Influence of Cu content on high temperature oxidation behavior of AlCoCrCuxFeNi high entropy alloys (x = 0; 0.5; 1)

Studies on the oxidation behavior of high entropy alloys from the Al-Co-Cr-Cu-Fe-Ni metallic system were conducted. Three different high entropy alloys were synthesized using arc-melting method: AlCoCrFeNi, AlCoCrCu_0.5FeNi and AlCoCrCuFeNi, with their crystal structures being respectively BCC, BCC and BCC + FCC. All alloys were oxidized in the air atmosphere at temperature of 1273 K for different time periods. Basing on the thermogravimetric result, the oxidation rate of the materials decreased with the increase of Cu content and the values of parabolic constants were on the level similar to those observed in Ni-Al intermetallic alloys. In all cases the oxide scale consisted of α-Al₂O₃, which exhibited poor adhesion to the surface during the cooling process, what was especially visible in alloys with Cu addition. In all oxidized samples a tendency towards formation of the Al-depleted FCC phase directly beneath the scale was observed. The phase constitution of all materials changed significantly during the oxidation process, with multiple new phases appearing in the system.     

The mechanical properties of intermetallic Ni50−xTi50Alx alloys (x = 6,7,8,9)

High-strength, heat- and oxidation-resistant low density Ti–Ni–Al intermetallic alloys have recently attracted attention competing with some conventional high temperature structural superalloy such as Ni-based superalloy. In the present study, the mechanical properties of Ti-rich Ni_50−xTi₅₀Al_x (x = 6,7,8,9) alloys were examined by compression tests at room temperature and at high temperature from 400 °C to 800 °C. X-ray diffraction, scanning electron microscopy as well as microhardness tester were utilized to characterize the microstructure as well as the structural evolution with the increasing Al additions. The systematic analyses of the mechanical behavior were made according to compression test at different temperatures. A yield stress of 1800 MPa and more than 10% of compression strain were achieved at room temperature; and a yield stress of 400 MPa at 800 °C. It is suggested that controlling the shape, the volume percent and the distribution of second phases in the matrix is most important to obtain good mechanical properties in these alloys. The strengthening mechanism of aluminum addition on the mechanical properties was discussed systemically according to the microstructure evolution and solution hardening and precipitation hardening upon Al addition.     

Magnetic and transport properties of Laves phase Dy1−xMmxCo2 (x = 0–0.5) alloys

The influence of Mm substitution (Mm = mischmetal) on structural, transport and magnetic properties of (Dy_1?xMm_x)Co₂ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys has been investigated by means of X-ray diffraction (XRD), temperature dependent electrical resistivity (ρ(T)), ac susceptibility (χ(T)) and thermopower (S(T)). XRD patterns show the formation of solid solutions crystallizing with cubic Laves (C15) type structure at room temperature. The pronounced discontinuities in the resistivity and thermopower at Curie temperature (T_C) are explained based on the suppression of the spin-fluctuation contribution. The gradual decrease in T_C and sharpness of discontinuities in ρ(T) and S(T) with increasing Mm substitution has been discussed.     

Hydrogen sorption in the LaNi5-xAlx-H system (0 ≤ x ≤ 1)

A thermodynamic assessment of hydrogen sorption in LaNi_5-xAl_x-H (0 ≤ x ≤ 1) alloys was performed combining experimental and theoretical investigations. The occurrence of sloped plateaux in Pressure Composition Isotherms (PCI) is discussed on the basis of compositional inhomogeneities and of possible re-distribution of metallic elements between the hydrogen rich and the hydrogen poor phases.In order to provide input for the thermodynamic assessment, literature data were reviewed and PCI experiments were performed at increasing temperatures on alloys with different Al contents. The presence of a compositional distribution was investigated using Rietveld refinement of X-Ray diffraction patterns, volumetric and calorimetric measurements, both on as received and annealed samples with LaNi_4.8Al_0.2 nominal composition. Moreover, ab initio energies of formation for LaNi₅, LaAl₅ and the respective hydrides were calculated. A full CALPHAD assessment of the Gibbs energy for LaNi_5-xAl_xH_y (0 ≤ x ≤ 1, 0 ≤ y ≤ 7) phase was obtained, showing a good agreement between experimental and calculated results.The presence of sloped and flat plateaux has been related to the occurrence of ortho- and para-equilibrium conditions.     

Structure and magnetic properties of SmxCo72−xFe16Zr7B4Cu1 (x = 8, 12, 15) alloys

Sm_xCo_72−xFe₁₆Zr₇B₄Cu₁ (x = 8, 12, 15) alloys with low level of Sm and mixed addition of several elements (Fe, Zr, B, Cu) were prepared via arc melting and rapid quenching technique. The influences of Sm content and cooling rate on the microstructure and magnetic properties of alloys were investigated. The ribbons melt-spun at 40 m/s present a novel composite microstructure with nanocrystalline phases embedded in amorphous matrix. Such a unique structure endows the x = 12 ribbons a coercivity as high as ∼20,000 Oe. Ferromagnetism of the amorphous matrix, the pinning effect of amorphous phase during the movement of domain-walls and exchange interactions among adjacent grains may contribute to the high coercivity performance.     

Microstructure and shape-memory characteristics of Ni56Mn25−xCoxGa19 (x = 4, 8) high-temperature shape-memory alloys

NiMnCoGa alloys were studied as candidate of high-temperature shape-memory alloys with Mn substituted by Co. The results show that the hot workability and room-temperature ductility of NiMnGa alloys can be greatly improved by forming ductile γ phase through Co addition. When the amount of γ phase is about 19%, the tensile stress and strain are 491 MPa and 8.2%, respectively. When the amount of γ phase increased to about 43%, the tensile stress and strain are 729.3 MPa and 14.1%, respectively. The reversible strain due to shape-memory effect is 2.1% under a prestrain of 5.3% for x = 4, and the shape-memory ability almost disappears for x = 8 due to large amount of γ phase, which seriously hampers the reorientation of martensitic variants and/or detwinning.     

First-principles investigation of the structural and electronic properties of Cu6−xNixSn5 (x = 0, 1, 2) intermetallic compounds

Cu_6−xNi_xSn₅ is an important intermetallic compound (IMC), which was known to greatly improve the reliability of the solder joints in integrated circuits. However, the improvement mechanisms and even the crystal structure of the IMC were not fully understood. In this paper, the first-principles calculations were performed to determine the stable structure of Cu_6−xNi_xSn₅ IMC. The structural and electronic properties of Cu_6−xNi_xSn₅ (x = 0, 1, 2) IMCs have been calculated. The results show that Ni atoms preferentially occupied 8f sites (Cu2) and formed Cu₄Ni₂Sn₅. The results of energy calculation and density of states demonstrate that this Cu₄Ni₂Sn₅ IMC had a more stable structure than Cu₆Sn₅. These results are also expected to account for the improvement in the reliability of the solder joint.     

Formation of bulk metallic glasses in Cu45Zr48−xAl7REx (RE = La,Ce, Nd,Gd and 0 ≤ x ≤ 5 at.%)

We report a series of bulk metallic glass-forming alloys of compositions (Cu₄₅Zr_48−xAl₇RE_x, RE = La, Ce, Nd, Gd and 0 ≤ x ≤ 5 at.%). By using a conventional copper mold sucking method, alloys with diameters ranging from 5 to 10 mm can be readily solidified into an amorphous structure without detectable crystallites. The best glass-forming ability is obtained for the alloys Cu₄₅Zr₄₆Al₇RE₂. Possible effects of RE addition on the glass-forming ability are discussed. In addition, the compositional effect on mechanical properties of Zr–Cu–Al–Gd alloys is presented.     

Study of Ni50+xMn25Ga25−x (x = 2–11) as high-temperature shape-memory alloys

Ni_50+xMn₂₅Ga_25−x (x = 2–11) alloys were studied as high-temperature shape-memory alloys, with regard to their microstructure, martensitic transformation behavior and high-temperature shape-memory effect. Single phase of martensite with tetragonal structure was present for x < 7, and dual phases containing martensite and γ were observed for x ⩾ 7. The martensitic transformation peak temperatures M_p increase monotonically from 39.1 °C for x = 2 to 443.8 °C for x = 7, then remain almost constant at 440 °C for x ⩾ 7. The shape-memory strains of the alloys decreased gradually from 6.1% for x = 4 to 2.8% for x = 8 and 0% for x = 11 under the same pre-strain. The variations of the martensitic transformation temperatures and the shape-memory effects with Ni contents correlate with changes in size factor, electron concentration and precipitation of γ phase.     

Thermal vacancy formation in Co-based Heusler-type alloys Co2MnZ (Z = Si, Ge, Sn)

Thermal vacancy formation was studied for the Heusler-type ferromagnetic alloys Co₂MnZ (Z = Si, Ge, Sn) as a function of temperature (773–1273 K) by the density, electrical resistivity and positron annihilation measurements. The vacancy concentration increased with increase in quenching temperature and particularly, a high vacancy concentration exceeding 2% was observed in Co₂MnGe and Co₂MnSn. Estimated vacancy formation and migration energies were comparable with those for B2-type FeAl and CoGa alloys with high vacancy concentration. Further, the vacancy type and the vacancy site were examined for alloys quenched from 773 K. As a result, it was suggested that the mono-vacancies are randomly distributed over the lattice sites.     

Lithiation and magnesiation of R5Sn3 (R = Y and Gd) alloys

The lithiation and magnesiation of binary R₅Sn₃ (R = Y and Gd) phases has been studied by XRD, SEM, EPMA and electrochemical methods. The formation of R₅Sn₃Li_x and R₅Sn₃Mg_x ternary hexagonal phases as the results of insertion of Li/Mg into octahedral voids were observed during the first stage of electrochemical lithiation and magnesiation respectively. During the second stage of the electrochemical lithiation and magnesiation is the decomposition of the hexagonal phase into new phases and substitution of tin atoms by Li/Mg are observed.     

Structural variations in the ternary system HfAl2−xCux (x = 0.2–1.0)

Synthesized ternary intermetallic phases HfAl_2?xCu_x in a series with (x = 0.49, 0.88, 1.04) are characterized as Laves phase structures. X-ray diffraction revealed homogeneity within ranges 0.2 ≤ x ≤ 0.5 and 0.7 ≤ x ≤ 0.9 for structure types MgCu₂ and MgNi₂, respectively. When Cu atoms gradually replaced the Al atoms, the structure type altered in the sequence MgCu₂ → MgNi₂ → MgZn₂, and the Kagomé nets were distorted with varied bond lengths. Measurements of physical properties revealed these phases to be metallic, with resistances 4.35 (x = 0.5), 5.85 (x = 0.7), and 6.50 (x = 0.9) mΩ cm, respectively, at temperature 300 K. The coloring schemes reveal that, upon increasing the proportion of Cu atoms, the stability of these phases correlated with the arrangements of the Al and Cu atoms. Calculated electronic structures indicate that the bonding character is consistent with the experimentally observed phase width.     

Observation of passive films on Fe–20Cr–xNi (x = 0, 10, 20 wt.%) alloys using TEM and Cs-corrected STEM–EELS

The structure and composition of passive film formed on Fe–20Cr–xNi (x = 0, 10, 20 wt.%) alloys in deaerated pH 8.5 borate buffer solution was examined by transmission electron microscope and Cs-corrected scanning transmission electron microscope-electron energy loss spectroscopy. Thickness of the passive film on each alloy was measured to be 2.5–2.7 nm and the passive film was enriched with Cr. The passive film formed on the alloys exhibited an amorphous structure, as confirmed by the lack of diffraction contrast and by the fast Fourier transform images taken within a region of the passive film on each alloy.     

Role of crystalline precipitates on the mechanical properties of (Cu0.50Zr0.50)100−xAlx (x = 4, 5, 7) bulk metallic glasses

The mechanical behaviour upon compression of (Cu_0.50Zr_0.50)_100?xAl_x (x = 4, 5, 7) rods with 2 and 3 mm diameter was systematically studied and compared with the literature data. Fully amorphous bulk metallic glasses (x = 5, 7) show little permanent deformation and reproducible yield stress values. The remarkable fracture strain, observed for some apparently X-ray diffraction amorphous samples (x = 4), was found to be due to significant amounts (at least 20%) of the B2-CuZr crystalline phase. The effect of possible flaws on the external surface of the rods was evaluated by mechanical testing of either as cast and machined samples.     

First-principles study of Ti3AC2 (A = Si,Al) (0 0 1) surfaces

We have systematically studied the mechanical properties and surface properties of (1 × 1) Ti₃AC₂ (A = Si, Al) (0 0 1) using the density functional theory (DFT). The calculated cleavage energy for each possible cleavage site shows that Ti–Si and Ti–Al are the weakest layers, while the Ti–C layer is the strongest one. It reveals that the main difference between Ti₃SiC₂ and Ti₃AlC₂ is that the Ti–Si bond is stronger than the Ti–Al bond. This shows the different mechanical and surface properties between them. The surface rumpling and surface energy of Ti₃SiC₂ and Ti₃AlC₂ are calculated. The study shows that the cleavage energy affects both the surface rumpling and the surface energy. The higher the cleavage energy, the larger the surface energy and surface rumpling. Furthermore, the most stable surface structures are predicted for different experimental conditions. The predicted surface structures agree with the available experimental results.     

Crystal structure of τ5–TiNi2−xAl5 (x = 0.48) and isotypic {Zr,Hf}Ni2−xAl5−y

The crystal structure of the compound in the Al-rich region of the Ti–Ni–Al system, τ₅–TiNi_2?xAl₅, x = 0.48, has been derived from X-ray powder and single crystal, neutron powder and electron diffraction (space group I4/mmm, a = 0.3984(2) nm, c = 1.4073(3) nm, R_F2 = 0.0133). Titanium atoms were unambiguously located from neutron powder data. τ₅ is isotypic with the crystal structure of ZrNi₂Al₅. Detailed transmission electron microscopy (TEM) in several crystallographic directions confirmed the lattice parameters and crystal symmetry. Although occupancy of Ni in the 4e site revealed a defect (occ. = 0.76), no significant homogeneity region was observed for this phase at 1020°C. Rietveld analyses of X-ray powder diffraction data for the Zr- and Hf-homologues confirmed for both compounds isotypism and revealed defects in the Ni sites and to a lesser extent also in the Al sites: ZrNi_2?xAl_5?y, x = 0.4, y = 0.4 and HfNi_2?xAl_5?y, x = 0.5, y = 0.2. The crystallographic relations among the structure types of Cu, TiAl₃, ZrNi₂Al₅ and Zr(Ni,Ga)₇ have been defined in terms of a Bärnighausen scheme.     

Microstructures of crystallized Ti51.5Ni48.5−xCux (x = 23.4–37.3) thin films

The microstructures of Ti_51.4Ni_25.2Cu_23.4, Ti_51.3Ni_21.1Cu_27.6, Ti_51.2Ni_15.7Cu_33.1, and Ti_51.4Ni_11.3Cu_37.3 thin films annealed at 773, 873 and 973 K for 1 h were investigated. The Ti_51.2Ni_15.7Cu_33.1 and Ti_51.4Ni_11.3Cu_37.3 films showed very small grain sizes (120 and 50 nm, respectively) compared with the Ti_51.4Ni_25.2Cu_23.4 and Ti_51.3Ni_21.1Cu_27.6 films (1 and 0.3 μm, respectively). They had no precipitates within the B2 grains. On the other hand, the Ti_51.4Ni_25.2Cu_23.4 films annealed at 773 and 873 K showed GP zones and Ti₂Cu precipitates, respectively, and the Ti_51.3Ni_21.1Cu_27.6 film annealed at 773 K showed TiCu precipitates in the grain interiors. The formation of precipitates in the grain interior was discussed in terms of the lattice mismatch between the precipitates and the matrix. The difference in grain size was attributed to different crystallization processes.     

Preparation and electrochemical properties of MgNi–MB (M = Co, Ti) composite alloys

Alloys MgNi–MB (M = Co, Ti) were successfully synthesized by means of mechanical alloying (MA). The XRD spectroscopy suggested that the alloys were amorphous. The discharge capacity and cycle life of these alloys were tested, showing that as borides were introduced, the cycling life of the alloys became much better than MgNi alloy. For instance, 10 h composite MgNi–CoB and MgNi–TiB retained 53.2% and 54.1% of the initial capacity after 30 cycles, while the MgNi alloy kept only 23.3%. The exchange impedance spectroscopy and the potentiodynamic polarization curves proved that the electrochemical properties of the composite alloys were improved significantly. MgNi–CoB was used as an example to study the mechanism of electrochemical hydrogen storage.