固相合金中的C-Me偏聚理论在陶瓷中的应用

氧化锆陶瓷由于性能优异,已得到了广泛的应用.氧化锆陶瓷的相变影响其性能,为控制相变进而控制性能,相变机理的研究至关重要.用固体与分子经验电子理论计算了c-ZrO2、t-ZrO2和m-ZrO2的价电子结构,得到形成它们强键骨架的共价键上的总共价电子对数分别为3.19184、3.45528和3.79625.按固相合金中的C-Me偏聚理论的思想推测ZrO2从高温到低温的相变顺序应为液相→c相→t相→m相.从价电子结构进行的推断与实验结果完全一致,说明合金相变的电子理论可以扩展到陶瓷材料中.

固相合金中的C-Me偏聚理论

近代固溶体的微观不均匀性理论指出,当异类原子AB间的结合力大于同类原子AA、BB间的结合力时,溶质原子将按固定的规则呈短程有序分布。本文称这种短程有序为固相合金中的C-Me偏聚。如果用余瑞璜的“固体与分子经验电子理论”中的共价键上共用电子对的值n_α表示AB、AA、BB原子间结合倾向的大小,那么对固相合金可提出如下假设:

Role of tensile forces in hot tearing formation of cast Al-Si alloy

The instrumented applied rod casting apparatus （ARCA） was developed to investigate the effects of tensile forces in the hot tearing formation of cast AI-Si alloys. The obtained data of tensile forces/temperature was used to identify hot tearing initiation and propagation and the fracture surface of samples was also investigated. The result shows that the applied tensile forces have a complex effect on load onset for the hot tearing initiation and propagation. During the casting solidification, the tensile forces are gradually increased with the increase of solid fraction. Under the action of tensile forces, there will appear hot tearing and crack propagation on the surface of the sample. When the tensile forces exceed the inherent strength of alloys, there will be fractures on the sample. As for the A356 alloy, the critical fracture stress is about 0.1 MPa. The hot tearing surface morphology shows that the remaining intergranular bridge and liquid films are thick enough to allow the formation of dendrite-tip bumps on the fracture surface.

Effect of silicon on phase selection of ternary compounds during solidification of ZA84 magnesium alloy

Effect of silicon on the phase selection between τ phase（Mg32（Al,Zn）49） and φ phase（Al2Mg5Zn2） in ZA84（Mg-8Zn-4Al-0.3Mn） magnesium alloy produced by steel mold cast was studied using X-ray diffractometer,scanning electron microscope and differential scanning calorimeter.The results show that with increasing Si addition in ZA84 alloy,the liquidus temperatures of the alloys and the solidification temperature ranges decrease.The ternary compound in ZA84 alloy is mainly τ phase and a little φ phase.When adding Si to ZA84 alloy,the preferential precipitation sequence of the ternary compounds changes,φ phase preferentially forms,whereas τ phase is suppressed.The solidification kinetics study of phase selection indicates that there is a critical degree of undercooling of the melt.If the undercooling exceeds the critical degree,τ phase preferentially forms while φ phase is suppressed;otherwise,φ phase preferentially forms while τ phase is suppressed.

Numerical simulation for solid- liquid phase change of metal sodium in combined wick

Based on the finite volume method and the enthalpy-porous model the solid-liquid phase change of sodium in the combined wick is numerically studied.The one-temperature model is used since the thermal conductivity of sodium is close to that of the combined wick materials.The non-Darcy law and natural convection in the melting process are taken into account.The results show that a thin metal fiber felt in the combined wick can result in a faster melting rate of the sodium and a shorter time for the molten sodium to reach the maximum velocity which can shorten the time for the high-temperature heat pipe startup.A thick metal fiber felt in the combined wick can result in a uniform temperature distribution in the vertical heating wall and a small wall temperature difference which can reduce the possibility of an overheat spot.

TEM microstructure of rapidly solidified Mg-6Zn-1Y-1Ce alloy

Rapidly solidified（RS） Mg-6Zn-1Y-1Ce ribbons were prepared by single roller melt-spinning technique.Transmission electron microscopy and energy dispersive X-ray spectroscopy were employed to characterize the microstructure of RS ribbons.The results show that there is high density of particles distributed within grains and at grain boundaries in the region near wheel side.The particle density is decreased in the middle region and free surface region.The alloy is predominantly composed of supersaturated--Mg solid solution,T phase and W phase;meanwhile,a few icosahedral quasicrystalline and Mg4Zn7 particles are also observed.The T phase is confirmed having a body-centered orthorhombic structure that is transformed from the body-centered tetragonal structure Mg12Ce phase due to the partial substitution of Mg atoms by Zn.

Rapid solidification of Cu_(60)Co_(30)Cr_(10) alloy under different conditions

Metastable liquid phase separation and rapid solidification in a metastable miscibility gap were investigated on the Cu60Co30Cr10 alloy by using the electromagnetic levitation and splat-quenching.It is found that the alloy generally has a microstructure consisting of a（Co,Cr）-rich phase embedded in a Cu-rich matrix,and the morphology and size of the（Co,Cr）-rich phase vary drastically with cooling rate.During the electromagnetic levitation solidification processing the cooling rate is lower,resulting in an obvious coalescence tendency of the（Co,Cr）-rich spheroids.The（Co,Cr）-rich phase shows dendrites and coarse spheroids at lower cooling rates.In the splat quenched samples the（Co,Cr）-rich phase spheres were refined significantly and no dendrites were observed.This is probably due to the higher cooling rate,undercooling and interface tension.

Mechanochemical redox-based synthesis of highly porous CoxMn1-xOy catalysts for total oxidation

A mechanochemical redox reaction between KMnO4 and CoCl2 was developed to obtain a CoxMn1-xOy catalyst with a specific surface area of 479 m^2 g^-1,which was higher than that obtained using a co-precipitation(CP)method(34 m2 g^-1),sol-gel(SG)method(72 m^2 g^-1),or solution redox process(131 m^2 g^-1).During catalytic combustion,this CoxMn1-xOy catalyst exhibited better activity(T100 for propylene=~200℃)than the control catalysts obtained using the SG(325℃)or CP(450℃)methods.The mechanical action,mainly in the form of kinetic energy and frictional heating,may generate a high degree of interstitial porosity,while the redox reaction could contribute to good dispersion of cobalt and manganese species.Moreover,the as-prepared CoxMn1-xOy catalyst worked well in the presence of water vapor(H2O 4.2%,>60 h)or SO2(100 ppm)and at high temperature(400℃,>60 h).The structure MnO2·(CoOOH)2.93 was suggested for the current CoxMn1-xOy catalyst.This catalyst could be extended to the total oxidation of other typical hydrocarbons(T90=150°C for ethanol,T90=225°C for acetone,T90=250℃for toluene,T90=120℃for CO,and T90=540℃for CH4).Scale-up of the synthesis of CoxMn1-xOy catalyst(1 kg)can be achieved via ball milling,which may provide a potential strategy for real world catalysis.

Conception of tooling adapted to thixoforging of high solid fraction hot-crack-sensitive aluminium alloys

Thixoforging is a type of semi-solid metal processing at high solid fraction (0.7<φs<1), which involves the processing of alloys in the semi-solid state.Tooling has to be adapted to this particular process to benefit shear thinning and thixotropic behaviour of such semi-solid material.Tooling parameters, such as the forming speed and tool temperature, have to be accurately controlled because of their influence on thermal exchanges between material flow and tool.These thermal exchanges influence the high-cracking tendency and the rheology of the semi-solid material during forming, which affects parts properties and therefore their quality.Extrusion tests show how thermal exchanges influence quality of thixoforged parts made of 7075 aluminium alloys at high solid fraction by modifying process parameters like forming speed, tool temperature and tool thermal protector.Thus an optimum in terms of thermal exchanges has to be found between surface quality and mechanical properties of the part.A direct application is the evaluation of surface quality of thixoforged thin wall parts made of 7075 aluminium alloy.

Sustainable solid-state synthesis of uniformly distributed PdAg alloy nanoparticles for electrocatalytic hydrogen oxidation and evolution

New sustainable syntheses based on solid-state strategies have sparked enormous attention and provided novel routes for the synthesis of supported metallic alloy nanocatalysts(SMACs).Despite considerable recent progress in this field,most of the developed methods suffer from either complex operations or poorly controlled morphology,which seriously limits their practical applications.Here,we have developed a sustainable strategy for the synthesis of PdAg alloy nanoparticles(NPs)with an ultrafine size and good dispersion on various carbon matrices by directly grinding the precursors in an agate mortar at room temperature.Interestingly,no solvents or organic reagents are used in the synthesis procedure.This simple and green synthesis procedure provides alloy NPs with clean surfaces and thus an abundance of accessible active sites.Based on the combination of this property and the synergistic and alloy effects between Pd and Ag atoms,which endow the NPs with high intrinsic activity,the PdAg/C samples exhibit excellent activities as electrocatalysts for both the hydrogen oxidation and evolution reactions(HOR and HER)in a basic medium.Pd9Ag1/C showed the highest activity in the HOR with the largest j0,m value of 26.5 A g Pd^–1 and j0,s value of 0.033 mA cmPd^–2,as well as in the HER,with the lowest overpotential of 68 mV at 10 mA cm^–2.As this synthetic method can be easily adapted to other systems,the present scalable solid-state strategy may open opportunity for the general synthesis of a wide range of well-defined SMACs for diverse applications.

Cellular/dendritic transition,dendritic growth and microhardness in directionally solidified monophasic Sn-2%Sb alloy

Horizontal directional solidification experiments were carried out with a monophasic Sn-2%Sb（mass fraction） alloy to analyze the influence of solidification thermal parameters on the morphology and length scale of the microstructure. Continuous temperature measurements were made during solidification at different positions along the length of the casting and these temperature data were used to determine solidification thermal parameters, including the growth rate（VL） and the cooling rate（TR）. High cooling rate cells and dendrites are shown to characterize the microstructure in different regions of the casting, with a reverse dendrite-to-cell transition occurring for TR5.0 K/s. Cellular（λc） and primary dendrite arm spacings（λ1） are determined along the length of the directionally-solidified casting. Experimental growth laws relating λc and λ1 to VL and TR are proposed, and a comparative analysis with results from a vertical upward directional solidification experiment is carried out. The influence of morphology and length scale of the microstructure on microhardness is also analyzed.

Vertical section phase diagrams of La−Fe−B ternary system

The vertical sections of the La−Fe−B system were investigated using electron probe microanalysis and differential thermal analysis.Based on the microstructures and phase compositions of the as-cast and equilibrium alloys,together with their heat flow−temperature curves,phase diagrams for three vertical sections were drawn:La_(x)Fe_(82)B_(y)(x+y=18),La_(x)Fe_(70)B_(y)(x+y=30)and La_(x)Fe_(53)B_(y)(x+y=47),where x and y represent mass fraction of La and B,respectively,%.Additionally,according to the phase diagrams,the compound La2Fe14B was identified as a stable phase at high temperatures.It was found to be stable between 926.2 and 792.6℃;at low temperatures,however,it decomposed into α-La,α-Fe and LaFe_(4)B_(4),according to the reaction La_(2)Fe_(14)B→α-Fe+α-La+LaFe_(4)B_(4).

Interface kinetics modeling of binary alloy solidification by considering correlation between thermodynamics and kinetics

By considering collision-limited growth mode and short-range diffusion-limited growth mode simultaneously,an extended kinetic model for solid−liquid interface with varied kinetic prefactor was developed for binary alloys.Four potential correlations arising from effective kinetics coupling the two growth modes were proposed and studied by application to planar interface migration and dendritic solidification,where the linear correlation between the effective thermodynamic driving force and the effective kinetic energy barrier seems physically realistic.A better agreement between the results of free dendritic growth model and the available experiment data for Ni−0.7at.%B alloy was obtained based on correlation between the thermodynamics and kinetics.As compared to previous models assuming constant kinetic prefactor,a common phenomenon occurring at relatively low undercoolings,i.e.the interface migration slowdown,can be ascribed to both the thermodynamic and the kinetic factors.By considering universality of the correlation between the thermodynamics and kinetics,it is concluded that the correlation should be considered to model the interface kinetics in alloy solidification.

Refinement and strengthening mechanism of Mg−Zn−Cu−Zr−Ca alloy solidified under extremely high pressure

Mg−Zn−Cu−Zr−Ca samples were solidified under high pressures of 2-6 GPa.Scanning electron microscopy and electron backscatter diffraction were used to study the distribution of Ca in the microstructure and its effect on the solidification structure.The mechanical properties of the samples were investigated through compression tests.The results show that Ca is mostly dissolved in the matrix and the Mg_(2)Ca phase is formed under high pressure,but it is mainly segregated among dendrites under atmospheric pressure.The Mg_(2)Ca particles are effective heterogeneous nuclei ofα-Mg crystals,which significantly increases the number of crystal nuclei and refines the solidification structure of the alloy,with the grain size reduced to 22μm at 6 GPa.As no Ca segregating among the dendrites exists,more Zn is dissolved in the matrix.Consequently,the intergranular second phase changes from MgZn with a higher Zn/Mg ratio to Mg7Zn3 with a lower Zn/Mg ratio.The volume fraction of the intergranular second phase also increases to 22%.Owing to the combined strengthening of grain refinement,solid solution,and dispersion,the compression strength of the Mg-Zn-Cu-Zr-Ca alloy solidified under 6 GPa is up to 520 MPa.

Effects of semi-solid isothermal process parameters on microstructure of Mg-Gd alloy

The effects of semi-solid isothermal process parameters on the microstructure evolution of Mg-Gd rare earth alloy produced by strain-induced melt activation(SIMA)were investigated.The formation mechanism of the particles in the process of the isothermal treatment was also discussed.The results show that the microstructure of the as-cast alloy consists ofα-Mg solid solution, Mg5RE and Mg24RE5(Gd,Y,Nd)phase.After being extruded with an extrusion ratio of 14:1 at 380℃,the microstructure of Mg-Gd alloy changes from developed dendrites to near-equiaxed grains.The liquid volume fraction of the semisolid slurry gradually increases with elevating isothermal temperature or prolonging isothermal time during the partial remelting.To obtain an ideal semisolid slurry,the optimal process parameters for the Mg-Gd alloy should be 630℃for isothermal temperature and 30 min for the corresponding time,respectively,where the volume fraction of the liquid phase is 52%.

Synthesis and Re-refinement of Cu_3PSe_4

The title compound Cu3PSe4 was synthesized by the reaction of CuCl, P2Se5 and Se in a molar ratio of 1:1:1 at 500 C and structurally characterized by X-ray crystallography. The crystal belongs to orthorhombic, space group Pmn21 with cell parameters: a = 7.685(2), b = 6.656(1), c = 6.377(1) , V = 326.2(1) 3, Z = 2, Dc = 5.472 g/cm3, Mr = 537.43, F(000) = 476, m = 32.12 mm-1, R = 0.0642, wR = 0.1481 and S = 1.037. The 3-D structure can be regarded as constructed from the alternately stacking of [Cu(2)Se4] tetrahedral layers and Cu(1)PSe tetrahedral layers along the b direction, in which the Cu(2)Se layer is comprised of corner-sharing [Cu(2)Se4] tetrahedra along the a and c directions, and the Cu(1)PSe layer is consisted of alternately corner-sharing [Cu(1)Se4] tetrahedra and [PSe4] tetrahedra along the a and c directions.

Effect of magnesium on dispersoid strengthening of Al-Mn-Mg-Si(3xxx) alloys

The effects of magnesium addition on the dispersoid precipitation as well as mechanical properties of 3xxx alloys wereinvestigated. The microstructures in as-cast and heat-treated conditions were evaluated by optical microscopy and transmissionelectron microscopy. The results reveal that Mg has a strong influence on the distribution and volume fraction of dispersoids duringprecipitation heat treatment. The microhardness and yield strength at ambient temperature increase with increasing Mg content. Thesolid solution and dispersoid strengthening mechanisms of materials after heat treatment are quantitatively analyzed. Dispersoidstrengthening for the alloys is the predominant strengthening mechanism after precipitation heat treatment. An analytical model isintroduced to predict the evolution of ambient-temperature yield strength.

Modeling and Measurements of Heat Transfer Phenomena in Two-Phase PbSn Alloy Solidification in an External Magnetic Field

The main aim of this work is to study numerically the influence of an external magnetic field on the solidification processes of two-component materials. Based on the continuum model of two-phase flow a mathematical model for the directional solidification of a binary alloy in a magnetic field is presented. The model includes mass, momentum, energy and species mass conservation equations written in compressible form and additional relationships describing the temperature-solute coupling. The geometry under study is a cylindrical mold with adiabatic walls and cooled bottom. The macroscale transport in the solidification of alloys is governed by the progress of the two-phase mushy zone, which is treated by means of a porous medium approach. The volume fraction of liquid and solid phases, respectively, is calculated from a 2D approximation of the phase diagram. The results of calculation are compared with experimental data.