Single-phase Region Research Articles

Hot pack rolling the Ti2AlNb alloy prepared by SPS: Testing of the high-temperature properties of the alloy directly without the prior heat treatment

Ti2AlNb alloy holds large research significance as a lightweight material for high-temperature structural applications in the next generation of aero-engines. In this work, pre-alloyed Ti2AlNb powders were spark plasma sintered to fabricate the raw compact, which was then hot rolled at different phase fields to fabricate sheets. The high-temperature properties of the sheets were tested directly without the prior heat treatment. Results show that the Ti2AlNb sheets have good high-temperature strengths even without the heat treatment prior to testing. For the sheet rolled at the single B2 phase region, the maximum tensile strength was up to 931.4 MPa at 650 °C. The sheet mainly shows intergranular fracture mode due to the weak bonding at grain boundaries. The sheets rolled at the O + B2 phase region have low tensile strengths at different testing temperatures. For the sheet rolled at α2+O + B2 phase region, the tensile strength gains its maximum strength of 956.6 MPa at 650 °C, and the sheet shows transcrystalline fracture mode. The fracture mechanisms of the sheets rolled at different phase regions are analyzed in detail and proved by experiment. This work shows the potential of fabricating Ti2AlNb sheet by using the spark plasma sintered compact.

Enhancing ferromagnetism in the Sm(Co, Mn)5 system: Impact on phase stability and magnetic properties

This work demonstrates that Mn can be a beneficial candidate for the performance modulation of permanent magnetic materials like SmCo5. Theoretical calculations reveal that Mn has a large positive contribution to the magnetization of SmCo5, owing to the strong ferromagnetic exchange interaction between Co and Mn atoms. It is also found that Mn could occupy both 2c and 3g crystal sites because of the minute energy difference between them can be easily overcome through thermal activation. Co atoms on the same site with Mn have increasing moments with the increase in Mn content, whereas those on the other sites show decreasing moments. The experimental substitution of Mn for Co leads to a contraction of the SmCo5 single-phase region, and thus pure Sm(Co, Mn)5 phase could only be obtained through subtle control of Sm content by annealing. Significantly, a 13 % increase in saturation magnetization of SmCo5 is achieved through Mn substitution. A slight rise in the Curie temperature is also obtained, suggesting that Mn substitution does not strongly disturb the exchange interactions of the Co sublattice. This study offers a new option for enhancing magnetic properties in Sm–Co systems and introduces novel physical phenomena resulting from Mn substitution, which is worthy of further investigation.

Solubility of carbon dioxide in Triton X-100 – Water – ethaline solvent mixtures at 298.15 K

Deep eutectic solvents (DESs) and liquid surfactants have been known as safe and low risk materials to prepare appropriate media for various physical and chemical processes. These compounds have demonstrated manifest properties while playing the solvent role. The solubility of carbon dioxide (CO2) were determined in aqueous solutions Triton X-100 at 298.15 K as a function of CO2 pressure. Also, ternary solvent mixtures including Triton X-100 as an additive in aqueous solutions of ethaline were evaluated for the capacity of CO2 capture. Triton X-100, DES, and water showed a three-component liquid–liquid equilibrium with a limited single-phase region. The determination of CO2 solubility in the single-phase region of ternary mixtures revealed that the Triton X-100 as an additive increases the solubility of CO2 in the aqueous solutions of ethaline. The effect of Triton X-100 on CO2 solubility in binary mixtures of ethaline and water appeared differently based on the concentration of Triton X-100.

The isothermal section of the phase diagram of Gd-Mn-Ga (≤50 at.%Ga) ternary system at 873K

The phase diagram of the Gd-Mn-Ga ternary system is a very important tool for the exploration and development of rare earth Heusler alloys with excellent physical properties, such as magnetic properties, half-metallic properties, ferromagnetic shape memory effect, magnetocaloric effect, ect. In this study, the alloy samples were prepared using a vacuum arc melting furnace, and the 873 K isothermal section of the Gd-Mn-Ga (≤50 at.%Ga) ternary alloy phase diagram was determined by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy spectrometry (EDS). The isothermal section consists of 22 single-phase regions, 44 two-phase regions, and 23 three-phase regions with the presence of 6 ternary compounds, GdMnGa, Gd2MnGa6, Gd2Mn15Ga2, Gd2Mn11Ga6, GdMn0.56Ga1.44, and GdMn0.37Ga1.63. The solid solubility ranges of Ga in β-Mn and Gd2Mn15Ga2 are 5.0–18.1 at.%Ga and 10.5–25.0 at.% Ga. The maximum solid solubility of Ga in α-Mn, GdMn12, and Gd6Mn23 are 2.0, 2.7, and 7.3 at.% Ga, respectively. The maximum solid solubility of Mn in GdGa2 is determined to be 13.1 at.% Mn.

Study on phase behavior of [C16mim] [br]/CAHS/n-butanol/n-hexane/water microemulsions: synergistic effect of ionic liquid-zwitterionic surfactants

The synergistic effect of ionic liquids (ILs) and anionic surfactants, ILs and cationic surfactants, and ILs and nonionic surfactants to form microemulsions has been studied by other researchers, while the synergistic effect of ionic liquids and zwitterionic surfactants to form microemulsions has not been reported. Here, we report the synergistic effect of imidazolium ionic liquid, 1-hexadecyl-3-methylimidazolium bromide [C16mim] [Br], and betaine-type zwitterionic surfactant, cocamidopropyl hydroxysultaine (CAHS), as surfactants, to form microemulsions with n-butanol, n-hexane and water. Phase behavior of the systems with different mass ratios of two surfactants, varied mass ratios of surfactant to cosurfactant (Km), and different temperatures are investigated by pseudo-ternary phase diagrams. The single-phase region of the microemulsion is maximized when the ratio of CAHS to [C16mim] [Br] is 1:4 and Km is 1.5, with little temperature dependence. The size and the polarity of microemulsions are explored. The synergistic effect of [C16mim] [Br] and CAHS is confirmed by surface tension and interface tension. A universal preparation method to form microemulsions by combining ionic liquids with zwitterionic betaine-type surfactants is provided and it play a pivotal role in broadening the potential uses of ILs in microemulsions.

Effect of TiO2 addition on microstructure and mechanical properties of Al2O3 ceramic joints brazed with Ag‐CuO fillers

AbstractIn our work, 0.5 mol.% TiO2 was added to Ag‐xCuO (x = 2, 4, 8 mol.%) fillers whose compositions at 1050°C were in the L2 single liquid phase region, the boundary of the miscible phase region, and the L1+L2 miscible phase region, respectively. The role of TiO2 was derived by comparing the quenching microstructure of the fillers on the Al2O3 ceramics and the effect on the interfacial microstructure and shear strength of the joints before and after the addition of TiO2. The results showed that the TiO2 would dissolve into the CuO‐rich L1 liquid phase of the filler, leading it to spread outside the molten droplet. When brazing with Ag‐4CuO‐0.5TiO2 filler, the CuO at the joint interface almost completely disappeared, and the shear strength of the joint was significantly reduced. In contrast, the joint strength showed a small increase when using Ag‐8CuO‐0.5TiO2 filler, as the excess CuO at the interface of the brazed joint was reduced. When the filler composition was in the L2 single liquid phase region, CuO was diffusely distributed throughout the molten droplet. TiO2 was hardly contacted with CuO and its addition had no significant effect on the interfacial microstructure and the shear strength of the joints.

Microstructure evolution during high temperature rolling and effect on the mechanical properties of TiBw/Ti65 composites with network structure

In this paper, the rolled TiB whisker (TiBw)/Ti65 composites with different rolling reductions were obtained by high temperature (HT) rolling in the β single-phase region. The changes in TiBw, silicide and grain size after rolling deformation were analyzed by microstructure characterization. Based on this, the strengthening mechanism and HT deformation behavior of rolled composites were analyzed in combination with room temperature and HT tensile experiments. The results show that TiBw fractured after rolling deformation, leading to a decrease in the aspect ratio and an increase in matrix connectivity. Silicide with an average diameter of 80 nm appeared in the rolled composites. The grain was continuously refined as the rolling reduction increased. A T-type texture with the crystal C-axis parallel to the transverse direction appeared at 40% rolling reduction. The strength and plasticity of sintered TiBw/Ti65 composites were increased after rolling. With an increase of rolling reduction, the composites exhibited fine-grain strengthening below 650 °C. When tensile above 700 °C, a softening effect occurred with the decrease of grain size, that is, the yield strength decreased for grain size <3.6 μm. The strengthening mechanisms of rolled TiBw/Ti65 composites include fine grain strengthening, load transfer strengthening of TiBw, Orowan strengthening of silicide, and dislocation density strengthening. Among them, fine grain strengthening and load transfer strengthening predominated and increased as the rolling reduction increased. After tensile at 750 °C, many recrystallized grains with large misorientation appeared around the initial coarse grains and TiBw.

Re-entrant percolation in active Brownian hard disks.

Non-equilibrium clustering and percolation are investigated in an archetypal model of two-dimensional active matter using dynamic simulations of self-propelled Brownian repulsive particles. We concentrate on the single-phase region up to moderate levels of activity, before motility-induced phase separation (MIPS) sets in. Weak activity promotes cluster formation and lowers the percolation threshold. However, driving the system further out of equilibrium partly reverses this effect, resulting in a minimum in the critical density for the formation of system-spanning clusters and introducing re-entrant percolation as a function of activity in the pre-MIPS regime. This non-monotonic behaviour arises from competition between activity-induced effective attraction (which eventually leads to MIPS) and activity-driven cluster breakup. Using an adapted iterative Boltzmann inversion method, we derive effective potentials to map weakly active cases onto a passive (equilibrium) model with conservative attraction, which can be characterised by Monte Carlo simulations. While the active and passive systems have practically identical radial distribution functions, we find decisive differences in higher-order structural correlations, to which the percolation threshold is highly sensitive. For sufficiently strong activity, no passive pairwise potential can reproduce the radial distribution function of the active system.

Optimization of the three-component Al-Fe-Si system composition

Three-component Al-Fe-Si systems with an equiatomic composition are high-strength alloys that are poorly resistant to plastic deformation. To increase the material ductility, a phase diagram of the Al-Fe-Si ternary system was constructed using ThermoCalc software. The conditions for the formation of the AL8FE2SI phase, which is an α-phase and has a hexagonal lattice, have been identified. At solidus temperatures, α-AL8FE2SI and β-AL9FE2SI2 are competitive. The conditions for the developing a single-phase region consisting of 97% α-AL8FE2SI have been identified. α-AL8FE2SI is a high-temperature phase; the system will perceive plastic deformation in the temperature range of 675–612 °C, with further phase conversion upon cooling.

Na(Co,Ti)O2 Cathodes for Na-Ion Batteries: A Proper Choice

Increased attention to sodium-containing materials during the last several years is caused by the rapid development of sodium-ion batteries (NIBs), considered as a potential successor for lithium-ion batteries (LIBs) [1]. Especially layered sodium oxides have gained large interest due to their potential applicability as electrode materials (both, as cathodes and anodes). Similar to LiCoO2 in LIBs, NaCoO2 shows an immense potential as a cathode in NIBs. A partial replacement of Co by Ti in a Na(Co,Ti)O2 cathode can stabilize the crystal structure during (de)sodiation and reduce the number of phase transformations. Although the phase diagram of the Na-Co-Ti-O system has not been studied yet, existence of single-phase and multi-phase regions in dependence on the Na-content and temperature is reported in the literature [2]. Typically, layered Na,Co,Ti-oxides adopt three very close structure types P2, O3 and P3, which differ in the oxygen surrounding for Na-cations (octahedral or prismatic) and in the number of the formula units in the crystal unit cell (2 or 3). For application in Na-batteries, however, a trial-and-error approach is mostly needed to evaluate a composition with outstanding electrochemical performance.The redox activity of Co and Ti cations is usually characterized by two well-separated cell potentials. The Co-oxidation (Co2+ → Co3+ and Co3+→ Co4+) occurs mostly at potentials between 3 and 4 V vs. Na+/Na, qualifying the material as cathode while Ti4+/Ti3+ redox process takes place below 1.5 V, as common for anode materials. Based on our experimental studies of layered Na,Co,Ti-oxides with different compositions as NaCo0.5Ti0.5O2 (O3) [3], Na0.65Co0.5Ti0.5O2 (P3) [3], Na0.67Co0.33Ti0.67O2 (P2- and O3-structures) [4] and Na0.8Co0.8Ti0.2O2 (P2) [5] as well as on literature report [6], we can conclude that the most stable cycling behavior can be realized, if the composition in the initial (discharged) state contains mainly Co3+ and not Co2+. The reason for this is a first-order spin-state transition of high-spin HS-Co2+ to low-spin LS-Co3+ and back upon (de)sodiation, which is accompanied by a phase transformation, leading to a huge potential hysteresis of about 2 V. This hysteresis only slightly depends on the temperature between 0 °C and 40 °C. Among the compositions mentioned above, Na0.8Co0.8Ti0.2O2 with the P2-structure is the best candidate for application as a cathode in Na-batteries. However, a deep discharge of the material below 1.6 V to reach the Na1.0Co0.8Ti0.2O2 composition is detrimental as well, since some amount of Co2+ appears, negatively influencing the cycling behavior.

Pressure Drop Characteristics of Subcooled Water in a Hypervapotron under High and Non-Uniform Heat Fluxes

To study the pressure drop characteristics of hypervapotron, which was designed as a water-cooling structure in the divertor dome of the fusion reactor, the pressure drop tests of subcooled water were carried out in a vertically upward hypervapotron. To simulate the one-side radiant heating condition in the engineering application, the non-uniform heat fluxes were obtained by using the off-center electrically heating method. The system parameters were as follows: mass flux G = 2000–5000 kg·m−2·s−1, inlet pressure p = 2–4 MPa, and equivalent one-side radiating heat flux qe = 0–5 MW·m−2. The effects of the parameters on the pressure drop were discussed in detail. It was observed that in the single-phase (SP) region, the pressure drop was little influenced by the inlet fluid temperature (Tb,in). However, in the subcooled boiling region, the pressure drop increased rapidly with the increasing Tb,in. A higher G leads to a high pressure drop. In the SP region, the influence of p on the pressure drop is not obvious, and the pressure drop decreased with the increasing qe. The test data are used to evaluate the typical pressure drop correlation, and the results show that none of these correlations can predict the pressure drop well under the test conditions. Therefore, a new pressure drop correlation is proposed for subcooled water in a hypervapotron under high and non-uniform heat fluxes. The new correlation has a high prediction accuracy for the test data, and the mean relative error (MRE) and root mean square error (RMSE) are 0.72% and 4.33%, respectively. The test results have a reference value for the design of the water-cooling structure of the diverter.

Experimental Determination of Phase Equilibria in the Mn-Zn Binary System

Phase equilibria in the Mn-Zn binary system were experimentally determined by chemical composition examination, crystal structure determination, and thermal analysis. Major changes were detected for the β, ε, and δ phases. The β-B2 single-phase region could not be confirmed in the studied system because a disordered body-centered cubic structure, which is identical to the δMn phase, was confirmed in a quenched sample from the previously proposed region of β phase. The ε phase has been controversial whether the phase is separated into ε, ε1, and ε2 phases or not. By studying a diffusion couple and several alloy compositions, it was established that the ε, ε1, and ε2 phases are not separate and comprise a single ε phase. Furthermore, the δ phase is not present in the Zn-rich region of the system because the corresponding invariant reactions were not detected via thermal analysis.

First-Principle Studies on Local Lattice Distortions and Thermodynamic Properties in Non-Stoichiometric Thorium Monocarbide

Thorium monocarbide (ThC) is interesting as an alternative fertile material to be used in nuclear breeder systems and thorium molten salt reactors because of its high thermal conductivity, good irradiation performance, and wide homogeneous composition range. Here, the influence of carbon vacancy site and concentration on lattice distortions in non-stoichiometric ThC1−x (x = 0, 0.03125, 0.0625, 0.125, 0.1875, 0.25, or 0.3125) is systematically investigated using first-principle calculations by the projector augmented wave (PAW) method. The energy, mechanical parameters, and thermodynamic properties of the ThC1-x system are calculated. The results show that vacancy disordering has little influence on the total energy of the system at a constant carbon vacancy concentration using the random substitution method. As the concentration of carbon vacancies increases, significant lattice distortion occurs, leading to poor structural stability in ThC1−x systems. The changes in lattice constant and volume indicate that ThC0.75 and ThC0.96875 represent the boundaries between two-phase and single-phase regions, which is consistent with our experiments. Furthermore, the structural phase of ThC1−x (x = 0.25–0.3125) transforms from a cubic to a tetragonal structure due to its ‘over-deficient’ composition. In addition, the elastic moduli, Poisson’s ratio, Zener anisotropic factor, and Debye temperature of ThC1-x approximately exhibit a linear downward trend as x increases. The thermal expansion coefficient of ThC1−x (x = 0–0.3125) exhibits an obvious ‘size effect’ and follows the same trend at high temperatures, except for x = 0.03125. Heat capacity and Helmholtz free energy were also calculated using the Debye model; the results showed the C vacancy defect has the greatest influence on non-stoichiometric ThC1−x. Our results can serve as a theoretical basis for studying the radiation damage behavior of ThC and other thorium-based nuclear fuels in reactors.

Microstructure evolution and mechanical properties of Ti-1023 alloy during β slow cooling process

Microstructure evolution and mechanical properties of Ti-1023 alloy during cooling from single-phase region to dual-phase region with the cooling rate of 1 °C/min were systematically investigated. As the final cooling temperature decreased, grain boundary α′ martensite (α′GB), grain boundary α phase (αGB), Widmanstatten grain boundary α phase (αWGB), and Widmanstatten intragranular α phase (αWI) were sequentially precipitated. Correspondingly, mechanical properties and deformation mechanisms of Ti-1023 alloy changed significantly. When the final cooling temperature was higher than 700 °C, tensile curve showed an obvious double-yielding phenomenon with elongation >20%. Whereas the final cooling temperature was lower than 700 °C, and the double-yielding phenomenon disappeared. This was mainly attributed to the fact that β stability of Ti-1023 alloys was enhanced with the increase of α(α′) precipitated phases. And the room temperature deformation mechanism gradually changed from stress-induced α″ martensitic phase transformation to dislocation slip. Furthermore, strengthening mechanism of Ti-1023 alloy was analyzed in combination with grain size and precipitated phase effects.

A comparative analysis of a unique DRX process in achieving a homogeneous distribution of β grains in the single phase region of as-cast Ti644321 alloy

This study aims to investigate the distinctive characteristics of a specialized dynamic recrystallization (DRX) process, which results in a fine and uniform distribution of β grains within the single phase region of as-cast Ti644321 alloy. The formation of deformation bands (DB) and flow localization (FL) is a result of the instability region. On the other hand, the stability region exhibits significant dynamic recrystallization (DRX). The phenomenon of DRX encompasses continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX), and the coupling mechanism of DDRX and CDRX, which was discovered in this study. The occurrence of DDRX is attributed to grain boundary bulging, while CDRX is a result of the progressive rotation of subgrains. A strain rate of 0.1 s−1 was found to form a fine and uniform distribution of β grains. The formation of this microstructure is facilitated by a coupling mechanism that combines CDRX and DDRX, which involves the grain boundary strain concentration effect that triggers DDRX and the subgrain rotational nucleation process of CDRX. The deformation mechanism of the alloy during hot deformation varies depending on the η value. In the high η region, the primary deformation mechanism is mainly DDRX+CDRX, while in the medium η region, it transitions to CDRX+DRV. Ultimately, in the low η region, the primary deformation mechanism is DB + FL. The presence of a high proportion of DBs hinders DRX. This study provides novel insights into the mechanisms of DRX in β titanium alloys and emphasizes the significance of considering different forms of DRX.

Experimental study of the phase relations of the Fe–Pt–Ho ternary system at 500 °C

Abstract The phase relations of the Fe–Pt–Ho ternary system at 500 °C have been studied by using X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques. The Ho3Pt4 phase is stable at 500 °C, and the introduction of Fe does not cause the Ho3Pt4 phase to be decomposed into the two neighbouring phases HoPt and HoPt2. The single phase ranges of α-Fe, Fe3Pt, FePt, FePt3 and Pt in Fe–Pt binary system are from 0 to 10 at.% Pt, 14 to 32 at.% Pt, 33 to 63 at.% Pt, 66 to 78 at.% Pt, and 82 to 100 at.% Pt, respectively. The highest solid solubility of Ho in the α-Fe, Fe3Pt, FePt, FePt3 and (Pt, Fe) phases is less than 1.5 at.% Ho, 2 at.% Ho, 2 at.% Ho, 1.5 at.% Ho and 1.5 at.% Ho, respectively. The isothermal section at 500 °C of the Fe–Pt–Ho ternary alloy phase diagram has been constructed, which consists of 19 single-phase regions, 35 two-phase regions and 17 three-phase regions. No new ternary compounds were found.

Phase Equilibria of the Ag-Al-Au Ternary System and Interfacial Reactions in the Au-xAg/Al Couples at 450 °C.

The phase equilibria of the Ag-Al-Au ternary system and the solid-state reaction couple for the Au-xAg/Al system were investigated isothermally at 450 °C. By investigating the Ag-Al-Au ternary system and its isothermal section, this study aims to provide a clearer understanding of the phase stability and interfacial reactions between different phases. This information is crucial for designing materials and processes in electronic packaging, with the potential to reduce costs and improve reliability. There were seven single-phase regions, thirteen two-phase regions, and six three-phase regions, with no ternary intermetallic compound (IMC) formed in the isothermal section of the Ag-Al-Au ternary system. When the Au-25 wt.% Ag/Al couple was aged at 450 °C for 240-1500 h, the AuAl2, Au2Al, and Au4Al phases formed at the interface. When the Ag contents increased to 50 and 75 wt.%, the Ag2Al, AuAl2, and Au4Al phases formed at the interface. When the aging time increased from 240 h to 1500 h, the total IMC thickness in all Au-xAg/Al couples became thicker, but the types of IMCs formed at the interface did not change. The total IMC thickness also increased with the increase in the Ag content. When the Ag content was greater than 25 wt.%, the Au2Al phase was converted into the Ag2Al phase. The IMC growth mechanism in all of the couples followed a reaction-controlled process.

Experimental determination and high-throughput calculation of the interdiffusion coefficient matrix and atomic mobility in Ag-rich fcc Ag–Sn–Zn alloys

In this paper, 10 groups of ternary Ag–Sn–Zn diffusion couples were prepared under 873, 973 and 1073 K within the fcc single-phase region. The element distribution across the diffusional interface was measured by EPMA, and a high-throughput determination of atomic mobilities in Ag-rich fcc Ag–Sn–Zn alloys was performed by using HitDIC (High-Throughput Determination of Interdiffusion Coefficients, https://hitdic.com/) software in the framework of the numerical inverse method. A self-consistent set of atomic mobility parameters was obtained. Reliability of the evaluated atomic mobility parameters were further verified by reasonably reproducing the experimental composition profiles. Insight into the composition- and temperature-dependence of various diffusion properties in fcc Ag–Sn–Zn alloy were subsequently retrieved and discussed. Furthermore, through the related Arrhenius information, the diffusion frequency factors and diffusion activity energies within the corresponding temperature and the composition range were obtained.

Influence of cooling manners on microstructure and mechanical properties of AISI 430 ferritic stainless steel

This paper investigated the microstructure and mechanical properties of AISI 430 ferritic stainless steel (FSS) after annealing in either the single-phase or two-phase region, followed by different cooling manners: water cooling, air cooling and furnace cooling. After annealing at 840 °C in the single-phase region, the sample consisted primarily of ferrite, with M23C6 and M2(C, N) precipitates aligned mainly along the rolling direction (RD). For the water-cooled and air-cooled samples annealed at 950 °C in the double-phase region, they formed a dual-phase structure comprising ferrite and martensite, with precipitations primarily located within the martensite lath. Conversely, the sample subjected to annealing at 950 °C followed by furnace cooling displayed a single ferrite structure, owing to the decomposition of austenite. In this case, the precipitations were mainly found along the grain boundaries, with some also forming within the ferrite grain. The paper delved into the analysis of mechanical properties and strengthening mechanism through tensile experiments and fracture morphology examination. The influence of heating range and cooling manner on the yield characteristics of FSS was discussed. The results indicated that the air-cooled and furnace-cooled samples annealed at 840 °C displayed a clear yield platform in their tensile curves. Conversely, the samples treated with other heat treatments exhibited continuous yielding or no obvious yield platform. This difference may be attributed to the influence of heating temperature range and cooling manner on the distribution of dislocations and interstitial atoms within the ferrite grains.

The stability and homogeneity range of the TCP phases in the Co-Fe-Ta system

The topologically closed phases (TCP) in the Co-Fe-Ta system were studied. The phase equilibria of the Co-Fe-Ta system were investigated for the first time. The Co-Fe-Ta isothermal section at 1100 °C was measured through 23 alloys and 5 diffusion couples. The continuous μ single-phase region through the entire Ta compositions has formed. All the other binary phases show large homogeneity ranges in the Co-Fe-Ta system. The enthalpies of formation for all end-members of the topologically closed phases (TCP), such as C14, C15 and μ, were calculated by density functional theory (DFT). The DFT results were used to evaluate the thermodynamic parameters of the C14, C15 and μ phases. The isothermal section at 1100 °C for the Co-Fe-Ta ternary system was accurately predicted by applying the DFT results to the CALPHAD assessment. Statistical analysis of the entry of different atoms into the lattices of the C14 and μ phases revealed that only four end-members were critical to obtain a quantitative CALPHAD prediction. This significantly reduces the work of the DFT calculation. A comprehensive CALPHAD description was performed finally considering all the available data. The calculated isothermal section at 1100 °C is in good agreement with the experimental data. The metastable Co-Fe-Ta FCC-L12 phase diagram has been studied. The predicted results show that the solvus temperature was decreased by the addition of Fe.