brittle nanophase materials, resulting from the coexistence of brittle grains and soft amorphous grain boundary phases. Simulations predict a crossover from intergranular continuous deformation to intragrain discrete deformation at a critical indentation depth. The crossover arises from the interplay between cooperative grain sliding, grain rotations, and intergranular dislocation formation similar to stick-slip behavior. The crossover is also manifested in switching from deformation dominated by indentation-induced crystallization to deformation dominated by disordering, leading to amorphization. This interplay between deformation mechanisms is critical for the design of ceramics with superior mechanical properties.     

A maximum in the strength of nanocrystalline copper

We used molecular dynamics simulations with system sizes up to 100 million atoms to simulate plastic deformation of nanocrystalline copper. By varying the grain size between 5 and 50 nanometers, we show that the flow stress and thus the strength exhibit a maximum at a grain size of 10 to 15 nanometers. This maximum is because of a shift in the microscopic deformation mechanism from dislocation-mediated plasticity in the coarse-grained material to grain boundary sliding in the nanocrystalline region. The simulations allow us to observe the mechanisms behind the grain-size dependence of the strength of polycrystalline metals.     

Ultrahigh strength in nanocrystalline materials under shock loading

Molecular dynamics simulations of nanocrystalline copper under shock loading show an unexpected ultrahigh strength behind the shock front, with values up to twice those at low pressure. Partial and perfect dislocations, twinning, and debris from dislocation interactions are found behind the shock front. Results are interpreted in terms of the pressure dependence of both deformation mechanisms active at these grain sizes, namely dislocation-based plasticity and grain boundary sliding. These simulations, together with new shock experiments on nanocrystalline nickel, raise the possibility of achieving ultrahard materials during and after shock loading.     

Hardening by annealing and softening by deformation in nanostructured metals

We observe that a nanostructured metal can be hardened by annealing and softened when subsequently deformed, which is in contrast to the typical behavior of a metal. Microstructural investigation points to an effect of the structural scale on fundamental mechanisms of dislocation-dislocation and dislocation-interface reactions, such that heat treatment reduces the generation and interaction of dislocations, leading to an increase in strength and a reduction in ductility. A subsequent deformation step may restore the dislocation structure and facilitate the yielding process when the metal is stressed. As a consequence, the strength decreases and the ductility increases. These observations suggest that for materials such as the nanostructured aluminum studied here, deformation should be used as an optimizing procedure instead of annealing.     

Superplastic extensibility of nanocrystalline copper at room temperature

A bulk nanocrystalline (nc) pure copper with high purity and high density was synthesized by electrodeposition. An extreme extensibility (elongation exceeds 5000%) without a strain hardening effect was observed when the nc copper specimen was rolled at room temperature. Microstructure analysis suggests that the superplastic extensibility of the nc copper originates from a deformation mechanism dominated by grain boundary activities rather than lattice dislocation, which is also supported by tensile creep studies at room temperature. This behavior demonstrates new possibilities for scientific and technological advancements with nc materials.     

High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys

The dimensionless thermoelectric figure of merit (ZT) in bismuth antimony telluride (BiSbTe) bulk alloys has remained around 1 for more than 50 years. We show that a peak ZT of 1.4 at 100 degrees C can be achieved in a p-type nanocrystalline BiSbTe bulk alloy. These nanocrystalline bulk materials were made by hot pressing nanopowders that were ball-milled from crystalline ingots under inert conditions. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, ZT is about 1.2 at room temperature and 0.8 at 250 degrees C, which makes these materials useful for cooling and power generation. Cooling devices that use these materials have produced high-temperature differences of 86 degrees , 106 degrees , and 119 degrees C with hot-side temperatures set at 50 degrees, 100 degrees, and 150 degrees C, respectively. This discovery sets the stage for use of a new nanocomposite approach in developing high-performance low-cost bulk thermoelectric materials.     

Shape and temperature memory of nanocomposites with broadened glass transition

Shape-memory polymers can revert to their original shape when they are reheated. The stress generated by shape recovery is a growing function of the energy absorbed during deformation at a high temperature; thus, high energy to failure is a necessary condition for strong shape-memory materials. We report on the properties of composite nanotube fibers that exhibit this particular feature. We observed that these composites can generate a stress upon shape recovery up to two orders of magnitude greater than that generated by conventional polymers. In addition, the nanoparticles induce a broadening of the glass transition and a temperature memory with a peak of recovery stress at the temperature of their initial deformation.     

In situ measurement of grain rotation during deformation of polycrystals

Texture evolution governs many of the physical, chemical, and mechanical properties of polycrystalline materials, but texture models have only been tested on the macroscopic level, which makes it hard to distinguish between approaches that are conceptually very different. Here, we present a universal method for providing data on the underlying structural dynamics at the grain and subgrain level. The method is based on diffraction with focused hard x-rays. First results relate to the tensile deformation of pure aluminum. Experimental grain rotations are inconsistent with the classical Taylor and Sachs models.     

Atomic pillar-based nanoprecipitates strengthen AlMgSi alloys

Atomic-resolution electron microscopy reveals that pillarlike silicon double columns exist in the hardening nanoprecipitates of AlMgSi alloys, which vary in structure and composition. Upon annealing, the Si2 pillars provide the skeleton for the nanoparticles to evolve in composition, structure, and morphology. We show that they begin as tiny nuclei with a composition close to Mg2Si2Al7 and a minimal mismatch with the aluminum matrix. They subsequently undergo a one-dimensional growth in association with compositional change, becoming elongated particles. During the evolution toward the final Mg5Si6 particles, the compositional change is accompanied by a characteristic structural change. Our study explains the nanoscopic reasons that the alloys make excellent automotive materials.     

火锅底料在不同容器中熬煮铅、砷、镉、铝含量变化

研究火锅底料在不锈钢锅和铝锅中熬煮铅、砷、镉、铝含量的变化。结果：用不锈钢锅熬煮可使汤料中的铅、砷、铝分别增加3倍、23．89％和24．52％，可使油脂中的铅、砷、镉、铝分别增加4倍、80％，40％和15．16％；用铝锅熬煮可使汤料中的铅、砷、铝分别增加2．4倍、14．91％和4．7倍，可使油脂中的铅、砷、锅分别增加2.3倍、62.16％和7％。结论：从健康的角度出发，不锈钢锅和铝锅可能都不适合作火锅烹调器具。     

Synthesis of zeolite as ordered multicrystal arrays

Zeolites are crystalline nanoporous aluminosilicates widely used in industry. In order for zeolites to find applications as innovative materials, they need to be organized into large two- and three-dimensional (2D and 3D) arrays. We report that uniformly aligned polyurethane films can serve as templates for the synthesis of uniformly aligned 2D and possibly 3D arrays of silicalite-1 crystals, in which the orientations of the crystals are controlled by the nature of the polymers. We propose that the supramolecularly organized organic-inorganic composites that consist of the hydrolyzed organic products and the seed crystals are responsible for this phenomenon.     

Deformation twins in hornblende

Hornblende deformation twins with twin planes parallel to ( 101) are produced experimentally in single crystals by compression parallel to the c axis. Twinning occurs at confining pressures from 5 to 15 kilobars and temperatures from 400 degrees to 600 degrees C (strain rate, 10(-5) per second).     

Mg/Al ordering in layered double hydroxides revealed by multinuclear NMR spectroscopy

The anion-exchange ability of layered double hydroxides (LDHs) has been exploited to create materials for use in catalysis, drug delivery, and environmental remediation. The specific cation arrangements in the hydroxide layers of hydrotalcite-like LDHs, of general formula Mg2+(1-x)Al3+(x)OH2(Anion(n-)(x/n)).yH2O, have, however, remained elusive, and their elucidation could enhance the functional optimization of these materials. We applied rapid (60 kilohertz) magic angle spinning (MAS) to obtain high-resolution hydrogen-1 nuclear magnetic resonance (1H NMR) spectra and characterize the magnesium and aluminum distribution. These data, in combination with 1H-27Al double-resonance and 25Mg triple-quantum MAS NMR data, show that the cations are fully ordered for magnesium:aluminum ratios of 2:1 and that at lower aluminum content, a nonrandom distribution of cations persists, with no Al3+-Al3+ close contacts. The application of rapid MAS NMR methods to investigate proton distributions in a wide range of materials is readily envisaged.