The metastable liquid miscibility gap in Cu-Co-Fe alloys

The metastable liquid-phase separation (MLPS) in the Cu-Co-Fe system was investigated using an electromagnetic levitation melting and solidification technique. It was found that when ternary alloys containing more than 10wt.% (12 at.%)Co and 10wt.% (11at.%)Fe were undercooled below a certain temperature, T _sep, the homogeneous melt separated into two liquid phases. In alloys containing more than 54 to 57wt.% (49 to 54at.%)Cu (depending on the Co and Fe content), the phase separation generally appeared as dispersed (Fe, Co)-rich droplets (L₁) in a Cu-rich matrix, whereas for alloys containing less copper, the separation resulted in Cu-rich droplets (L₂) in a (Fe, Co)-rich matrix. The metastable liquid miscibility gap boundary of the Cu-Co-Fe ternary was determined using the measured T _sep and the composition of the separated phases. The ternary liquid-phase separated boundaries were found to be consistent with a cross-sectioned phase diagram in which one axis represents pure copper and the other Fe + Co.     

Liquid phase separation in immiscible Cu–Fe alloys

Cu–Fe binary alloys containing 20–50wt. % Fe were studied by the combination of the melt fluxing and cyclic superheating technique. The microstructural evolution of Cu–Fe alloys was investigated with scanning electron microscopy. When the undercooling was larger than the critical undercooling, the Fe-rich spheroids were embedded into a Cu-rich matrix and the metastable phase separation was observed in microstructures. The size of separated particles in the Cu-35wt.% Fe alloy was larger than that of other Cu–Fe alloys with different compositions and the size of separated droplets was related to the ΔT_S, which was equal to the undercooling (ΔT) minus the critical undercooling (ΔT_C). Moreover, a large undercooling tended to promote the coagulation of the separated droplets, so the size of the separated Fe-rich spheroids in the microstructure of the immiscible Cu–Fe alloys increased with the increase in the undercooling.     

不同冷却条件下Cu60Co30Cr10合金的快速凝固（英文）

利用电磁悬浮和快淬实验研究Cu60Co30Cr10合金在亚稳不混溶区的液相分离和快速凝固特征。结果表明,合金的显微组织为富(Co,Cr)相分布在富Cu相基体中,且富(Co,Cr)相颗粒的形状和大小随着冷却速率的变化而有明显的区别。在悬浮凝固条件下冷却速率较低,富(Co,Cr)相较粗大,有明显的聚集粗化趋势,富(Cu)相中有大量富(Co,Cr)相枝晶。而在快淬凝固条件下富(Co,Cr)相明显细化,富(Cu)相中未发现富(Co,Cr)相枝晶形成,这可能与较高的冷却速率、较大的过冷度和较高的界面张力有关。     

Critical-point wetting at the metastable chemical binodal in undercooled Fe–Cu alloys

Complementary results of differential thermal analysis and microstructural examination on Fe–Cu alloys provide the first evidence for critical-point wetting occurring at a completely metastable miscibility gap. The perfect wetting conditions hold for a composition range of 50–65 at.% Fe in the vicinity of the critical concentration. For samples encased with a glass slag, the Cu-rich liquid completely wets the glass upon undercooling to the metastable miscibility gap. In the perfect-wetting range, the metastable homogeneous liquid phase exhibited phase separation without undercooling below the chemical binodal. At deep undercooling, solidification of alloys with phase separated liquids results in a coarse scaled two-phase microstructure. In contrast, the homogeneous liquid phase of samples with compositions outside the perfect wetting range did undercool below the equilibrium onset of the metastable phase separation reaction. The phase separation in these samples occurred on a much finer scale. For samples without a glass encasement and thus in the presence of the Al₂O₃ crucible and an iron oxide layer, perfect wetting occurred near the consolute point on both sides of the metastable miscibility gap. This demonstrates that critical-point wetting is independent of the surface environment, but the wetting phase selected is surface sensitive.     

The corrosion behavior of Fe-Cr alloys containing Co,Mn, and/or Ni and of a Co-base alloy in the presence of molten (Li,K)-carbonate

The corrosion behavior or commercial Fe ana Co base alloys and Fe-Cr model alloys with different contents of Co and/or Mn was investigated by continuous exposure tests in the presence of a thin carbonate film. All alloys studied form multi-layered corrosion scales consisting of outer Li containing oxides and inner Cr rich oxides, i.e. spinels or LiCrO₂. The LiCrO₂ is formed on alloys with high Cr contents (≤ 20 wt.%), whereas mixed (Fe,M)_3-x Cr_xO₄ spinels (M = Co, Mn, Ni) were found on alloys with lower Cr content (15–20 wt.%). Insoluble Cr containing oxides occur only in the inner layers of the corrosion scale, whereas on the surface of corroded specimens soluble chromates were detected. Alloys with Mn contents greater than 15 wt.% form Mn₂O₃ in the initial stages of the experiments, this oxide reacts with the melt and formation of Li₂MnO₃ takes place. In exposure tests up to 500 h Fe-Cr alloys with low contents of Mn and Co (10 wt.% Co or Mn) form iron rich oxides (LiFeO₂ and LiFe₅O₈) with varying amounts of dissolved Mn or Co. In the later corrosion stages outward diffusion of Mn and/or Co takes place and LiCoO, and Li₂MnO₃ are formed on top of LiFeO₂, whereby the concentration of Mn and/or Co in the inner layers (LiFeO₂ and spinel) decreases. The outer Li containing oxides LiFeO₂, LiCoO₂ and Li₂MnO₃ are nearly insoluble in the melt and when present at the surface protect the metallic material from further corrosive attack. Fe-Cr model alloys containing Co and Mn form multi-layered corrosion layers after 2000 h of exposure. These layers consist of four oxides in the following sequence from the metal-scale to the scale-melt interface: (Fe,Cr,Co,Mn)₃O₄ spinel, LiFeO₂, Li₂MnO₃ and LiCoO₂.     

Oxidation of Two Ternary Fe–Cu–5 at.% Al Alloys in 1 atm of Pure O2 at 800°C

The oxidation of two two-phase ternary Fe–Cu–Al alloys containing about 5 at.% aluminium, one Fe-rich and one Cu-rich, has been studied at 800°C under 1 atm O₂. The Fe-rich alloy (Fe–15Cu–5Al) shows two parabolic stages, with a large decrease of the parabolic rate constant after about 2 hr. The presence of 5 at.% Al reduces significantly the oxidation rate of this alloy with respect to a binary Fe-Cu alloy of similar composition by forming an external alumina scale. Moreover, the addition of 15 at.% Cu is able to reduce the critical aluminium content needed to form alumina scales with respect to binary Fe–Al alloys. On the contrary, the Cu-rich Fe–85Cu–5 Al alloy presents a single parabolic stage and forms a thick and porous external scale composed of an outermost layer of copper oxides and an inner region containing a mixture of copper and Fe–Al oxides, coupled to the internal oxidation of iron and aluminium. As a result, the oxidation of the Cu-rich ternary alloy at 800°C is much faster than that of the Fe-rich ternary alloy.     

High temperature oxidation behaviour of Ni–Fe–Co anodes for aluminium electrolysis

The oxidation behaviour of ternary Ni_xFe_yCo_z alloys (where x/y (wt) = 1 and 1.85; z = 0, 10, 30 and 50 wt.%) was studied in air at 800 °C. Alloys were found to follow complex oxidation kinetics, with the highest oxidation rates observed for alloys having 50 wt.% Co. Significant improvements in oxidation resistance were achieved by addition of 10 and 30 wt.% Co to the Ni–Fe system. The decrease in oxidation rate was associated with suppression of Fe₂O₃ formation in preference for (Co,Ni)_xFe_3−xO₄ growth. The results were discussed in light of the materials requirements for inert anodes for aluminium electrolysis.     

Liquid–liquid demixing and microstructure of Co–Cu–Zr alloys with low Zr content

Liquid–liquid phase separation and its effect on the microstructure has been investigated along the quasi-binary line (Co₄₀Cu₆₀)_100−xZr_x with x = 2, 4, 6, 9 and additionally for (Co₅₀Cu₅₀)₉₄Zr₆ and (Co₆₀Cu₄₀)₉₄Zr₆. The elemental distributions and the microstructures were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy for samples that were (i) processed by thermal cycling in alumina crucibles at 10, 20 and 30 K/min with simultaneous differential thermal analysis, (ii) rapidly quenched by single-roller melt spinning and (iii) quenched after having been electromagnetically levitated at various temperatures. The metastable miscibility gap of the binary Co–Cu system with phase separation into Co- and Cu-rich liquids transforms into a stable miscibility gap for Zr contents 3 < x < 7.5 with separation into Co/Zr-rich and Cu-rich liquids. In contrast to the binary Co–Cu system where the Cu-rich liquid phase always surrounds the Co-rich phase, the Zr addition modifies the surface tension energies and/or wetting behavior in a peculiar way so that the Co/Zr-rich phase always encloses the Cu-rich liquid phase concerning the ternary Co–Cu–Zr system in that compositional range. The macrosegregation morphologies of the liquid phase separation that built up via Ostwald ripening, gravity-driven convection, collision, coalescence and wetting effects proceed on a very short time scale and even samples that have been prepared by rapid quenching techniques still exhibit phase separated regions in the micrometer regime.     

Effects of Ag,Co, and Ge additions on microstructure and mechanical properties of Be-Al alloy fabricated by investment casting

The effects of Ag, Co, and Ge additions on microstructure and mechanical properties of Be-35Al (wt.%) alloys fabricated by investment casting were studied. The results reveal that the trace metals 1.5wt.% Ag, 0.7wt.% Co, and 0.8wt.% Ge additions do not change the nucleation temperature of Be phase. However, the nucleation temperature of the Al phase decreases from 642 °C to 630 °C by DSC due to the Ge addition. The strength of the alloys sharply increases due to the dissolution of the Ag and Ge solutes into the Al phase and the Co into the Be phase characterized by SEM and EDS. Obviously, the strength of Be-Al-Ag-Co-Ge alloy is improved by the solution strengthening. Furthermore, a few Ag₃Al particles contribute to the strength of the Al phase. Be-Fe-Al ternary intermetallic compounds which can effectively reduce the negative effect of an impurity element Fe on the mechanical properties of Be-Al alloys are also found by XRD and EDS.

     

Rapid solidification and liquid-phase separation of undercooled CoCrCuFexNi high-entropy alloys

Fluxing and cyclic superheating technique was used to investigate the rapid solidification behavior of CoCrCuFe_xNi (x = 1.0, 1.5, 2.0, molar concentration) high-entropy alloys in this study. The microstructures of CoCrCuFe_xNi (x = 1.0, 1.5, 2.0) high-entropy alloys solidified at different undercoolings (ΔT) were investigated. Liquid-phase separation leading to Cu-rich and Cu-depleted regions, were obtained in the solidified microstructure from highly undercooled melt. This occurs when the melt undercooling exceeds a critical undercooling (ΔT_crit) of 160 K for CoCrCuFeNi, 190 K for CoCrCuFe_1.5Ni and 293 K for CoCrCuFe₂Ni alloy. However, typical dendrites and interdendritic regions were observed in rapid-solidified CoCrCuFe_xNi alloys prepared from melts with a small undercooling (ΔT < ΔT_crit). Conversely, a large amount of Cu-rich spheres and even egg-type structures were observed in alloys solidified from melts with large degree of undercooling, ΔT in excess of the critical value, ΔT_crit. A large amount of Cu-rich nano-phases were found in the matrix, possibly, due to the precipitation of Cu-rich phase from the supersaturated solid solution obtained during solidification. The positive enthalpies of mixing between Cu and the other elements in the multi-component alloys resulted in the occurrence of liquid-phase separation prior to the liquid–solid transformation starts. The sluggish diffusion effect of high-entropy alloys and rapid solidification play an important part in the precipitation of nanophase during the solid-state transformation in the Cu-based matrix. Similar to other immiscible alloys, liquid-phase separation occurred when a critical undercooling was exceeded. Differently, nanophases were found in the microstructures of multi-component CoCrCuFe_xNi (x = 1.0, 1.5, 2.0) alloys.     

Cu-Fe64Ni32Co4合金显微组织及热物理性能研究

本研究利用相图计算的CALPHAD方法和真空电弧熔炼技术,设计并制备了Cu_x(Fe_0.64Ni_0.32Co_0.04)_100-x(x=30, 45, 60, wt. %)系列合金。实验研究了该系列合金在不同热处理工艺时的显微组织,热导率以及热膨胀系数。研究结果表明：Cu-Fe₆₄Ni₃₂Co₄系列合金在600 °C和800 °C时效处理后均为fcc富铜相和fcc富因瓦(铁镍钴)相组成的各向同性的多晶合金。该系列合金在1000 °C淬火并在600 °C时效处理50 h后,其热膨胀系数变化范围为6.88~12.36×10_-6 K_-1;热导率变化范围为22.91~56.13 W.m_-1.K_-1;其热导率明显高于因瓦合金,其中Cu₃₀(Fe_0.64Ni_0.32Co_0.04)₇₀与 Cu₄₅(Fe_0.64Ni_0.32Co_0.04)₅₅合金的热膨胀系数可以与电子封装中半导体材料的热膨胀系数相匹配。     

Experimental Investigation and Thermodynamic Calculation of the Phase Equilibria in the Co-Cu-V Ternary System

The phase equilibria of the Co-Cu-V ternary system at 900, 1000, 1100 and 1200 °C have been experimentally determined by optical microscopy and electron probe micro-analysis of the equilibrated alloys. The phase transformations were investigated by means of the differential scanning calorimetry. Based on the experimental data of phase equilibria and thermodynamic properties, the thermodynamic assessment of the Co-Cu-V ternary system was carried out by using the calculation of phase diagrams method. A consistent set of the thermodynamic parameters leading to reasonable agreement between the calculated results and experimental data was obtained in the Co-Cu-V ternary system. Meanwhile, the calculated results show that the critical temperature of metastable magnetically induced miscibility gap of (α _f Co) and (α _p Co) phases in the Co-V system gradually decreases with increasing Cu composition in the range of 0-3 wt.% additions.     

深过冷Cu40Co40Ti20合金的亚稳相分离与快速凝固

利用电磁悬浮技术和熔体快淬使Cu40Co40Ti20合金熔体达到深过冷,发生亚稳相分离.经悬浮冷却凝固的Cu40Co40Ti20发生液相分离,富Cu相中出现富(Co,Ti)相枝晶,富(Co,Ti)相中形成二次析出的富Cu相;经纯铜快淬处理的合金形成外层为富(Co,Ti)相,内部为富Cu相的双层球状结构.两种方法得到的组织中,富(Co,Ti)相内部均出现了二次析出的富Cu相小球.对样品进行X射线衍射分析,相组成主要有(Cu)和Co2 Ti相.     

Galvanic Corrosion in Molten Salts: A Discussion of the Corrosion Mechanism of Two-Phase Ni–20Cr–20/30Cu Alloys in Eutectic (Li,K)2CO3 at 650°C

The corrosion of a Ni–20Cr and three ternary Ni–20Cr–Cu alloys containing 10, 20 and 30 wt.% Cu, respectively, in a eutectic (Li, K)₂CO₃ melt was studied at 650°C under air. Ni–20Cr and Ni–20Cr–10Cu are solid-solution alloys, while both Ni–20Cr–20Cu and Ni–20Cr–30Cu are two-phase alloys composed of a Cu-depleted matrix together with a small amount of a Cu-rich phase. The results indicate that Ni–20Cr and Ni–20Cr–10Cu have similar corrosion rates, forming a scale of external NiO and inner Cr-rich oxides. The two-phase Ni–20Cr–20/30 Cu alloys corrode rapidly, producing a mixture of Ni–Cr–rich oxides and a Ni–Cu metallic phase, which may be ascribed to the accelerated corrosion of the Cu-depleted matrix coupled with the cathodic Cu-rich phase of the alloys. As compared to Ni–20Cr–30Cu, Ni–20Cr–20Cu suffers from more-severe corrosion, due to a smaller area fraction of the cathodic Cu-rich phase in the alloy. The galvanic corrosion mechanism is also discussed.     

深过冷条件下Cu—Co合金的二次液相分解与合金的凝固

采用熔融玻璃深过冷技术和金相分析法,对Cu-(10%-30%)Co合金液态深过冷条件下的二次液相分解和合金的凝固行为进行了分析,研究表明,由于液相分解后富C液相的溶解度随温度下降而降低,使Cu原子不断从富Cu原子不断从富C液相中分解出来,如果分解出的Cu原子不能扩散到富Cu原子不能扩散到富Cu液相中,会在富Co液相内形成富Cu液滴。但合金凝固后富Co相中富Cu相并非都来自于干净人液相分解,一部分是来自于富Co液相长大和富Co相选分结晶,冷却曲线的测定结果和分析显示,Cu-Co合金液相分解后,富Co液相的结晶温度与合金成分和实验条件有关,富Co液滴结晶完成后发生了包晶转变。     

Miscibility gap of B2 phase in NiAl to Cu3Al section of the Cu–Al–Ni system

The phase separation in the bcc phase of the Cu–Al–Ni system at 600–700 °C was investigated mainly by energy dispersion X-ray spectrometry (EDS) and differential scanning calorimetry (DSC). The compositions of the β₁ (A2 or B2: Cu-rich), β₂ (B2: NiAl-rich) and γ (γ-brass type) phases in equilibrium were determined. It was found that there is a β₁+β₂ miscibility gap in the β phase region as previously reported by Alexander. It was confirmed by means of high temperature in situ TEM observation that this miscibility gap consists of the B2+B2 phases but not the A2+B2 phases which is sometimes observed in many other Ni–Al and Co–Al base ternary bcc alloys. Thermodynamic calculation was performed which indicates that this characteristic feature suggests that the β₁ (B2)+β₂ (B2) miscibility gap is a part of a Cu-rich B2+NiAl-rich B2 miscibility gap island formed around the center of the composition triangle of the isothermal section. The phase separation in the β phase region and the stability of the ordered bcc aluminide are presented and discussed.     

Formation of amorphous phase with crystalline globules in Fe–Cu–Si–B and Fe–Cu–Zr–B immiscible alloys

The microstructure of rapidly solidified melt-spun ribbons of (Fe_0.75M_0.10B_0.15)_100−xCu_x (M = Si, Zr) alloys was investigated focusing on amorphous-phase formation and the solidification structure. In this study, Fe–Cu–Si–B and Fe–Cu–Zr–B alloys were designed to show amorphous-phase formation and liquid-phase separation simultaneously. Amorphous-phase formation was confirmed in both Fe–Cu–Si–B and Fe–Cu–Zr–B alloys. Minor exceptions in a combination map of mixing enthalpy and quaternary predicted phase diagram are acceptable range for designing a quaternary Fe–Cu-based alloy system that shows liquid-phase separation in Fe-based and Cu-based liquids and the formation of an Fe-based amorphous phase.     

Effect of semisolid microstructure on solidified phase content in 1xxx Al alloys

Melt-spun high purity Al–0.3 wt% Fe–0.1 wt% Si alloys, containing V and Al–5Ti–1B grain refiner, were melted at 2 K/min from 635°C to different temperatures above the solidus in a differential scanning calorimeter (DSC). Quenched and slow cooled (at 2 K/min) microstructures were examined using scanning electron microscopy (SEM). Melting was observed in a hot-stage reflected light microscope. Al–Fe₄Al₁₃ melted first, forming cell boundary liquid films. Rapid coarsening occurred at <4 K above the solidus via motion of these boundaries. Intracellular liquid droplets formed at 4–6 K above the solidus. These droplets have been demonstrated to be sites of metastable Al–FeAl_m formation in V and Al–5Ti–1B containing alloys on solidification. With increasing temperature the liquid cell boundaries thickened and consumed the droplets, until there were none left at 10 K above the solidus, when the microstructure was fully molten. Consequently Al–FeAl_m only formed in V and Al–5Ti–1B containing alloys when solidified following melting to 4–10 K above the solidus.     

Annealing effects on the microstructure and properties of bulk high-entropy CoCrFeNiTi0.5 alloy casting ingot

Most previous researches focused on small casting ingots prepared by arc melting, when studying high-entropy alloys. Large sized ingots were also necessary in exploring the existence of volume effects in the multi-principal element alloys. During the experiments, a large sized CoCrFeNiTi_0.5 alloy casting ingot was prepared by a medium frequency induction melting furnace. A slight volume effect occurred, reflecting mainly in the growth of crystalline grains and the increase of alloy hardness in the ingot. To investigate the effect of annealing temperature on microstructure and properties of CoCrFeNiTi_0.5 alloy, several samples taken from the ingot were annealed at 600 °C, 700 °C, 800 °C and 1000 °C respectively for 6 h. Almost no effects were found to the crystalline structure and elemental distribution when the samples were annealed below 1000 °C. The crystalline structure of CoCrFeNiTi_0.5 alloy was composed of one principal face-centered cubic (FCC) solid-solution matrix and a few intermetallic phases in the form of interdentrite. Dendrite contained approximately equivalent amount of Co, Cr, Fe, Ni and a smaller amount of Ti. When annealed below 1000 °C, the interdendrite stayed in (Ni, Ti)-rich phase, (Fe, Cr)-rich phase and (Co, Ti)-rich phase. After 1000 °C annealing, (Co, Ti)-rich phase disappeared, while (Ni, Ti)-rich phase and (Fe, Cr)-rich phase grew. The microhardness of the as-cast CoCrFeNiTi_0.5 alloy was 616.80 HV and the macrohardness was 52 HRC. The hardness of the samples stayed generally unchanged after annealing. This indicated a high microstructure stability and excellent resistance to temper softening that the CoCrFeNiTi_0.5 alloy exhibited.