High-temperature deformation of single-crystal Ni<Subscript>3</Subscript>Al-Based alloys

Comparative studies of the structure and strength properties of VKNA-1V and VKNA-4U 〈001〉 single-crystal alloys were performed using tensile tests at 1200 and 1250°C. Dynamic recovery is the principal relaxation mechanism. At 1250°C, the strength properties of the VKNA-4U alloy are higher than those of the VKNA-1V alloy.     

Coarsening behavior of Ni<Subscript>3</Subscript>Ga precipitates in Ni-Ga alloys: Dependence of microstructure and kinetics on volume fraction

Coarsening of Ni₃Ga precipitates in binary Ni-Ga alloys containing 14.89 and 18.31 at. pct Ga, aged at 628 °C, was investigated using transmission electron microscopy and magnetic analysis. The equilibrium volume fractions, f _e , of Ni₃Ga were 0.053 and 0.486, respectively. The rate constant for the kinetics of average particle growth decreases anomalously as f _e increases. Ni₃Ga precipitates with concave interfaces are observed at small sizes (45 to 55 nm) in the 14.89 pct Ga alloy. Elastic interactions lead to rapid coalescence in the 18.31 pct Ga alloy, producing rod-shaped precipitates; this shape has not been observed before in Ni-base alloys aged under stress-free conditions. The small concave-cuboidal precipitates and rapid coalescence are attributed to the large elastic self- and interaction energies, respectively, associated with the large precipitate-matrix lattice mismatch in this alloy system. Analysis of the present and previous data yields a Ni₃Ga/matrix interfacial-free energy of 3.91±1.12 mJ/m², which is the smallest value in all the binary Ni-base systems investigated to date. A reasonable value of the chemical diffusion coefficient is also obtained from the data.     

Influence of rare-earth metals on the high-temperature strength of Ni<Subscript>3</Subscript>Al-based alloys

The influence of the content of reaction- and surface-active alloying elements (rare-earth metals (REMs)) and the method of their introduction into cast high-temperature γ′-Ni₃Al-based intermetallic alloys, which are thermally stable natural eutectic composites, on their structure-phase state and the mechanical properties is studied. The life of low-alloy heterophase γ′ + γ cast high-temperature light Ni₃Al-based alloys is shown can be increased at temperatures exceeding 0.8T _m (T _m is the melting temperature of Ni3Al) due to additional stabilization of the single-crystal structure of these alloys with submicron and nanometer-sized particles of the phases formed by refractory and active REMs. It is also shown that stage-by-stage fractional introduction of all components into alloys during vacuum induction melting with allowance for their reaction activities (most refractory metals are introduced in the form of low-melting-point master alloys at the first stage of vacuum induction melting, and lanthanum is introduced with a master alloy in the optimal contents of 0.1–2 wt % into the charge of VKNA-1V and VKNA-25 alloys at the final stage) leads to the formation of a modified structure stabilized by nanoprecipitates of nickel and aluminum lanthanides and the phases formed by refractory metals. This method increases the life of VKNV-1V-type alloys (0.5 wt % Re) at 1000–1200°C by a factor of ∼1.7 and that of VKNA-25-type alloys (1.2 wt % Re and Co) by a factor of ∼3.     

Phase analysis of multicomponent alloys using an Ni<Subscript>50</Subscript>Co<Subscript>25</Subscript>Mo<Subscript>25</Subscript> alloy as an example

method for phase analysis of three-component alloys is proposed. It is based on a pair interaction model and an experimental determination of the sign of pair chemical interaction energy and includes an electron-microscopic investigation of microstructures above and below the ordering–separation phase transition temperature for each diffusion couple. This method is used to study an Ni₅₀Co₂₅Mo₂₅ alloy. The phases that precipitate in this alloy over the entire heating temperature range, including the liquid state, are detected.     

Ordering transformations in Ni<Subscript>75</Subscript>Mo<Subscript>15</Subscript>Mn<Subscript>10</Subscript> alloy

Effect of partial substitution of Mo in Ni₃Mo by Mn on its ordering behaviour is studied here using Transmission Electron Microscopy (TEM). The change in alloy composition does not change the nature of the short range ordered (SRO) state, which shows intensity maxima at {1½ 0} and equivalent positions. However, prolonged aging at 973 K leads to transformation of the Ni₇₅Mo₁₅Mn₁₀ alloy, studied here, almost entirely into the ordered D0₂₂ phase unlike in the case of the binary Ni-Mo alloy. Curved antiphase boundaries (APBs), with wetting of the interdomain D0₂₂ regions by the disordered phase, characterize the microstructure of the aged alloy. A small volume fraction of a sigma-like topologically close packed (TCP) phase that appears in the as quenched alloy seems to remain unaffected by ageing.     

Electronic structure and ferromagnetic effect in Ni<Subscript>2</Subscript>MnGa alloy

By using the first-principles discrete variational method (DVM), we have investigated the magnetic contribution to the binding energy, the Fermi energy level, the interatomic energy, the bond order (BO), the total density of states (DOS), and the charge density distribution for varying c/a in Ni₂MnGa alloy. The binding energy curves for the ferromagnetic (FM) phase for varying c/a are more complicated than those in the nonferromagnetic (NM) phase whose energy difference displays the magnetic contribution to the binding of the crystal. The interactions between the central atom Mn and the surrounding atoms, including the interatomic energies and the BOs, are different from each other, which leads to crystal anisotropy. The finding suggests that the interatomic energy difference between the FM and NM phases will be the origin of magnetic anisotropy. The difference in electron density on the different distortions for the FM and NM phases varying with c/a shows the different redistribution of the magnetization for the Mn and Ni in the [110] plane. This work was supported by the National Science Foundation of China under Grant No. 50301011.     

Kinetics of Ni<Subscript>3</Subscript>S<Subscript>2</Subscript> sulfide dissolution in solutions of sulfuric and hydrochloric acids

The kinetics of Ni₃S₂ sulfide (heazlewoodite) dissolution in solutions of hydrochloric and sulfuric acids is studied. The process under study in the temperature range of 30–90°C is found to occur in a kinetic regime and is controlled by the corresponding chemical reactions of the Ni₃S₂ decomposition by solutions of inorganic acids (E _a = 67–92 kJ/mol, or 16–22 kcal/mol). The only exception is the Ni₃S₂-HCl system at elevated temperatures (60–90°C). In this case, the apparent activation energy decreases sharply to 8.8 kJ/mol (2.1 kcal/mol), which is explained by the catalytic effect of gaseous chlorine formed under these conditions. The studies performed are related to the physicochemical substantiation of the hydrometallurgical processing of the copper-nickel converter mattes produced in the industrial cycle of the Norilsk Mining Company.     

Rare-earth metals in nickel aluminide-based alloys: III. Structure and properties of multicomponent Ni<Subscript>3</Subscript>Al-based alloys

The possibility of increasing the life of heterophase cast light Ni₃Al-based superalloys at temperatures higher than 0.8T _m of Ni₃Al is studied when their directional structure is additionally stabilized by nanoprecipitates, which form upon additional alloying of these alloys by refractory and active metals, and using special methods for preparing and melting of an alloy charge. The effect of the method of introducing the main components and refractory reaction-active and surface-active alloying elements into Ni₃Al-based cast superalloys, which are thermally stable natural composite materials of the eutectic type, on the structure-phase state and the life of these alloys is studied. When these alloys are melted, it is necessary to perform a set of measures to form particles of refractory oxide cores covered with the β-NiAl phase and, then, γ′_prim-Ni₃Al phase precipitates during solidification. The latter phase forms the outer shell of grain nuclei, which provides high thermal stability and hot strength of an intermetallic compound-based alloy. As a result, a modified structure that is stabilized by the nanoprecipitates of nickel and aluminum lanthanides and the nanoprecipitates of phases containing refractory metals is formed. This structure enhances the life of the alloy at 1000 °C by a factor of 1.8–2.5.     

Effect of Ce<Subscript>2</Subscript>O<Subscript>3</Subscript> on Structure,Viscosity, and Crystalline Phase of CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Li<Subscript>2</Subscript>O-Ce<Subscript>2</Subscript>O<Subscript>3</Subscript> Slags

Aiming at devising new mold flux for Ce-bearing stainless steel, a fundamental investigation on the effect of Ce₂O₃ on properties of the CaO-Al₂O₃-Li₂O-Ce₂O₃ slag was provided by the present work. The results show that adding Ce₂O₃ could decrease the viscosity of the slag due to its effects on decreasing the polymerization of the slag. The crystalline process was restrained by increasing the content of Ce₂O₃, and the crystalline phases also can be influenced by the slag structure. The crystalline phases were transferred from LiAlO₂ and CaO to LiAlO₂ and CaCeAlO₄ with the addition of Ce₂O₃ to the slag, which could be well confirmed by the structure of the unit cell of the crystals.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Concentration on Density and Structure of (CaO-SiO<Subscript>2</Subscript>)-xAl<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag

The effect of Al₂O₃ concentration on the density and structure of CaO-SiO₂-Al₂O₃ slag was investigated at multiple Al₂O₃ mole percentages and at a fixed CaO/SiO₂ ratio of 1. The experiments were conducted in the temperature range of 2154 K to 2423 K (1881 °C to 2150 °C) using the aerodynamic levitation technique. In order to understand the relationship between density and structure, structural analysis of the silicate melts was carried out using Raman spectroscopy. The density of each slag sample investigated in this study decreased linearly with increasing temperature. When the Al₂O₃ content was less than 15 mole pct, density decreased with increasing Al₂O₃ content due to the coupling of Si (Al), whereas above 20 mole pct density of the slag increased due to the role of Al³⁺ ion as a network modifier.     

Reduction of Fe<Subscript>2</Subscript>MoO<Subscript>4</Subscript> by hydrogen gas

In the present work, the reduction kinetics of iron molybdate (Fe₂MoO₄) by hydrogen gas was investigated by thermogravimetric analyses (TGA). Both isothermal and nonisothermal experiments were conducted. By using fine particles, very shallow powder bed, and high hydrogen flow rate, the study could be focused on the chemical reaction. The activation energy obtained from the isothermal experiments was found to be 173.5 kJ/mol, which was in reasonable agreement with the value of 158.3 kJ/mol obtained from the nonisothermal experiments. The reduction product was found to be an intermetallic compound, Fe₂Mo, of microcrystalline structure.     

Effects of Annealing and Pressure on Devitrification and Mechanical Properties of Amorphous Al<Subscript>87</Subscript>Ni<Subscript>7</Subscript>Gd<Subscript>6</Subscript>

Annealing studies at different temperatures, as well as those conducted with 940 MPa hydrostatic pressure, were conducted on amorphous ribbons of Al₈₇Ni₇Gd₆. The studies were performed to investigate the evolution of structure under different conditions and to particularly examine the effects of superimposed hydrostatic pressure during annealing. This amorphous alloy devitrifies at low temperatures via the precipitation of nano-crystalline α-Al particles. The effects of these various exposures on the amount of devitrification have been quantified using a variety of analytical techniques (i.e., X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM)). In addition, the effects of devitrification on the mechanical properties have been quantified using microhardness indentation and uniaxial tension tests. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.

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Shape memory effect in a rapidly quenched Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> alloy

The structural and thermomechanical properties of rapidly quenched layered amorphous–crystalline Ti₅₀Ni₂₅Cu₂₅ composite materials with various ratios of amorphous and crystalline phases are studied. These layered composite materials are shown to exhibit the two-way shape memory effect accompanied by bending deformation without additional thermomechanical treatment. The ratio of amorphous and crystalline phases is found to affect the reversible change in the shape of the composite material.