Multi-scale Microstructure Research Articles

Tool path modeling and fabrication of multi-boundary lens array by tool offset end-fly-cutting

Lens arrays are widely used in advanced industrial fields, such as national defense, medical care, and aerospace, because of their exceptional optical performance. Single-point diamond turning based on fast/slow tool servo technology is a common machining method for fabricating lens arrays; however, the constant-angle or constant-arc-length sampling of its spiral tool trajectory leads to an uneven cutting speed, and thus, the machined morphology of the lens arrays deviates from their theoretical morphology. This shortcoming seriously restricts the high-efficiency production of large-scale lens arrays, as well as reduces design accuracy. Accordingly, a novel method is proposed herein for the fabrication of multi-boundary lens arrays by tool offset end-fly-cutting. First, a mathematical model was established to generate a uniform tool trajectory for the subsequent fabrication of square and hexagonal boundary lens arrays. From a comparison of the measured and simulated results, the root-mean-square error of the sectional curves of the multi-boundary lens array is below 1 μm, proving the effectiveness of the production method. The developed method provides a new technical scheme for the fabrication of multi-boundary lens arrays and a theoretical reference for machining multiscale microstructures.

Tracheid-inspired nanoarchitectured carbon-based aerogels with ultra-compressibility for wearable piezoresistive sensors

Carbon-based aerogels are difficult to accomplish satisfactory mechanical strength and fatigue resistance due to the brittleness of framework, limiting their application in piezoresistive sensors. In this work, inspired by tracheid structure in plants, flexible piezoresistive sensors were prepared based on stable and elastic carbon-based aerogels with a long-range oriented multiscale matrix structure. The carbon-based aerogel was generated by bidirectional freezing and further assembled to i) 2D morphology Ti3C2Tx MXene as conductive framework and ii) cellulose nanofibers (CNF) offering abundant hydroxyl groups as flexible substrate. Besides, positively charged chitosan was introduced to serve as “bridging junction reinforcement element”, which tightly connects CNF and Ti3C2Tx, contributing to improved strength and stability of the carbon-based aerogel. According to finite element simulation, the bonding and supporting effect of chitosan is confirmed, and the aligned carbon layer and elastic supporting microstructure can be directly modulated. Interestingly, the piezoresistive sensor exhibits super-compressibility which can withstand 10,000 cycles under 50% strain, showing a fast response time (6.3 ms) and high sensitivity (177.08 kPa−1). Briefly, this work provides an innovative strategy to obtain nanoarchitecture functional materials integrating multiscale microstructures into carbon-based aerogels, demonstrating their potential to improve the sensing characteristics and functionality of wearable piezoresistive sensors.

New insight into tailorable eutectic high entropy alloys with remarkable strength–ductility synergy and ample shaping freedom fabricated using laser powder bed fusion

Eutectic high entropy alloys (EHEAs) are typical heterostructured materials comprising ductile and hard phases that can be easily tailored to fabricate components with desirable structures and properties. Herein, an AlCoCrFeNi 2.1 EHEA with dual-phase nanolamellar structure and high strength and ductility (yield strength ( σ YS ) = 1210 MPa, ultimate tensile strength ( σ UTS ) = 1414 MPa, and elongation ( ε ) = 16%) was produced using laser powder bed fusion (LPBF). The AlCoCrFeNi 2.1 EHEA presented fine-grain structure with nanoprecipitates (L1 2 and BCC phases) embedded within alternating FCC and B2 nano-scale lamellae. We demonstrated that the high strength of the EHEA originated primarily from the high back stress strengthening of fine grains and high-density heterophase interfaces, and high dislocation density caused by rapid solidification as well as dispersed nanoparticles provided extra strengthening. Furthermore, the cellular structure comprising nearly square FCC cells and surrounding B2 phases was observed, which was induced by the presence of oxides on the surface of the feedstock powder. AlCoCrFeNi 2.1 EHEA with two-hierarchical dual-phase structure, that is, a mixture of lamellar and cellular structures, was obtained. Such heterogeneous structure presented outstanding strength–ductility synergy ( σ YS = 1042 MPa, σ UTS = 1303 MPa, and ε = 26%). These results promote the development of high-performance materials with manufacturing flexibility using additive manufacturing approaches to tailor their multiscale microstructure.

Laser Powder Bed Fusion Processing of Soft Magnetic Fe–Ni–Si Alloys: Effect of Hot Isostatic Pressing Treatment

Laser powder bed fusion (L-PBF)-processed high-silicon steel has great advantages in freely designed electric engines, and various studies have been conducted in this field. However, the analysis of both the mechanical and magnetic properties, focusing on the multiscale microstructure under as-fabricated and heat-treated conditions, which is indispensable for industrial applications, has not been performed. In this study, an Fe–Ni–Si sample was fabricated using the L-PBF process. Subsequently, the following hot isotropic pressing (HIPing) process was employed as a post heat treatment step for the Fe–Ni–Si alloys. The effects of HIPing on the microstructure were investigated, focusing on the metastable stable phase transformation in the Fe–Ni–Si system. X-ray diffraction results showed single-phase fcc γ (Fe, Ni) in the L-PBF-processed samples before and after HIPing. Moreover, the acicular Ni/Si-rich structure (formed in the as-fabricated L-PBF sample because of its high cooling rates) transformed to the equilibrium austenite, Ni3Si, and FeNi3 phases during HIPing. After HIP, the compressive modulus and strength increased from 11 GPa and 650 MPa to approximately 18 GPa and 900 MPa, respectively. The magnetic properties were evaluated via a hysteresis loop, and the coercivity increased from 1.8 kA/m and to 2.9 kA/m after the HIPing process.

FDM 3D-printed volcanic-shaped structure for ultrafast solar-driven interfacial evaporation and efficient energy utilization

In this study, we fabricated a volcanic-shaped 3D solar evaporator for ultrafast solar interfacial evaporation and high-efficient energy utilization by using fused deposition modeling (FDM) technology. Benefiting from the multiscale microstructure and polydopamine coating on the evaporation surface, the printed evaporator shows excellent water supply capacity, light utilization, and the Marangoni effect. Adjusting the high/diameter (H/D) ratio of the evaporator can further improve the evaporation performance. Under simulated sunlight irradiation, the optimized 3D evaporation system reaches ultrafast interfacial water evaporation (4.02 kg m−2 h−1) and efficient energy harvesting (220.34 %). Good water supply and the Marangoni effect endow the evaporator with excellent stability and self-cleaning properties against salt deposition for long-time seawater desalination (3.5 wt% NaCl solution). Meanwhile, the prepared 3D evaporator exhibits huge applied potential in the treatment and recycling of organic wastewater, especially the acid solution (pH = 1). The design strategy for the volcanic-shaped 3D evaporator provides novel ideas for developing efficient solar interfacial evaporators.

Exploring the potential of FSW-ed Al–Zn–Mg–Cu-based composite reinforced by trace in-situ nanoparticles in manufacturing workpiece with customizable size and high mechanical performances

Al–Zn–Mg–Cu alloys that are prone to segregation and hot cracks always encounter difficulty in manufacturing large-size workpiece with desirable strength-plasticity synergy, especially when the tonnage of the extruder exceeds 6000 tons. This work puts forward a novel idea to address this challenge by joining extruded profiles of Al–Zn–Mg–Cu-based composite reinforced by trace in-situ TiC–TiB2 nanoparticles through friction stir welding (FSW). The extruded profiles of Al–Zn–Mg–Cu-based composite along transverse direction (TD) achieved superior yield strength (σY) of 827 MPa and large plastic strain (εp) of 10.2%. The σY of FSW-ed joint along TD reached 847 MPa, and the εp was maintained at 2.8%. It was found that trace nanoparticles can weaken the grain boundary segregation of the composite ingot significantly, effectively avoiding the hot cracking that may occur during hot extrusion and FSW. In extruded profiles, nanoparticles promoted the nucleation of η′ precipitates, promoted the dislocation multiplication, and increased the proportions of low-angle grain boundaries (LAGBs) within 2°–15° and high-angle grain boundaries (HAGBs), thereby enhancing the strength. Meanwhile, nanoparticles increased the density of LAGBs and facilitated the precipitation of Guinier–Preston (GP) zones, which facilitated to maintain high plasticity of the extruded profiles of composite. Additionally, nanoparticles can effectively inhibit the abnormal grain growth in nugget zone (NZ) during post-weld heat treatment (PWHT), which strengthened the FSW-ed joint. This work elaborated the multiscale microstructure evolution mechanisms, offering guidance to fabricate Al–Zn–Mg–Cu workpieces with infinitely extensive size as required in actual manufacturing and fantastic mechanical performances.

Mechanical Properties of Ultra-High Performance Concrete with Coal Gasification Coarse Slag as River Sand Replacement.

Coal gasification coarse slag (CGCS) is a by-product of coal gasification. Despite its abundance, CGCS is mostly used in boiler blending, stacking, and landfill. Large-scale industrial applications of CGCS can be environment-friendly and cost saving. In this study, the application of CGCS as a substitute for river sand (RS) with different replacement ratios in ultra-high performance concrete (UHPC) was investigated. The effects of CGCS replacement ratios on the fluidity and mechanical properties of specimens were examined, and the effect mechanisms were explored on the basis of hydration products and the multi-scale (millimetre-scale and micrometre-scale) microstructure analysis obtained through X-ray diffraction (XRD), scanning electron microscopy, and X-ray energy-dispersive spectroscopy. With an increase in the CGCS replacement ratio, the water-binder ratio (w/b), flexural strength, and compressive strength decreased. Specimens containing CGCS of ≤25% can satisfy the strength requirement of non-structural UHPC, with flexure strength of 29 MPa and compressive strength of 111 MPa at day 28. According to the XRD results and multi-scale microstructure analysis, amorphous glass beads in CGCS positively influenced ettringite generation due to the pozzolanic activity. Porous carbon particles in CGCS showed strong interfacial bonding with cement slurry due to internal hydration; this bonding was conducive to improving the mechanical strength. However, CGCS hindered hydration in the later curing stage, leading to an increase in the unreacted cement and agglomeration of fly ash; in addition, at a CGCS replacement ratio of up to 50%, an apparent interfacial transition zone structure was observed, which was the main contributor to mechanical strength deterioration.

Tailoring multi-scale hierarchical microstructure parameters to achieve super-high strength and considerable plasticity in a dual-phase Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr alloy

In the present study, traditional treatment was designed specially and carried out on a dual-phase Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr alloy, and a multi-scale hierarchical microstructure differed from the ordinary microstructure of titanium alloys was obtained. During the solution-treatment at 900 ℃ for 1 h followed by furnace cooling (FC), air cooling (AC), water cooling (WC) and at 920℃for 1 h followed by WC, three equiaxed microstructures composed of primary equiaxed α phase (αp) and β phase with different fractions, and a typical bimodal microstructure containing primary αp and acicular secondary α phase (αs) were formed. During the aging-treatment at 550 ℃ for 6 h followed by AC after solution-treatment at 900 ℃ for 1 h followed by WC, a multi-scale hierarchical microstructure composed of equiaxed αp, acicular αs with a thickness of dozens of nanometers, smaller acicular α phase (αs-s) with a thickness of ten nanometers, distributed in the space of two αs, and whisker β phase (βw) in the transformed β (βt) regions, was obtained. Moreover, with the solution temperature increased to 920 ℃, it was deliberately designed that equiaxed αp was more refined, and finer and denser acicular αs-s precipitated in the alloy. Contributing to the equiaxed αp refinement and nano-scale acicular α phases, the hierarchical microstructure exhibited super-high yield strength of 1228 MPa and ultimate tensile strength of 1389 MPa. Meanwhile, the refined equiaxed αp could also offset the negative effect of acicular α phases on plasticity, and the plasticity was maintain at an considerable level of 8%.

Multidirectional Gradient Feature With Shape Index for Effective Texture Classification

Recently, local gradient microstructure of image textures has become an important field of texture classification, but it is generally to investigate the multiscale local microstructures of image gradient, and rarely consider the multidirectional and multiscale local microstructure of image gradient. The proposed algorithm first extracts the two-order gradient feature of the image from different orthogonal directions and further constructs the multiple shape index of the image, and then calculates the histogram feature vectors of the shape index on different orthogonal directions and scales, and finally connects all histogram feature vectors on different orthogonal directions and scales to obtain the final matching feature vector of the image. To further enhance the discriminant ability of feature vector generated by multidirectional shape index schemes, the weight of each block of images is also considered. Experiments on two texture databases and one palmprint database have fully confirmed the effective of proposed algorithm.

Enhancing the yield strength of Ni–Co–Cr–Fe–Al as-cast hypoeutectic high-entropy alloys by introducing γ′ precipitation

It is still a challenge to further increase the yield strength of eutectic high-entropy alloys, a promising multiple principal alloy system. By modifying the Ni: Co ratio to introduce the γ′ phase, we developed a series of Ni60-xCoxCr10Fe10Al18Mo2 (x = 30, 20, 10, 0) casting hypoeutectic high-entropy alloys with improved yield strength and ductility. The increase of Ni concentration via decreasing Co concentration promotes the amount of FCC phase with nano γ′ particles in it. The as-cast Ni60Cr10Fe10Al18Mo2 HEA with FCC/γ'+B2 phase composition exhibits a superior σ0.2–831 MPa due to the formation of the γ′ phase. The results provide a feasible route for developing high-performance Ni–Co–Cr–Fe–Al casting HEAs with multiscale microstructure.

Fabrication of micro-nano multi-scale hierarchical porous structure on the surface of Inconel718 nickel-base superalloy by one-step method

A new high-efficiency composite process consisting of laser machining and abrasive belt grinding was proposed to prepare multi-scale graded microstructures on the surface of Inconel718 nickel-base superalloy for aviation. The scheme is based on the machining characteristics of abrasive belt grinding, low damage molding and high-efficiency grinding and polishing, the effective combination of ultrashort pulse laser-induced surface microstructure growth, and the thermal effect in the process of high-energy beam machining, so as to realize the efficient preparation of hierarchical microstructure. In the one-step forming process, the accuracy of the material surface is ensured by abrasive belt grinding. The porous microstructure on the surface is catalyzed by the thermal effect produced by the high-energy beam laser. The scale of the porous structure is 3 µm–6 µm, the pore surface will generate nanoparticles, the size is below 100 nm, and dense nanoparticles will be generated on the surface. The porous structure can be super hydrophilic and adsorbed to form a liquid membrane. The formation of rib-like microstructures on the surface induced by ultrashort pulse laser makes the liquid film have directional spreading properties. Rib-like microstructure height is 50 µm–150 µm and it can be adjusted by setting the scanning line width. The one-step method for preparing hierarchical microstructures proposed in this study can efficiently process large-area microstructure surfaces with different wetting properties. • The porous structure, nanoparticles, and rib-like bulges can be efficiently fabricated. • The microstructure can be easily regulated.

Microstructure of NiTi superelastic alloy manufactured by selective laser melting

Nitinol is used in dental tools for its superelasticity, shape memory and biocompatibility. Selective laser melting (SLM) on a powder bed is a potential process for manufacturing such as dental tools. Microstructural studies are conducted on NiTi-SLM using a parallel strip strategy and 90° laser beam rotation. The multi-scale (nano-, micro- and mesoscopic) microstructure is revealed. Similar “cast” type microstructure features are found. The SLM melt is casted in a “micro-mold”. Fine micro-grains with random crystallographic orientation are formed near the mold walls and served as seats for the rapid growth of columnar dendritic grains along the 〈0 0 1〉 direction with epitaxial crystallographic relationships. The growth of the columnar grains curves towards the center of the “micro-mold”, coinciding with the center of the laser beam. It is proposed that SLM processing resembles “Metallic Self Micro-Mold Casting” (MSµMC) and/or “Self Micro-Molding” (S2M).

Heterogeneous precipitate microstructure in titanium alloys for simultaneous improvement of strength and ductility

• We propose a step-aging method to improve the comprehensive mechanical properties of titanium alloys by designing heterogeneous microstructures through simulations and experiments. • The formation of heterogeneous microstructure can be attributed to the integration of conventional nucleation and growth, and non-conventional pseudospinodal decomposition mechanisms. • Ti-1023 (Ti-10V-2Fe-3Al) alloy with heterogeneous microstructure has shown enhanced ductility by ∼50% and strength by ∼10% comparing with that of homogeneous microstructure. • Both deformation twin and dislocation slip for heterogeneous microstructure have been activated and contributed to the enhanced mechanical properties. The design of alloys with simultaneous high strength and high ductility is still a difficult challenge. Here, we propose a new approach to designing multi-phase alloys with a synergistic combination of strength and ductility by engineering heterogeneous precipitate microstructures through the activation of different transformation mechanisms. Using a two-phase titanium alloy as an example, phase field simulations are carried out firstly to design heat treatment schedules that involve both conventional nucleation and growth and non-conventional pseudospinodal decomposition mechanisms, and the calculated microstructures have been evaluated by crystal plasticity finite element modeling. According to simulations, we then set a two-step heat treatment to produce bimodal α + β microstructure in Ti-10V-2Fe-3Al. Further mechanical testing shows that the ductility of the alloy is increased by ∼50% and the strength is increased by ∼10% as compared to its unimodal counterpart. Our work may provide a general way to improve the mechanical properties of alloys through multiscale microstructure design.

Effect of microstructure on fatigue crack deviation in AA2050-T84

Precipitation hardened AA2050 exhibits an unusual crack deviation phenomenon during fatigue testing: when the intended crack propagation direction is not parallel to macroscopic rolling direction, after initial propagation perpendicular to the loading direction, the crack deviates and further abnormally propagates in other direction which can be parallel to the direction of applied loading. Such a behaviour renders difficult the prediction of cracking in AA2050 and, thus, of AA2050 fatigue lifetime. In the present work using multi-scale microstructure characterization (SEM/EBSD/EDS, transmission electron microscopy (TEM) and atom probe tomography (APT)) of AA2050-T84 after fatigue tests, the effect of different microstructure parameters on crack deviation is carefully investigated keeping constant the macroscopic mechanical conditions (sample geometry, loading direction and amplitude, frequency). The respective influence of grain structure (including that of grain boundaries), intermetallics and precipitate-free zones is evaluated. The grain boundaries with misorientation angle ranging from 40 to 60° are observed to be the origin of the deviation phenomenon: the principal crack deviates at grain boundaries and further propagates along grain boundary. Numerous Fe- and Mn-rich intermetallics found in the volume of AA2050-T84 do not represent the major cause for the crack deviation, although, fostering it (only 25% of cracks were observed to deviate in the regions containing the intermetallics). The cause of failure at grain boundaries was deduced from APT results and TEM observation of precipitate free zones comparing T84 and T34 states (the latter do not reveal the crack deviation). In addition, it was observed that because of unrecrystallized state of AA2050-T84, the grains feature “layered” strained structure favourable to deviation inside the grains when a critical internal misorientation is reached along the crack path.

Multibioinspired Soft Grasping Actuators with Laser‐Induced Multiscale Microstructures

Soft robotics has been widely adopted in numerous applications of soft grippers, which utilize compliance to achieve superior grasping performances with excellent simplicity, adaptability, and robustness. The critical concerns for soft grippers are insufficient grasping ability and the limitation of functions. Herein, a multibionic soft gripper with multiscale microstructures is demonstrated, featuring light weight (12 mg), versatility, and endurance to heavy dust. The multibionic gripper mimics the grasping structure of eagle claws, the friction‐increasing structure of gecko feet, and the shining structural color on butterfly wings. External laser stimuli allow the grasping of various objects and precise measurement of the target sizes. The soft actuator realizes a systematic bionic function rather than a single bionic function, providing new possibilities for miniaturization, environmental adaptation, and multifunctionality of grasping actuators.

Entropy engineering enhances the thermoelectric performance and microhardness of (GeTe)1−x(AgSb0.5Bi0.5Te2)x

As an endogenous material genetic property, entropy is an emerging fingerprint factor in the material genome. By designing multicomponent thermoelectric materials with high configuration entropy, the lattice thermal conductivity can be significantly reduced via severe lattice distortion, and the Seebeck coefficient can be improved by enhancing the crystal symmetry. However, high entropy also causes the deterioration of carrier mobility, restraining the improvement of the dimensionless figure of merit zT. Herein, we design the (GeTe)1−x(AgSb0.5Bi0.5Te2)x, aka TABGS alloys, via replacing half of the Sb by Bi in the well-known (GeTe)1−x-(AgSbTe2)x, aka TAGS alloys, to attest the efficacy of entropy engineering. In view of the low carrier mean free path of TAGS alloys approaching to the Mott-Ioffe-Regel limit, further Bi substitution and increased configuration entropy cannot impair the carrier mobility any longer. In addition, the suppressed rhombohedral-cubic phase transition via high configuration entropy and the reduced carrier concentration contribute to the substantially improved Seebeck coefficient. Furthermore, the elicited multiscale microstructures and the decreased sound velocity upon AgSb0.5Bi0.5Te2 alloying effectively restrain the lattice thermal conductivity. As a result, (GeTe)0.80(AgSb0.5Bi0.5Te2)0.20 achieves a high zT of 1.60 at 723 K and an average zTave of 1.23 from 300 to 773 K. Meantime, its room-temperature Vickers hardness reaches 2.21 GPa due to solid solution hardening. These results highlight the efficacy of entropy engineering in enhancing the thermoelectric performance, especially for numerous materials with intrinsically low carrier mobility.

The Effects of Magnesium Borate Whiskers on the Mechanical Performance of Oil Well Cement

Quasi-brittle behaviors of cement-based material can be reinforced through diverse methods due to its unique multiscale features. Among them, reinforcing agents are the most direct and effective means, such as slag, metakaolin, and fly ash. These materials can improve the performance of cement stone to a certain extent, but their chemical composition is still mainly silica, which does not in essence change the defects of cement. Therefore, in this experiment, magnesium borate whisker (Mg2B2O5) was used as reinforcer of 90°C class G oil well cement and magnesium borate whisker reinforced cement-based material (MBWRC) was prepared. On the one hand, the performance of mechanical strength was controlled, and the resistance to compression, traction, and bending was included. Furthermore, static stress-strain behaviors analysis and toughness behaviors analysis (dynamic multicycle loading test) were further conducted. Second, mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) were used to test the characteristics of pores and interfaces of cement-based materials. Through multiscale microstructure analysis, MBWRC was found to have excellent 90°C mechanical performances when compared to control sample’s, for which tensile strength increased to 235% of controlled sample’s, and flexural strength increased to 130%, plus a healthy developed compressive performance, and MBWRC showed much denser pore structure, in which harmless micropore increased from 13.3% to 14.40%, porosity decreased from 17.01% to 16.20%, and permeability decreased from 0.2533% to 0.0205%. Furthermore, MBWRC showed resistance capability to mechanical loading, which can be attributed to the formation of denser pore structure and more excellent mechanical performance.

Hierarchical multiscale crystal plasticity framework for plasticity and strain hardening of multi-principal element alloys

The multi-principal element alloys (MPEAs) exhibit the unprecedented combinations of the excellent mechanical properties, especially high strength and good ductility. However, the accurate and reasonable models for describing the mechanical behavior of MPEAs are still scarce due to their distinctive serious lattice distortion effects, which limit the performance prediction. Here, we develop a new general framework by combining the atomic simulation, discrete dislocation dynamics, and crystal plasticity finite element method, to study the strain-hardening behavior for MPEAs, which achieves the influence of the complex cross-scale factors, including the lattice distortion at the nanoscale and the dislocation hardening at the microscale, on the plastic deformation. Compared with the classic crystal plasticity finite element, the bottom-up hierarchical multiscale model could couple the underlying physical mechanisms from the nano-micron-meso scales and captures the inhomogeneous strain field induced by the serious lattice distortion, thus showing the high accuracy and ubiquitous availability for MPEAs. The result shows that the prediction of the strain-stress curve in the polycrystal MPEAs agrees well with the experimental result at the quasi-static tension, which verifies the accuracy of the proposed method. In addition to the dislocation evolution, the heterogeneous strain distribution combined with the significant change from the orientation of some grains could be an important reason for the enhanced strength at the micron scale. The present work not only gives an insight into the relationship between the multiscale microstructure and strain hardening considering the mechanistic linkages of the lattice distortion, dislocation behavior, and grain structure, but also provides a general approach to physically predict the mesoscopic mechanical response in MPEAs.

Numerical investigation of multiscale lateral microstructures enhancing passive micromixing efficiency via secondary vortex flow

Passive micromixing can efficiently mix laminar flows through molecular and convective diffusion. Microstructures are expected to be efficient, easily integrated into micromixers, and suitable for micromixers over a wide range of Re. This paper presents the enhancement effects of the multiscale lateral microstructures on the flow field characteristics and mixing efficiency through numerical simulations at Re = 0.01–50. Inspired by the regulation of lateral microstructures on the local flow field, cross-scale staggered baffles (CSBs) were established and applied in typical passive micromixers. For low-Re conditions, the paired trapezoidal microstructures (PTMs) of the CSBs improved the mixing effect by increasing the local streamline tortuosity. For high-Re conditions, the PTMs of CSBs increased the number of expanding vortices in the microchannel, which could increase the size of the fluid interfaces, and an optimal mixing index with relatively little pressure drop was achieved. Moreover, the CSBs were applied to the serpentine curved channel, which caused large expanding vortices on the inner side of the curved channel, and then the state of the Dean vortices on the cross section of the curved channel changed. Therefore, compared with the conventional micromixer channel structure, lateral microstructures regulate the local flow field through the enhancement of the streamlines and the secondary flow effects, and lateral microstructures have great potential to improve the mixing efficiency over a wide range of Re.

High-temperature failure mechanism and defect sensitivity of TC17 titanium alloy in high cycle fatigue

• Internal crack initiation with nanograins is found in TC17 alloy at 400 °C. • Oxygen-rich layer formed at high temperature affects surface crack initiation. • Fatigue strength of notched specimens is insensitive to high temperature. • A fatigue strength model is proposed including effect of temperature and defect. Crack initiation is an essential stage of fatigue process due to its direct effect on fatigue failure. However, for titanium alloys in high-temperature high cycle fatigue (HCF), the crack initiation mechanisms remain unclear and the understanding for the defect sensitivity is also lacking. In this study, a series of fatigue tests and multi-scale microstructure characterizations were conducted to explore the high-temperature failure mechanism, and the coupled effect of temperature and defect on TC17 titanium alloy in HCF. It was found that an oxygen-rich layer (ORL) was produced at specimen surface at elevated temperatures, and brittle fracture of ORL at surface played a critical role for surface crack initiation in HCF. Besides, internal crack initiation with nanograins at high temperatures was a novel finding for the titanium alloy. Based on energy dispersive spectroscopy, electron backscatter diffraction and transmission electron microscope characterizations, the competition between surface and internal crack initiations at high temperatures was related to ORL at surface and dislocation resistance in inner microstructure. The fatigue strengths of smooth specimens decreased at elevated temperatures due to the lower dislocation resistance. While the fatigue strengths of the specimens with defect were not very sensitive to the temperatures. Finally, a fatigue strength model considering the coupled effect of temperature and defect was proposed for TC17 titanium alloy.