Study of Hollow Cathode Arc Welding Technology for Ti Alloys

In this paper, technology margin experiment of hollow cathode fusion arc welding (FAW) and penetrating arc welding (PAW) is presented in detail. The advantages of PAW. compared with FAW, are better weld formation, narrower welds, as well as a larger welding technology margin. Technological experiment and hydraulic test for the Ti alloy pressure vessels with PAW are described. Also, the weld formation control on Ti alloys is preliminarily analyzed. Finally, conclusions are addressed.

Crystallography of precipitates in Mg alloys

Crystallography of precipitates in Mg alloys is indispensable to explain and predict alloy microstructures and properties.In order to obtain a global understanding of diversified experimental results,a general theory of singular interface is introduced,which provides the physical base and calculation methodology for interpreting precipitate morphology and orientation relationship(OR),especially useful for understanding irrational facets and ORs.Guided by the theory,recent experimental findings are systematically summarized,including thermally stable and metastable precipitates with various crystal structures.Then,theoretical advances inspired by the findings are introduced,which deepens our understanding on OR selection and preference of irrational facets.At last,future research directions in this field are proposed.

Preparation and properties of CMAS resistant bixbyite structured high-entropy oxides RE_(2)O_(3)(RE=Sm,Eu,Er,Lu,Y,and Yb):Promising environmental barrier coating materials for Al_(2)O_(3)f/Al_(2)O_(3) composites

Y_(2)O_(3) is regarded as one of the potential environmental barrier coating(EBC)materials for Al_(2)O_(3)f/Al_(2)O_(3)ceramic matrix composites owing to its high melting point and close thermal expansion coefficient to Al_(2)O_(3).However,the relatively high thermal conductivity and unsatisfactory calcium-magnesium-aluminosilicate(CMAS)resistance are the main obstacles for the practical application of Y_(2)O_(3).In order to reduce the thermal conductivity and increase the CMAS resistance,four cubic bixbyite structured high-entropy oxides RE_(2)O_(3),including(Eu_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3),(Sm_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3),(Sm_(0.2)Eu_(0.2)Er_(0.2)Y_(0.2)Yb_(0.2))2O_(3),and(Sm_(0.2)Eu_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3)were designed and synthesized,among which(Eu_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3)and(Sm_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3)bulks were prepared by spark plasma sintering(SPS)to investigate their mechanical and thermal properties as well as CMAS resistance.The mechanical properties of(Eu_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3)and(Sm_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3) are close to those of Y_(2)O_(3) but become more brittle than Y_(2)O_(3).The thermal conductivities of(Eu_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3) and(Sm_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb02)2O_(3)(5.1 and 4.6 W·m^(-1)·K^(-1))are only 23.8%and 21.5%respectively of that of Y_(2)O_(3)(21.4 W·m^(-1)·K^(-1)),while their thermal expansion coefficients are close to those of Y_(2)O_(3) and A12O_(3).Most importantly,HE RE_(2)O_(3) ceramics exhibit good CMAS resistance.After being attacked by CMAS at 1350℃for 4 h,the HE RE_(2)O_(3) ceramics maintain their original morphologies without forming pores or cracks,making them promising as EBC materials for Al_(2)O_(3)f/Al_(2)O_(3) composites.

等离子火焰下C/C复合材料的SiC/ZrC-ZrB2双层涂层的耐烧蚀性能及其机理

为了提高碳/碳(C/C)复合材料的耐烧蚀性能,采用包埋法和料浆刷涂法制备了SiC/ZrC-ZrB2双层涂层。在2300°C以上的等离子火焰下测试了SiC/ZrC-ZrB2涂层的耐烧蚀性能。结果表明,SiC/ZrC-ZrB2双层涂层的耐烧蚀性能优于ZrC-ZrB2单层涂层。等离子火焰烧蚀20 s后,ZrC-ZrB2涂层的质量烧蚀率和线烧蚀率分别为0.89 mg/s和15.3μm/s,而SiC/ZrC-ZrB2涂层的质量烧蚀率和线烧蚀率分别为0.09 mg/s和24.15μm/s,烧蚀过程中,SiC内层产生SiO2玻璃相,形成ZrO2-SiO2熔融膜。与ZrC-ZrB2涂层表面形成的ZrO2熔融膜相比,具有较低氧扩散速率和黏度的ZrO2-SiO2熔融膜使SiC/ZrC-ZrB2涂层具有较好的自愈性。因此,SiC/ZrC-ZrB2涂层耐烧蚀性能的提高可归因于等离子烧蚀过程中形成的致密的ZrO2-SiO2熔融膜。

Synthesis and mechanical and elevated temperature tribological properties of a novel high-entropy(TiVNbMoW)C_(4.375) with carbon stoichiometry deviation

High-entropy carbides are a nascent group of ceramics that are promising for high-temperature applications due to the combination of good stability,high hardness(H),high strength,and superior creep resistance that they display.Due to high melting points and low lattice diffusion coefficients,however,the high-entropy carbides are usually difficult to consolidate to a nearly full density.To cope with this challenge,herein,binary carbides including TiC,V_(8)C_(7),NbC,Mo_(2)C,and WC with different carbon stoichiometry were used to prepare dense high-entropy(TiVNbMoW)C_(4.375),and the influence of carbon vacancy on formation ability and mechanical properties of carbon-deficient high-entropy(TiVNbMoW)C_(4.375) were investigated.Intriguingly,although the starting binary carbides have different crystal structures and carbon stoichiometry,the as-prepared high-entropy material showed a rock-salt structure with a relatively high density(98.1%)and good mechanical properties with hardness of 19.4±0.4 GPa and fracture toughness(KIC)of 4.02 MPa·m^(1/2).More importantly,the high-entropy(TiVNbMoW)C_(4.375) exhibited low coefficient of friction(COF)at room temperature(RT)and 800℃.Wear rate(W)gradually increased with the temperature rising,which were attributed to the formation of low-hardness oxidation films at high temperatures to aggravate wear.At 800℃,lubricating films formed from sufficient oxidation products of V_(2)O_(5) and MoO_(3) effectively improved tribological behavior of the high-entropy(TiVNbMoW)C_(4.375).Wear mechanisms were mainly abrasive wear resulting from grain pullout and brittle fracture as well as oxidation wear generated from high-temperature reactions.These results are useful as valuable guidance and reference to the synthesis of high-entropy ceramics(HECs)for sliding parts under high-temperature serving conditions.

Development and testing of a wireless smart toolholder with multi-sensor fusion

The smart toolholder is the core component in the development of intelligent and precise manufacturing.It enables in situ monitoring of cutting data and machining accuracy evolution and has become a focal point in academic research and industrial applications.However,current table and rotational dynamometers for milling force,vibration,and temperature testing suffer from cumbersome installation and provide only a single acquisition signal,which limits their use in laboratory settings.In this study,we propose a wireless smart toolholder with multi-sensor fusion for simultaneous sensing of milling force,vibration,and temperature signals.We select force,vibration,and temperature sensors suitable for smart toolholder fusion to adapt to the cutting environment.Thereafter,structural design,circular runout,dynamic balancing,static stiffness,and dynamic inherent frequency tests are conducted to assess its dynamic and static performance.Finally,the smart toolholder is tested for accuracy and repeatability in terms of force,vibration,and temperature.Experimental results demonstrate that the smart toolholder accurately captures machining data with a relative deviation of less than 1.5%compared with existing force gauges and provides high repeatability of milling temperature and vibration signals.Therefore,it is a smart solution for machining condition monitoring.

Modeling of the process-induced stress, damage, microstructure, and deformation evolution during the pyrolysis process manufacturing CMCs

An insightful understanding of the formation mechanism of process-inherent defects anddeformation is increasingly important for the property evaluation and structural design of ceramicmatrix composites (CMCs). For this purpose, a coupled thermal–diffusive–mechanical modelingapproach was proposed by considering three important phenomena that occur during the pyrolysisprocess for manufacturing CMCs: variations of the physical and mechanical properties of theconstituents, generation and diffusive of pyrolysis gas, and multiple thermal deformations. Thesynergistic effects of these three phenomena on the stress, damage development, microstructuralmorphology, and process deformation of SiC matrix composites were investigated using finiteelement simulations. This new approach was validated by comparing the simulation and experimentalresults. Significant volume shrinkage of the matrix during the polymer-to-ceramic transformationresulted in large tensile stresses and subsequent highly fragmented microstructure in CMCs. Thepyrolysis-gas-induced expansion on the matrix under a damage state may yield a positive processdeformation of CMCs at the macroscale, overcoming the effects of the volume shrinkage of the bulkmatrix at the microscale. The modeling approach is expected to guide high-quality manufacturing ofCMCs and comprehensive studies of structure–processing–property relationships.

Liquid crystal elastomer composites for soft actuators

Liquid crystal elastomers are active materials that combine the anisotropic properties of liquid crystals with the elasticity of polymer networks.The LCEs exhibit remarkable reversible contraction and elongation capabilities in response to external stimuli,rendering them highly promising for diverse applications,such as soft robotics,haptic devices,shape morphing structures,etc,However,the predominant reliance on heating as the driving stimulus for LCEs has limited their practical applications.This drawback can be effectively addressed by incorporating fllers,which can generate heat under various stimuli.The recent progress in LCE composites has significantly expanded the application potential of LCEs.In this minireview,we present the design strategies for soft actuators with LCE composites,followed by a detailed exploration of photothermal and electrothermal LCE.composites as prominent examples.Furthermore,we provide an outlook on the challenges and opportunities in the feld of LCE composites.

智能材料形状记忆合金研究进展

As one of the newly developing intelligent/smart materials,shape memory alloys(SMAs)have become an important material and have broad application prospects.With smart structures,the integration of SMAs as actuators and sensors in structural components,has drawn significant attention and interest in the aerospace field.In this paper,the research status of SMAs at home and abroad in recent years was reviewed,including the characteristics,classification,investigation progress and applications in the aerospace field.Finally,the development trend prospects for SMAs was also presented.

Nb-and Ta-doped(Hf,Zr,Ti)C multicomponent carbides with enhanced oxidation resistance at 2500℃

Enhancing oxidation resistance of multicomponent carbides above 2000℃is critical for their thermal protection applications.For this purpose,novel Nb-and Ta-doped(Hf,Zr,Ti)C multicomponent carbides were designed to improve their oxidation resistance at 2500℃.The results demonstrated that Nb and Ta doping reduced the oxidation rate constant by 16.67%and 25.17%,respectively,thereby significantly improving the oxidation resistance of(Hf,Zr,Ti)C.This enhancement was attributed to the changes in oxycarbide composition and distribution within the oxide layer by adding Nb and Ta.Owing to the different oxidation tendencies of the constituent elements,a distinctive structure was formed in which(Hf,Zr)O_(2)served as a skeleton,and various oxycarbides were dispersed throughout the oxide layer.The doped Nb and Ta were retained within oxycarbides,retarding the diffusion of oxygen into the lattice.More importantly,the addition of Nb and Ta reduced the size of oxycarbides,decreasing both size and quantity of the pores in the oxide layer and facilitating the formation of a more effective oxygen barrier.

Theoretical prediction on thermal and mechanical properties of high entropy(Zr(0.2)Hf(0.2)Ti(0.2)Nb(0.2)Ta(0.2))C by deep learning potential

High entropy materials(HEMs, e.g. high entropy alloys, high entropy ceramics) have gained increasing interests due to the possibility that they can provide challenge properties unattainable by traditional materials. Though a large number of HEMs have emerged, there is still in lack of theoretical predictions and simulations on HEMs, which is probably caused by the chemical complexity of HEMs. In this work,we demonstrate that the machine learning potentials developed in recent years can overcome the complexity of HEMs, and serve as powerful theoretical tools to simulate HEMs. A deep learning potential(DLP) for high entropy(Zr(0.2) Hf(0.2) Ti(0.2) Nb(0.2) Ta(0.2))C is fitted with the prediction error in energy and force being 9.4 me V/atom and 217 me V/?, respectively. The reliability and generality of the DLP are affirmed,since it can accurately predict lattice parameters and elastic constants of mono-phase carbides TMC(TM = Ti, Zr, Hf, Nb and Ta). Lattice constants(increase from 4.5707 ? to 4.6727 ?), thermal expansion coefficients(increase from 7.85×10-6 K^(-1) to 10.58×10-6 K^(-1)), phonon thermal conductivities(decrease from 2.02 W·m-1·K^(-1) to 0.95 W·m-1·K^(-1)), and elastic properties of high entropy(Zr(0.2) Hf(0.2) Ti(0.2) Nb(0.2) Ta(0.2))C in temperature ranging from 0°C to 2400°C are predicted by molecular dynamics simulations. The predicted room temperature properties agree well with experimental measurements, indicating the high accuracy of the DLP. With introducing of machine learning potentials, many problems that are intractable by traditional methods can be handled now. It is hopeful that deep insight into HEMs can be obtained in the future by such powerful methods.

Effect of reaction routes on the porosity and permeability of porous high entropy(Y0.2Yb0.2Sm0.2Nd0.2Eu0.2)B6 for transpiration cooling

Transpiration cooling technique is a reusable and high-efficiency thermal protection system(TPS),which is potential to improve the reusability and security of re-entry space vehicle.Relatively low density,high permeability and high porosity are general requirements for porous media of transpiration cooling systems.In this work,a new porous high entropy metal hexaboride(Y0.2Yb0.2Sm0.2Nd0.2Eu0.2)B6 is designed and prepared by the in-situ reaction/partial sintering method.Two reaction routes are designed to synthesize(Y0.2Yb0.2Sm0.2Nd0.2Eu0.2)B6,including boron thermal reduction and borocarbon thermal reduction.The as-prepared porous HE(Y0.2Yb0.2Sm0.2Nd0.2Eu0.2)B6 ceramics possess homogeneous microstructure and exhibit low density,high porosity,high compressive strength and high permeability.The combination of these properties makes porous HE(Y0.2Yb0.2Sm0.2Nd0.2Eu0.2)B6 promising as a candidate porous media for various transpiration cooling applications.

One-step synthesis and electromagnetic absorption properties of high entropy rare earth hexaborides(HE REB6)and high entropy rare earth hexaborides/borates(HE REB6/HE REB03)composite powders

Considering the emergence of severe electromagnetic interference problems,it is vital to develop electromagnetic(EM)wave absorbing materials with high dielectric,magnetic loss and optimized impedance matching.However,realizing the synergistic dielectric and magnetic losses in a single phase material is still a challenge.Herein,high entropy(HE)rare earth hexaborides(REB6)powders with coupling of dielectric and magnetic losses were designed and successfully synthesized through a facial one-step boron carbide reduction method,and the effects of high entropy borates intermedia phases on the EM wave absorption properties were investigated.Five HE REB6 ceramics including(Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6,(Ce0.2Hu0.2Sm0.2Er0.2Yb0.2)B6,(Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6,(Ce0.2Ya2Sm0.2Eu0.2Yb0.2)B6,and(Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6 possess CsCl-type cubic crystal structure,and their theoretical densities range from 4.84 to 5.25 g/cm^(3).(Ce02Y0.2Sm0.2Er0.2Yb02)B6 powders with the average particle size of 1.86 jim were found to possess the best EM wave absorption properties among these hexaborides.The RLmin value of(Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6 reaches-33.4 dB at 11.5 GHz at thickness of 2 mm;meanwhile,the optimized effective absorption bandwidth(EAB)is 3.9 GHz from 13.6 to 17.5 GHz with a thickness of 1.5 mm.The introduction of HE REB03(RE=Ce,Y,Sm,Eu,Er,Yb)as intermediate phase will give rise to the mismatching impedance,which will further lead to the reduction of reflection loss.Intriguingly,the HEREB6/HEREB03 still possess wide effective absorption bandwidth of 4.1 GHz with the relative low thickness of 1.7 mm.Considering the better stability,low density,and good EM wave absorption properties,HE REB6 ceramics are promising as a new type of EM wave absorbing materials.

Electromagnetic wave absorbing properties of TMCs(TM=Ti,Zr,Hf,Nb and Ta)and high entropy(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C

Electromagnetic wave(EMW)absorbing materials play a vital role in modern communication and information processing technologies to inhibit information leakage and prevent possible damages to environment and human bodies.Currently,most of EMW absorbing materials are either composites of two or more phases or in the form of nanosheets,nanowires or nanofibers in order to enhance the EMW absorption performance through dielectric loss,magnetic loss and dielectric/magnetic loss coupling.However,the combination of complex shapes/multi phases and nanosizes may compound the difficulties of materials processing,composition and interfaces control as well as performance maintenance during service.Thus,searching for single phase materials with good stability and superior EMW absorbing properties is appealing.To achieve this goal,the EMW absorbing properties of transition metal carbides TMCs(TM=Ti,Zr,Hf,Nb and Ta)and high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C which belong to ultrahigh temperature ceramics,were investigated in this work.Due to the good electrical conductivity and splitting of d orbitals into lower energy t2glevel and higher energy eglevel in TMC6octahedral arrangement,TMCs(TM=Ti,Zr,Hf,Nb and Ta)exhibit good EMW absorbing properties.Especially,Hf C and Ta C exhibit superior EMW absorbing properties.The minimum reflection loss(RLmin)value of Hf C is-55.8 d B at 6.0 GHz with the thickness of 3.8 mm and the effective absorption bandwidth(E_(AB))is 6.0 GHz from 12.0 to 18.0 GHz at thickness of 1.9 mm;the RL_(minvalue)of Ta C reaches-41.1 d B at 16.2 GHz with a thickness of 2.0 mm and the EABis 6.1 GHz with a thickness of 2.2 mm.Intriguingly,the electromagnetic parameters,i.e.,complex permittivity and permeability are tunable by forming single phase solid solution or high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C.The R_(Lminvalue)of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C is-38.5 d B at 9.5 GHz with the thickness of 1.9 mm,and the EABis 2.3 GHz(from 11.3 to 13.6 GHz)at thickness of 1.5 mm.The significance of this work is that it opens a new window to design single phase high performance EMW absorbing materials by dielectric/magnetic loss coupling through tuning the conductivity and crystal field splitting energy of d orbitals of transition metals in carbides,nitrides and possibly borides.

Achieving strong microwave absorption capability and wide absorption bandwidth through a combination of high entropy rare earth silicide carbides/rare earth oxides

Developing electromagnetic(EM) wave absorbing materials with low reflection coefficient and optimal operating frequency band is urgently needed on account of the increasingly serious EM pollution. However, the applications of common EM absorbing materials are encumbered by poor high-temperature stability, poor oxidation resistance, narrow absorption bandwidth or high density. Herein, the strong EM absorption capability and wide efficient absorption bandwidth of high entropy ceramics are reported for the first time, which are designed by a combination of the novel high entropy(HE) rare earth silicide carbides/rare earth oxides(RE3 Si2 C2/RE2 O3). Three HE powders, i.e., HERSC-1(HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2),HERSC-2 HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2/HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)2 O3) and HERSC-3(HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2/HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)2 O3), are synthesized. Although HERSC-1 exhibits a limited absorption effect(the minimum reflection loss(RLmin) is-11.6 d B at 3.4 mm) and a relatively narrow effective absorption bandwidth(EAB) of 1.7 GHz, the optimal absorption RLminvalue and EAB of HERSC-2 and HERSC-3 are-40.7 d B(at 2.9 mm), 3.4 GHz and-50.9 d B(at 2.0 mm), 4.5 GHz,respectively, demonstrating strong microwave absorption capability and wide absorption bandwidth.Considering the better stability, low density and strong EM absorption effect, HE ceramics are promising as a new type of EM absorbing materials.

Temperature Dependent Thermal and Elastic Properties of High Entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2):Molecular Dynamics Simulation by Deep Learning Potential

High entropy diborides are new categories of ultra-high temperature ceramics,which are believed promising candidates for applications in hypersonic vehicles.However,knowledge on high temperature thermal and mechanical properties of high entropy diborides is still lacking unit now.In this work,variations of thermal and elastic properties of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) with respect to temperature were predicted by molecular dynamics simulations.Firstly,a deep learning potential for Ti-Zr-Hf-Nb-Ta-B diboride system was fitted with its prediction error in energy and force respectively being 9.2 meV/atom and 208 meV/A,in comparison with first-principles calculations.Then,temperature dependent lattice constants,anisotropic thermal expansions,anisotropic phonon thermal conductivities,and elastic properties of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) from 0℃to 2400℃were evaluated,where the predicted room temperature values agree well with experimental measurements.In addition,intrinsic lattice distortions of(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) were analyzed by displacements of atoms from their ideal positions,which are in an order of 10^(-3) A and one order of magnitude smaller than those in(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C.It indicates that lattice distortions in(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) is not so severe as expected.With the new paradigm of machine learning potential,deep insight into high entropy materials can be achieved in the future,since the chemical and structural complexly in high entropy materials can be well handled by machine learning potential.

Theoretical prediction,synthesis,and crystal structure determination of new MAX phase compound V2SnC

Guided by the theoretical prediction,a new MAX phase V2SnC was synthesized experimentally for the first time by reaction of V,Sn,and C mixtures at 1000°C.The chemical composition and crystal structure of this new compound were identified by the cross-check combination of first-principles calculations,X-ray diffraction(XRD),energy dispersive X-ray spectroscopy(EDS),and high resolution scanning transmission electron microscopy(HR-STEM).The stacking sequence of V2C and Sn layers results in a crystal structure of space group P63/mmc.The a-and c-lattice parameters,which were determined by the Rietveld analysis of powder XRD pattern,are 0.2981(0)nm and 1.3470(6)nm,respectively.The atomic positions are V at 4f(1/3,2/3,0.0776(5)),Sn at 2d(2/3,1/3,1/4),and C at 2a(0,0,0).A new set of XRD data of V2SnC was also obtained.Theoretical calculations suggest that this new compound is stable with negative formation energy and formation enthalpy,satisfied Born-Huang criteria of mechanical stability,and positive phonon branches over the Brillouin zone.It also has low shear deformation resistance c44(second-order elastic constant,cij)and shear modulus(G),positive Cauchy pressure,and low Pugh’s ratio(G/B=0.500<0.571),which is regarded as a quasi-ductile MAX phase.The mechanism underpinning the quasi-ductility is associated with the presence of a metallic bond.

Improved thermal stability and infrared emissivity of high-entropy REMgAl_(11)O_(19)and LaMAl_(11)O_(19)(RE=La,Nd,Gd,Sm,Pr,Dy;M=Mg,Fe,Co,Ni,Zn)

LaMgAl_(11)O_(19)(LMA),characterized by high melting point,low density and thermal conductivity as well as good infrared emissivity,is regarded as a potential candidate for the thermal protection of hypersonic vehicles.Nevertheless,the unsatisfied phase stability at high temperature results in declining of the emissivity below 6μm,which limits the extensive applications of LaMgAl_(11)O_(19).In order to overcome this obstacle,three dense bulk high-entropy ceramics,(La_(0.2)Nd_(0.2)Gd_(0.2)Sm_(0.2)Pr_(0.2))MgAl_(11)O_(19)(HE LMA-1),(La_(0.2)Nd_(0.2)Gd_(0.2)Sm_(0.2)Dy_(0.2))Mg Al_(11)O_(19)(HE LMA-2)and La(Mg_(0.2)Fe_(0.2)Co_(0.2)Ni_(0.2)Zn_(0.2))Al_(11)O_(19)(HE LMA-3),were designed and successfully prepared through solid state reaction at 1700℃for 4 h in one step.XRD analyses show that the phase compositions of HE LMA-1,HE LMA-2 and HE LMA-3 are single-phase solid solutions with the relative density of 95.61%,95.49%and 94.31%,respectively.Heat treatment experiments demonstrate that the phase composition of HE LMA-1 remains a single phase after high-temperature heating,while second phase appears in other two samples.The stability of HE LMA-1 is attributed to small average size differenceδ(~4%)of constitute elements.Intriguingly,the average emissivity of HE LMA-1 in the range of 3-6μm reaches 0.9,which is significantly higher than that of LMA and other two HE LMA samples.The emissivity of all samples remains above 0.95 from 6 to 10μm.In the far infrared region(10-14μm),although the emissivity of these specimens decreases slightly,it still exceeds 0.85.The UV-Vis absorption spectra indicate that the formation of many discrete impurity energy levels with small intervals in HE LMA-1 promotes the f electrons to transit between adjacent impurity energy levels and conduction band,which enhances the infrared emission of HE LMA-1 below 6μm.In a word,with improved phase stability and thermal emissivity in infrared range,high-entropy REMgAl_(11)O_(19),especially(La_(0.2)Nd_(0.2)Gd_(0.2)Sm_(0.2)Pr_(0.2))MgAl_(11)O_(19)(HE LMA-1),is a promising candidate in thermal protection coatings of hypersonic vehicles.

Diamond wheel grinding characteristics of 3D orthogonal quartz fiber reinforced silica ceramic matrix composite

Fiber-reinforced silica ceramic matrix composites(SiO_(2) f/SiO_(2))have gained extensive attention in recent years for its applications in aeronautics field such as radar radome and window.However,the machining properties and mechanism of the material remain unclear.The features and mechanical properties of the material itself have a significant influence on both its machining characteristics and surface integrity.Thus,a full-factor grinding experiment is conducted using a 3 D orthogonal SiO_(2) f/SiO_(2) aiming to obtain its machining characteristics.The effects of grinding parameters and tools on the grinding force,surface roughness,and material damage type are investigated using a dynamometer,Scanning Electron Microscope(SEM),and Acoustic Emission(AE)analysis.The AE frequency band is analyzed,and a semi-analytical force model is established to study the difference between a single grain and wheel grinding.It was found that the changes in surface roughness correlate with the changes in grinding force,with fiber fracture being the main reason behind the increase in grinding force.Finally,the material removal mechanism was studied based on the AE analysis.It was found that the removal mechanism is fiber fracture dominated with matrix crack and debonding,and the primary sources of energy consumption are fiber fracture and friction.

Oxidation behavior of non-stoichiometric(Zr,Hf,Ti)C_(x) carbide solid solution powders in air

Multi-component solid solutions with non-stoichiometric compositions are characteristics of ultra-high temperature carbides as promising materials for hypersonic vehicles.However,for group IV transition-metal carbides,the oxidation behavior of multi-component non-stoichiometric(Zr,Hf,Ti)C_(x)carbide solid solution has not been clarified yet.The present work fabricated four kinds of(Zr,Hf,Ti)C_(x)carbide solid solution powders by free-pressureless spark plasma sintering to investigate the oxidation behavior of(Zr,Hf,Ti)C_(x)in air.The effects of metallic atom composition on oxidation resistance were examined.The results indicate that the oxidation kinetics of(Zr,Hf,Ti)C_(x)are composition dependent.A high Hf content in(Zr,Hf,Ti)C_(x)was beneficial to form an amorphous Zr-Hf-Ti-C-0 oxycarbide layer as an oxygen barrier to enhance the initial oxidation resistance.Meanwhile,an equiatomic ratio of metallic atoms reduced the growth rate of(Zr,Hf,Ti)O_(2)oxide,increasing its phase stability at high temperatures,which improved the oxidation activation energy of(Zr,Hf,Ti)C_(x).