Phase Diagrams of High-Entropy Alloy System Al-Co-Cr-Fe-Mo-Ni

The high-entropy alloy system Al-Co-Cr-Fe-Mo-Ni has been previously designed based on the Al _x CoCrCuFeNi system and was demonstrated to have great improvements in toughness at high hardness level and in softening resistance at elevated temperatures. For this promising system, it is important to have phase diagrams for further efficient alloy design and research. The current study combines the results from scanning electron microscopy, transmission electron microscopy, room-temperature and high-temperature x-ray diffractometry, and differential thermal analysis to construct approximate phase diagrams by varying the content of each composing element. Thermodynamic calculation in considering both mixing enthalpy and entropy was also used to justify the simple phase fields in these phase diagrams.     

Isothermal Phase Diagrams of Binary Alloys and Two Oxidants

An example of an isothermal thermodynamic phase diagram of a quaternary system composed of a binary alloy and two oxidants has been calculated under appropriate values of the stability of the corresponding compounds, assuming that each metal forms only one compound with each oxidant. Initially, two-dimensional sections of the complete three-dimensional diagram made along planes corresponding to constant values of one of the variables involved were examined in detail. The actual structure of this type of three-dimensional diagram was then discussed by analyzing the shape and the boundaries of the various regions corresponding to the conditions of stability of each phase taken separately or in combination with other phases.     

Thermodynamic Database for Phase Diagrams of Mn-RE Binary Alloy Systems

Thermodynamic assessment of Mn-RE (RE: Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu) binary systems has been carried out by means of the calculation of phase diagrams method on the basis of experimental data including phase equilibria and thermodynamic properties. The Gibbs free energies of liquid phase and solid solution phases in the Mn-RE systems were all described by the subregular solution model with the Redlich–Kister formulation, whereas those of intermetallic compounds were described by the sublattice model. Sets of thermodynamic functions with self-consistent parameters leading to satisfactory agreement between calculated results and experimental data were eventually obtained. A primary thermodynamic database for the phase diagrams of the Mn-RE (RE: Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu) binary systems was developed from these essential thermodynamic parameters. This database can be used not only to calculate the phase equilibria and thermodynamic properties of binary Mn-RE systems but also to extrapolate to higher-order systems, which can provide theoretical guidance for design and development of high-performance Mn-RE alloy materials.     

贵金属二元合金相图研究的新进展和展望

对贵金属二元合金相图研究现状及其可靠性进行了全面的分析,把贵金属与周期表中各元素的相互作用分成连续固溶体、简单包晶型、简单共晶型、简单偏晶型、生成化合物的复杂体系和在固态和液态都不互溶的6种类型,并把它们放在元素周期表中进行研究后,定性总结了它们的若干规律;评价了二元相图坐标的热力学表示原理;评价了二元相图的半径验计算法和中间相的IUPAC（国际理论化学和应用化学联合会）和MSIT（国际材料科学组织）表示法,强调半经验的热力学计算法比较成熟,而按第一原理的理论计算法尚处在快速发展中;展望了贵金属二元合金相图的发展方向。     

Alloy Design and Properties Optimization of High-Entropy Alloys

This article reviews the recent work on the high-entropy alloys (HEAs) in our group and others. HEAs usually contain five or more elements, and thus, the phase diagram of HEAs is often not available to be used to design the alloys. We have proposed that the parameters of ?? and ?? can be used to predict the phase formation of HEAs, namely ??????1.1 and ??????6.6%, which are required to form solid-solution phases. To test this criterion, alloys of TiZrNbMoV _x and CoCrFeNiAlNb _x were prepared. Their microstructures mainly consist of simple body-centered cubic solid solutions at low Nb contents. TiZrNbMoV _x alloys possess excellent mechanical properties. Bridgman solidification was also used to control the microstructure of the CoCrFeNiAl alloy, and its plasticity was improved to be about 30%. To our surprise, the CoCrFeNiAl HEAs exhibit no apparent ductile-to-brittle transition even when the temperatures are lowered from 298?K to 77?K.     

The Phase Competition and Stability of High-Entropy Alloys

Phase competition and stability of several typical high-entropy alloys (HEAs) were studied, and the effects of alloying additions and processing conditions on phase formation in these alloys were discussed. Alloying with chemically incompatible elements having a large difference in either the atomic size or enthalpy of mixing with constituting components in HEAs, e.g., Cu and Al in the FeCoNiCr alloy system, inevitably induced phase separation and stimulated formation of duplex solid-solution phases and even intermetallic compounds. The solid-solution phase in the as-cast FeCoNiCrMn HEA is extremely stable due to the good chemical compatibility among constituent components, but in the FeCoNiCrAl and (FeCoNiCrAl)₉₉Si₁ HEAs with the incompatible elements Al and Si, pretreatment and annealing processes could induce phase transitions and the formation of new phases, indicating that the as-cast solid-solution phases were destabilized by quenched-in chemical segregation, resulting from additions of the dissimilar elements.     

Alloy Design Strategies and Future Trends in High-Entropy Alloys

High-entropy alloys (HEAs) are newly emerging advanced materials. In contrast to conventional alloys, HEAs contain multiple principal elements, often five or more in equimolar or near-equimolar ratios. The basic principle behind HEAs is that solid-solution phases are relatively stabilized by their significantly high entropy of mixing compared to intermetallic compounds, especially at high temperatures. This makes them feasibly synthesized, processed, analyzed, and manipulated, and as well provides many opportunities for us. There are huge numbers of possible compositions and combinations of properties in the HEA field. Wise alloy design strategies for suitable compositions and processes to fit the requirements for either academic studies or industrial applications thus become especially important. In this article, four core effects were emphasized, several misconceptions on HEAs were clarified, and several routes for future HEA research and development were suggested.     

非均匀组织FeMnCoCr高熵合金的微观结构和力学性能

提出了一种简单的高熵合金加工工艺,即对Fe-Mn系高熵合金采用中等形变量冷轧和中温短时退火相结合的方法,获得了由晶粒尺寸为数十微米的形变晶粒和超细尺度再结晶晶粒组成的非均匀结构.通过向合金中同时引入由高密度位错、晶粒细化、析出相、ε-马氏体、α-马氏体和回复孪晶等微观结构特征及变形过程中持续发生的形变孪生、ε-马氏体相...     

Phase Composition and Superplastic Behavior of a Wrought AlCoCrCuFeNi High-Entropy Alloy

A cast AlCoCrCuFeNi high-entropy alloy was multiaxially forged at 950°C to produce a fine homogeneous mixture of grains/particles of four different phases with the average size of ~2.1 μm. The forged alloy exhibited unusual superplastic behavior accompanied by a pronounced softening stage, followed by a steady-state flow stage, during tensile deformation at temperatures of 800°C–1000°C and at strain rates of 10^?4–10^?1 s^?1. Despite the softening stage, no noticeable strain localization was observed and a total elongation of up to 1240% was obtained. A detailed analysis of the phase composition and microstructure of the alloy before and after superplastic deformation was conducted, the strain rate and temperature dependences of the flow stress were determined at different stages of the superplastic deformation, and the relationships between the microstructure and properties were identified and discussed.     

Thermodynamic Phase Diagrams of Ternary Alloys Exposed to a Single Oxidant

Oxidation of Metals - A simple example of an isothermal thermodynamic phase diagram for a quaternary system composed of a ternary A–B–C alloy reacting with a single oxidant (O) has been...     

Soft-Magnetic High-Entropy AlCoFeMnNi Alloys with Dual-Phase Microstructures Induced by Annealing

The simultaneous enhancement of magnetic and mechanical properties is desirable but challenging for soft-magnetic materials.A fabrication strategy to meet this requirement is therefore in high demand.Herein,bulk equiatomic dual-phase AlCoFeMnNi high-entropy alloys were fabricated via a magnetic levitation induction melting and casting process followed by annealing at 700-1000℃,and their microstructures as well as mechanical and magnetic properties were investigated.The as-cast alloy possessed a single metastable B2-ordered solid solution that decomposed upon annealing into a dual-phase structure comprising an Al-and Ni-rich body-centered cubic(BCC) matrix and Fe-and Mn-rich face-centered cubic(FCC)precipitates both in the grain interior and along the grain boundaries.The magnetic and mechanical properties were closely related to the relative volume fraction of FCC in the alloy.The FCC volume fraction could be increased by increasing the annealing temperature,thereby offering tunable properties.The optimal annealing temperature for balanced magnetic and mechanical properties was found to be 800℃.The alloy annealed at this temperature had an average BCC grain size of 12±3μm and FCC volume fraction of 41±4%.Correspondingly,the s aturation magnetization and coercivity reached 82.57 Am~2/kg and 433 A/m,respectively.The compressive yield strength and fracture strength were 1022 and 2539 MPa,respectively,and the plasticity was 33%.Owing to its adjustable microstructure and properties,the AlCoFeMnNi alloy has potential for use as a multi-functional soft-magnetic material.     

Achieving a High-Strength CoCrFeNiCu High-Entropy Alloy with an Ultrafine-Grained Structure via Friction Stir Processing

High-entropy alloys(HEAs) are a new class of materials with a potential engineering application,but how to obtain ultrafine or nano-sized crystal structures of HEAs has been a challenge.Here,we first presented an equiatomic CoCrFeNiCu HEA with excellent mechanical properties obtained via friction stir processing(FSP).After FSP,the Cu element segregation in the cast CoCrFeNiCu HEA was almost eliminated,and the cast coarse two-phase structure(several micrometers) was changed into an ultrafine-grained single-phase structure(150 nm) with a large fraction of high-angle grain boundaries and nanoscale deformation twins.This unique microstructure was mainly attributed to the severe plastic deformation during FSP,and the sluggish diffusion effect in dynamics and the lattice distortion effect in crystallography for HEAs.Furthermore,FSP largely improved the hardness and yield strength of the CoCrFeNiCu HEA with a value of 380 HV and more than 1150 MPa,respectively,which were 1.5 times higher than those of the base material.The great strengthening after FSP was mainly attributed to the significant grain refinement with large lattice distortion and nano-twins.This study provides a new method to largely refine the microstructure and improve the strength of cast CoCrFeNiCu HEAs.     

Phase Selection and Microhardness of Directionally Solidified AlCoCrFeNi2.1 Eutectic High-Entropy Alloy

In the present work, the solidi?cation behaviors and microhardness of directionally solidi?ed AlCoCrFeNi_2.1 eutectic highentropy alloy (EHEA) obtained at different growth velocities are investigated. The microstructure of the as-cast AlCoCrFeNi _2.1 EHEA is composed of bulky dendrites (NiAl phase) and lamellar eutectic structures, indicating that the actual composition of the alloy slightly deviates from the eutectic point. However, it is interesting to observe that the full...     

Thermodynamic Characteristic and Phase Evolution in Immiscible Cr–Mo Binary Alloys

This paper systematically reports the thermodynamic characteristic and phase evolution of immiscible Cr–Mo binary alloy during mechanical alloying(MA) process. The Cr–35Mo(in at%) powder mixture was milled at 243 and258 K, respectively, for different time. For comparative study, Cr–15Mo and Cr–62Mo powder mixtures were milled at 243 K for 18 h. Solid solution Cr(Mo) with body-centered cubic(bcc) crystal structure and amorphous Cr(Mo) alloy was obtained during MA process caused by high-energy ball milling. Based on the Miedema's model, the free-energy change for forming either a solid solution or an amorphous in Cr–Mo alloy system is positive but small at a temperature range between 200 and 300 K. The thermodynamical barrier for forming alloy in Cr–Mo system can be overcome when MA occurs at 243 K, and the supersaturated solid solution crystal nuclei with bcc structure form continually, and three supersaturated solid solutions of Cr–62Mo, Cr–35Mo and Cr–15Mo formed. Milling the Cr–35Mo powder mixture at 258 K, the solid solution Cr(Mo) forms firstly, and then the solid solution Cr(Mo) transforms into the amorphous Cr(Mo)alloy with a few of nanocrystallines when milling is prolonged. At higher milling temperature, it is favorable for the formation of the amorphous phase, as indicated by the thermodynamical calculation for immiscible Cr–Mo alloy system.     

Mechanical Properties and Phase Stability of WTaMoNbTi Refractory High-Entropy Alloy at Elevated Temperatures

High-temperature structural metals remain in high demand for aerospace aircraft,gas turbine engines,and nuclear power plants.Refractory high-entropy alloys (RHEAs) with superior mechanical properties at elevated temperatures are promising candidates for high-temperature structural materials.In this work,a WTaMoNbTi RHEA with adequate room temperature plasticity and considerable strength at 1600 ℃ was fabricated by vacuum arc-melting.The room temperature fracture strain of the as-cast WTaMoNbTi RHEA was 7.8％,which was about 5.2 times that of the NbMoTaW alloy.The alloy exhibited a strong resistance to high-temperature softening,with a high yield strength of 173 MPa and compressive strength of 218 MPa at 1600 ℃.The WTaMoNbTi RHEA possessed excellent phase stability in the range of room temperature to 2000 ℃.The dendritic grains grew into equiaxed grains after compression test at 1600 ℃ due to the dynamic recrystallization process at high temperature.This work presents a promising high-temperature structural material that can be applied at 1600 ℃.     

铸造镁合金的枝晶生长模拟

根据hcp晶体学结构和优先生长方向,建立了铸造镁合金晶体生长的物理模型,提出了一种新的随机性模拟方法--虚拟生长中心计算模型.模型考虑了枝晶生长动力学、各向异性和二次枝晶臂粗化,采用枝晶形状函数揭示了一次枝晶和二次枝晶的生长演化过程.引入坐标变换技术可更快速准确计算任意晶向枝晶的生长捕获.耦合了微观溶质场计算,得到了更准确的枝晶生长形貌和溶质分布情况.对Mg-Al合金定向凝固和等轴晶生长的模拟验证了本模型的正确性.     

Microstructures and Mechanical Properties of NiTiFeAlCu High-Entropy Alloys with Exceptional Nano-precipitates

Three novel NiTiFeAlCu high-entropy alloys, which consist of nano-precipitates with face-centered cubic structure and matrix with body-centered cubic structure, were fabricated to investigate microstructures and mechanical properties. With the increase in Ni and Ti contents, the strength of NiTiFeAlCu alloy is enhanced, while the plasticity of NiTiFeAlCu alloy is lowered. Plenty of dislocations can be observed in the Ni₃₂Ti₃₂Fe₁₂Al₁₂Cu₁₂ high-entropy alloy. The size of nano-precipitates decreases with the increase in Ni and Ti contents, while lattice distortion becomes more and more severe with the increase in Ni and Ti contents. The existence of nano-precipitates, dislocations and lattice distortion is responsible for the increase in the strength of NiTiFeAlCu alloy, but it has an adverse influence on the plasticity of NiTiFeAlCu alloy. Ni₂₀Ti₂₀Fe₂₀Al₂₀Cu₂₀ alloy exhibits the substantial ability of plastic deformation and a characteristic of steady flow at 850 and 1000 °C. This phenomenon is attributed to a competition between the increase in the dislocation density induced by plastic strain and the decrease in the dislocation density due to the dynamic recrystallization.     

Microstructure and Mechanical Properties of a Refractory CoCrMoNbTi High-Entropy Alloy

In this work, a new refractory high-entropy alloy, the Co-Cr-Mo-Nb-Ti system, was proposed as a family of candidate materials for high-temperature structural applications. CoCrMoNbTi _x (x values in terms of molar ratios, x = 0, 0.2, 0.4, 0.5 and 1.0) alloys were prepared by vacuum arc melting. The effects of variations in the Ti content on the phase constituents, microstructure and mechanical properties of the alloys were investigated using x-ray diffractometry, scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy and compressive testing. The results showed that the CoCrMoNbTi_0.4 alloy possessed a typical cast dendritic microstructure consisting of a single body-centered cubic (BCC) solid solution. Laves phases (Cr₂Nb and Co₂Ti) were formed in other alloys with different Ti contents. The results were discussed in terms of the mixing enthalpy, atomic size difference, electronegativity difference and valance electron concentrations among the elements within alloys. The alloy hardness exhibited a slightly decreasing trend as the Ti content increased, resulting from the coarser microstructure and reduced amount of Laves phases. Augmented Ti content increased the compressive strength, but decreased the ductility. Particularly, for the CoCrMoNbTi_0.2 alloy, the hardness, compressive strength and fracture strain were as high as 916.46 HV_0.5, 1906 MPa and 5.07%, respectively. The solid solution strengthening of the BCC matrix and the formation of hard Laves phases were two main factors contributing to alloy strengthening.     

二元铁合金系液态结构与特性的研究进展

从结构和性质两个方面论述了近年来有关液态二元铁合金的研究现状、研究工作听 不足和今后应予以加强的研究方向。     

相场参数对强制流动下等温合金枝晶生长的研究

基于相场方法对强制流动下的等温合金的凝固过程进行了数值模拟,研究了流速、过饱和度、耦合参数、各向异性参数对枝晶生长形貌的影响.结果表明,强制流动下的晶核生长为非对称的枝晶,上游方向的枝晶臂尖端生长得到促进、下游和垂直流方向的受到抑制,且垂直臂随着流速的增加逐渐向上游方向倾斜生长;此外,结果还表明,在一定流速下,随着过冷度、耦合参数、各向异性值的增大,上游、垂直流和下游方向的枝晶臂尖端稳态速度均呈递增趋势.