L12 Structure Research Articles

Contrasting response on magnetocaloric effect and refrigeration capacity due to Ni or Mn substitution by Fe in Ni-Mn-In-Co-Fe alloys

We set the composition of two Ni-Mn-In-Co-Fe alloys as (Ni46.8Fe1.2)Co2Mn36.5In13.5 and Ni48Co2(Mn34.5Fe2)In13.5 in order to display the martensitic transformation near to room temperature. Under a magnetic field change of 5 T the polycrystalline low Co content alloys displayed a large magnetocaloric effect ΔSMag of 15.5 and 20.0 Jkg-1K-1; and giant refrigeration capacity RCFWHM up to 422 and 292 Jkg-1 for (Ni46.8Fe1.2)Co2Mn36.5In13.5 and Ni48Co2(Mn34.5Fe2)In13.5, respectively. The XRD patterns showed cubic L21 structure with space group 225 in both alloys and via Rietveld refinement negligible differences were found in lattice parameter of 0.598(7) and 0.598(9) nm for (Ni46.8Fe1.2)Co2Mn36.5In13.5 and Ni48Co2(Mn34.5Fe2)In13.5, respectively. Nevertheless, the magnetocaloric response in the alloys exhibited remarkable dissimilarities. Whilst the applied magnetic field shifted the martensitic transformation − 3.4 KT-1 in the Ni48Co2(Mn34.5Fe2)In13.5 alloy this sensibility increased up to − 6.8 KT-1 in the (Ni46.8Fe1.2)Co2Mn36.5In13.5 alloy; consequently the span in the Full Width Half Maximum of the ΔSMag(T) curve was 17 and 32 K, respectively.

Influence of self-substitution on the thermoelectric Fe2VAl Heusler alloy

The microstructure and the thermoelectric properties were systematically determined in the Fe2V1+xAl1-x, Fe2+xVAl1-x, Fe2-xV1+xAl series to investigate the influence of self-substitution on the Fe2VAl Heusler alloy. In the explored range of compositions (−0.1 < x < 0.1), all these series are solid solutions, which form anti-site defects to accommodate the off stoichiometry. They all crystallize in the cubic L21 structure, but their lattice parameter unusually increases with x. A Bader analysis based on Density Functional Theory calculations indicates that these uncommon lattice parameter changes arise from variations in the interatomic electron transfer. The antisite defects behave like dopants that control the conduction type and charge carrier concentration. This leads to large thermoelectric power factor (PF) in the Fe2V1+xAl1-x series, which displays the largest electronic mobility. PF = 6.7 mW m−1 K−2 at 250 K and PF = 3.2 mW m−1 K−2 at 325 K are reached in n-type Fe2V1.03Al0.97 and p-type Fe2V0.985Al1.015 respectively. The lattice thermal conductivity systematically decreases upon self-substitution, but with differences among the series which can be traced back to the interatomic electron transfer unveiled by the Bader analysis. Finally, the figure of merit is improved to ZT = 0.06 at 500 K in p-type Fe2V0.93Al1.07 and ZT = 0.15 at 420 K in n-type Fe2V1.08Al0.92.

A unified DFT exploration on transport and thermodynamic properties of L21 structure of Rh2XZn (X = Mn, Fe) ferromagnets

Rh2XZn (X = Mn, Zn), full-Heusler compounds have been investigated with respect to their transport and thermodynamic properties using the first-principles approach. The full potential linearized augmented plane wave method with different exchange potentials is used to find the structural stability within considered L21 and XA structures. Moreover, Rh2XZn (X = Fe, Zn) are found to be ferromagnetic with magnetic moments of ∼3 and ∼4µB, which are compatible with the Pauling-Slater rule. To further seek the application of the material in the transport phenomena, figure of merit has been estimated by using Boltzmann theory. Our calculations lead to a 0.15 value of figure of merit at 1200 K for Rh2FeZn. Furthermore, the thermodynamic stability has been examined by considering various parameters within 0 to 1200 K temperature range. One of the novelties lies within the high value of Debye temperature that surmised the materials to be used as hard materials.

Electronic Properties and Chemical Bonding in V2FeSi and Fe2VSi Heusler Alloys

First-principles calculations of the stability, electronic, and magnetic properties of full-Heusler compound V2FeSi and Fe2VSi in regular (L21) and inverse (XA) structures have been performed using density functional theory within an SCAN meta-GGA functional. It is found that the XA crystal lattice is energetically more favorable for V2FeSi, while Fe2VSi forms the L21 structure. In both cases, the electronic structure of the energetically stable modifications corresponds to half-metallic ferrimagnets. Magnetic properties of energetically favorable structures obey the Slater–Pauling rule. All considered properties of the studied structures are explained within the crystal orbital Hamilton population analysis.

Precipitation sequence in Al-Sc-Zr alloys revisited

Sc and Zr additions to Al provide significant strengthening through the formation of Al3(Sc,Zr) L12 dispersoids. Fe impurities are always present in commercial Al alloys and the effect of these impurities on precipitation mechanisms in Al-Sc-Zr alloys remain poorly understood. This is particularly relevant when considering recycled aluminium as recycling results in the build-up of impurities. We used a combination of resistivity and thermo-electric power (TEP) measurements to monitor the evolution of solid solution during ageing of an Al-Sc, Al-Zr and Al-Sc-Zr alloy. Sc first comes out of solution to form Al3Sc dispersoids, followed by Zr to form Al3Zr. A range of Al3Sc-core/Al3Zr-shell faceted morphologies were observed with high angular annular dark field scanning transmission electron microscopy (HAADF-STEM). The presence of a new class of Fe-rich plate-like precipitates in the {100}Al was revealed via HAADF-STEM. The ordered positions of the precipitate and matrix reflections in the selected area diffraction pattern and in high-resolution TEM images reveal that these precipitates display a multi-layered structure. The centre of the plates is pseudo-icosahedral with apparent five-fold symmetry, and is rich in Fe. The outside layer has a L12 structure and is rich in Sc and Zr. The layered structure allows complete coherency with the aluminium matrix. These precipitates are highly anisotropic with a thickness of 5-10 nm and a length up to ∼1000 nm. This is the first report of the formation of this type of coherent pseudo-icosahedral precipitates in isothermal conditions which is named the D-phase.

Stability of FePt, FePt3 Nanoclusters of Different Habits

Calculations of the total energy of Fe-Pt nanoclusters, corresponding in the phase diagram to the compositions of FePt, FePt3 intermetallics and possessing either characteristic structures L10 and L12, respectively, or non-characteristic disordered structure A1, as well as various particle habits (cuboctahedra, icosahedra) are carried out by molecular dynamics for the first time. The dependences of cluster stability on their size and temperature are plotted, along with the schemes of temperature transformations of cluster morphology and the dependence of the melting points of the clusters with these structures and habits on their size. The size range (2–8 nm) corresponds to the sizes of particles observed by high-resolution electron microscopy. It is shown that the species play an essential part in the phase transformations proceeding under heating in the nanostructured system Fe-Pt and leading to the formation of nanocrystals with highly ordered L10 structure possessing giant coercivity are cubic nanoclusters with the ordered structures L10 and L12. With an increase in cluster size, their stability and melting points increase, tending to saturation of the dependencies within the size range above 10 nm. The least stable clusters are those of intermetallics with non-characteristic disordered structure A1 and icosahedral habit.

The effects of discontinuous precipitation and ordering on the age-hardening in Au-20Ag-30Cu (wt.%) alloy

Discontinuous precipitation (DP) and ordering have been typically considered to influence the mechanical properties of many alloy systems during the aging process. In the present study, the microstructural evolution and the effect of discontinuous precipitation and ordering on the age-hardening behavior in Au-20Ag-30Cu (wt.%) ternary alloy were investigated. Au-20Ag-30Cu (wt.%) alloy is found to be typically age-hardened. Thermodynamic calculation forecasts the discontinuous precipitation forming Cu-rich α1 phase and Ag-rich α2 phase, companied by the disorder-order transformation forming AuCu3 phase with L12 structure for this Au-Ag-Cu alloy. The DP is experimentally observed to form at grain boundaries consisting of alternative α1 and α2 two-phase lamellae. And the L12-type AuCu3 ordered phase, which presents nanometric spherical particle, forms at grain interior and the lamellar α1 phase of DP. The DP and L12-type AuCu3 nanoprecipitates both grow with the elevated aging temperature. The DP at grain boundaries is found to deleteriously affect the overall age-hardening with different deformation mechanisms between these two lamellar phases of DP. High-density dislocations are observed in the lamellar α1 phase of DP around the hardness indentation, which indicates that the interior L12-type AuCu3 nanoprecipitates in α1 phase can effectively impede the movement of dislocations to boost the age-hardening. The overall age-hardening is determined by the competitive effect of the DP and L12-type AuCu3 nanoprecipitates. This study offers theoretical foundation for controlling the microstructures and optimizing the mechanical properties of Au-Ag-Cu alloys.

Ferromagnetic resonance in FePt thin films at elevated temperatures

Understanding of damping processes in ferromagnetic thin films at elevated temperatures has significant implications for heat-assisted magnetic recording, spin-transfer torque memory, and magnetic sensors operating at high temperatures. Through cavity-based high-temperature ferromagnetic resonance (FMR) measurements, this work examined the FMR linewidth and damping properties of continuous cubic FePt thin films at elevated temperatures. The data show that the FMR linewidth and the Gilbert damping constant both increase monotonically when temperature is increased from room temperature toward the Curie temperature. This temperature dependence is opposite to that observed previously in FePt thin films that are granular, rather than continuous, and have L10 structure, rather than cubic structure; in those films, the FMR linewidth decreases monotonically with an increase in temperature [PR Applied 10, 054046 (2018)]. These opposite results originate from the difference in the crystalline structure and microstructure of the films. In the previous work, the granular L10-order FePt films hold dense material imperfection and thereby may host strong two-magnon scattering (TMS); the TMS-produced damping decreases with an increase in temperature, giving rise to reduced FMR linewidths at high temperatures. In the current work, the continuous cubic FePt films have much less imperfection and thereby host weak TMS, and the dominant damping mechanism is spin-flip magnon-electron scattering (SF-MES). The SF-MES process becomes stronger with an increase in temperature, giving rise to larger linewidth and higher damping at high temperatures. This work and the previous work together demonstrate that for a given thin-film material, the temperature dependence of the FMR linewidth critically relies on the structural properties of the film. They also indicate that one can engineer damping in magnetic thin films through the control of the structural properties of the films.

The site preference and doping effect on mechanical properties of Ni3Al-based γ′ phase in superalloys by combing first-principles calculations and thermodynamic model

The fundamental aspects of site preference of alloying elements on sublattice of the strengthen γ′ phase with L12 structure have not been well understood, which hinders the optimized design of advanced Ni-based high-temperature alloys. In this contribution, the temperature- and composition-dependent site occupying preferences of the binary, ternary, and quaternary of Ni3Al-based γ′ phase alloyed with Mi where Mi represents the additional transitional metals Co, Cr, Cu, Fe, Mn, Mo, Re, Ta, Ti, V, or W atoms (arranged in alphabetical order) chosen frequently, were studied using a two-sublattice thermodynamic model (Ni, Al, Mi)1a(Ni, Al, Mi)3c. The site occupying fractions (SOFs) were calculated based on a thermodynamic database established in this work, where the thermodynamic data of the end-members involved were obtained using first-principles calculations and phonon spectrum calculations. The calculated SOFs results show that there is an obvious site preference for stoichiometry binary Ni3Al, and its site configuration changes from (Al)1a(Ni)3c at room temperature to (Al0.9984Ni0.0015)1a (Al0Ni0.9994)3c at 1273 K. For the γ′ phase with the composition 78Ni-26Al-4Mi (atom ratio and xNi/xAl = 3:1), Mo atoms always preferred to occupy the 1a sublattice (Al site), Co, Mn, and Ti atoms always prefer the 3c sublattice (Ni site) in the whole temperature range, while the site preference of Cr, Cu, Fe, Re, Ta, V, or W atom is affected by temperature. For example, when the heat treatment temperature is lower than 700 K, Cr, Cu, Fe, Ta, V, and W atoms occupy the 1a and 3c sublattice randomly, and Re atoms prefer to 3c sublattice, while when the heat treatment temperature is higher than 1273 K, Cr, Cu, and W atoms prefer 3c sublattice, Fe and Ta atoms prefer to 1a sublattice, while all Re atoms occupy the 3c sublattice exclusively, and all V atoms occupy the 1a sublattice exclusively, respectively. Likewise, the site preference of the quaternary system with selective compositions 78Ni-26Al-2 M1-2 M2 was also predicted. Based on calculated SOFs results, the mechanical and thermodynamic properties were studied at the ground state. It has been revealed that Cr, Re, and V doping can improve the microhardness of Ni3Al alloys; in particular, the effect of Cr is extraordinary; and all elements, except Mn, Mo, and Ti, would enhance the bulk modulus of Ni3Al-based γ′ phase, in which Mn have the greatest influence on reducing the bulk and shear modulus, respectively. Furthermore, all the B/G ratios of the computed Ni3Al-based γ′ phase are >1.75, showing inherent ductility. Only Cr doping significantly enhances the Debye temperature of the Ni3Al-based γ′ phase.

Highly dispersed L12-Pt3Fe intermetallic particles supported on single atom Fe-Nx-Cy active sites for enhanced activity and durability towards oxygen reduction

Highly active and durable oxygen reduction reaction (ORR) catalysts with sufficient activity and stability of Pt are beneficial for the commercialization of proton exchange membrane fuel cells. Here we report an effective approach to prepare a composite catalyst comprising of ordered L12-Pt3Fe intermetallic nanoparticles interact with single atom Fe-Nx-Cy active sites. The addition of Fe and the confinement effect of hierarchical porous structure limit the growth of intermetallic particle size (around 2.5 nm). The ligand effect of the electron transfer from Fe to Pt and the synergistic interaction between L12-Pt3Fe and Fe-Nx-Cy work together to reduce oxygen intermediates adsorption and improve kinetics process. Experimentally, the L12-Pt3Fe/CFe-N-C catalyst shows high mass activity and specific activity at 1.010 A/mgPt and 1.166 mA/cm2, respectively, which are 5.8 and 5.1 times higher than those of commercial Pt/C (0.174 A/mgPt and 0.230 mA/cm2). Thanks to the more stable L12 structure, L12-Pt3Fe/CFe-N-C exhibits better durability (14 mV E1/2 loss of L12-Pt3Fe/CFe-N-C and 33 mV E1/2 loss of commercial Pt/C) after 30,000 cycles accelerated stress tests. The strategy to design and prepare small particle Pt-based intermetallic alloys coordinated with M-N-C active sites provides a new direction to obtain low-cost and easily prepared effective ORR catalysts.

Effect of L21 and XA ordering on structural, martensitic, electronic, magnetic, elastic, thermal and thermoelectric properties of Co2FeGe Heusler alloys

Based on density functional theory (DFT), the structural, martensitic transition, band structure, density of states, elastic, magnetic, and thermoelectric properties of L21 and XA ordering of Co2FeGe compound were investigated. The calculated total energy shows that the most stable phase of these compounds is the L21 type with ferromagnetic state using GGA + U approach. In addition, the possible martensitic transition is occurring for L21 phase which has minimum energy localized at c/a = 1.02. The electronic density of states (DOS) and band structure of Co2FeGe exhibit a ferromagnetic metallic for both L21 and XA phases. From the calculated elastic constants show that the studied phases are elastically anisotropic and satisfies Born′s criteria for mechanical stability. Moreover, through the quasi-harmonic Debye model applied in the Gibbs codeand the thermodynamic analyses have also been estimated. Also, the thermoelectric properties for the both L21 and XA structures are examined and discussed based on the Seebeck, electrical conductivity and thermal conductivity coefficients.

Development of a high strength Zr/Sc/Hf-modified Al-Mn-Mg alloy using Laser Powder Bed Fusion: Design of a heterogeneous microstructure incorporating synergistic multiple strengthening mechanisms

Laser Powder Bed Fusion (L-PBF) provides great advantages in creating supersaturated solid solutions due to its intrinsic ultrafast cooling and high solidification rate, which is particularly desired for enhanced solid solution strengthening and precipitation strengthening. In the present work, a novel high strength Zr/Sc/Hf-modified Al-Mn-Mg alloy with a good L-PBF processability is investigated. The as-built alloy exhibits a heterogeneous microstructure featuring a bi-modal grain structure with coarse columnar and fine equiaxed grains and heterogeneously distributed nanometer and submicrometer-sized cuboid-shaped primary Al3(Sc,Zr,Hf) particles. The Al3(Sc,Zr,Hf) phase, exhibiting a homogeneous elemental distribution of Zr, Sc, and Hf, adopts a cubic L12 structure with a lattice parameter of 0.4044 ± 0.009 nm, showing a good coherency with α-Al. Additionally, the rapid L-PBF solidification enables the formation of submicrometer-sized elongated and globular primary Al6Mn and a supersaturated Al matrix with a high dislocation density. This results in a good combination of strength (yield strength of 438 ± 3 MPa and ultimate tensile strength of 504 ± 2 MPa) and ductility (elongation at fracture of 10.9 ± 1.4 %), with a moderate work hardening strength of 113 ± 12 MPa. Direct-ageing at 325 °C for 10 h promotes the formation of a large amount of rod-shaped Al6Mn precipitates and a few spherical Al3(Sc,Zr,Hf) nanoprecipitates. The heat treatment increases the hardness from 166 ± 2 HV in as-built condition to 173 ± 4 HV and enhances the tensile strength (yield strength of 487 ± 2 MPa and ultimate tensile strength of 542 ± 3 MPa) but slightly reduces the ductility to 7.4 ± 0.7 %. The high strength was achieved by the synergistic effect of grain boundary strengthening, solid solution strengthening, and Orowan strengthening mechanisms.

B2-disorder effects on the structural, electronic and magnetic properties of Co2MnAl Heusler alloy

Co-based Heusler alloys are mostly studied for their half metallicity, resulting in 100% spin polarization and a very high Curie temperature, which are the prerequisites for spintronics devices. As the site (swap) disorder has a significant impact on these properties, understanding of the impact of site disorder on these properties is of great interest for the spintronic community. First principle calculations within density functional theory (DFT) have been carried out on the structural, electronic, and magnetic properties of the B2 disordered Co2MnAl full Heusler alloy along with that of L21 ordered structure for comparison purpose. The plane wave pseudopotential method, within the Generalized Gradient approximation for exchange correlation functional was used to study B2 disordered Co2MnAl Heusler alloy. The degree of site (swap) disorder is systematically varied from 0% to 100% in steps of 12.5% using the supercell approach. DFT calculations reveal no considerable changes in the structural and magnetic properties of Co2MnAl by the presence of the disorder and are found to be the same as for the L21 structure. However, on the other hand, electronic properties such as, the spin polarization increased from 76% to 100% in the case of B2-25% and B2-75% disorder from the L21 structure resulting in half-metallic character. The study further suggests that static-magnetization and structural properties of Co2MnAl Heusler alloy are robust against site disorder.

Elimination of room-temperature brittleness of Fe–Ni–Co–Al–Nb–V alloys by modulating the distribution of Nb through the addition of V

Compared with Ti–Ni-based and Cu–Al-based alloys, Fe–Ni–Co–Al-based superelastic alloys have received great attention in recent years because of their low cost and excellent performance. However, the precipitation of brittle precipitates on grain boundaries in Fe–Ni–Co–Al-based polycrystalline alloys significantly reduces the mechanical properties. In this paper, the effects of V on the formation of brittle precipitates on both grain boundaries and inside grains and mechanical properties in Fe–Ni–Co–Al–Nb alloy were investigated systematically. The results show that γ′(Ni3Al) precipitate with L12 structure precipitate out on grain boundaries and inside grains in the Fe–Ni–Co–Al–Nb alloy during aging without V addition. The γ′ precipitate with L12 structure on the grain boundaries grew to form a heavily-coarsened L12 precipitate (HCLP γ′ precipitate) due to the accumulation of Nb on the grain boundaries, resulting in a significant decrease in elongation. With the addition of V, intragranular Nb and V can combine together with γ′ to form Ni3(Al, Nb, V) (nano-sized γ′ precipitate) composite precipitates, which improves the strength. More important, since the diffusion coefficient of V was larger than that of Nb, V preferentially occupied the lattice positions on grain boundaries, repelling Nb from the grain boundaries to the matrix. Thus, the coarsening of HCLP γ′ precipitate on the grain boundaries was reduced, and the elongation of Fe–Ni–Co–Al–Nb–V alloy was significantly improved.

Theoretical prediction of high entropy intermetallic compound phase via first principles calculations, artificial neuron network and empirical models: A case of equimolar AlTiCuCo

Understanding the role of high entropy intermetallic compounds is essential for the fundamental description of high entropy alloys. Here, we present a theoretical study to predict the structure of equimolar AlTiCuCo using ab-initio calculations, agricultural neuron network and empirical models. Based on Miedema's theory, the potential reasonable configurations of the compound with L12 and B2 structure are predicted by ab-initio calculations. The mixing energy result indicates that the L12-type intermetallic compound is more likely to form. The calculated elastic moduli and phonon dispersion show that the predicated structures are mechanical and dynamical stable. The correlation coefficient between the elastic moduli and the input features for phase structure prediction is obtained. It reveals that average bulk modulus related features of the components have a strong effect on the elastic properties of the Al–Ti–Cu–Co intermetallic compound system. Finally, thermo-physical properties, electronic density of states and Bader's net charge are computed and compared.

First-Principles Calculation of Transport Properties of Heusler Alloy Co2MnAl at Finite Temperatures

We develop a framework for first-principles calculations of the transport properties of magnetic materials at finite temperatures within the coherent potential approximation. First, we calculate the grand potential by the tight-binding linearized muffin-tin orbital method and apply it to the Kubo formula to obtain the temperature dependence of the conductivities. Next, we demonstrate the computation for the magnetic moments M(T), longitudinal and transverse electrical conductivities, σxx(T), σxy(T), Seebeck coefficients Sxx(T), and anomalous Nernst coefficients Nxy(T) of Co2MnAl of both L21 and B2 structures. It is found that σxx(T) of L21 and B2 structures exhibit slow relaxation with temperature compared with most transition metal alloys, which reflects a transition of the system from a half-metallic to metallic system with increasing temperature. The σxy(T = 0) of the B2 structure is much smaller than that of the L21 structure, while σxx(T) of the B2 structure exhibits slower relaxation with temperature than the L21 structure. The Sxx(T) and Nxy(T) exhibit nonmonotonic variation in the low temperature region and the values above room temperature are in agreement with the experimental data.

Effect of Er and Zr additions and aging treatment on grain refinement of aluminum alloy fabricated by laser powder bed fusion

In this work, Al-0.88Er-0.78Zr (wt%) alloy was fabricated via laser powder bed fusion (LPBF). The microstructure, precipitates distribution, electrical conductivity and hardness of as-built and aging treatment specimens were carefully characterized. Results show that LPBF process greatly expand the solid solubility of Er and Zr in Al. The addition of Er can significantly refine the grain size and form a bimodal grain structure consisting of fine equiaxed grains (grain size ~0.53 ± 0.15 µm) at the boundary of molten pool together with coarse columnar grains (width 2.57 ± 0.84 µm) at the center of molten pool. After 375 ℃ for 3 h aging treatment, the particles on the grain boundary inhibit the grain growth; at the same time, a large number of Al3(Er,Zr) particles with L12 structure and size of 2.25 ± 0.3 nm are precipitated, which plays a major role in enhancing the hardness with the peak hardness of about 89.24 ± 3.77 HV. Er and Zr elements are proved to be an alternative way of the additive elements for developing a new 3D printing high-strength aluminum alloy.

Synthesis and characterization of Co-Ti-Sn alloys: Co1.5TiSn as half-metal based on the Fe1.5TiSb layered Heusler prototype

The Co-Ti-Sn family of alloys from their half- to full-Heusler compositions are synthesized. Co1.5TiSn, a compound predicted by our theoretical calculations, is successfully synthesized in the L21 structure with cobalt vacancies in the tetrahedral sites. The saturated magnetic moment at 5 K is measured to be ~ 0.6 μB∕formula unit, consistent with density functional theory (DFT) predictions of half-metallicity in the compound. A cluster expansion-based analysis of the alloy system indicates the Co1.5TiSn compound to be nearly on the convex hull, within DFT accuracy. Our experimental realization of the Co1.5TiSn compound suggests that it is either kinetically or entropically stabilized.

Experimental investigation and thermodynamic description of the Co–Nb–Ti system

The solidification microstructures of as-cast alloys and the equilibria microstructure of annealed alloys at 1273 K in the Co–Nb–Ti system were determined by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD), and the liquidus surface projection and partial isothermal section at 1273 K were constructed. Nine primary solidification regions, bcc(Nb,Ti), fcc(Co), Co3Ti, CoTi, CoTi2, λ1, λ2, λ3, and μ and five ternary invariant reactions, liq. → λ1 + λ2 + μ, liq. → CoTi + λ2 + μ, liq. → bcc(Nb,Ti) + CoTi + μ, liq. + fcc(Co) + λ3 → Co3Ti, and liq. + CoTi → bcc(Nb,Ti) + CoTi2 were experimentally determined or deduced in the liquidus surface projection. Three three-phase regions, CoTi + λ2 + μ, bcc(Nb,Ti) + CoTi + μ, and bcc(Nb,Ti) + CoTi + CoTi2 and six two-phase regions, Co3Ti + λ3, λ2 + μ, CoTi + λ2, CoTi + μ, bcc(Nb,Ti) + μ, and bcc(Nb,Ti) + CoTi at 1273 K were experimentally determined. The continuous compound λ2 with a C15 structure from the Co–Nb side to Co–Ti side was formed at 1273 K. The solubilities of Ti in μ and Nb in CoTi were determined to be ~11.1 at% and ~18.4 at% at 1273 K, respectively. And no new phases were found. According to the available experimental data from the present work and literatures, the Co–Nb–Ti system was optimized by CALPHAD (CALculatiuon of PHAse Diagram) method. Four solution phases (liquid, bcc, fcc, and hcp) were described using the substitutional solution model. A two-sublattice model (Co,Nb,Ti)2(Co,Nb,Ti)1 was used to described three compounds λ1, λ2, and λ3 according to their crystal structures and homogeneity ranges. Co7Nb2 and CoTi2 were treated as Co7(Nb,Ti)2 and Co(Nb,Ti)2 by a two-sublattice model, respectively. μ was treated as (Co,Nb,Ti)1(Nb,Ti)4(Co,Nb,Ti)2(Co,Nb,Ti)6 by a four-sublattice model. Co3Ti with L12 structure and CoTi with B2 structure were treated as the ordered phases of fcc and bcc solutions using the thermodynamic models (Co,Nb,Ti)0.75(Co,Nb,Ti)0.25Va1 and (Co,Nb,Ti,Va)0.5(Co,Nb,Ti,Va)0.5Va3, respectively. A set of self-consistent thermodynamic parameters of the Co–Nb–Ti system was obtained.

Phase diagram of the Cu–Ni3Al pseudo-binary system

Cu–Ni–Al alloys with a Ni to Al ratio of approximately 3.0 are of particular interest for the development of electrically conductive products; thus, investigating their phase behavior is essential for achieving specific properties in these materials. In this study, the phase diagram of the Cu–Ni3Al pseudo-binary system was experimentally determined using cast and fully heat-treated Cu–Ni3Al alloys with a wide range of compositions, via microstructural observation and different quantitative analyses. The Cu–Ni3Al system undergoes a eutectic reaction Liquid ↔ α-Cu solid solution (Cuss) with an FCC structure +γ′-Ni3Al with an ordered FCC L12 structure. As the combined Ni and Al (Ni3Al) content increased, the microstructure transitioned from a single α-Cuss phase to two eutectic phases (α-Cuss and γ′-Ni3Al), to a primary γ′-Ni3Al phase and two eutectic phases, and to a single γ′-Ni3Al phase. The solidus and liquidus temperatures increased with the (Ni+Al) content. The solid solubility of (Ni+Al) in the α-Cuss phase (C(Ni+Al)) could be described by an Arrhenius-type equation (ln C(Ni+Al) = 5.50 – 2637/T), indicating a significant temperature dependence. In contrast, the solubility of Cu in the γ′-Ni3Al phase did not significantly decrease with the temperature. The phase diagram of the Cu–Ni3Al system was constructed from these results. The phase diagram will be useful in the development of Cu–Ni–Al alloys with exceptional strength and electrical conductivity.