Magnetically induced phase separation in the fcc phase and thermodynamic calculations of phase equilibria of the Co-Cr system

Two-phase equilibria between the ferromagnetic fcc and the paramagnetic fcc phase from 800 ℃ to 900 ℃ in the Co-rich region have been detected by the diffusion couple technique. Two phase separation region of the fcc has been confirmed along the Curie temperature.The phase equilibria including the present results and the thermodynamic data of the Co-Cr system reported in the literature were analyzed on the basis of the thermodynamic evaluation. A set of thermodynamic values for the liquid, fcc, hcp, bcc, sigma phases was obtained. The calculated phase equilibria were in good agreement with most of the experimental data.

Phase transition and thermodynamic properties of BiFeO_3 from first-principles calculations

The first-principles projector-augmented wave method employing the quasi-harmonic Debye model,is applied to investigate the thermodynamic properties and the phase transition between the trigonal R3c structure and the orthorhombic Pnma structure.It is found that at ambient temperature,the phase transition from the trigonal R3c phase to the orthorhombic Pnma phase is a first-order antiferromagnetic-nonmagnetic and insulator-metal transition,and occurs at 10.56 GPa,which is in good agreement with experimental data.With increasing temperature,the transition pressure decreases almost linearly.Moreover,the thermodynamic properties including Grneisen parameter,heat capacity,entropy,and the dependences of thermal expansion coefficient on temperature and pressure are also obtained.

Thermodynamic calculation of high zinc-containing Al-Zn-Mg-Cu alloy

Phase fraction and solidification path of high Zn-containing Al-Zn-Mg-Cu series aluminum alloy were calculated by calculation of phase diagram （CALPHAD） method. Microstructure and phases of Al-9.2Zn-1.7Mg-2.3Cu alloy were studied by X-ray diffraction （XRD）, differential scanning calorimetry （DSC） and scanning electron microscopy （SEM）. The calculation results show that η（MgZn2） phase is influenced by Zn and Mg. Mass fractions of η（MgZn2） in Al-xZn-1.7Mg-2.3Cu are 10.0%, 9.8% and 9.2% for x=9.6, 9.4, 8.8 （mass fraction, %）, respectively. The intervals of Mg composition were achieved for θ（Al2Cu）＋η（MgZn2）, S（Al2CuMg）＋η（MgZn2） and θ（Al2Cu）＋S（Al2CuMg）＋η（MgZn2） phase regions. Al3Zr, α（Al）, Al13Fe4, η（MgZn2）, α-AlFeSi, Al7Cu2Fe, θ（Al2Cu）, Al5Cu2MgsSi6 precipitate in sequence by no-equilibrium calculation. The SEM and XRD analyses reveal that α（Al）, η（MgZn2）, Mg（Al,Cu,Zn）2, θ（Al2Cu） and Al7Cu2Fe phases are discovered in Al-9.2Zn-1.7Mg-2.3Cu alloy. The thermodynamic calculation can be used to predict the major phases present in experiment.

Experimental Study and Thermodynamical Calculation on the Formation of Eutectic Phase of MnS-RE_2S_3 and(Mn,Ca)S-RE_2S_3 in Sulfur-Containing Free-Cutting Steel

The free-cutting phase in RE or Ca-RE treated sulfur-containing free-cutting steel is the eutectic phases of MnS-RE_2S_3 and (Mn,Ca)S-RE_2S_3,respectively.The atomic ratio of RE/S needed to modify all the MnS into the eutectic phase is higher than 1.48 or 1.41-1.37 Ca/S,when RE or Ca-RE is used as the modifiz- er in the sulfur-containing free-cutting steel.Moreover,the thermodynamical calculation shows that the eutectic temperature is lower than the solidifying temperature,which is the key condition for the eutectic phase to keep globual during solidifying.

Thermodynamic Calculation of Si_3N_4-SiC Phase Equilibrium

SisN4 and SiC phase stability via gas phase reactions among SiO, CO/CO2 and N2 has been calculated based on thermochemical equilibrium. The influences of carbon activity (αC), and the partial pressure of SiO (PSiO), CO (PCO) and N2 (PN2) on the Si3N4-SiC stability have been studied and the related phase diagrams have been constructed. Result shows that the lowering αC and PCO/PSiO ratio and the increasing PN2 greatly elevate the Si3N4-SiC equilibrium temperature. Some previously observed experimental results related to Si3N4 and SiC formation at different temperature from the gas phase reactions have been discussed and some guides for sintering and synthesis Of the Si3N4 materials have been proposed

Thermodynamic Calculation of Carbide Precipitate in Niobium Microalloyed Steels

On the basis of regular solution sublattice model, thermodynamic equilibrium of austenite/carbide in Fe-Nb-C ternary system was investigated. The equilibrium volume fraction, chemical driving force of carbide precipitates and molar fraction of niobium and carbon in solution at different temperatures were evaluated respectively. The volume fraction of precipitates increases, molar fraction of niobium dissolved in austenite decreases and molar fraction of carbon increases with decreasing the niobium content. The driving force increases with the decrease of temperature, and then comes to be stable at relatively low temperatures. The predicted ratio of carbon in precipitates is in good agreement with the measured one.

Phase stability,electronic,elastic and thermodynamic properties of Al-RE intermetallics in Mg-Al-RE alloy:A first principles study

Electronic structure and elastic properties of Al_(2)Y,Al_(3)Y,Al_(2)Gd and Al_(3)Gd phases were investigated by means of first-principles calculations from CASTEP program based on density functional theory(DFT).The ground state energy and elastic constants of each phase were calculated,the formation enthalpy(ΔH),bulk modulus(B),shear modulus(G),Young's modulus(E),Poisson's ratio(ν)and anisotropic coefficient(A)were derived.The formation enthalpy shows that Al_(2)RE is more stable than Al_(3)RE,and Al-Y intermetallics have stronger phase stability than Al-Gd intermetallics.The calculated mechanical properties indicate that all these four intermetallics are strong and hard brittle phases,it may lead to the similar performance when deforming due to their similar elastic constants.The total and partial electron density of states(DOS),Mulliken population and metallicity were calculated to analyze the electron structure and bonding characteristics of the phases.Finally,phonon calculation was conducted,and the thermodynamic properties were obtained and further discussed.

Calculation of Cu-rich part of Cu-Ni-Si phase diagram

The thermodynamic calculation of phase equilibria in the Cu-Ni-Si alloy system was carried out using the CALPHAD method. The calculations show that there are three two-phase areas and two three-phase areas in the Cu-rich parts of the isothermal section of the phase diagram at 300-600 ℃,and the three two-phase areas are FCC-A1(Cu-rich)+γ-Ni5Si2,FCC-A1(Cu-rich)+δ-Ni2Si and FCC-A1(Cu-rich)+ε-Ni3Si2,two three-phase areas are FCC-A1(Cu-rich)+γ-Ni5Si2+δ-Ni2Si and FCC-A1(Cu-rich)+δ-Ni2Si+ε-Ni3Si2. For this reason,an alloy located in the Cu-rich portion may precipitate γ-Ni5Si2,δ-Ni2Si or ε-Ni3Si2;the proportion of each phase depends on the alloy composition and aging temperature. The transmission electron microscope analysis of the Cu-3.2Ni-0.75Si alloy indicates that the precipitates are mainly δ-Ni2Si with only a few γ-Ni5Si2 phase particles,which agrees well with the thermodynamic calculations of phase equilibria.

Phase transition, thermodynamic and elastic properties of ZrC

First principles calculation and quasi-harmonic Debye model were used to obtain more physical properties of zirconium carbide under high temperature and high pressure.The results show that the B1structure of ZrC is energetically more favorable with lower heat of formation than the B2structure,and that mechanical instability and positive heat of formation induce the inexistence of the B2structure at normal pressure.It is also found that the B1structure would transform to the B2structure under high pressure below the critical point of V/V0=0.570.In addition,various thermodynamic and elastic properties of ZrC are obtained within the temperature range of0-3000K and the pressure range of0-100GPa.The calculated results not only are discussed and understood in terms of electronic structures,but also agree well with corresponding experimental data in the literature.

Thermodynamics for the non-conventional synthesizing of out-of-plane ordered double-transition metal“312”and“413”MAX phases(o-MAX):A high throughput linear programing first-principles calculation

The reaction thermodynamics for synthesizing the“312”and“413”o-MAX phases using the powder met-allurgy are investigated using a linear programing optimization algorithm based on the high-throughput first principles phonon calculations.The validity and reliability of the current methodology are verified by correctly predicting the impurities in four experimentally known o-MAX systems including Cr-Ti-Al-C,Cr-V-Al-C,Mo-Sc-Al-C and Mo-Ti-Al-C.The formability of each investigated o-MAX phase is evaluated by means of formation enthalpy and formation Gibbs free energy in a temperature range from 0 K to 1700 K.It is revealed that the thermodynamic stability of the“413”o-MAX structure is no better than that of the“312”phase.The formability of“413”o-MAX is also reduced at high sintering temperature,compared to that of“312”phase.The optimal synthetic routes are predicted for all thermodynamically stable“312”and“413”o-MAX phases.It is found that most o-MAX phases considered could be prepared as the single phase using the non-conventional synthetic routes from the aspect of reaction thermodynamics.Few of them including Cr_(2)TaAlC_(2),Nb_(2)HfAlC_(2),Nb_(2)TaAlC_(2),Nb_(2)Hf_(2)AlC_(3),Nb_(2)Ta_(2)AlC_(3),Mo_(2)V_(2)AlC_(3)and Mo_(2)Ta_(2)AlC_(3)are predicted to be either destabilized at high temperature or overwhelmed by the most competing side reaction.

NEW METHODS FOR CALCULATING ACTIVITIES OF COMPONENTS FROM PHASE DIAGRAMS IN BINARY PERITECTIC SYSTEMS

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Thermodynamic optimization of the DyCl_3-KCl and DyCl_3-CaCl_2 systems

From the measured phase diagram data and experimental thermochemical properties, the DyCl3-KCl and DyCl3-CaCl2 phase diagrams were optimized and calculated by the CALPHAD technique. The Gibbs energies of liquid phase in the two systems has been optimized and calculated by new modified quasi-chemical model in the pair-approximation for short-range ordering, and a series of thermodynamic functions has also been optimized based on an interactive computer-assisted analysis. The results showed that the calculated phase diagrams and thermodynamic data were serf-consistent.

A thermodynamic assessment of Ce-Al system

The optimized descriptions of the phase diagram and thermodynamic properties for Ce-Al system have been obtained from experimental thermodynamic and phase diagram data by means of the computer program THERMO-CALC based on the least squares method, using models for the Gibbs energy of individual phases. The system contains five intermetallic compounds. The calculated standard enthalpies of Ce_3Al, CeAl, CeAl_2, CeAl_3 and Ce_3Al_ 11 are -26.7, -48.9, -48.4, -44.0 and -41.7 kJ/mol, respectively. A consistent set of thermodynamic parameters was derived. The optimized and experimental data are in good agreement.

Optimization and Calculation of the Al-Be-Si System

Lattice stability of Be is estimated,and interaction parameters for the liquids in the Al-Be and Be-Si systems are obtained using phase dia- gram data from literatures.Using the obtained parameters and the lattice stabilities of Al and Si given in the literatures,the Al-Be and Be-Si phase diagrams are calculated.By means of Kohler's for- mula the Gibbs energy for the liquid phase in the Al-Be-Si ternary system is extrapolated.The calcu- lation shows that no excess ternary term is necessa- ry for the thermodynamic description of the system. The liquidus projection,isothermal section at 1273 K,as well as vertical sections at 94 and 88 wt-% Al and 2 wt-% Be are calculated.The calcu- lated results are in reasonable agreement with the experimental data available.

Thermodynamic Analysis of ZnO Crystal Growth from the Melt

Recently, the efforts in solid-state materials include developing technologies capable of producing ZnO wafers in large dimensions and good quality based device applications. High quality bulk crystals are obtained by growing from high purity the melt. However, the thermochemical properties of ZnO (high melting point and high vapor pressure) make the growth of single crystals difficult. The thermodynamic calculations show that ZnO crystals can be grown from the melt if a suitable dynamic atmosphere composition is used. The oxygen requirement with increasing the temperature can be fulfilled by adding the NO-NO2 gases into the CO2 atmosphere. At ZnO melting point, the oxygen partial pressure of gas mixtures containing CO2-NO-CO-NO2 at Pt = 5 atm reaches to PO2 = 0.29 atm. According to this new thermodynamic result, it would be expected that ZnO crystal could be grown from the melt at lower total pressure comparing to pure CO2.

Pressure-induced structure,electronic,thermodynamic and mechanical properties of Ti_(2)AlNb orthorhombic phase by first-principles calculations

Effects of pressure on lattice parameters, electronic, thermodynamic and mechanical properties of the fully ordered Ti_(2)AlNb orthorhombic phase were studied using first-principles calculations based on density functional theory(DFT). The bonding nature for ordering orthorhombic Ti_(2)AlNb was revealed quantitatively through the electronic structure analyzing. The external pressures play limited roles in the elastic anisotropy of the alloy due to the outstanding dynamical and mechanical stabilities under pressure. However, the shear modulus of O phase manifests anisotropic, where {010} shear planes are the easiest planes to cleave among the principal planes under all pressures.The heat capacities, volume expansions and thermal expansion coefficients were calculated using the quasi-harmonic approximation model based on the phonon dispersion curves. Meanwhile, the bulk modulus, Young’s modulus,shear modulus and the hardness are promptly enhanced under pressure. The predicted results give hints to design Ti_(2)AlNb-based alloy as high-pressure applications.

Au-Ag-Al系相图的热力学优化和计算

使用计算相图方法(CALPHAD)计算Au-Ag-Al的三元相图时,三个边际二元相图(Au-Al,Au-Ag,Ag-Al)的准确性对三元相图的计算有很大的影响,目前Au-Al边际二元相图存在着一定的矛盾。在综合评估Au-Al体系实验数据的基础上,优化和计算了Au-Al合金体系的平衡相图,使用置换式溶体溶液模型描述Au-Al体系中液相、Bcc和Fcc相的吉布斯自由能,分别用亚点阵模型(Al)(Au)_(4)、(Al)_(3)(Au)_(8)、(Al)(Au)_(2)、(Al)(Au)和(Al,Au)_(2)(Al,Au)描述AlAu_(4)、Al_(3)Au_(8)、AlAu_(2)、AlAu和Al_(2)Au相,通过Pandat软件优化得到一组各相的热力学参数,计算得到的Au-Al相图与实验数据和热力学数据相吻合。结合Au-Ag和Ag-Al的热力学参数,计算了Au-Ag-Al液相面投影图和等温截面图,液相面投影图显示该三元系存在8个四相平衡反应,对Au-Ag-Al合金的研究具有指导意义。

基于稳定性分析的含酸/醇烃水体系相平衡计算

油气开发的井流物多以油气水混相形式存在。对于酸性油气田,还常伴有硫化氢或二氧化碳等酸性气体。为了避免水合物生成堵塞管道,加注醇类防冻剂是常用的方法。因而油气田井流物易于呈现含酸、含醇的多相烃水混输状态。含酸/醇烃水体系相平衡的准确计算,是预测其物性特征的基础,也是支持该复杂体系多相混输流动仿真的关键。但目前被广泛应用的基于经典立方型状态方程计算油气水体系相平衡的方法,存在对液烃及水相分布预测准确性不高的问题。为此,引入稳定性分析的方法,构建适用于含酸/醇烃水体系的稳定性检验方式,结合SRK-HV状态方程,建立了耦合稳定性分析的含酸/醇烃水多相多组分相平衡计算模型及算法。经多角度验证,本研究所提模型具有较高的准确性与稳定性。

高熵碳化物陶瓷第一性原理计算研究进展与展望

过渡族难熔金属元素和碳组成的高熵碳化物陶瓷因具有优异的耐高温性能,在高熵材料领域引起了关注。由于涉及复杂的成分范围和服役环境,利用传统试错法研发高熵碳化物陶瓷周期长且成本高,而采用第一性原理计算方法可以准确高效地预测高熵碳化物陶瓷的相稳定性、弹性性能、电子结构和热力学等性质。本文对第一性原理计算方法进行了简述,并对第一性原理计算在高熵碳化物陶瓷研究中的应用成果进行了综述,最后展望了第一性原理计算在高熵碳化物陶瓷领域的发展方向。

CE3MN双相不锈钢铸件裂纹成因与预防

针对30″-300 Lb上装式球阀阀体用CE3MN铸造超级双相不锈钢材质,利用FactSage8.2热力学计算软件以及FSstel钢铁数据库,并结合光学显微镜、扫描电镜和能谱仪以及X射线衍射仪等试验手段,研究分析了CE3MN材质铸件在固溶处理之前切割铸态冒口时所产生的裂纹原因。结果表明:CE3MN铸件在脆性相析出的敏感温度范围以内缓慢加热或缓慢冷却过程中,由于铸造成分偏析严重而产生大量富Cr、Mo的有害金属间化合物σ相,主要以短棒状、细针状弥散分布于铁素体基体内或以长链状、条带状、珊瑚状、(σ+γ_(2))胞状以及魏氏组织长针状等形式分布于α/γ相界处。生产实践表明:在气割CE3MN铸件冒口之前,毛坯预先进行固溶处理以消除铸态脆性σ相,抑制裂纹形成和扩展,或者利用锯床先冷锯切铸态冒口后再进行固溶处理,可从根本上规避产生裂纹的温度因素。