Single Γ-phase Research Articles

Systematic Study on the Microstructures of Biomedical Co–20Cr–15W–10Ni Alloys with Carbon Contents Ranging from 0 to 0.2 mass pct

Herein, the effect of carbon content on the microstructures of Co–20Cr–15W–10Ni (mass pct, CCWN) alloys was systematically studied. For this, CCWN alloys with carbon contents of 0, 0.05, 0.10, and 0.20 mass pct, i.e., 0C, 0.05C, 0.10C, and 0.20C alloys, respectively, were prepared using an induction melting furnace. The as-cast alloys were solution treated at 1523 K for 7.2 ks, followed by cold swaging and heat treatment at 1173 K–1473 K for 0.15–7.2 ks. Consequently, η-phase (M6C-M12C type, M: metallic element) precipitates were detected in the as-cast 0.10C and 0.20C alloys, whereas no precipitates were observed in the 0C and 0.05C alloys. These precipitates were dissolved via a solution treatment. After cold swaging, the ε-phase formed through a strain-induced martensitic transformation in the 0C and 0.05C alloys. Following heat treatment, a single γ-phase matrix was observed in all the alloys, and μ-phase (Co7W6-type) and η-phase precipitates were observed in the 0C and carbon-added alloys, respectively. The precipitation temperature range of the η-phase increased with increasing carbon content. The stability of the precipitates qualitatively conformed with that of the calculated phase diagram. This is the first paper that reports the microstructural changes in CCWN alloys with varying carbon contents.

Effects of the electrolytes on the electrochemical behavior of nickel-base superalloys prepared by laser directed energy deposition

The electrochemical behavior of Inconel 718 prepared by laser directed energy deposition with single γ-phase and dual (γ + γ″)-phase was investigated using 10 wt% NaNO3, 5 wt% NaNO3 + 5 wt% NaCl and 10 wt% NaCl, respectively. It is found that the Cl- ion weakens the inhibitive effect of the transpassive film, resulting in a uniform electrochemical dissolution for deposits with the single γ-phase, but an inferior surface quality for deposits with the dual (γ + γ″) phases due to the γ″ phase inducing uneven pits expansion. Two qualitative models are proposed to demonstrate their electrochemical dissolution mechanism.

Improved Mechanical Properties of EBM Biomedical Co–28Cr–6Mo–0.11N Alloy by the Dispersed Precipitation Control of ε-HCP Phase Based on ε ↔ γ Phase Transformation

Mechanical properties of electron-beam-melted biomedical Co–Cr–Mo–N alloys can be improved by the grain refinement from reverse transform treatment, which transforms a low-temperature strong ε-phase into a high-temperature ductile γ-phase. Although mechanical properties of alloys consisting of a single ε- or γ-phase have been previously reported on, those comprising mixed ε- and γ-phases have not yet been investigated. Herein, the heat treatment conditions of the Co–28Cr–6Mo–0.11N alloy were determined to control the phase fraction while obtaining fine grains in the mixed phases with superior mechanical properties. The phase transformation behavior was analyzed. Superior mechanical properties were observed in the mixed phases containing 70 pct γ-phase and 30 pct ε-phase. The tensile and yield strengths were higher, and the elongation was approximately the same, compared to that of the single γ-phase. Moreover, the 30 pct ε-phase mixed-phase material obtained during the γ → ε heat treatment had a smaller overall average grain size and showed superior mechanical properties than that obtained during the ε → γ heat treatment. This study is expected to facilitate the application of biomedical Co–Cr–Mo–N alloys with fine grains and superior mechanical properties obtained via heat treatment.

Influence of Nb, Ta and Zr on the Interdiffusion Coefficients and Solid Solution Strengthening of γ-TiAl Single Phase Alloys

The alloying elements Nb, Ta and Zr improve the creep properties of fully lamellar γ/α2 titanium aluminides significantly. Since high temperature deformation mainly occurs in the γ-phase of γ/α2 titanium aluminides, the diffusivity and the solid solution hardening effect of these three elements in the γ-phase is studied by analyzing the concentration gradients of the alloying elements and the resulting hardness across the interdiffusion zone of diffusion couples by energy dispersive X-ray diffraction and nanoindentation. The results reveal that Zr has the highest interdiffusion coefficient but also the largest solid solution hardening coefficient. The mechanical properties of single γ-phase Ti-54Al-5X alloys were investigated by strain rate jump tests. The addition of 5 at.% Nb or Ta lead to an increased strength compared to a binary γ-Ti-54Al alloy. The Zr-containing γ-TiAl alloy reveals the highest strength at 750 °C and 900 °C, which is discussed to be due to the strong solid solution hardening effect of Zr. However, in comparison to the other alloys, Ti-54Al-5Zr shows quite brittle behavior up to 900 °C. The lower diffusivity of Ta compared to Nb leads to a higher strength of the Ta-modified alloy at 900 °C.

Electrodeposition of Single γ-phase Zn–Ni Alloy from Deep Eutectic Solvents using Metal Oxides as Precursors

The electrodeposition behavior of Zn–Ni alloy on a Cu electrode has been studied in the choline chloride-urea-ethylene glycol (ChCl-urea-EG) based deep eutectic solvent (DES). The nickel and zinc sources are introduced into the solvent by the dissolution of nickel oxide (NiO) and zinc oxide (ZnO), respectively. Cyclic voltammetry indicates that the electroreduction of Ni(II) or Zn(II) species in the DES is a diffusion-controlled process and the diffusion coefficient of Zn(II) species is an order of magnitude larger than that of Ni(II) species. Under the examined condition, the anomalous codeposition of Zn–Ni alloy is observed, which is different from normal codeposition of Zn–Ni alloy in similar DESs using metal chlorides as precursor. The deposition potential is found to play an important role in controlling the composition and surface morphology of the resultant Ni-Zn alloys. The phase structure of the deposits depends on the Ni content, and changes from a mixture of η-phase and γ-phase to single γ-phase with (411) plane orientation. A mechanism for this electrodeposition process has been proposed on the basis of the formation of complex ions [NiO·urea·Cl]−, [ZnO∙urea·Cl]− and [ChCl·O·urea]2− in ChCl-urea-EG-ZnO-NiO electrolyte during the electrodeposition, emphasizing that the metal oxides were dissolve in DES by forming metal complex ions, the cathodic reduction of metal complex ions generates complex ions [ChCl·O·urea]2− with alloys (NixZny), and [ChCl·O·urea]2− is oxidized to release O2 at anode.

Constrained Dendritic Growth and Solute Concentration Effects in Rapidly Solidified Co-Cr Alloys

Growth temperature and dendrite tip radius equations were employed in this work in order to predict the experimental conditions under which a Co-Cr alloy was produced having a single γ-phase microstructure free from interdendritic segregation. The outcome of these predictions indicated that in Co alloys containing 20 to 25 wt pct Cr, single γ-phase dendritic microstructures are achieved independently of the implemented solidification growth velocities. Moreover, these alloys are found to be essentially free from interdendritic segregation. Co alloys containing 30 to 35 wt pct Cr exhibit both the γ-phase and the eutectic constituent when solidification rates (V) are below 0.1 and 0.5 mm/s, respectively. Alternatively, the eutectic constituent is suppressed when the solidification growth velocities exceed these values. In particular, it is found that a single-phase microstructure can be achieved when the solidification growth rates exceed the transition velocity between constitutional supercooling and the absolute stability conditions.

Development and tribological characterisation of nanostructured Zn-Ni and Zn-Co coatings: a comparative study

ABSTRACTZn-Ni and Zn-Co alloy coatings were electrodeposited on mild steel from sulphate-based baths. The morphology, microstructure, microhardness and tribological behaviours of the coatings have been studied and discussed. While the Zn-5wt-% Co layers presented a nanocrystalline simple nodular structure (45 ± 5 nm), the Zn-14wt-% Ni showed a particular structure called cauliflower morphology (30 ± 7 nm). The X-ray diffraction analysis showed that each of the electrodeposits was formed from zinc solid solution with a uniform zinc-cobalt intermetallic phase γ2 (CoZn13) for Zn-5wt-% Co alloy. However, a single γ-phase (intermetallic compound Ni5Zn21) was presented for the Zn-14wt-% Ni alloys. The Zn-14wt-% Ni films were found to be harder and rougher than the Zn-5wt-% Co layers. Plastic deformation and oxide layers production were the main wear mechanisms for the investigated coatings. The Zn-14wt-% Ni coatings were found to have the best wear resistance due to their microhardness and particular structure.

Magnetic and microwave properties of Fe18Ni82 nanoparticles with close to zero magnetostriction

Magnetic nanoparticles (MNPs) with nominal Fe18Ni82 composition were prepared by the electrophysical technique of the electrical explosion of wire. Although we were not able to experimentally estimate the magnetostriction coefficient for MNPs, this iron–nickel ratio is usually associated to close to zero magnetostriction. Different values of the overheating rate K between 0.9 and 2.0, where K is the ratio between the electrical energy ejected into the wire and the sublimation energy of Fe18Ni82 alloy, were used for the fabrication of the MNPs. Structural characterization (X-ray diffraction, scanning electron microscopy, low temperature nitrogen adsorption), magnetic and microwave measurements were used for the comparative analysis of the properties of MNPs. The increase of the overheating rate led to the increase of the specific surface value (Ssp) of the MNPs and the decrease of their average size. The fabricated MNPs were spherical in shape with a very high degree of sphericity and very weakly aggregated with the average weighted diameter in the range of 94–301 nm depending on the specific surface value for each batch. For 0.9 ≤ K ≤ 1.2 the MNPs were in the γ-phase, cubic (S.G: Fm-3 m) face-centered cubic lattice and very a small amount of material had a cubic Pm-3 m structure with lost symmetry. For 1.2 < K ≤ 2.0 they were single γ-phase, cubic (S.G: Fm-3 m) face cubic centered structure. Magnetic measurements and microwave absorption showed that the MNPs of all batches have high saturation magnetization and exhibit both microwave phenomena: significant zero field microwave absorption and resonant absorption in the vicinity of the ferromagnetic resonance field. Contrary to previous studies of FeNi MNPs with non-zero magnetostriction, the field for ferromagnetic resonance was very close to the one expected for MNPs with small contributions arising from magnetocrystalline anisotropy, strains and the deviations from sphericity. The magnetic and microwave characteristics in combination with a very high value of the single batch are promising for technological applications of these MNPs including the development of composites consisting of a polymer matrix filled with MNPs.

High-pressure and high-temperature synthesis of heavy lanthanide sesquisulfides Ln2S3 (Ln=Yb and Lu)

Detailed pressure-temperature phase diagrams of heavy lanthanide sesquisulfides Ln2S3 (Ln = Yb and Lu) have been investigated by in-situ x-ray diffraction experiments under high pressure and high temperature using synchrotron radiation and multi-anvil press. Based on the results of the in-situ observation, the single γ-phase (Th3P4-type structure, I4¯3d) samples of Ln2S3 (Ln = Yb and Lu) have been synthesized under high pressure. The physical properties of the compounds were studied by electrical resistivity, specific heat, and magnetic susceptibility measurements between 2 K and 300 K.

The equilibrium between hydrogen isotopes and TiVCr alloys

TiVCr alloys are treated as candidate materials for hydrogen isotope separation because of their prominent inverse isotope effects and hydrogen storage properties. In this work, the relationship between the separation factor (αH–D) that characterizes the isotope effects and the ratio of equilibrium pressures (RP) of the isotopes was investigated. It was found that αH–D in the plateau region was independent on RP of the α-phase if the phase transitions during hydrogenating are merely dependent on the hydrogen content and not on the type of hydrogen isotopes. In order to deal with the gradient plateau region that originates from the inhomogeneous compositions of TiVCr alloys, a model relating αH–D in the single γ-phase to RP in the gradient plateau region was formulated. Furthermore, the values of αH–D were also determined by experiments for Ti0.3V0.4Cr0.3–hydrogen systems, and compared with the prediction results, providing validations to the formulated model.

Chemical Synthesis and Characterization of γ-Co2NiGa Nanoparticles with a Very High Curie Temperature

We report the first study on the chemical preparation, structural characterization, and magnetic properties of Co2NiGa nanoparticles (NPs) of a single γ-phase (γ-Co2NiGa). The γ-Co2NiGa NPs were prepared by impregnation in colloidal silica followed by high temperature annealing under hydrogen atmosphere. The crystal structure of tetragonal γ-Co2NiGa was confirmed by selected area electron and X-ray diffraction studies. Energy-dispersive X-ray spectroscopy mapping and X-ray absorption near edge structure data provided evidence for the successful preparation of the intermetallic NPs and the absence of Co, Ni, Co–Ni, or metal-containing impurity phases. Extended X-ray absorption fine structure spectroscopy data confirmed the formation of the γ-Co2NiGa phase by examining the atomic environments surrounding Co, Ni, and Ga. The Co2NiGa NPs are ferromagnetic with a high saturation magnetization, which is consistent with the theoretical model. γ-Co2NiGa NPs exhibit a very high Curie temperature (≈1139 K), which m...

Characterization and understanding of residual stresses in a NiCoCrAlY bond coat for thermal barrier coating application

The residual stresses in a NiCoCrAlY bond coat deposited on a Ni-base superalloy substrate after oxidation at 1150°C were studied by X-ray diffraction using the sin2Ψ technique. The stresses were found to be tensile; they first increased and then decreased with oxidation time. High temperature stress measurement indicated that the stress developed and built up upon cooling, predominantly within the temperature range from 1150°C to 600°C. Microstructural examination suggested that, due to the limited penetration depth into the bond coat, the X-ray only probed the stress in a thin surface layer consisting of the single γ-phase formed through Al depletion during oxidation. Quantitative high temperature X-ray diffraction analysis revealed that, above 600°C, the volume fraction of the β-phase in the bond coat increased with decreasing temperature. The mechanisms of stress generation in the bond coat were examined and are discussed based on the experiments designed to isolate the contribution of possible stress generation factors. It was found that the measured bond coat stresses were mainly induced by the volume change of the bond coat associated with the precipitation of the β-phase upon cooling.

Electrodeposition of nanocrystalline Zn–Ni coatings with single gamma phase from an alkaline bath

Nanocrystalline Zn–Ni (grain size about 26nm) alloy coatings were electrodeposited on carbon steel substrates in alkaline bath with 5,5′-dimethylhydantoin (DMH) as the complexing agent. The coatings with 13wt.%–16wt.% Ni content and the electrolyte with high current efficiency (>85%) were achieved at the optimized parameters, i.e., Ni2+/(Zn2++Ni2+) ratio 0.32, current density 2A·dm−2, temperature 50°C and agitation speed 1000rpm. The electrochemical behavior of the bath has been studied by cyclic voltammetry (CV), chronopotentiometry (CE) and cathodic polarization. Results show that the deposition of Zn–Ni alloys occurs at moderate overpotential and the rotation speed and bath temperature have a strong effect on cathodic polarization curves. The effects of Ni2+/(Zn2++Ni2+) ratio, current density, temperature and agitation speed on Ni content and cathode current efficiency were investigated. Studies indicate that the phase structure, grain size, microhardness and corrosion resistance of deposits are directly dependent on Ni content in deposits. The phase structure of deposits changes from a mixture of η-phase and γ-phase to single γ-phase with (411) plane orientation. Furthermore, the grain size decreases with the increase of Zn content in deposits and the increase in current density can also lead to the decrease in grain size. The increase in microhardness of deposits is strongly affected by the increase of Ni content in deposits rather than the Hall–Petch relationship. Zn–Ni alloy coatings with about 13wt.% Ni content present best corrosion resistance and the bath has a good stability.

Pressure dependence of the photoluminescence from γ-In2Se3 thin films prepared using MOCVD with a single-source precursor

Single γ-phase In2Se3 films were prepared by using metal-organic chemical-vapor deposition(MOCVD) with a single-source precursor [(Me)2In(μ-SeMe)]2. The basic physical properties of the grown films were examined by using X-ray diffraction, Raman spectroscopy, and photoluminescence spectroscopy at room temperature. The pressure dependence of the photoluminescence spectrum of the In2Se3 films was measured at room temperature. At 1 atm, 2 PL peaks were observed, one at 1.88 eV due to a bound exciton transition and the other at 1.50 eV due to a bound-to-free transition. While the pressure coefficients, \(\frac{{dE}} {{dp}}|\begin{array}{*{20}c} {} \\ T \\ \end{array} \ \) at pressures below 1.4 GPa were nearly zero, the pressure coefficients of both PL peaks at pressures above 1.4 GPa were −25 meV/GPa.

Effect of Mo Addition on Structure and Magnetocaloric Effect in γ-FeNi Nanocrystals

Nanocrystalline powders of (Fe70Ni30)100−xMox (x = 1 to 4) were produced by high-energy (SPEX) mechanical alloying. Increasing the Mo content was found to stabilize the face-centered cubic phase in mechanically alloyed nanopowders. To obtain a single γ-phase, a powdered sample was solution annealed in the γ-phase field and water quenched. The Curie temperature, TC, of the alloys was lowered with Mo addition, without decreasing the refrigeration capacity (RC), due to the additional temperature broadening of the magnetic entropy change. Based on previous study on the role of disorder, the additional temperature broadening was attributed to increased positional disorder and changes in the distribution of ferromagnetic exchange bonds introduced by Mo addition into the γ-FeNi system. (Fe70Ni30)97Mo3 and (Fe70Ni30)96Mo4 alloys have RCFWHM values of ∼440 J/kg and ∼432 J/kg at 5 T, comparable to other prominent magnetic refrigerants operating near room temperature. The economic viability of these alloys, along with their competitive magnetocaloric properties and potential for scalable production, make them good candidate magnetic refrigerants without critical rare-earth materials.

Tuning the Curie temperature in γ-FeNi nanoparticles for magnetocaloric applications by controlling the oxidation kinetics

Mechanically alloyed Fe70Ni30 and Fe72Ni28 alloys were characterized in terms of their structural and magnetic properties. Previous studies showed that single phase FCC γ-FeNi alloys with ∼26-30 at. % Ni have Curie temperatures, Tc, near room temperature. Having Tc near room temperatures along with large magnetization makes γ-FeNi alloys attractive for room temperature magnetocaloric cooling technologies. To obtain a single γ-phase, particles were solution annealed in the γ-phase field and water quenched. The preferential oxidation of Fe during ball milling was used as a means to tune the Curie temperature, Tc, of the alloy. Refrigeration capacities, RCFWHM, of the Fe70Ni30 and the Fe72Ni28 alloys were calculated to be ≈470 J/kg and 250 J/kg at 5 T, with peak temperatures ≈363 K and ≈333 K, respectively. The RCFWHM for the Fe70Ni30 is higher than the previously reported Nanoperm (Fe70Ni30)89Zr7B4 type alloy and on the same order of magnitude with other Fe-based alloys. The maximum magnetic entropy change values observed for the Fe70Ni30 and the Fe72Ni28 are 0.65 and 0.5 J kg−1 K−1, respectively, at a field of 5 T. These are smaller than those of rare earth magnetic refrigerants showing first order transformation behavior. The larger RCFWHM value results mainly from the width of the magnetic entropy curve in these types of materials. We discuss the economic advantage of these rare earth free refrigerants.

Near Room Temperature Magnetocaloric Response of an (FeNi)ZrB Alloy

Mechanical alloying of (Fe70Ni30)89Zr7B4 powders leads to the formation of a γ-FeNi phase. In order to obtain single γ-phase, a powdered sample was solution annealed in the γ-phase field and water quenched. The Curie temperature of this powder was slightly higher than room temperature. The refrigerant capacity calculated for this alloy, RCFWHM ~ 330 J·kg-1 for 5 T, is comparable to other prominent room temperature magnetocaloric materials, making this alloy a good candidate for magnetic refrigeration near room temperature with additional benefits that is non-rare earth containing and less expensive than many alternatives.

Density-functional study of U–Mo and U–Zr alloys

The ground-state properties of U–Mo solid solutions are studied by density functional theory and compared to the similar U–Zr system. We discuss how the heat of formation in both alloys correlates with the charge transfer between the alloy components, and how the specific behavior of the density of states in the vicinity of the Fermi level promotes the stabilization of the U 2Mo compound. Our calculations suggest that, due to the existence of a single γ-phase over the typical fuel operation temperatures, γ-U–Mo alloys should indeed have much lower constituent redistribution than γ-U–Zr alloys for which binodal decomposition causes a high degree of constituent redistribution.

Compositional Variations between Different Generations of γ′ Precipitates Forming during Continuous Cooling of a Commercial Nickel-Base Superalloy

The compositional and microstructural evolution of different generations of precipitates of the ordered γ′ phase during the continuous cooling, followed by isothermal aging, of a commercial nickel-base superalloy, Rene 88DT, has been characterized by three-dimensional atom probe (3DAP) tomography coupled with energy-filtered transmission electron microscopy (EFTEM) studies. After solutionizing in the single γ-phase field, during continuous cooling at a relatively slow rate (~24 °C/min), the first-generation primary γ′ precipitates, forming at relatively higher temperatures, exhibit near-equilibrium compositions, while the smaller-scale secondary γ′ precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often containing an excess of Co and Cr while being depleted in Al and Ti content. The compositions of the γ matrix near these precipitates also exhibit similar trends, with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates. Subsequent isothermal aging at 760 °C leads to coarsening of the primary γ′ precipitates without affecting their composition significantly. In contrast, the composition of the secondary γ′ precipitates is driven toward equilibrium during the isothermal aging process.

Structure, magnetostrictive, and magnetic properties of heat-treated Mn 42Fe 58 alloys

The newly discovered large magnetostriction in antiferromagnetic γ-Mn–Fe alloy is of interest to develop a new kind of ductile and low-cost magnetostrictive materials. This work presents a detailed study on the structures, magnetostrictive, and magnetic properties of a heat-treated Mn 42Fe 58 alloy by X-ray diffraction (XRD), transmission electronic microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometer. The single γ-phase separates into a mixture containing a face-centered-cubic (fcc) γ-phase, a hexagonal close-packed (hcp) ɛ-phase, and a body-centered-cubic (bcc) α-phase after isothermal heat treatment for 24 h at 1100 °C. Such phase separation in Mn 42Fe 58 alloy not only leads to a slight increase in lattice parameter a of the γ-phase and Néel temperature T N, but also results in an obvious enhancement in magnetization due to the presence of ferromagnetic bcc phase. The magnetostrictive performance, however, deteriorates accompanying with the phase separation.