Tracking Regulatory Mechanism of Trace Fe on Graphene Electromagnetic Wave Absorption

Polarization and conductance losses are the fundamental dielectric attenuation mechanisms for graphene-based absorbers, but it is not fully understood in revealing the loss mechanism of affect graphene itself. For the first time, the reduced graphene oxide(RGO) based absorbers are developed with regulatory absorption properties and the absorption mechanism of RGO is mainly originated from the carrier injection behavior of trace metal Fe nanosheets on graphene. Accordingly, the minimum reflection loss(RLmin) of Fe/RGO-2composite reaches-53.38 dB(2.45 mm), and the effective absorption bandwidth achieves 7.52 GHz(2.62 mm) with lower filling loading of 2 wt%. Using off-axis electron hologram testing combined with simulation calculation and carrier transport property experiments, we demonstrate here the carrier injection behavior from Fe to graphene at the interface and the induced charge accumulation and rearrangement, resulting in the increased interfacial and dipole polarization and the conductance loss. This work has confirmed that regulating the dielectric property of graphene itself by adding trace metals can not only ensure good impedance matching, but also fully exploit the dielectric loss ability of graphene at low filler content,which opens up an efficient way for designing lightweight absorbers and may be extended to other types materials.

Recent developments on corrosion behaviors of Mg alloys with stacking fault or long period stacking ordered structures

Corrosion is one of the most drawbacks which restricts the wide applications of Mg alloys.In the last decade,the corrosion behaviors of Mg alloys with stacking fault(SF)and/or long period stacking ordered(LPSO)structures have obtained increasing attention.However,the corrosion mechanism of the SF–or LPSO–containing Mg alloys has not been well illustrated and even reverse results have been reported.In this paper,we have reviewed recent reports on corrosion behaviors of SF–or LPSO–containing Mg alloys to better clarify and understand the significance and mechanism.Moreover,some deficiencies are presented and advises are proposed for the development of corrosion resistant Mg alloys with SF or LPSO structures.

Nucleation potency prediction of LaB_6 with E2EM model and its influence on microstructure and tensile properties of Al-7Si-0.3Mg alloy

The edge-to-edge matching(E2EM)crystallographic model was used to predict the orientation relationships(ORs)between LaB6 and Al.Three different possible ORs can be predicted between LaB6 and Al,which are(100)Al||(100)LaB6,[001]Al||[001]LaB6;(110)Al||(110)LaB6,[001]Al||[001]LaB6;and(111)Al||(111)LaB6,[011]Al||[011]LaB6.The prediction results are perfectly confirmed through TEM analysis and prove the nucleation potency of LaB6.The refining efficacy of Al-2La-1B refiner and its influence on the tensile properties were investigated in the as-cast Al-7Si-0.3Mg alloy.According to the results,LaB6 has higher nucleation potency than TiB2,leading to better grain refining efficacy of Al-2La-1B refiner in the as-cast Al-7Si-0.3Mg alloy.Regarding the mechanical performances,tensile properties of the as-cast Al-7Si-0.3Mg casting alloy are prominently improved after addition of Al-2La-1B refiner,due to the refined microstructures.

Effect of pre-straining on structure and formation mechanism of precipitates in Al−Mg−Si−Cu alloy

The effect of pre-straining on the structure and formation mechanism of precipitates in an Al−Mg−Si−Cu alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM).Elongated and string-like precipitates are formed along the dislocations in the pre-strained Al−Mg−Si−Cu alloy.The precipitates formed along the dislocations exhibit three features:non-periodic atomic arrangement within the precipitate;Cu segregation occurring at the precipitate/α(Al)interface;different orientations presented in one individual precipitate.Four different formation mechanisms of these heterogeneous precipitates were proposed as follows:elongated precipitates are formed independently in the dislocation;string-like precipitates are formed directly along the dislocations;different precipitates encounter to form string-like precipitates;precipitates are connected by other phases or solute enrichment regions.These different formation mechanisms are responsible for forming different atomic structures and morphologies of precipitates.

Effects of postweld aging on the microstructure and properties of bobbin tool friction stir-welded 6082-T6 aluminum alloy

Samples of 6082-T6 aluminum alloy were subjected to bobbin tool friction stir welding (BT-FSW), and the joints were treated by postweld natural aging (PWNA) and postweld artificial aging (PWAA). The microstructure, microhardness, and tensile properties of the aged and as-welded specimens were investigated. Transmission electron microscopy (TEM) observations revealed that a large number of Guinier–Preston (GP) zones precipitated in the form of a network on the stir zone (SZ) after PWNA for 60 d, and a large number of β'' phases precipitated in the matrix for after PWAA for 6 h. As the aging time increased, the microhardness of the SZ and the thermomechanically affected zone (TMAZ) increased significantly, and the hardness of the SZ after PWAA for 6 h was close to that of the base metal (BM). With increasing PWNA time, the strength and strain increased slightly. When the PWAA time increased, the strength clearly increased, with a maximum value of 279.9 MPa after 6 h, while the strain decreased.

Effects of Nd on microstructure and mechanical properties of as-cast Mg-12Gd-2Zn-xNd-0.4Zr alloys with stacking faults

In order to study the effects of Nd addition on microstructure and mechanical properties of Mg-Gd-Zn-Zr alloys,the microstruc-ture and mechanical properties of the as-cast Mg-12Gd-2Zn-xNd-0.4Zr(x=0,0.5wt%,and 1wt%)alloys were investigated by using optical microscope,scanning electron microscope,X-ray diffractometer,nano indentation tester,microhardness tester,and tensile testing machine.The results show that the microstructures mainly consist ofα-Mg matrix,eutectic phase,and stacking faults.The addition of Nd plays a significant role in grain refinement and uniform microstructure.The tensile yield strength and microhardness increase but the compression yield strength decreases with increasing Nd addition,leading to weakening tension-compression yield asymmetry in reverse of the Mg-12Gd-2Zn-xNd-0.4Zr alloys.The highest ultimate tensile strength(194 MPa)and ultimate compression strength(397 MPa)are obtained with 1wt%Nd addition of the alloy.

Effects of ultrasonic dispersion on structure of electrodeposited Ni coating on AZ91D magnesium alloy

To obtain the refined electrodeposited nickel layer on AZ91D magnesium alloy,ultrasonic technology was applied in the processes of pre-treatment and electrodeposition.The phases of pre-treatment layer and the nickel coating were analyzed by X-ray diffractometry(XRD)and X-ray photoelectron spectroscopy(XPS),and the microstructure was observed by scanning electron microscopy(SEM).Then,the effects of ultrasonic dispersion on the microstructure of pre-treatment layer and the grain refinement of electrodeposited nickel layer were discussed.The results showed that the pre-treatment electrodeposited Cu-Sn layer with compact microstructure could be synthesized in alkaline copper-tin liquid with ultrasonic agitation,as a result,smooth and refined nickel coating formed on AZ91D magnesium alloy.On the other hand,preferred orientation in the coating decreased because of the refined grains.

Effects of cold rolling deformation on microstructure,hardness,and creep behavior of high nitrogen austenitic stainless steel

Effects of cold rolling deformation on the microstructure, hardness, and creep behavior of high nitrogen austenitic stainless steel （HNASS） are investigated. Microstructure characterization shows that 70% cold rolling deformation results in significant refinement of the microstructure of this steel, with its average twin thickness reducing from 6.4 μm to 14 nm. Nanoindentation tests at different strain rates demonstrate that the hardness of the steel with nano-scale twins （nt-HNASS） is about 2 times as high as that of steel with micro-scale twins （mt-HNASS）. The hardness of nt-HNASS exhibits a pronounced strain rate dependence with a strain rate sensitivity （m value） of 0.0319, which is far higher than that of mt-HNASS （m = 0.0029）. nt-HNASS shows more significant load plateaus and a higher creep rate than mt-HNASS. Analysis reveals that higher hardness and larger m value of nt-HNASS arise from stronger strain hardening role, which is caused by the higher storage rate of dislocations and the interactions between dislocations and high density twins. The more significant load plateaus and higher creep rates of nt-HNASS are due to the rapid relaxation of the dislocation structures generated during loading.

Achieving gradient heterogeneous structure in Mg alloy for excellent strength-ductility synergy

Most metals including Mg alloys have a longstanding dilemma of strength-ductility trade-off,which is hindering their wider applications.In this study,we propose a gradient heterogeneous grain(GHG)structure for evading this trade-off dilemma and ultrasonic severe surface rolling is attempted to construct this novel structure in ZE41 Mg alloy.Here,the GHG structure combine the benefits of gradient structure and heterogeneous grain structure and introduce large microstructural heterogeneities.Compared to the coarse-grain and heterogeneous-grain structured alloys,the GHG structured one exhibits dramatical enhancement in strength,ductility,and strain hardening capability.To the best of our knowledge,its strength becomes much higher than that of common ZE41 Mg alloys at no reduction in ductility.These unique mechanical properties stem from not only the individual contribution of the heterogeneous structure components including the fine/ultrafine grains and deformed coarse grains but also their synergistic effect via hetero-deformation induced strengthening and hardening effects.In summary,our study provides a feasible way to develop new Mg alloys with high strength and good ductility.

In vitro bio-corrosion behaviors of biodegradable AZ31B magnesium alloy under static stresses of different forms and magnitudes

Biomedical degradable materials would be subjected to different degrees and forms of static stress after being implanted in the human body.In this work,the biocorrosion behaviors of AZ31B magnesium alloy under different stress forms with different magnitudes(20~150MPa)were studied.It was found that the corrosion behaviors at stressed conditions were severer than those at unstressed conditions and corrosion rates were obviously accelerated.The biocorrosion behaviors are more sensitive to the effects of tensile loads than to compressive loads.A biocorrosion numerical model on the degradation process of Mg alloy under static loads was established.The corrosion current density(i_(corr))of Mg alloy and the applied static stress(σ)matches a linear relationship of ln i_(corr)~σwell during the early stage(within 24 hrs)while deviated gradually in the latter period of corrosion.This work could provide a guidance and theoretical reference for further researches on the biocorrosion behaviors and practical clinical applications of the biomedical materials subjected to physiological loads.

Stress corrosion cracking of magnesium alloys:A review

Magnesium(Mg)alloys have been widely used in automobile,aviation,computer,and other fields due to their lightweight,high specific strength and stiffness,low pollution,and good electromagnetic shielding performance.However,the chemical stability of Mg alloys is poor,especially in the corrosive medium environment with high stress corrosion sensitivity,which causes sudden damage to structural components and restricts their application field.In recent years,owing to the increasing failure rate of engineering structures caused by stress corrosion of Mg alloys,it has become necessary to understand and pay more attention to the stress corrosion cracking(SCC)behavior of Mg alloys.In this paper,the SCC mechanisms and test methods of Mg alloys have been summarized.The recent research progress on SCC of Mg alloys has been reviewed from the aspects of alloying,preparation process,surface modification,corrosive medium,and strain rate.More importantly,future research trends in the field of SCC of Mg alloys have also been proposed.

Structure design and property adjustment of new cage rich-nitrogen pentazolyltetraazacubanes as potential high energy density compounds

In this study,based on two attractive energetic compounds pentazole(PZ) and tetraazacubane(TAC),a new family of high energy and high nitrogen compounds pentazolyltetraazacubanes were designed.Then,a different number of NH2 or NO2 groups were introduced into the system to further adjust the property.The structures,properties,and the structure-property relationship of designed molecules were investigated theoretically.The results showed that all nine designed compounds have extremely high heat of formation(HOF,1226-2734 kJ/mol),good density(1,73-1.88 g/cm3),high detonation velocity(8.30-9.35 km/s),high detonation pressure(29.8-39.7 GPa) and acceptable sensitivity(△V:41-87 A3).These properties could be effectively positive adjusted by replacing one or two PZ rings by NH2 or/and NO2 groups,especially for the energy and sensitivity performance,which were increased and decreased obviously,respectively.As a result,two designed pentazolyltetraazacubanes were predicted to have higher energy and lower sensitivity than the famous high energy compound in use 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane,while two others have better co mbination property than 1,3,5-Trinitro-1,3,5-triazacyclohexane.In all,four new pentazolyltetraazacubanes with good combination performance were successfully designed by combining PZ with TAC,and the further property adjustment strategy of introducing a suitable amount of NH2/NO2 groups into the system.This work may help develop new cage energetic compounds.

Accurate prediction method for the microstructure of amorphous alloys without non-metallic elements

A new structural parameter of amorphous alloys called atomic bond proportion was proposed, and a topological algorithm for the structural parameter was proven feasible in the previous work. In the present study, a correction factor, λ,is introduced to optimize the algorithm and dramatically improve the calculation accuracy of the atomic bond proportion.The correction factor represents the ability of heterogeneous atoms to combine with one another to form the metallic bonds and it is associated with the uniformity of the master alloy, mixing enthalpy, cooling rate during preparation, and annealing time. The correction factor provides a novel pathway for researching the structures of the amorphous alloys.

Time-resolved ultra-small-angle X-ray scattering beamline(BL10U1)at SSRF

The construction of a new beamline,BL10U1,was completed at the Shanghai synchrotron radiation facility in 2020.This multipurpose beamline was designed to provide X-ray scattering techniques such as ultra-small-angle X-ray scattering(USAXS),small-angle X-ray scattering(SAXS),wide-angle X-ray scattering,and microfocus SAXS(μSAXS)for a broad user community.To realize fast time-resolved USAXS experiments,the beamline adopted an in-vacuum undulator with a total length of 1.6 m as the photon source.An in-house cryogenic-cooled double multilayer monochromator was installed to deliver a photon flux of approximately 10^(13) photons/s at a photon energy of 10 keV.The three-year successful operation of this beamline demonstrated that the monochromator operated smoothly,as expected.BL10U1 has three end stations in succession:USAXS end station,μSAXS end station,and end station for industrial applications.The minimum scattering vector q~0.0042 nm^(-1) at 10 keV can be achieved at the USAXS end station equipped with a 28 m-long and 1.8 m-diameter vacuum flight tube.At theμSAXS end station,a beam spot of less than 10×8μm was achieved for micro-SAXS experiments.In contrast,in situ experimental instruments up to 5 m high and 8 m wide can be mounted at the industrial application end station,which offers industrial scientists the opportunity to use their large industrial equipment.BL10U1 opens up a new capability to investigate phenomena such as non-equilibrium and dynamic processes of materials with a wide length scale from angstroms to micrometers with millisecond time resolution.In this paper,we also report beamline design considerations and commissioning results.

Effect of current density on corrosion resistance of micro-arc oxide coatings on magnesium alloy

Oxide coatings were prepared on magnesium alloys in electrolyte solution of Na2SiO3 at different current densities(3,4 and 5 A/cm 2 )with micro-arc oxidation process.X-ray diffractometry(XRD)results show that the oxide coatings formed on magnesium alloys are mainly composed of MgO and MgAl2O4 phases;in addition,the content of MgO increases with increasing the current density.The morphology and surface roughness of the coatings were characterized by confocal laser scanning microscopy (CLSM).The results show that the surface roughness(Ra)decreases with increasing the current density.Moreover,the electrochemical corrosion results prove that the MgO coating produced in the electrolyte Na2SiO3 at current density of 5 A/cm 2 shows the best corrosion resistance.

The microscopic mechanical performance for nonuniform welded joint of nickel-based alloy with nanoindentation

To quantify the nonuniform micromechanical performance of welded joint,the load-displacement curves by nanoindentation test were introduced to examine different zones including base metal,coarse grained heat affected zone,partially melted zone,weld metal near the fusion boundary and weld metal center.The results showed that the strengthening effect of weld metal was more obvious than that of heat affected zone for nickel based welded joint and especially in coarse grained heat affected zone,the hardening resulted from overheating was not apparent.Nickel based weld metal with high content of alloying elements which were often segregated at interdendritic regions or precipitated in grain interior under nonequilibrium solidification contributed to the characteristics that differ from conventional low alloy steel welded joint.

Corrosion behavior of Mg-3Gd-1Zn-0.4Zr alloy with and without stacking faults

To develop biodegradable magnesium alloy with desirable corrosion properties,a low Gd-containing Mg-3Gd-1Zn-0.4Zr(wt%,GZ31K)alloy was prepared.The as-cast ingot was solution treated and then hot extruded.Microstructures were characterized by scanning electron microscopy(SEM).Corrosion behavior of the alloy under each condition was studied by hydrogen evolution and quasi in-situ corrosion methods.It has been found that the as-cast alloy is composed ofα-Mg,stacking faults(SFs)at the outer edge of the matrix grains,and eutectic phase along the grain boundaries.After solution treatment,the SFs disappear and precipitates rich in Zn and Zr elements form in the grain interior and boundaries.The microstructure is significantly refined after extrusion.Hydrogen evolution tests show that the as-cast alloy exhibits the best corrosion resistance,and the solution-treated alloy has the worst corrosion resistance.Corrosion rate of the alloy under each condition decreases first and then increases with prolonging immersion time.Corrosion experiments demonstrate thatα-Mg was corroded preferentially,the eutectic phase and precipitates exhibit better corrosion resistance.The as-extruded alloy demonstrates uniform corrosion due to fine and homogeneous microstructure.

Effect of solution concentration on sealing treatment of Mg-Al hydrotalcite film on AZ91D Mg alloy

Cerium-based sealing treatment was developed for Mg-Al hydrotalcite film on AZ91D Mg alloy,and the influence of cerium salt solution was investigated to modify the surface integrity and corrosion resistance.Scanning electron microscope(SEM)and X-ray diffraction(XRD)measurements were carried out to analyze the surface morphology and phase composition.The corrosion resistance of Mg-Al hydrotalcite film after sealing treatment was evaluated by the polarization curve and electrochemical impedance spectroscopy(EIS)tests.The results showed that lower concentration of Ce-containing solution was beneficial to seal the micro-cracks on Mg-Al hydrotalcite film,and improve the surface integrity and corrosion resistance;higher concentration of Ce-containing solution could seal fewer micro-cracks,and the corrosion resistance was decreased owing to the disintegration of Mg-Al hydrotalcite film.

Friction and wear behaviors of biodegradable Mg-6Gd-0.5Zn-0.4Zr alloy under simulated body fluid condition

The friction and wear behaviors of biodegradable Mg-6Gd-0.5Zn-0.4Zr(wt%,GZ60K)alloy were evaluated under simulated body fluid(SBF)condition using ball-on-disk configuration and compared with those under dry sliding condition.The results show that under dry sliding and SBF conditions,the friction coefficient declines with increasing applied load and keeps stable with prolonging sliding time.The friction coefficient of the alloy effectively decreases in SBF as compared to dry sliding due to lubrication caused by SBF.The real wear rates under SBF condition are lower than those under dry sliding condition for each parameter.Nevertheless,the nominal wear rates are higher in SBF which are attributed to the more mass loss caused by corrosion but not wear.Both the nominal wear rate in SBF and the dry sliding wear rate increase with increasing applied load,and they decline firstly and then keep stable with prolonging sliding time.It is concluded that the wear of the alloy is restricted by the SBF,but the corrosion of the alloy is aggravated by the wear.

Thermoviscoelastic Modeling Approach for Predicting the Recovery Behaviors of Thermally Activated Amorphous Shape Memory Polymers

A thermoviscoelastic modeling approach is developed to predict the recovery behaviors of the thermally activated amorphous shape memory polymers(SMPs)based on the generalized finite deformation viscoelasticity theory.In this paper,a series of moduli and relaxation times of the generalized Maxwell model is estimated from the stress relaxation master curve by using the nonlinear regression(NLREG)method.Assuming that the amorphous SMPs are approximately incompressible isotropic elastomers in the rubbery state,the hyperelastic response of the materials is well modeled with a hyperelastic model in Ogden form.In addition,the Williams-Landel-Ferry(WLF)equation is used to describe the horizontal shift factor obtained with time-temperature superposition principle(TTSP).The finite element simulations show good agreement with the experimental thermomechanical behaviors.Moreover,the possibility of developing a temperature-responsive intravascular stent with the SMP studied here is investigated in terms of its thermomechanical property.Therefore,it can be concluded that the model has good prediction capabilities for the recovery behaviors of amorphous SMPs.