Mechanics of Tunable Helices and Geometric Frustration in Biomimetic Seashells

Helical structures are ubiquitous in nature and engineering, ranging from DNA molecules to plant tendrils, from sea snail shells to nanoribbons. While the helical shapes in natural and engineered systems often exhibit nearly uniform radius and pitch, helical shell structures with changing radius and pitch, such as seashells and some plant tendrils, adds to the variety of this family of aesthetic beauty. Here we develop a comprehensive theoretical framework for tunable helical morphologies, and report the first biomimetic seashell-like structure resulting from mechanics of geometric frustration. In previous studies, the total potential energy is everywhere minimized when the system achieves equilibrium. In this work, however, the local energy minimization cannot be realized because of the geometric incompatibility, and hence the whole system deforms into a shape with a global energy minimum whereby the energy in each segment may not necessarily be locally optimized. This novel approach can be applied to develop materials and devices of tunable geometries with a range of applications in nano/biotechnology

Microstructure and Mechanical Properties of Low-Density, B2-Ordered AlNbZrTi_x Multi-Principal Element Alloys

Low-density multi-principal element alloys (MPEAs) combining a high specific strength and considerable ductility have remained a research hotspot, due to their promising prospects for energy-saving industrial applications. Light Ti-containing AlNbZrTix (x = 1−3) MPEAs were designed and prepared by induction melting and annealing. As the Ti content increases, the microstructure of these MPEAs evolves from dual phase (B2-ordered and Zr5Al3-type structure) into a single-phase B2-ordered structure, while the density reduces by ~8.7%, from ~5.85 g·cm−3 (x = 1) to ~5.34 g·cm−3 (x = 3). Unexpectedly, the AlNbZrTix (x = 1, 2, 3) alloys possess high specific yield strengths of ~270 kPa·m3·kg−1, ~221 kPa·m3·kg−1, >208 kPa·m3·kg−1, along with excellent fracture strains of ~17.8%, 21.8%, and >50%, respectively. These combined compressive properties are superior to the reported data of most BCC/B2-dominant MPEAs. The deformation mechanism of the B2-ordered structure is explained as a dislocation-based mechanism, accompanied by antiphase domains. Here, the effect of Ti on the microstructure and compressive properties of AlNbZrTix MPEAs was investigated, providing scientific support for the development of advanced low-density materials

Design and Preparation of CNTs/Mg Layered Composites

In order to effectively solve the problem of strength and ductility mismatch of magnesium (Mg) matrix composites, carbon nanotubes (CNTs) are added as reinforcement. However, it is difficult to uniformly disperse CNTs in a metal matrix to form composites. In this paper, electrophoretic deposition (EPD) was used to obtain layered units, and then the CNTs/Mg layered units were sintered by spark plasma sintering to synthesize layered CNTs/Mg composites. The deposition morphology of the layered units obtained by EPD and the microstructure, damping properties, and mechanical properties of the composite material were analyzed. The results show that the strength and ductility of the composite sample sintered at 590 °C were improved compared with the layered pure Mg and the composite sample sintered at 600 °C. Compared with pure Mg, the composites rolled by 40% had a much higher strength but no significant decrease in ductility. The damping properties of the CNTs/Mg composites were tested. The damping–test-temperature curve (tanδ~T) rose gradually with increasing temperature in the range of room temperature to 350 °C, and two internal friction peaks appeared. The damping properties of the tested composites at room temperature decreased with increasing frequency. The layered structure of the CNTs/Mg had ultra-high strengthening efficiency and maintained its ductility. The layered units prepared by EPD can uniformly disperse the CNTs in the composites

Heterogeneous Microstructure and Tensile Properties of an Austenitic Stainless Steel

Stainless steel (SS) exhibits excellent ductility; however, its low strength hinders its practical applications. To achieve good synergy between strength and ductility, a heterogeneous structure was introduced into a newly developed nitrogen-alloyed low-nickel austenitic steel, QN1803. The received QN1803 was cold-rolled and annealed at 993 K for different durations, and the microstructural evolution and tensile mechanical properties were investigated. The yield strength (1130 MPa) of the QN1803 annealed at a temperature of 993 K for 15 min was approximately three times higher than that of the as-received sample (314 MPa). The short annealing time of 15 min yielded a heterogeneous structure with grain size distributions ranging from nanoscale to micron-scale. The annealed QN1803 exhibited typical dislocation cells and dislocation walls caused by slipping after cold rolling. During annealing, a step-like lamellar structure is formed. The high yield strength was obtained from the large number of twins and hard ultrafine grains. The good ductility is due to the large number of dislocations generated in the soft grains and the GNDs around the heterogeneous interfaces. Additionally, the lamella structure of the material also contributes to improved ductility to a certain degree. The aim of this paper is to develop new materials with both high yield strength and excellent toughness based on more economical materials cost

GPa grade cryogenic strength yet ductile high-entropy alloys prepared by powder metallurgy

The carbon-doped Fe50Mn30Co10Cr10 HEAs with excellent cryogenic mechanical properties were prepared by powder metallurgy, and their tensile deformation behavior and strain-hardening mechanism were investigated. The yield strength of the HEAs at 77 K improved from 489.7 to 1087.0 MPa as the carbon content increased from 0 to 3 at. %, which mainly stems from the increased lattice friction caused by the addition of carbon and the cryogenic environment; the ultimate tensile strength of the HEAs doped with 2 and 3 at. % carbon reached 1.2 and 1.4 GPa, respectively, while the elongation to fracture reached 31.6 % and 16.8 %, respectively, which is mainly attributed to the joint activation of microbands, twinning, and HCP phase. The deformation mechanism gradually changed from deformation-induced phase transformation to microbands and twinning with increasing carbon content under cryogenic conditions. This study provides a meaningful reference for the design and development of powder metallurgy HEAs with excellent performance in cryogenic applications

The effect of grain size on the annealing-induced phase transformation in an Al0.3CoCrFeNi high entropy alloy

The annealing-induced phase transformation was investigated in coarse-grained and severely deformed nanocrystalline face-centered cubic (FCC) high entropy alloys (HEA). Increasing temperature leads to phase transformations from the FCC phase to an L12 phase and finally to a B2 phase in the coarse-grained HEA. By contrast, direct transformation from the FCC phase to the B2 phase was observed in the nanocrystalline HEA at a lower temperature

On the Microstructure and Mechanical Properties of CrNx/Ag Multilayer Films Prepared by Magnetron Sputtering

CrNx/Ag multilayer coatings and a comparative CrNx single layer were deposited via reactive magnetron sputtering. In multilayer coatings, the thickness of each CrNx layer was constant at 60 nm, while that of the Ag layer was adjusted from 3 to 10 nm. Microstructure of the films was characterized by X-ray diffraction and transmission electron spectroscopy. The results suggest that the film containing 3 nm of Ag layer presents a nanocomposite structure comprising fine nano-grains and quasi-amorphous clusters. With Ag layer thickness reaching 4.5 nm and above, Ag grains coalesce to produce continuous an Ag layer and exhibit (111) preferential crystallization. Hardness of the films was detected by nanoindentation and it reveals that with increasing the Ag layer thickness, the hardness continuously decreases from 30.2 to 11.6 GPa. Wear performance of the films was examined by the ball-on-disk test at 500 °C. The result suggests that the out-diffusion of Ag towards film surface contributes to the friction reduction, while the wear performance of films depends on the thickness of the Ag layer

Effect of Al Addition on Microstructure and Properties of CoCrNi Medium-Entropy Alloy Prepared by Powder Metallurgy

Powder metallurgy possesses the advantages of low energy consumption, less material consumption, uniform composition, and near-final forming. In order to improve the mechanical properties and high-temperature oxidation resistance of CoCrNi medium-entropy alloy (MEA), CoCrNiAlX (X = 0, 0.1, 0.3, 0.5, 0.7) MEAs were prepared using mechanical alloying (MA) and spark-plasma sintering (SPS). The effect of aluminum content on the microstructure and properties of the MEAs was investigated. The results show that the CoCrNi MEA is composed of face center cubic (fcc) phase and some carbides (Cr23C6). With the increase in Al content, there exists Al2O3 precipitation. When the Al content is increased to Al0.5 and Al0.7, the body center cubic (bcc) phase begins to precipitate. The addition of aluminum significantly enhances the properties of the alloys, especially those containing fcc+bcc dual-phase solid solutions. The yield strength, compressive strength, and hardness of CoCrNiAl0.7 alloy are as high as 2083 MPa, 2498 MPa, and 646 HV, respectively. The high-temperature resistance also reaches the oxidation resistance level. Different oxides include Cr2O3, Al2O3, and (Co, Ni) Cr2O4 and NiCrO3 spinel oxides formed on the surface of alloys. The formation of an Al2O3 oxidation film prevents the further erosion of the matrix by oxygen elements

Assimilation of middepth velocities from Argo floats in the western South China Sea

Previous studies are mainly limited to temperature and salinity (T/S) profiling data assimilation, while data assimilation based on Argo float trajectory information has received less research focus. In this study, a new method was proposed to assimilate Argo trajectory data: The middepth (indicates the parking depth of Argo floats in this study, ~1200 m) velocities are estimated from Argo trajectories and subsequently assimilated into the Regional Ocean Model System (ROMS) using four-dimensional variational data assimilation (4DVAR) method. This method can avoid a complicated float trajectory model in direct position assimilation. The 2-month assimilation experiments in South China Sea (SCS) showed that this proposed method can effectively assimilate Argo trajectory information into the model and improve middepth velocity field by adjusting the unbalanced component in the velocity increments. The assimilation of the Argo trajectory-derived middepth velocity with other observations (satellite observations and T/S profiling data) together yielded the best performance, and the velocity fields at the float parking depth are more consistent with the Argo float trajectories. In addition, this method will not decrease the assimilation performance of other observations [i.e., sea level anomaly (SLA), sea surface temperature (SST), and T/S profiles], which is indicative of compatibility with other observations in the 4DVAR assimilation system.Mathematical Physic

Erratum: Mechanics of tunable helices and geometric frustration in biomimetic seashells

Original article: EPL, 105 (2014) 6400