Overcoming strength-ductility tradeoff with high pressure thermal treatment.

Conventional material processing approaches often achieve strengthening of materials at the cost of reduced ductility. Here, we show that high-pressure and high-temperature (HPHT) treatment can help overcome the strength-ductility trade-off in structural materials. We report an initially strong-yet-brittle eutectic high entropy alloy simultaneously doubling its strength to 1150 MPa and its tensile ductility to 36% after the HPHT treatment. Such strength-ductility synergy is attributed to the HPHT-induced formation of a hierarchically patterned microstructure with coherent interfaces, which promotes multiple deformation mechanisms, including dislocations, stacking faults, microbands and deformation twins, at multiple length scales. More importantly, the HPHT-induced microstructure helps relieve stress concentration at the interfaces, thereby arresting interfacial cracking commonly observed in traditional eutectic high entropy alloys. These findings suggest a new direction of research in employing HPHT techniques to help develop next generation structural materials.

Grain boundary plasticity initiated by excess volume.

Grain boundaries (GBs) serve not only as strong barriers to dislocation motion, but also as important carriers to accommodate plastic deformation in crystalline solids. During deformation, the inherent excess volume associated with loose atomic packing in GBs brings about a microscopic degree of freedom that can initiate GB plasticity, which is beyond the classic geometric description of GBs. However, identification of this atomistic process has long remained elusive due to its transient nature. Here, we use Au polycrystals to unveil a general and inherent route to initiating GB plasticity via a transient topological transition process triggered by the excess volume. This route underscores the general impact of a microscopic degree of freedom which is governed by a stress-triaxiality-based criterion. Our findings provide a missing perspective for developing a more comprehensive understanding of the role of GBs in plastic deformation.

Controllable Pseudoelasticity in Metallic Nanocrystals by Grain Boundary Engineering.

Materials with pseudoelasticity can recover from large strains exceeding their elastic limits during unloading, making them promising damage-tolerant building blocks for advanced nanodevices. Nevertheless, a practical approach to realize controllable pseudoelastic behavior at nanoscale remains challenging. Here, we proposed a grain boundary (GB) engineering approach to endow metallic nanocrystals with a controllable pseudoelasticity. Both in situ nanomechanical testing and atomistic simulations demonstrate that such controllable pseudoelasticity is governed by the extension and contraction of an inherent stacking fault array at the GB. By precisely tuning GB misorientation and inclination, our simulation results reveal that metallic nanocrystals can exhibit tailored pseudoelastic performance across a broad spectrum of GBs in different face-centered cubic metals. These findings enrich our understanding of the intrinsic pseudoelasticity of GBs and provide a GB engineering approach toward metallic materials with reversible deformability.

Inhibition of Heterogeneous Nucleation in Water by Hydrogel Coating.

Heterogeneous nucleation plays a critical role in the phase transition of water, which can cause damage in various systems. Here, we report that heterogeneous nucleation can be inhibited by utilizing hydrogel coatings to isolate solid surfaces and water. Hydrogels, which contain over 90% water when fully swelled, exhibit a high degree of similarity to water. Due to this similarity, there is a great energy barrier for heterogeneous nucleation along the water-hydrogel interface. Additionally, hydrogel coatings, which possess polymer networks, exhibit higher fracture energy and more robust adhesion to solid surfaces compared to water. This high fracture and adhesion energy acts as a deterrent for fracture nucleation within the hydrogel or along the hydrogel-solid interface. With a hydrogel layer approximately 100 µm thick, the boiling temperature of water under atmospheric pressure can be raised from 100 to 108 °C. Notably, hydrogel coatings also result in remarkable reductions in cavitation pressure on multiple solid surfaces. We have demonstrated the efficacy of hydrogel coatings in preventing damages resulting from acceleration-induced cavitation. Hydrogel coatings have the potential to alter the energy landscape of heterogeneous nucleation on the water-solid interface, making them an exciting avenue for innovation in heat transfer and fluidic systems.

Hierarchical twinning governed by defective twin boundary in metallic materials.

Dense networks of deformation twins endow metals and alloys with unprecedented mechanical properties. However, the formation mechanism of these hierarchical twin structures remains under debate, especially their relations with the imperfect nature of twin boundaries (TBs). Here, we investigate the intrinsic deformability of defective TBs in face-centered cubic metallic materials, where the inherent kinks on a set of primary TBs are demonstrated to facilitate the formation of secondary and hierarchical nanotwins. This defect-driven hierarchical twinning propensity is critically dependent on the kink height, which proves to be generally applicable in a variety of metals and alloys with low stacking fault energies. As a geometric extreme, a fivefold twin can be constructed via this self-activated hierarchical twinning mechanism. These findings differ from the conventional twinning mechanisms, enriching our understanding of twinning-mediated plasticity in metallic materials.

Extra plasticity governed by shear band deflection in gradient metallic glasses.

Inspired by gradient materials in nature, advanced engineering components with controlled structural gradients have attracted substantial research interests due to their exceptional combinations of properties. However, it remains challenging to generate structural gradients that penetrate through bulk materials, which is essential for achieving enhanced mechanical properties in metallic materials. Here, we report practical strategies to design controllable structural gradients in bulk metallic glasses (BMGs). By adjusting processing conditions, including holding time and/or controlling temperatures, of cryogenic thermal cycling and fast cooling, two different types of gradient metallic glasses (GMGs) with spatially gradient-distributed free volume contents can be synthesized. Both mechanical testing and atomistic simulations demonstrate that the spatial gradient can endow GMGs with extra plasticity. Such an enhanced mechanical property is governed by the gradient-induced deflection of shear deformation that fundamentally suppresses the unlimited shear localization on a straight plane that would be expected in BMGs without such a gradient.

Upconversion-nanoparticle-functionalized Janus micromotors for efficient detection of uric acid.

We report enzyme-powered upconversion-nanoparticle-functionalized Janus micromotors, which are prepared by immobilizing uricase asymmetrically onto the surface of silicon particles, to actively and rapidly detect uric acid. The asymmetric distribution of uricase on silicon particles allows the Janus micromotors to display efficient motion in urine under the propulsion of biocatalytic decomposition of uric acid and simultaneously detect uric acid based on the luminescence quenching effect of the UCNPs modified on the other side of SiO2. The efficient motion of the motors greatly enhances the interaction between UCNPs and the quenching substrate and improves the uric acid detection efficiency. Overall, such a platform using uric acid simultaneously as the detected substrate and motion fuel offers considerable promise for developing multifunctional micro/nanomotors for a variety of bioassay and biomedical applications.

Twinning-assisted dynamic adjustment of grain boundary mobility.

Grain boundary (GB) plasticity dominates the mechanical behaviours of nanocrystalline materials. Under mechanical loading, GB configuration and its local deformation geometry change dynamically with the deformation; the dynamic variation of GB deformability, however, remains largely elusive, especially regarding its relation with the frequently-observed GB-associated deformation twins in nanocrystalline materials. Attention here is focused on the GB dynamics in metallic nanocrystals, by means of well-designed in situ nanomechanical testing integrated with molecular dynamics simulations. GBs with low mobility are found to dynamically adjust their configurations and local deformation geometries via crystallographic twinning, which instantly changes the GB dynamics and enhances the GB mobility. This self-adjust twin-assisted GB dynamics is found common in a wide range of face-centred cubic nanocrystalline metals under different deformation conditions. These findings enrich our understanding of GB-mediated plasticity, especially the dynamic behaviour of GBs, and bear practical implication for developing high performance nanocrystalline materials through interface engineering.

Constitutive Description of Extra Strengthening in Gradient Nanotwinned Metals.

Gradient nanotwinned (GNT) metals exhibit extra strengthening and work hardening behaviors, which endow them impressive potentials in engineering applications. The increased strength is attributed to the dense interactions between dislocations and boundaries in the grain interiors. However, a constitutive model elucidating the extra strengthening effect is currently lacking. Here, we propose a theoretical framework to describe the mechanical response of GNT metals, especially the unusual extra strengthening behavior. The model captures the deformation mechanisms of GNT metals and coincides well with the reported experiment. The constitutive description developed in this work presents a tool to guide the structural design for developing gradient metallic materials.

Penta-Twin Destruction by Coordinated Twin Boundary Deformation.

Penta-twinned nanomaterials often exhibit unique mechanical properties. However, the intrinsic deformation behavior of penta-twins remains largely unclear, especially under the condition of high shear stress. In this study, we show that the deformation of penta-twins often subject to a structural destruction via dislocation-mediated coordinated twin boundary (TB) deformation, resulting in a reconstructed pentagon-shaped core. This reconstructed core region is mainly induced by the coordinated TB migration along different directions (for the nucleation and growth) and accelerated by the TB sliding (for the growth). The destructed penta-twin core can effectively accommodate the intrinsic disclination of the penta-twin, which further collapses beyond a critical size, as predicted by an energy-based criterion. These intrinsic deformation behaviors of penta-twins would enable the possibility of controlling the morphology of penta-twinned nanomaterials with unique properties.

Revealing extreme twin-boundary shear deformability in metallic nanocrystals.

Metals containing abundant coherent twin boundaries (TBs) are able to sustain substantial plastic deformation without fracture due to shear-induced TB migration and sliding. Retaining ductility in these metals, however, has proven difficult because detwinning rapidly exhausts TB migration mechanisms at large deformation, whereas TB sliding was only evidenced for loading on very specific crystallographic orientations. Here, we reveal the intrinsic shear deformability of twins in nanocrystals using in situ nanomechanical testing and multiscale simulations and report extreme shear deformability through TB sliding up to 364%. Sliding-induced plasticity is manifested for orientations that are generally predicted to favor detwinning and shown to depend critically on geometric inhomogeneities. Normal and shear coupling are further examined to delineate a TB orientation-dependent transition from TB sliding to TB cracking. These dynamic observations reveal unprecedented mechanical properties in nanocrystals, which hold implications for improving metal processing by severe plastic deformation.

Self-powered soft robot in the Mariana Trench.

The deep sea remains the largest unknown territory on Earth because it is so difficult to explore1-4. Owing to the extremely high pressure in the deep sea, rigid vessels5-7 and pressure-compensation systems8-10 are typically required to protect mechatronic systems. However, deep-sea creatures that lack bulky or heavy pressure-tolerant systems can thrive at extreme depths11-17. Here, inspired by the structure of a deep-sea snailfish15, we develop an untethered soft robot for deep-sea exploration, with onboard power, control and actuation protected from pressure by integrating electronics in a silicone matrix. This self-powered robot eliminates the requirement for any rigid vessel. To reduce shear stress at the interfaces between electronic components, we decentralize the electronics by increasing the distance between components or separating them from the printed circuit board. Careful design of the dielectric elastomer material used for the robot's flapping fins allowed the robot to be actuated successfully in a field test in the Mariana Trench down to a depth of 10,900 metres and to swim freely in the South China Sea at a depth of 3,224 metres. We validate the pressure resilience of the electronic components and soft actuators through systematic experiments and theoretical analyses. Our work highlights the potential of designing soft, lightweight devices for use in extreme conditions.

Metallic nanocrystals with low angle grain boundary for controllable plastic reversibility.

Advanced nanodevices require reliable nanocomponents where mechanically-induced irreversible structural damage should be largely prevented. However, a practical methodology to improve the plastic reversibility of nanosized metals remains challenging. Here, we propose a grain boundary (GB) engineering protocol to realize controllable plastic reversibility in metallic nanocrystals. Both in situ nanomechanical testing and atomistic simulations demonstrate that custom-designed low-angle GBs with controlled misorientation can endow metallic bicrystals with endurable cyclic deformability via GB migration. Such fully reversible plasticity is predominantly governed by the conservative motion of Shockley partial dislocation pairs, which fundamentally suppress damage accumulation and preserve the structural stability. This reversible deformation is retained in a broad class of face-centred cubic metals with low stacking fault energies when tuning the GB structure, external geometry and loading conditions over a wide range. These findings shed light on practical advances in promoting cyclic deformability of metallic nanomaterials.

Extra strengthening and work hardening in gradient nanotwinned metals.

Gradient structures exist ubiquitously in nature and are increasingly being introduced in engineering. However, understanding structural gradient-related mechanical behaviors in all gradient structures, including those in engineering materials, has been challenging. We explored the mechanical performance of a gradient nanotwinned structure with highly tunable structural gradients in pure copper. A large structural gradient allows for superior work hardening and strength that can exceed those of the strongest component of the gradient structure. We found through systematic experiments and atomistic simulations that this unusual behavior is afforded by a unique patterning of ultrahigh densities of dislocations in the grain interiors. These observations not only shed light on gradient structures, but may also indicate a promising route for improving the mechanical properties of materials through gradient design.

History-independent cyclic response of nanotwinned metals.

Nearly 90 per cent of service failures of metallic components and structures are caused by fatigue at cyclic stress amplitudes much lower than the tensile strength of the materials involved. Metals typically suffer from large amounts of cumulative, irreversible damage to microstructure during cyclic deformation, leading to cyclic responses that are unstable (hardening or softening) and history-dependent. Existing rules for fatigue life prediction, such as the linear cumulative damage rule, cannot account for the effect of loading history, and engineering components are often loaded by complex cyclic stresses with variable amplitudes, mean values and frequencies, such as aircraft wings in turbulent air. It is therefore usually extremely challenging to predict cyclic behaviour and fatigue life under a realistic load spectrum. Here, through both atomistic simulations and variable-strain-amplitude cyclic loading experiments at stress amplitudes lower than the tensile strength of the metal, we report a history-independent and stable cyclic response in bulk copper samples that contain highly oriented nanoscale twins. We demonstrate that this unusual cyclic behaviour is governed by a type of correlated 'necklace' dislocation consisting of multiple short component dislocations in adjacent twins, connected like the links of a necklace. Such dislocations are formed in the highly oriented nanotwinned structure under cyclic loading and help to maintain the stability of twin boundaries and the reversible damage, provided that the nanotwins are tilted within about 15 degrees of the loading axis. This cyclic deformation mechanism is distinct from the conventional strain localizing mechanisms associated with irreversible microstructural damage in single-crystal, coarse-grained, ultrafine-grained and nanograined metals.

Torsional Detwinning Domino in Nanotwinned One-Dimensional Nanostructures.

How to maintain sustained deformation in one-dimensional nanostructures without localized failure is an important question for many applications of nanotechnology. Here we report a phenomenon of torsional detwinning domino that leads to giant rotational deformation without localized failure in nanotwinned one-dimensional metallic nanostructures. This mechanism is demonstrated in nanotwinned Cu nanorods via molecular dynamics simulations, where coherent twin boundaries are transformed into twist boundaries and then dissolved one by one, resulting in practically unlimited rotational deformation. This finding represents a fundamental advance in our understanding of deformation mechanisms in one-dimensional metallic nanostructures.

A jogged dislocation governed strengthening mechanism in nanotwinned metals.

Atomistic simulations reveal a new and unique strengthening mechanism in nanotwinned metals governed by the collective motion of multiple necklace-like extended jogged dislocations. This mechanism prevails in a columnar-grained nanotwinned metal subject to an external stress parallel to the twin planes, provided the twin boundary spacing falls below a critical value. A theoretical model based on the depinning of unit jogs on twin planes is proposed to determine the flow stress associated with this deformation mechanism and is shown to be in agreement with atomistic simulations.

Blocking effect of twin boundaries on partial dislocation emission from void surfaces.

Recent discovery that nanoscale twin boundaries can be introduced in ultrafine-grained metals to improve strength and ductility has renewed interest in the mechanical behavior and deformation mechanisms of these nanostructured materials. By controlling twin boundary spacing, the effect of twin boundaries on void growth is investigated by using atomistic simulation method. The strength is significantly enhanced due to the discontinuous slip system associated with these coherent interfaces. Atomic-scale mechanisms underlying void growth, as well as the interaction between twin boundaries and the void, are revealed in details.

Hardening by twin boundary during nanoindentation in nanocrystals.

The atomistic deformation processes of nanocrystals embedded with nanoscale twin boundaries during nanoindentation are studied by molecular dynamics simulations. Load-displacement curves are obtained and the hardening mechanisms associated with the nanoscale twin boundaries are revealed. Johnson's theoretical indentation model is adopted to estimate the elastic stage of the nanoindentation. In addition, twin boundary-mediated dislocation nucleation is observed and analyzed.