Constituent Layers Research Articles

Effects of alloying, microstructure and texture on the strength–ductility trade-off in layered aluminum

Tailoring constituent layers can lead to improved mechanical properties of layered metals. In this work, we designed and fabricated two samples of layered Al. One sample with AA3003 and AA1060 layers and the other with only AA1060 layers. The effects of these designs on the deformation behaviors are revealed by in-situ monitoring of the local strain evolution. It was found that larger microstructural and textural variations can lead to a larger difference in transverse compressive strain between constituent layers under uniaxial tension, which indicates more significant defomation incompatibility and thus a larger strain gradient caused by the constraint effect. The effects of the texture on the transverse compressive strain levels was analysed by the Lankford values calculated by the visco-plastic self-consistent method. Furthermore, the fracture behaviors of the two samples are compared and discussed based on the cross-sectional fracture morphologies. This work illustrates the potential of tailoring layered metals for a superior strength–ductility combination.

Influences of defects on the propagation of transverse waves in periodic piezoelectric laminate structure with nanoscaled layers

By the stiffness matrix method based on the nonlocal elastic continuum theory, the transmission spectra of the transverse elastic waves propagating normally in nanoscaled periodic piezoelectric laminates with defects are investigated in detail in this paper. Defect is introduced by replacing a constituent layer or inserting another layer in a unit-cell. The thicknesses of all sub-layers and defect layer are at the nanoscale. Simultaneously, complex band structures and localization factors are calculated and employed to verify the band structures described by the transmission spectra. The influences of the defect type, the defect material, the volume fraction of the defect layer in the unit-cell, the piezoelectric constants of the defect layer, and the nanoscale size-effect on the transmission spectra are analyzed and discussed in detail. The present study provides a theoretical guide for the design and applications of the nanoscaled acoustic filters or transducers with defects.

Structural Evaluation by FWD In Flexible Pavement

Pavement performance evaluation an prediction are of great importance to facilitate pavement management system. Pavement evaluations are conducted to determine functional and structural conditions of a pavement section either for purposes of routine monitoring or planned corrective action. The Falling Weight Deflectometer is a device that is used to evaluate pavement and pavement layer stiffness. It is a trailer-mounted device that operates by dropping a weight on to the pavement and measuring the resulting pavement deflections. Various computations may be performed on the deflection data to evaluate the pavement and the stiffness of it and its constituent layers. During testing, a FWD subjects the pavement surface to a load pulse which simulates the load produced by a rolling vehicle wheel. The load pulse is produced by dropping a large weight onto a "buffer" which shapes the pulse, and then transmitted to the pavement through a circular load plate. Data are acquired from various sensors for use in post-test analysis of pavement properties. Data collected from FWD can be used for determination of structural capacity of in-service pavements for analytical analysis of pavement performance, predicting layer moduli for pavement component layers, remaining life of pavements and in deciding strengthening and rehabilitation measures to be adopted for meeting the requirements. Analyses of data are being carried out using standard software as supplied by the manufacturer. Information such as load, layer types and thicknesses, range of moduli and Poisson’s ratio etc. for different layer materials would also be needed for FWD deflection data analysis. While carrying out analyses of data, it assumes material to be homogeneous, isotropic, linear elastic, weightless and semi-infinite. This report contains evaluation on flexible pavement by FWD and according to resulting data, design of overlay will be done accordingly.

First-Principles Study of n*AlN/n*ScN Superlattices with High Dielectric Capacity for Energy Storage.

As a paradigm of exploiting electronic-structure engineering on semiconductor superlattices to develop advanced dielectric film materials with high electrical energy storage, the n*AlN/n*ScN superlattices are systematically investigated by first-principles calculations of structural stability, band structure and dielectric polarizability. Electrical energy storage density is evaluated by dielectric permittivity under a high electric field approaching the uppermost critical value determined by a superlattice band gap, which hinges on the constituent layer thickness and crystallographic orientation of superlattices. It is demonstrated that the constituent layer thickness as indicated by larger n and superlattice orientations as in (111) crystallographic plane can be effectively exploited to modify dielectric permittivity and band gap, respectively, and thus promote energy density of electric capacitors. Simultaneously increasing the thicknesses of individual constituent layers maintains adequate band gaps while slightly reducing dielectric polarizability from electronic localization of valence band-edge in ScN constituent layers. The AlN/ScN superlattices oriented in the wurtzite (111) plane acquire higher dielectric energy density due to the significant improvement in electronic band gaps. The present study renders a framework for modifying the band gap and dielectric properties to acquire high energy storage in semiconductor superlattices.

The influence of interface effect on the microstructure and mechanical behavior of tri-metal Ti/Al/Cu laminated metal composites

A new strategy via a combination of rolling bonding and isolation method was developed to evaluate the mechanical behavior of Ti/Al/Cu laminated metal composite (LMC) under the rule of mixture (ROM) condition, with a great emphasis on the effect of interface structure. The results showed that the development of the microstructure through the thickness of the Al layer was inhomogeneous after rolling bonding, which was attributed to the formation of shearing action caused by the difference in flow properties between the constituent layers. Additionally, the evolution of the crystallographic texture of the Al layer at different positions indicated that the severe shearing effect resulted in the appearance of a typical shear texture r-Cube {001}<110> component in the region near the Ti side, and the coexistence of deformation and recrystallization textures in the region near the Cu side was considered to be the result of dynamic recovery. The measured strengths of Ti/Al/Cu LMCs significantly deviated from the predicted strengths in the calculation scheme of Cu, Al and Ti single sheets by the ROM (about 22.2 MPa), while the Ti + Al/Cu and Cu + Al/Ti schemes exhibited a slight deviation for the predicted results (about 4.7 MPa and 9 MPa). It is found that the existence of the layer interface contributed to the development of mechanical properties as a result of crack nucleation and propagation during plastic deformation, resulting in a large deviation between the experimental results and the predicted values in the scheme of ignoring the interface effect.

A methodology for the use of alkyd paint in thermally aged easel painting reconstructions for mechanical testing

For the preservation of painted cultural heritage on wooden substrates, it is important to understand the fracture mechanisms in the multilayer system of which they are constructed and how the environment plays a role in the composites’ physical properties. Past research has investigated the material response of each constituent layer but much more needs to be done to represent the heterogeneous composite structure of easel paintings. In recent years fracture mechanics concepts have been applied to glue and glue/chalk multilayers. However, few experiments have been conducted on multilayers that include oil paint, due to its very long, and impractical drying time, which can be a few years up to decades depending on the type of study. The paper presents a methodology for the use of thermally aged alkyd paint in easel painting reconstructions for mechanical testing, specifically as a substitute for naturally aged traditional linseed oil paint. Elastic and failure properties of the paint have been obtained from environmentally-controlled tensile tests on thin free-film samples. To obtain the characteristic properties of increased elastic modulus and reduced ductility, a thermal ageing protocol has been experimentally developed. The results are compared with data from the published literature, theoretical models and with 30-year-old samples of cold-pressed linseed oil lead white paint tested within this research work. The final methodology provides the research community with a viable way to produce samples that can be used to understand the behaviour of a (simplified) but complete multilayer system.

Laminated plate-type acoustic metamaterials with Willis coupling effects for broadband low-frequency sound insulation

Acoustic metamaterials provide a powerful tool for designing effective noise-reducing means at subwavelength scales. This paper is concerned with the sound insulation characteristics of plate-type acoustic metamaterials under some pragmatic conditions, including the effect of structure scale on the sound insulation performance, the Willis coupling effect in multilayer plate-type acoustic metamaterials, and the acoustic coupling between plate-type acoustic metamaterials and existing structures. Herein, laminated plate-type acoustic metamaterials (LPAM) composed of multilayer plate-type acoustic metamaterials sandwiched with porous materials are constructed to effectively insulate broadband low-frequency noise. By a numerical study on the normal, oblique, and diffuse-field incident sound transmission loss (STL) of the proposed LPAM and its constituent layers, the presence of superior sound insulation performance and the governing physical mechanism are discussed. The effectiveness of numerical study is validated by the STL experiments performed in a sound impedance tube for small-scale samples and in a reverberation and anechoic chamber for large-scale samples. The proposed LPAM construction, analysis technique, and reported findings may contribute to the design and application of plate-type acoustic metamaterials for broadband low-frequency noise control.

Anomalous optical excitations from arrays of whirlpooled lattice distortions in moiré superlattices.

Moiré superlattices formed by stacking two-dimensional crystals have reinvigorated the pursuit for emergent functionalities of engineered superlattices. Unique optical characteristics can be realized from the interplay between the electronic excitations and the atomic rearrangements owing to their intrinsic softness. Although large-scale reconstructions have been identified at small twist angles, they have been treated as being rigid at large twist angles. Here, we report that moiré superlattices made from single layers of MoS2 and WSe2 exhibit a pair of torsional strains with opposite chirality irrespective of the twist angle. The whirlpool-shaped periodic lattice distortions introduce fuzziness in the Raman spectra and universal redshifts to the intralayer excitons for all twist angles. We show that both of these modulations become weaker as the twist angle increases but do not disappear, whereas they are turned off when the constituent layers are not tightly coupled, thus establishing an essential structure-property relationship for moiré superlattices.

Testing of Fabric Reinforced Cementitious Matrix in Shear without Substrate

This paper aims at investigating the matrix-to-textile stress transfer in a fabric reinforced cementitious matrix FRCM system, not bonded to any substrate, under shear loads. To this end, direct shear tests are performed on a basalt FRCM specimen introduced into an innovative properly designed four-hinge frame loaded by a universal testing machine. The role of the single components in the global shear behavior of the FRCM is experimentally analyzed. Digital image correlation (DIC) is adopted for evaluating both the displacement and strain fields as well as for detecting the damage. Furthermore, the shear response of the tested FRCM material is reproduced via an effective numerical approach that considers the nonlinear behavior of the mortar and the possible micro-mechanisms that arise between the textile and the matrix, introducing suitable interfaces joining the FRCM constituent layers, i.e. textile and mortar layers. Experimental outcomes highlighted the non-negligible influence of the matrix in the shear response of the composite, both in strength and stiffness. The proven DIC technique demonstrated to be suitable also for this novel test type, since it allows to obtain shear strains, location and amplitude of cracks with satisfying accuracy, such as to make direct shear tests results a benchmark to be used for numerical simulations. Numerical analyses are performed in order to verify the efficiency of the proposed model in reproducing the mechanical behavior of FRCM composites under shear loads and in describing the damage patterns during the loading process.

Electron delocalization enhances the thermoelectric performance of misfit layer compound (Sn1−x Bi x S)1.2(TiS2)2

The misfit layer compound (SnS)1.2(TiS2)2 is a promising low-cost thermoelectric material because of its low thermal conductivity derived from the superlattice-like structure. However, the strong covalent bonds within each constituent layer highly localize the electrons thereby it is highly challenging to optimize the power factor by doping or alloying. Here, we show that Bi doping at the Sn site markedly breaks the covalent bonds networks and highly delocalizes the electrons. This results in a high charge carrier concentration and enhanced power factor throughout the whole temperature range. It is highly remarkable that Bi doping also significantly reduces the thermal conductivity by suppressing the heat conduction carried by phonons, indicating that it independently modulates phonon and charge transport properties. These effects collectively give rise to a maximum ZT of 0.3 at 720 K. In addition, we apply the single Kane band model and the Debye–Callaway model to clarify the electron and phonon transport mechanisms in the misfit layer compound (SnS)1.2(TiS2)2.

Effect of washing on quality, breathability performance and reusability of disposable face masks

Disposable face masks are among the personal protective equipment (PPE) that highly contribute to protecting people in the context of the current COVID-19 pandemic. Health authorities recommend wearing a mask as a barrier measure to limit the spread of viral respiratory diseases. During the first waves of the pandemic, besides professional high-quality PPE, decontaminated disposable mask reuse and homemade cloth masks were allowed due to scarcities. This work introduces a simple method based on-time history of the differential pressure, and an easy to use the setup for the testing of different kinds of respiratory protective masks for the purposes of quality control and evaluation of air permeability performance. The standard mask testing method and the new proposed approach were then used to evaluate the effect of machine washing on the widely used type of disposable masks; namely the surgical (medical) face masks. The objective is to determine the number of acceptable washing cycles that this kind of mask can withstand before losing its performance in terms of breathability and airflow resistance. Other quality characteristics such as material (fibres) degradation and hydrophobicity are investigated. Degradation mechanisms due to washing cycles for the different mask constituent layers were studied by scanning electron microscopy (SEM) imaging. This work is an attempt to contribute to the determination of the reusability threshold of general-purpose disposable surgical type face masks thereby contributing to the reduction of environmental concerns. Results in terms of the studied above parameters suggest limiting the reuse of standard type surgical masks to only one machine washing cycle.

Comprehensive quantum transport analysis of M-superlattice structures for barrier infrared detectors

In pursuit of designing superior type-II superlattice barrier infrared detectors, this study encompasses an exhaustive analysis of utilizing M-structured superlattices for both the absorber and barrier layers through proper band engineering and discusses its potential benefits over other candidates. The electronic band properties of ideally infinite M-structures are calculated using the eight band k.p method that takes into account the effects of both strain and microscopic interface asymmetry to primarily estimate the bandgap and density-of-states effective mass and their variation with respect to the thicknesses of the constituent material layers. In contrast, for practical finite-period structures, the local density-of-states and spectral tunneling transmission and current calculated using the Keldysh non-equilibrium Green’s function approach with the inclusion of non-coherent scattering processes offer deep insights into the qualitative aspects of miniband and localization engineering via structural variation. Our key results demonstrate how to achieve a wide infrared spectral range, reduce tunneling dark currents, induce strong interband wavefunction overlaps at the interfaces for adequate absorption, and excellent band-tunability to facilitate unipolar or bipolar current blocking barriers. This study, therefore, perfectly exemplifies the utilization of 6.1 Å material library to its full potential through the demonstration of band engineering in M-structured superlattices and sets up the right platform to possibly replace other complex superlattice systems for targeted applications.

Electromagnetic-thermal-structure multi-layer nonlinear elastoplastic modelling study on epoxy-impregnated REBCO pancake coils in high-field magnets

• Multi-physics elastoplastic nonlinear FE model with main constituent materials of epoxy impregnated REBCO pancake winding is developed. • Discrete stresses appear on all the constituent materials. • Stress of each constituent material induced by thermal mismatch cannot be ignored. • Plastic failure mainly induced by hoop stress propagates from inner to outer turns with transport current increasing. • Failure of the superconducting layer occurs before yield in Hastelloy due to the direct action of Lorentz force. The elastoplastic mechanical behaviour of an epoxy-impregnated REBCO pancake winding under cryogenics and high magnetic field is investigated in the frame of finite element (FE) modelling. A two-dimensional axisymmetric electromagnetic-thermal-structure multi-physics multi-layer FE model with main layers of the coated conductor and insulation materials is developed. The radial stress and hoop stress on each constituent layer induced by thermal mismatch stress during cooling are investigated. The mechanical behaviour of each constituent material is also analysed by considering the thermal mismatch stress and electromagnetic force under 20 T background field. The results show that discrete stresses appear in all the constituent materials indicating that the multi-layer winding model containing the main constituent materials is necessary for the accurate stress analysis in an epoxy-impregnated REBCO winding. The stress of each constituent material induced by thermal mismatch during cooling process are too high to be ignored in the subsequent electromagnetic structure analysis. The mechanical-magnetic coupling analyses show that the stresses of all the constituent materials increase with the transport current. The plastic failure mainly induced by hoop stress successively occurs on copper stabilizer and Hastelloy substrate of the innermost turn, and the plastic region propagates from the inner turns to the outer turns with the increase of transport current. The failure of the superconducting layer occurs before the yield in Hastelloy due to the direct action of Lorentz force on the superconducting layers. The transverse tensile stress increases with the increasing transport current, indicating that the risk of transverse delamination failure increases with the increase of transport current. The mechanical failure modes including delamination within the conductor, plastic deformation in substrate and crack in superconducting layer should be seriously considered.

Effect of work hardening discrepancy on strengthening of laminated Cu/CuZn alloys

To clarify the work hardening discrepancy effect on the strengthening of laminated metals, we designed two laminated Cu/Cu4Zn and Cu/Cu32Zn samples with different layer thicknesses. Cu/Cu4Zn has larger work hardening discrepancy between two constituent layers relative to Cu/Cu32Zn, but the yield strengths of two CuZn constituents are comparable. Uniaxial tensile results suggest Cu/Cu4Zn with larger work hardening discrepancy exhibits a significant strengthening at early deformation stage while Cu/Cu32Zn possesses a better ductility. The underlying mechanisms for the strengthening effect are attributed to more geometrically necessary dislocations accumulated at interfaces and severer strain localization due to the larger work hardening discrepancy.

Interlayer exciton valley polarization dynamics in large magnetic fields

In van der Waals heterostructures (HS) consisting of stacked MoSe$_2$ and WSe$_2$ monolayers, optically bright interlayer excitons (ILE) can be observed when the constituent layers are crystallographically aligned. The symmetry of the monolayers allows for two different types of alignment, in which the momentum-direct interlayer transitions are either valley-conserving (R-type alignment) or changing the valley index (H-type anti-alignment). Here, we study the valley polarization dynamics of ILE in magnetic fields up to 30~Tesla by time-resolved photoluminescence (PL). For all ILE types, we find a finite initial PL circular degree of polarization ($DoP$) after unpolarized excitation in applied magnetic fields. For ILE in H-type HS, we observe a systematic increase of the PL $DoP$ with time in applied magnetic fields, which saturates at values close to unity for the largest fields. By contrast, for ILE in R-type HS, the PL $DoP$ shows a decrease and a zero crossing before saturating with opposite polarization. This unintuitive behavior can be explained by a model considering the different ILE states in H- and R-type HS and their selection rules coupling PL helicity and valley polarization.

Degradation of Phonons in Disordered Moiré Superlattices.

The elastic collective modes of a moiré superlattice arise not from vibrations of a rigid crystal but from the relative displacement between the constituent layers. Despite their similarity to acoustic phonons, these modes, called phasons, are not protected by any conservation law. Here, we show that disorder in the relative orientation between the layers and thermal fluctuations associated with their sliding motion degrade the propagation of sound in the moiré superlattice. Specifically, the phason modes become overdamped at low energies and acquire a finite gap, which displays a universal dependence on the twist-angle variance. Thus, twist-angle inhomogeneity is manifested not only in the noninteracting electronic structure of moiré systems, but also in their phononlike modes. More broadly, our results have important implications for the electronic properties of twisted moiré systems that are sensitive to the electron-phonon coupling.

Effect of lamellar structural parameters on the bending fracture behavior of AA1100/AA7075 laminated metal composites

• The Al/Al laminated composites with excellent interface state were fabricated by accumulative roll bonding. • Toughening mechanism of Al/Al laminated composites under the condition of various lamellar structure parameters was explored. • The change of thickness in soft layer was considered as a main reason for affecting bending properties. • Crack deflection and arresting contributed to the extra toughness enhancing in the laminated composite with higher layer number. The lamellar structure has an important impact on the mechanical properties of dissimilar laminated metal composites (LMCs), including the thickness ratio of dissimilar metal constituent layers and the number of layers. AA1100 and AA7075 with thickness ratios of 1:4 and 3:4 were fabricated for multilayer AA1100/AA7075 LMCs by hot accumulative roll bonding (ARB) technology. The bending fracture characteristics of AA1100/AA7075 LMCs with different thickness ratios and numbers of constituent layers were investigated. The research results indicated that AA1100/AA7075 LMCs with a low thickness ratio exhibited better bending ductility and toughness than those with a high thickness ratio, which was attributed to the crack growth resistance caused by the thickness of the soft AA1100 layer. The toughening mechanism introduced by crack deflection or arresting contributed to the enhancement in the toughness of the LMCs compared with that of the single 7075Al layer. The bonding interfaces of AA1100/AA7075 LMCs with different numbers of layers are continuous and straight due to the high ARB temperature. A decrease in bending toughness was observed as the number of layers increased. Unlike LMCs with a low number of layers, crack deflection or interface delamination is also considered a main toughening mechanism in dissimilar LMCs in addition to the thickness effect.

Growth of vertically aligned BN nanosheets via Fe-catalyzed thermal chemical vapor deposition

Hexagonal boron nitride nanosheets (BNNSs) demonstrated potential applications for ultraviolet (UV) nanoelectronics, catalytic support, electron field emission, and many other areas. However, their extensive industrial application is limited by the complicated conventional procedures for scalable preparations. In this study, vertically aligned BNNSs were directly synthesized without any substrates using Fe-catalyzed thermal chemical vapor deposition (CVD) method under a NH 3 gas flow at 1200 °C via the catalytic pyrolysis of the inorganic hybrid precursor. BNNSs grew via an Oswald ripening process assisted by oriented attachment mechanism under Fe catalyzed by analyzing the evolution process. Most nanosheets are <10 nm thick, and the number of constituent layers is typically between 10 and 20 nm. BNNSs improved crystallinity, had high UV light emission, and had excellent anti-oxidation behavior because of their unique structures. The development of a simple, high yield and environmentally friendly synthesis method for BNNSs allows for the exploration of industrial upscaling applications. • Vertically aligned BNNSs deposited by a catalytic-thermal CVD route without substrates. • The inorganic precursor was prepared by a sample wet chemical synthesis methods. • BNNSs exhibited enhanced crystallinity and UV light emission as well as remarkable oxidation resistance. • BNNSs grow via an Oswald ripening process assisted by the oriented attachment mechanism.

Superior strength-ductility synergy of layered aluminum under uniaxial tensile loading: The roles of local stress state and local strain state

A typical layered Al composed of alternating fine-grained layers (FLs) and coarse-grained layers (CLs), fabricated by hot pressing and hot rolling, exhibits a superior strength-ductility synergy under uniaxial tensile loading compared with monolithic Al. The mechanism behind the excellent mechanical properties was elucidated from the perspective of local stress state and local strain state. We found that the operative slip systems of CLs cannot be predicted by the classical Schmid factor, indicating a complex local stress state differing from global uniaxial tension. The so-called local multiaxial stress state, probably caused by the deformation incompatibility between constituent layers, was then proposed, and proved feasible for predicting the slip systems in CLs. On the other hand, the digital image correlation revealed that the strong strain partitioning leads to the transformation of local strain state on CLs from uniaxial to biaxial stretch despite under uniaxial tensile loading. Such the local strain state was incorporated into the Taylor ambiguity method, which can derive magnitudes of operative slip systems in the CLs grains with different orientations. The Taylor ambiguity results agreed well with the multiple microbands formation revealed by the quasi in-situ electron backscatter diffraction coupled with a novel misorientation mapping approach. The characteristic slip system activation and dislocation microbands governed by the local stress state and local strain state imply more latent hardening, hence enhancing the strain hardening capability of layered Al. Our work provides a new direction to achieve a superior strength-ductility synergy by designing local stress/strain and tailoring corresponding deformation mechanisms.

Interface-affected mechanical properties and strengthening mechanisms in heterogeneous high entropy alloy nanolaminates

Introducing heterogeneous interfaces by constructing laminated structure is a promising avenue to achieve the controllable strengthening behavior of high entropy alloys. In this work, the microstructural evolution and mechanical properties of magnetron sputtered Ni/Fe50Mn30Co10Ni10 nanolaminates with equal layer thickness h ranging from 5 to 150 nm were investigated systematically. With reducing h, the nanoindentation hardness of Ni/FeMnCoNi nanolaminates firstly increased and subsequently decreased, emerging a maximum value at the critical h of ~25 nm due to the transformation of constraining barrier for dislocation slipping from the heterogeneous interfaces to columnar grain boundaries. Microstructural observation demonstrated that the interfacial structure of Ni/FeMnCoNi transformed from incoherent to completely coherent at h below ~25 nm, and both the constituent layers made comparable contribution to the plastic deformation of Ni/FeMnCoNi nanolaminates. The strong h-dependent mechanical behavior could be rationalized in terms of the co-deformation of constituent layers and the structural evolution of Ni/FeMnCoNi interface. Additionally, the strengthening mechanisms were discussed based on the confined slip of dislocation within the constituent layers and columnar grains.