基于Miedema模型和Toop模型的Al-Si-Er合金热力学参数计算

运用Miedema混合焓模型计算了Al-Er、Si-Er和Al-Si二元合金的部分热力学性质,选择Er为非对称性组元,在此基础上结合Toop模型计算出Al-Si-Er三元合金的混合焓、过剩熵与过剩自由能。结果表明,Al-Si-Er三元合金的混合焓、过剩熵与过剩自由能在整个浓度范围内均为负值,在富Er和贫Er区域各热力学参数变化趋势较大。通过计算Al、Si和Er3种组元的偏摩尔过剩自由能得到Al-Si-Er三元合金各组元的等活度值曲线。曲线显示各组元随着摩尔分数的减少其活度值都急剧减小,在三元系成分三角形的中心部分Al、Si和Er的活度值都很小,这表明Al、Si和Er3种组元之间存在很强的相互作用,易形成三元金属间化合物,这一结论与Al-Si-Er三元相图的实际情况相符合。

Al-Cu-Zn合金溶质分凝的热力学模型

提出了计算多元合金溶质分凝行为的热力学模型,分析了活度法和浓度法两种计算平衡分凝因数方法的异同,应用提出的模型系统地研究了Al-Cu-Zn合金的平衡分凝因数,计算表明,活度法和浓度法的计算结果接近,溶质Cu和Zn的平衡分凝因数在凝固温度范围内随温度增加下降10％左右,Zn和Cu任何一种溶质元素的浓度增加都会降低另一种元素的平衡分凝因数,同时研究了杂质元素对三元Al-Cu基合金中Cu的平衡分凝固数的影响。

BT25y高温钛合金固溶处理及热暴露过程中硅化物的析出机制

TEM观察了BT25y高温钛合金硅化物在固溶处理以及热暴露过程中的析出行为,结合Calphad相图计算技术研究了该合金硅化物的析出机制。在经单相区990℃固溶处理后的个别样品和两相区940℃固溶处理后的部分样品中,观察到(Ti,Zr)5Si3硅化物颗粒,依据计算,分析这些硅化物可能是由于Si分布不均匀,局部浓度超过0 3%而析出的;在两相区940℃固溶处理后的样品中还观察到平衡析出的(Ti,Zr)6Si3硅化物颗粒。两种固溶处理后的样品,在700℃热暴露过程中都有大量细颗粒(Ti,Zr)6Si3硅化物弥散析出。

SmCo_7纳米晶合金晶粒组织热稳定性的热力学分析与计算机模拟

推导出了单相纳米晶合金的晶界过剩体积与晶粒尺寸之间的定量关系,建立了纳米晶合金的晶界热力学性质随温度和晶粒尺寸发生变化的确定性函数.针对SmCo_7纳米晶合金,通过纳米晶界热力学函数计算和分析,研究了单相纳米晶合金的晶粒组织热稳定性.研究表明,当纳米晶合金的晶粒尺寸小于对应于体系中品界自由能最大值的临界晶粒尺寸时,纳米晶组织处于相对稳定的热力学状态:当纳米晶粒尺寸达到和超过临界尺寸时,纳米晶组织将发生热力学失稳,导致不连续的快速晶粒长大.利用纳米晶合金热力学理论与元胞自动机算法相耦合的模型对SmCo7纳米晶合金在升温过程中的晶粒长大行为进行了计算机模拟,模拟结果与纳米晶合金热力学模型的计算预测结果一致,由此证实了关于纳米晶合金晶粒组织热稳定性的研究结论.

合金熔体结构和原子-分子热力学模型

对金属熔体结构的研究证实熔体中存在短程有序结构,而含金属间化合物的合金熔体中发现了原子-分子结合的团簇结构,即固态下的金属间化合物在熔体中并没有消失,而是以团簇结构(实际上就是液态的分子)存在,并与熔体中的自由原子存在化学平衡。从熔体结构出发,提出了适用于合金熔体的原子-分子热力学模型。然后以Ca-Mg合金熔体为例,介绍了该模型的构建和求解方法,并计算了1 010K下的Ca-Mg合金熔体中各物质的摩尔分数。最后,将计算得到的钙、镁摩尔分数与各自的实测活度值进行比较,发现两者吻合得较好。这从计算上证明了合金熔体中同时存在着金属原子和金属间化合物分子,两者处于动态化学平衡之中。且达到平衡时,金属原子的摩尔分数实际上就是各自的活度。因此,对于合金熔体而言,活度并不存在。

L1_2型Co_3Ti基金属间化合物有序化行为的热力学研究

采用亚晶格模型,辅助以第一性原理总能计算,研究了L12结构的Co3Ti基金属间化合物中元素的占位有序化行为。结果表明,Co3Ti化学计量比合金呈现完全有序化;对于xCo/xTi为3:1的Co72Ti24M4合金,第3合金组元M为Si或Ta时,M与Ti共同占据1a位置,M为Cu、Ni、Pd、Rh、V或W时,M与Co占据3c位置,而当M为Al、Cr、Ge、Mn、Sc或Y时,M在1a和3c位置的占位分数相同,这些元素的占位行为均不受温度影响;而当M为Fe、Hf、Mo、Nb、Ru和Zr的占位情况随温度发生变化。随原子核外层电子的增加,原子占位逐渐倾向于从1a亚晶格转向3c位置。亦预测了xCo/xTi偏离3:1的部分合金的占位分数随合金成分和温度的变化细节,预测结果与文献进行了比较,并澄清了文献上的分歧。

Phase field simulation for non-isothermal solidification of multicomponent alloys coupled with thermodynamics database

In order to quantitively model the real solidification process of industrial multicomponent alloys, a non-isothermal phase field model was studied for multicomponent alloy fully coupled with thermodynamic and diffusion mobility database, which can accurately predict the phase equilibrium, solute diffusion coefficients, specific heat capacity and latent heat release in the whole system. The results show that these parameters are not constants and their values depend on local concentration and temperature. Quantitative simulation of solidification in multicomponent alloys is almost impossible without such parameters available. In this model, the interfacial region is assumed to be a mixture of solid and liquid with the same chemical potentials, but with different composition. The anti-trapping current is also considered in the model. And this model was successfully applied to industrial A1-Cu-Mg alloy for the free equiaxed dendrite solidification process.

Effect of hot working on microstructure and mechanical properties of TC11/Ti_2AlNb dual-alloy joint welded by electron beam welding process

The influence of hot working on the microstructures of TC11/Ti2 Al Nb dual-alloy joints welded by electron beam welding(EBW) process was investigated. The tensile tests were performed at room temperature for specimens before and after thermal exposure. The results show that the fusion zone of TC11/Ti2 Al Nb dual-alloy joint welded by EBW is mainly composed of β phase. After deformation and heat treatment, the grain boundaries of the as-cast alloy are broken and the fusion zone mainly consists of β, α2and α phases. The fusion zone performs poor property in the tensile test. Specimens before and after thermal exposure all fail in this area under different deformation conditions. The ultimate tensile strength of specimens after heat treatment is up to 1190 MPa at room temperature. The joints by water quenching after deformation have better plasticity with an elongation up to 4.4%. After thermal exposure at 500 °C for 100 h, the tensile strength of the specimen slightly rises while the ductility changes a little. SEM observation shows that the fracture mechanism is predominantly transgranular under different deformation conditions.

Microstructure and mechanical properties of TC21 titanium alloy after heat treatment

Microstructure evolutions during different heat treatments and influence of microstmcture on mechanical properties of TC21 titanium alloy were investigated. The results indicate that the excellent mechanical properties can be obtained by adopting air cooling after forging followed by heat treatment of （900℃, 1 h, AC）＋（590 ℃, 4 h, AC）. Deformation in single β field produces pan-like prior fl grains, while annealing in single fl field produces equiaxed prior fl grains. Cooling rate after forging or annealing in single fl field and the subsequent annealing on the top of α＋β field determine the content and morphology of coarse a plates. During aging or the third annealing, fine secondary a plates precipitate. Both ultimate strength and yield strength decrease with the content increase of coarse a plates. Decreasing effective slip length and high crack propagation resistance increase the plasticity. The crisscross coarse a plates with large thickness are helpful to enhance the fracture toughness.

Thermodynamic assessment of Co-Cr-W ternary system

The Co-Cr-W ternary system was critically assessed using the CALPHAD technique.The solution phases including the liquid,γ-Co,ε-Co and α-Cr were described by a substitutional solution model.The σ,μ and R phases were described by three-sublattice models of（Co,W）8（Cr,W）4（Co,Cr,W）18,（Co,Cr,W）7W2（Co,Cr,W）4 and（Co,W）27（Cr,W）14（Co,Cr,W）12,respectively,in order to reproduce their homogeneity ranges.A self-consistent set of thermodynamic parameters for each phase was derived.The calculated isothermal sections at 1 000,1 200 and 1 350 ℃ are in good agreement with the experimental data.A eutectoid reaction of R μ＋γ-Co＋σ in this ternary system was predicted to occur at 1 022 ℃.

Effects of heat treatment on microstructure and mechanical properties of ZA27 alloy

The effects of heat treatment on the microstructure and mechanical properties of ZA27 alloy were studied by X-ray diffraction（XRD）,scanning electron microscopy（SEM） and mechanical characterization.The results indicated that the as-cast microstructure of the alloy was mainly composed of α,decomposed β,η and ε phases.After solid solution treatment at 365 ℃ for 1 h,α and η phases dissolved,and the microstructure of specimen was mainly composed of the supersaturated β phases.The phase decomposition of supersaturated ZA27 alloy is a two-stage phase transformation：the decomposition of the supersaturated β phase at the early stage of aging,and with the increase of aging time,ε phase decomposition through a four-phase transformation：α＋ε→T ＇＋ η.A good combination of high tensile elongation and reasonable strength can be achieved by suitable heat treatments.

Thermodynamic optimization of Bi-Ni binary system

Based on the available experimental data,the Bi-Ni binary system was optimized thermodynamically by the CALPHAD method.The solution phases,including liquid,fcc_A1（Ni） and rhombohedral_A7（Bi）,were described as substitutional solution phases,of which the excess Gibbs energies were expressed with the Redlich-Kister polynomial.The intermetallic compound,BiNi,was modeled using three sublattices（Bi）（Ni,Va）（Ni,Va） considering its crystal structure（NiAs-type） and the compatibility of thermodynamic database in the multi-component systems,while Bi3Ni was treated as a stoichiometric compound.Finally,a set of self-consistent thermodynamic parameters formulating the Gibbs energies of various phases in this binary system were obtained.The calculated results are in reasonable agreement with the reported experimental data.

Microstructure and mechanical properties of heat-treated Ti-5Al-2Sn-2Zr-4Mo-4Cr

The effects of heat treatment parameters on the microstructure,and mechanical properties and fractured morphology of Ti-5Al-2Sn-2Zr-4Mo-4Cr with the equiaxed,bi-modal and Widmanst？tten microstructures were investigated.The heating temperatures for obtaining the equiaxed,bi-modal and Widmanst？tten microstructures were 830,890 and 920 °C,respectively,followed by furnace cooling at a holding time of 30 min.The volume fraction of primary α phase decreased with increasing the heating temperature,which was 45.8% at 830 °C,and decreased to 15.5% at 890 °C,and then the primary α phase disappeared at 920 °C during furnace cooling.The variation of volume fraction of primary α phase in air cooling is similar to that in furnace cooling.The increase in heating temperature and furnace cooling benefited the precipitation and growth of the secondary α phase.The equiaxed microstructure exhibited excellent mechanical properties,in which the ultimate strength,yield strength,elongation and reduction in area were 1035 MPa,1011 MPa,20.8% and 58.7%,respectively.The yield strength and elongation for the bi-modal microstructure were slightly lower than those of the equiaxed microstructure.The Widmanst？tten microstructure exhibited poor ductility and low yield strength,while the ultimate strength reached 1078 MPa.The dimple fractured mechanism for the equiaxed and bi-modal microstructures proved excellent ductility.The coexistence of dimple and intercrystalline fractured mechanisms for the Widmanst？tten microstructure resulted in the poor ductility.

Effects of heat treatment on microstructure and mechanical properties of Mg-3Sn-1Mn magnesium alloy

The effects of heat treatment on the microstructure and mechanical properties of Mg-3Sn-1Mn alloy were preliminarily investigated by using optical and electron microscopy,X-ray diffraction（XRD） analysis,and tensile and creep test.The results indicate that the heat treatment has an obvious effect on the microstructure and mechanical properties of the Mg-3Sn-1Mn alloy.After the solid solution treatment at 420 ℃,a majority of the Mg2Sn phases in the alloy are dissolved into the matrix.However,after the further aging treatment at 250 ℃,lots of fine Mg2Sn phases in the aged alloy are precipitated at the grain boundaries and within the grains.As a result,the tensile and creep properties of the aged alloy are significantly improved.The mechanism for the higher tensile and creep properties of the aged alloy is related to the dispersive distribution of the Mg2Sn phase in the--Mg matrix.

Microstructures and properties of welded joint of aluminum alloy to galvanized steel by Nd:YAG laser + MIG arc hybrid brazing-fusion welding

According to the differences in melting point between aluminum alloy and steel, 6013-T4 aluminum alloy was joined to galvanized steel by large spot Nd：YAG laser ＋ MIG arc hybrid brazing-fusion welding with ER4043（AlSi5） filler wire. The microstructures and mechanical properties of the brazed-fusion welded joint were investigated. The joint is divided into two parts of fusion weld and brazed seam. There is a zinc-rich zone at fusion weld toe, which consists of α（Al）-Zn solid solution and Al-Zn eutectic. The brazed seam is the Fe-Al intermetallic compounds （IMCs） layer of 2-4μm in thickness, and the IMCs include FeAl2, Fe2Al5 and Fe4Al13. FeAl2 and Fe2Al5 are located in the compact reaction layer near the steel side, and Fe4Al13 with tongue shape or sawtooth shape grows towards the fusion weld. The tensile strength of the joint firstly increases and then decreases as the welding current and laser power increase, the highest tensile strength can be up to 247.3 MPa, and the fracture usually occurs at fusion zone of the fusion weld. The hardness is the highest at the brazed seam because of hard Fe-Al IMCs, and gradually decreases along the fusion weld and galvanized steel, respectively.

Rate controlling mechanisms in hot deformation of 7A55 aluminum alloy

The hot deformation behavior of 7A55 aluminum alloy was investigated at the temperature ranging from 300 ℃ to 450 ℃ and strain rate ranging from 0.01 s-1 to 1 s-1 on a Gleeble-3500 simulator. Processing maps were established in order to apprehend the kinetics of hot deformation and the rate controlling mechanism was interpreted by the kinetic rate analysis obeying power-law relation. The results indicated that one significant domain representing dynamic recrystallization （DRX） existed on the processing maps and lying in 410-450 ＆#176;C and 0.05-1 s-1. The conclusions of kinetic analysis correlated well with those obtained from processing maps. The apparent activation energy values calculated in the dynamic recrystallization （DRX） domain and the stability regions except dynamic recrystallization （DRX） domain were 91.2 kJ/mol and 128.8 kJ/mol, respectively, which suggested that grain boundary self-diffusion and cross-slip were the rate controlling mechanisms.

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.

Effect of heat treatment on microstructures and mechanical properties of sand-cast Mg-10Gd-3Y-0.5Zr magnesium alloy

The microstructure, mechanical properties and fracture behavior of sand-cast Mg-10Gd-3Y-0.5Zr alloy （mass fraction,%） under T6 condition （air cooling after solid solution and then aging heat treatment） were investigated. The optimum T6 heat treatments for sand-cast Mg-10Gd-3Y-0.5Zr alloy are （525 ℃, 12 h＋225 ℃, 14 h） and （525 ℃, 12 h＋250 ℃, 12 h） according to age hardening curve and mechanical properties, respectively. The ultimate tensile strength, yield strength and elongation of the Mg-10Gd-3Y-0.5Zr alloy treated by the two optimum T6 processes are 339.9 MPa, 251.6 MPa, 1.5%and 359.6 MPa, 247.3 MPa, 2.7%, respectively. The tensile fracture mode of peak-aged Mg-10Gd-3Y-0.5Zr alloy is transgranular quasi-cleavage fracture.

Effect of heat treatment on microstructure and tensile strength of NiCoCrAl alloy sheet fabricated by EB-PVD

The NiCoCrAl alloy sheet was fabricated by electron beam physical vapor deposition technique and the effects of the heat treatment on the microstructure and tensile strength of the NiCoCrAl alloy sheet were investigated. The heat treatment at 1050 °C is favorable to improve the interface bonding between the columnar structures due to the disappearance of the intergranular gaps. Comparing with the thin NiCoCrAl alloy sheet before heat treatment, the Ni3Al phase appears in the NiCoCrAl alloy sheet after heat treatment, which is favorable to improve the interface bonding between the columnar structures. The increase in the tensile strength and elongation is attributed to the improvement of the interface bonding between the columnar structures. The residual stress in the NiCoCrAl alloy sheet after heat treatment is reduced significantly, which also confirms that the interface bonding is improved by the heat treatment.

Thermodynamics analysis and precipitation behavior of fine carbide in K416B Ni-based superalloy with high W content during creep

The precipitation behavior of carbide in K416 B superalloy was investigated by means of creep measurement and microstructure observation. The results show that nanometer M6 C particles discontinuously precipitate in the γ matrix or along the γ/γ′ interface of the alloy during high temperature tensile creep. Thereinto, the amount of fine M6 C carbide increases as creep goes on, and the coherent interfaces of M6 C phase precipitating from the γ matrix are {100} and {111} planes. The thermodynamics analysis indicates that the solubility of element carbon in the matrix decreases when the alloy is deformed by the axial tensile stress during creep, so as to cause the carbon segregating in the regions of stress concentration and combining with carbide-forming elements M（W, Co）, which promotes the fine M6 C carbide to precipitate from the γ matrix.