The metastable constituent effects on size-dependent deformation behavior of nanolaminated micropillars: Cu/FeCoCrNi vs Cu/CuZr

Metastable high entropy alloys(HEAs) and amorphous metallic glasses(MGs), with the chemical disordered character, are intensively studied due to their excellent performance. Here, we introduce Cu to separately constrain these two metastable materials and comparatively investigate their deformation behaviors and mechanical properties of Cu/HEA Fe Co Cr Ni and Cu/MG Cu Zr nanolaminated micropillars in terms of intrinsic layer thickness h and extrinsic pillar diameter D. The metastable HEA layers, as the hard phase in Cu/HEA micropillars, are stable and dominate the deformation, while transformation(crystallization) occurs in MG which plays a minor role in deformation of Cu/MG micropillars. The h-controlled deformation mode transits from the D-independent homogenous-like deformation at large h to the Ddependent shear banding at small h in both Cu/HEA and Cu/MG micropillars. Although both Cu/HEA and Cu/MG micropillars exhibit a maximum strain hardening capability controlled by h, the former manifests much lower hardening capability compared with the latter. The intrinsic size h and extrinsic size D have a strong coupling effect on the strength of Cu/HEA and Cu/MG micropillars. The strength of strength of Cu/HEA micropillars exhibits the D-dependent transition from "smaller is stronger" to "smaller is weaker"with increasing h. By contrast, the strength of Cu/MG micropillars exhibits the transition from bulk-like D-independent behavior at large h to small volume D-dependent behavior(smaller is stronger) at small h.

Theoretical optimization of micropillar arrays for structurally stable bioinspired dry adhesives

Inspired by the excellent adhesion performances of setae structure from organisms,micro/nano-pillar array has become one of the paradigms for adhesive surfaces.The micropillar arrays are composed of the resin pillars for adhesion and the substrate with different elastic modulus for supporting.The stress singularity at the bi-material corner between the pillars and the substrate can induce the failure of the micropillar-substrate corner and further hinder the fabrication and application of micropillar arrays,yet the design for the stability of the micropillar array lacks systematical and quantitative guides.In this work,we develop a semi-analytical method to provide the full expressions for the stress distribution within the bi-material corner combining analytical derivations and numerical calculations.The predictions for the stress within the singularity field can be obtained based on the full expressions of the stress.The good agreement between the predictions and the FEM results demonstrates the high reliability of our method.By adopting the strain energy density factor approach,the stability of the pillar-substrate corner is assessed by predicting the failure at the corner.For the elastic mismatch between the pillar and substrate given in this paper,the stability can be improved by increasing the ratio of the shear modulus of the substrate to that of the micropillar.Our study provides accurate predictions for the stress distribution at the bi-material corner and can guide the optimization of material combinations of the pillars and the substrate for more stable bioinspired dry adhesives.

Advances in micropillar arrays in cellular biomechanics detectionand tissue engineering

Cellular biomechanical features contributed to the occurrence and development of various physiological andpathological phenomena. Micropillar arrays have emerged as an important tool for both the assessment andmanipulation of cellular biomechanical characteristics. This comprehensive review provides an in-depthunderstanding of the fabrication methodologies of micropillar arrays and their applications in deciphering and finetuning cellular biomechanical properties and the innovative experimental platforms including organ-on-a-chip andorganoids-on-a-chip. This review provides novel insights into the potential of micropillar technology, poised toupdate the landscape of stem cell research and tissue engineering.

Dynamic recrystallization-induced temperature insensitivity of yield stress in single-crystal Al1.2CrFeCoNi micropillars

High-entropy alloys, a new class of metallic materials, exhibit excellent mechanical properties at high temperatures. In spite of the worldwide interest, the underlying mechanisms for temperature dependence of mechanical properties of these alloys remain poorly understood. Here, we systemically investigate the mechanical behaviors and properties of Al_(1.2)CrFeCoNi(comprising a body-centered cubic phase) and Al_(0.3)CrFeCoNi(comprising a face-centered cubic phase) single-crystal micropillars with three orientations([100], [110], and [111]) at temperatures varying from 300 to 675 K by using in situ compression of micropillars inside a scanning electron microscope. The results show that the yield stresses of Al_(1.2)CrFeCoNi micropillars are insensitive to temperature changes, and their flow stresses and work hardening rates increase slightly with increasing temperature from 300 to550 K, which differs from the typical temperature dependence of yield/flow stresses in metals and alloys. In contrast,Al_(0.3)CrFeCoNi micropillars exhibit typical thermal softening. Furthermore, it is found that the Al_(1.2)CrFeCoNi micropillars exhibit a transition from homogenous deformation to localized deformation at a critical temperature, while the Al_(0.3)CrFeCoNi micropillars always maintain a well-distributed and fine slip deformation. Detailed transmission electron microscopy analyses reveal that dynamic recrystallization(involving dislocation tangles, and formation of dislocation cell structures and sub-grains)plays a key role in the observed temperature insensitivity of the yield stress and increasing flow stress(and work hardening rate)with increasing temperature in the Al_(1.2)CrFeCoNi micropillars, and that thermally activated dislocation slip leads to thermal softening of the Al_(0.3)CrFeCoNi micropillars. The differences in deformation modes and temperature dependence of the mechanical properties between Al_(1.2)CrFeCoNi and Al_(0.3)CrFeCoNi essentially originate from the differences in dislocation activities and slip systems since the two alloys adopt different phases. Our findings provide key insights in the temperature dependence of mechanical properties and deformation behaviors of high-entropy alloys with body-centered cubic and face-centered cubic phases.

A review of challenges and opportunities in micropillar compression studies in Mg alloys

This review paper provides an overview of the micropillar compression technique as applied to magnesium(Mg) and its alloys. It explores the influence of various factors, such as pillar size, shape, temperature, and strain rate on the mechanical properties of Mg.Additionally, the impact of alloying elements, aging, and precipitates in Mg alloys has been extensively examined, revealing their significant influence on mechanical performance. The study highlights the strength and strain hardening improvements in Mg with decreasing pillar size in micropillar compression. Furthermore, the role of precipitates as strengthening agents, affecting deformation mechanisms and overall mechanical response, is explored. These valuable insights are crucial for designing Mg-based materials with enhanced mechanical properties for advanced engineering applications.

Silicon micropillar electrodes of lithiumion batteries used for characterizing electrolyte additives

The 100 crystal-oriented silicon micropillar array platforms were prepared by microfabrication processes for the purpose of electrolyte additive identification. The silicon micropillar array platform was used for the study of fluorinated vinyl carbonate(FEC), vinyl ethylene carbonate(VEC), ethylene sulfite(ES), and vinyl carbonate(VC) electrolyte additives in the LiPF_6 dissolved in a mixture of ethylene carbonate and diethyl carbonate electrolyte system using charge/discharge cycles, electrochemical impedance spectroscopy, cyclic voltammetry, scanning electron microscopy, and x-ray photoelectron spectroscopy. The results show that the silicon pillar morphology displays cross-shaped expansion after lithiation/delithiation, the inorganic lithium salt keeps the silicon pillar morphology intact, and the organic lithium salt content promotes a rougher silicon pillar surface. The presence of poly-(VC) components on the surface of FEC and VC electrodes allows the silicon pillar to accommodate greater volume expansion while remaining intact. This work provides a standard, fast, and effective test method for the performance analysis of electrolyte additives and provides guidance for the development of new electrolyte additives.

Micropillar Cavity Design for 1.55-μm Quantum-Dot Single-Photon Sources

The 1.55-μm quantum-dot (QD) micropillar cavities are strongly required as single photon sources (SPSs) for silica-fiber-based quantum information processing. Theoretical analysis shows that the adiabatic distributed Bragg reflector (DBR) structure may greatly improve the quality of a micropillar cavity. An InGaAsP/InP micropillar cavity is originally difficult, but it becomes more likely usable with inserted tapered (thickness decreased towards the center) distributed DBRs. Simulation turns out that, incorporating adiabatically tapered DBRs, a Si/SiO2- InP hybrid micropillar cavity, which enables weakly coupling InAs/InP quantum dots (QDs), can even well satisfy strong coupling at a smaller diameter. Certainly, not only the tapered structure, other adiabatic designs, e.g., both DBR layers getting thicker and one thicker one thinner, also improve the quality, reduce the diameter, and degrade the fabrication difficulty of Si/SiO2-InP hybrid micropillar cavities. Furthermore, the problem of the thin epitaxial semiconductor layer can also be greatly resolved by inserting adiabatic InGaAsP/InP DBRs. With tapered DBRs, the InGaAsP/InP-air-aperture micro-pillar cavity serves as an efficient, coherent, and monolithically producible 1.55-μm single-photon source (SPS). The adiabatic design is thus an effective way to obtain prospective candidates for 1.55-μm QD SPSs.

Dissipative particle dynamics simulation of flow through periodic arrays of circular micropillar

Flow through arrays of micropillar embedded inside microfluidic chip systems is important for various microfluidic devices. It is critical to accurately predict the mass flow rate through pillar arrays based on the pillar design. This work presents a dissipative particle dynamics （DPD） model to simulate a problem of flow across periodic arrays of circular micropillar and investigates the permeability of two types of micropillar arrays. The flow fields including horizontal and vertical velocity fields, the number density field, and the streamline of the flow are analyzed. The predicted solid volumes by the presented DPD simulation of both types of arrays are quite close to the actual counterparts. These quantitative agreements show usefulness and effectiveness of the DPD model in simulating arrays of micropillar. By comparing two types of micropillar arrangement patterns, we find that the arrangement pattern of micropillar does not have significant influence on the permeability of the array.

Magnetized Micropillar-Enabled Wearable Sensors for Touchless and Intelligent Information Communication

The wearable sensors have recently attracted considerable attentions as communication interfaces through the information perception,decoding,and conveying process.However,it is still challenging to obtain a sensor that can convert detectable signals into multiple outputs for convenient,e cient,cryptic,and high-capacity information transmission.Herein,we present a capacitive sensor of magnetic field based on a tilted flexible micromagnet array(t-FMA)as the proposed interaction interface.With the bidirectional bending capability of t-FMA actuated by magnetic torque,the sensor can recognize both the magnitude and orientation of magnetic field in real time with non-overlapping capacitance signals.The optimized sensor exhibits the high sensitivity of over 1.3 T-1 and detection limit down to 1 mT with excellent durability.As a proof of concept,the sensor has been successfully demonstrated for convenient,e cient,and programmable interaction systems,e.g.,touchless Morse code and Braille communication.The distinguishable recognition of the magnetic field orientation and magnitude further enables the sensor unit as a high-capacity transmitter for cryptic information interaction(e.g.,encoded ID recognition)and multi-control instruction outputting.We believe that the proposed magnetic field sensor can open up a potential avenue for future applications including information communication,virtual reality device,and interactive robotics.

A shape-reconfigurable,light and magnetic dual-responsive shape-memory micropillar array chip for droplet manipulation

Droplet manipulation on an open surface has great potential in chemical analysis and biomedicine engineering.However,most of the reported platforms designed for the manipulation of water droplets cannot thoroughly solve the problem of droplet evaporation.Herein,we report a shape-reconfigurable micropillar array chip for the manipulation of water droplets,oil droplets and water-in-oil droplets.Water-in-oil droplets provide an enclosed space for water droplets,preventing the evaporation in an open environment.Perfluoropolyether coated on the surface of the chip effectively reduces the droplet movement resistance.The micropillar array chip has light and magnetic dual-response due to the Fe3O4 nanoparticles and the reduced iron powder mixed in the shape-memory polymer.The micropillars irradiated by a near-infrared laser bend under the magnetic force,while the unirradiated micropillars still keep their original shape.In the absence of a magnetic field,when the micropillars in a temporary shape are irradiated by the near-infrared laser to the transition temperature,the micropillars return to their initial shape.In this process,the surface morphology gradient caused by the deformation of the micropillars and the surface tension gradient caused by the temperature change jointly produce the driving force of droplet movement.

Significant“smaller is softer”in amorphous silicon via irradiation-me diate d surface modification

“Smaller is softer”is a reverse size dependence of strength,defying the“smaller is stronger”tenet.It usually results from surface-mediated displacive or diffusive deformation and is mainly found in some ultra-small-scale(below tens of nanometers)metallic materials.Here,making use of the surface modifi-cation via ion beam irradiation,we bring the“smaller is softer”into being in a covalently-bonded,hard,and brittle material-amorphous Si(a-Si)at a much larger size regime(<∼500 nm).It is manifested as the transition from the quasi-brittle failure to the homogeneous plastic deformation as well as the de-creasing yield stress with sample volume reduction at the submicron-scale regime.An analytical model of hard core/superplastic shell has been proposed to explain the artificially-controllable size-dependent softening.This surface engineering pathway via ion irradiation is not only of particular interest to tai-lor the strength and deformation behaviors in small-sized a-Si or other covalently-bonded amorphous solids but also of practical relevance to the utility of a-Si in microelectronics and microelectromechanical systems.

Superior strength-plasticity synergy in a heterogeneous lamellar Ti_(2)AlC/TiAl composite with unique interfacial microstructure

Improving the plasticity of TiAl alloys at room temperature has been a longstanding challenge for the de-velopment of next-generation aerospace engines.By adopting the nacre-like architecture design strategy,we have obtained a novel heterogeneous lamellar Ti_(2)AlC/TiAl composite with superior strength-plasticity synergy,i.e.,compressive strength of∼2065 MPa and fracture strain of∼27%.A combination of micropil-lar compression and large-scale atomistic simulation has revealed that the superior strength-plasticity synergy is attributed to the collaboration of Ti_(2)AlC reinforcement,lamellar architecture and heteroge-neous interface.More specifically,multiple deformation modes in Ti_(2)AlC,i.e.,basal-plane dislocations,atomic-scale ripples and kink bands,could be activated during the compression,thus promoting the plas-tic deformation capability of composite.Meanwhile,the lamellar architecture could not only induce sig-nificant stress redistribution and crack deflection between Ti_(2)AlC and TiAl,but also generate high-density SFs and DTs interactions in TiAl,leading to an improved strength and strain hardening ability.In addi-tion,profuse unique Ti_(2)AlC(1¯10¯3)/TiAl(111)interfaces in the composite could dramatically contribute to the strength and plasticity due to the interface-mediated dislocation nucleation and obstruction mecha-nisms.These findings offer a promising paradigm for tailoring microstructure of TiAl matrix composites with extraordinary strength and plasticity at ambient temperature.

Effect of grain size on iron-boride nanoglasses

Metallic nanoglasses are made of amorphous grains that are separated by lower-density amorphous boundaries,which have been proposed to enhance plasticity through the deflection of cracks and shear bands at interfaces.It has been difficult to experimentally control grain size and interfacial structure to understand their roles in plastic deformation.Here,we fabricate bulk nanoglasses via compaction and sintering of colloidally synthesized amorphous iron-boride nanoparticles.These nanoglasses have amor-phous grains with diameters from 116 nm to 576 nm and were tested using nanoindentation and mi-cropillar compressions.The nanoglass with a grain size of 576 nm shows the highest elastic modulus and hardness of 101 GPa and 7.4 GPa,respectively.Transmission electron microscopy reveals that nanocrys-tals form within the nanoglasses during compaction.Higher nanocrystal density correlates with higher nanoparticle crystallization enthalpy,an increase in plasticity,and a decrease in yield strength.Plastic strain of 5.0%,yield strength of 3.8 GPa,and ultimate compressive strength of 2.7-3.8 GPa were achieved.We show that the compaction of colloidal metallic glass nanoparticles results in robust bulk samples,with mechanical properties similar to that of other iron-based bulk metallic glasses.

Reveal Hydrogen Behavior at Grain Boundaries in Fe-22Mn-0.6C TWIP Steel via In Situ Micropillar Compression Test

In this study,the effect of hydrogen on dislocation and twinning behavior along various grain boundaries in a high-manganese twinning-induced plasticity steel was investigated using an in situ micropillar compression test.The compressive stress in both elastic and plastic regimes was increased with the presence of hydrogen.Further investigation by transmission electron backscatter diffraction and scanning transmission electron microscope demonstrated that hydrogen promoted both dislocation multiplication and twin formation,which resulted in higher stress concentration at twin-twin and twin-grain boundary intersections.

Molecular dynamics simulation and micropillar compression of deformation behavior in iridium single crystals

The compression behaviors of iridium single crystals with different crystalline orientations were investigated by micropillar compression tests and molecular dynamics(MD) simulations.The results indicated that the deformation process of iridium single crystals with [100]and [110] orientations was presented as the stacking faults expansion and the formation of Lomer-Cottrell locks.And the occurrence of Lomer-Cottrell locks was considered as the interaction of stacking faults on {111} planes by MD simulations.The evolution of crystal structure in compression indicated that the Lomer-Cottrell locks might contribute to the large plastic deformation of iridium single crystals.Moreover,the deformation features in MD simulations showed that the elastic modulus(E) and yield stress(σ_(s)) of iridium single crystals were significantly influenced by the temperature.The elastic modulus and yield stress gradually decreased with an increased temperature for all orientations.Meanwhile,the single crystal with a closely spaced lattice structure exhibited superior mechanical properties at a same temperature.

Suspended penetration wetting state of droplets on microstructured surfaces

When a water droplet on a micropillar-structured hydrophobic surface is submitted to gradually increased pressure, the CassieBaxter wetting state transforms into the Wenzel wetting state once the pressure exceeds a critical value. It has been assumed that the reverse transition(Wenzel-to-Cassie-Baxter wetting state) cannot happen spontaneously after the pressure has been removed.In this paper, we report a new wetting-state transition. When external pressure is exerted on a droplet in the Cassie-Baxter wetting state on textured surfaces with high micropillars to trigger the breakdown of this wetting state, the droplet penetrates the micropillars but does not touch the base of the surface to trigger the occurrence of the Wenzel wetting state. We have named this state the suspended penetration wetting state. Spontaneous recovery from the suspended penetration wetting state to the initial Cassie-Baxter wetting state is achieved when the pressure is removed. Based on the experimental results, we built models to establish the penetration depth that the suspended penetration wetting state could achieve and to understand the energy barrier that influences the equilibrium position of the liquid surface. These results deepen our understanding of wetting states on rough surfaces subjected to external disturbances and shed new light on the design of superhydrophobic materials with a robust wetting stability.

Exceptionally shear-stable and ultra-strong Ir-Ni-Ta high-temperature metallic glasses at micro/nano scales

Ir-Ni-Ta metallic glasses(MGs)exhibit an array of superior high-temperature properties,making them attractive for applications at high temperatures or in harsh environments.However,Ir-Ni-Ta bulk MGs are quite brittle and often fracture catastrophically even before plastic yielding,significantly undercutting their high-strength advantage.Here,we show that the Ir-Ni-Ta MGs are not intrinsically brittle,but rather malleable when the feature size is reduced to micro/nano-scales.All tested Ir-Ni-Ta MG micropillars with a diameter ranging from~500 nm to~5μm display a large plastic strain above 25%(the maximum up to 35%),together with a yield strength up to 7 GPa,well exceeding the strength recorded in most metallic materials.The intrinsic shear stability of Ir-Ni-Ta MGs,as characterized by the normalized shear displacement during a shear event,is much larger than those malleable Zr-and Cu-based MGs.Our results suggest that Ir-Ni-Ta MGs are excellent candidates for micro/nanoscale structural applications used at high-temperature or extreme conditions.

Deformation behaviors of Cu bicrystals with an inclined twin boundary at multiple scales

Cu bicrystals of different sizes with a sole twin boundary（TB） inclined at 45？with respect to the loading direction were deformed under unidirectional and cyclic loading, respectively. It is found that the slip bands（SBs） parallel to the TB can be activated near the TB at all scales without obeying the Schmid＇s law.It is concerned with the local stress enhancement in the macroscale while it is more closely related to the scarce dislocation sources in the microscale. Moreover, a wedge-shaped zone formed near the TB in the microscale ascribed to the limited specimen size.

Effect of hydrogen on deformation behavior of Alloy 725 revealed by in-situ bi-crystalline micropillar compression test

Nickel-based Alloy 725 bi-crystalline micropillars with different types of grain boundaries(GBs)were compressed in hydrogen-free and in-situ hydrogen-charged conditions to investigate the hydrogen effect on the deformation behavior of the selected GBs.In the presence of hydrogen,the compressive stresses on the micropillars increase regardless of the GB type.It was proposed that this hydrogen-induced hardening behavior is the synergistic effect of hydrogen-enhanced dislocation multiplication and interactions,the pinning effect of hydrogen on dislocation motion,and hydrogen-enhanced lattice friction.Transmission electron backscatter diffraction(t-EBSD)results demonstrate that both low-angle GBs and high-angle GBs can effectively suppress dislocation transmission through the GBs,resulting in dislocations pile up along the GBs in the hydrogen-charged condition.In contrast,this behavior was not observed in the micropillars with twin boundaries.

Mechanical size effect of eutectic high entropy alloy:Effect of lamellar orientation

The effect of lamellar orientation on the deformation behavior of eutectic high entropy alloy at the micrometer scale,and the roles of two rarely explored laminate orientations(i.e.,the lamellar orientation at~0°and 45°angles with the loading direction)in regulating size-dependent plasticity were investigated using in-situ micropillar compression tests.The alloy,CoCrFe NiTa_(0.395),consists of alternating layers of Laves and FCC phases.It was found that the yield stress of the 0°pillars scaled inversely with the pillar diameters,in which the underlying deformation mode was observed to transform from pillar kinking or buckling to shear banding as the diameter decreased.In the case of the 450 pillars with diameters ranging from 0.4 to 3μm,there exists a’weakest’diameter of~1μm,at which both constraint effect and dislocation starvation are ineffective.Irrespective of the lamellar orientations,the strain hardening rate decreased with decreasing pillar diameter due to the diminishing dislocation accumulation that originated from the softening nature of large shear bands in the 0°pillars,and the enhanced probability of dislocation annihilation at the increased free surfaces in the 45°pillars.The findings expand and deepen the understanding of the mechanical size effect in small-scale crystalline materials and,in so doing,provide a critical dimension for the development of high-performing materials used for nanoor microelectromechanical systems.