Effect of atomic ordering on the role of grain boundaries in the plastic deformation of Ni<Subscript>3</Subscript>Fe alloy

The structure of dislocations and the defect structure of grain boundaries and their parameters in Ni₃Fe alloy with short-range order (SRO) and long–range order (LRO) at different stages of plastic deformation are studied by means of transmission diffraction electron microscopy using thin foils and replicas. It is found that atomic ordering reduces the Σ3 twins plasticizing effect, increases the density of grain boundary defects, slows their annihilation during deformation, and intensifies the microstrains at the triple junctions of grain boundaries.     

Orientation dependence of creep in Ni<Subscript>3</Subscript>Ge single crystals

The creep of Ni₃Ge alloy single crystals oriented along [001], \([\bar 139]\), \([\bar 234]\) and \([\bar 122]\) strain axes is investigated. It is established that the transition from an octahedral to a cubic slip raises the alloy’s resistance to creep deformation. The slip in cube planes demonstrates the high stability of the rate of steady-state creep with respect to changes in the testing temperature.     

Electronic structure and energy ratios of Ni<Subscript>3</Subscript>Al clusters in TiNi nanoparticles with an impurity Al atom

The scattered-wave method is used to calculate electron spectra of central Ni ₈Ti, Ti₈Ni, and Ti ₈Al nanoclusters in nanoparticles of (Ni ₂₆Ti₆₄) Al₁ and (Ti ₂₆Ni₆₄) Al₁ alloys in the B2 structure. The single Al atom is shown to have a better chance for accommodation on the titanium sublattice because of high binding energy. __________ Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 10, pp. 67–72, October, 2005.     

Deposition of Ni<Subscript> 13</Subscript> and Cu<Subscript> 13</Subscript> clusters on Ni(111) and Cu(111) surfaces

The soft deposition of Ni₁₃ and Cu₁₃ clusters on Ni(111) and Cu(111) surfaces is studied by means of constant-energy molecular-dynamics simulations. The atomic interactions are described by the Embedded Atom Method. It is shown that the shape of the nickel clusters deposited on Cu(111) surfaces remains rather intact, while the copper clusters impacting on Ni(111) surfaces collapse forming double and triple layered products. Furthermore, it is found that for an impact energy of 0.5 eV/atom the structures of all investigated clusters show the lowest similarity to the original structures, except for the case of nickel clusters deposited on a Cu(111) surface. Finally, it is demonstrated that when cluster and substrate are of different materials, it is possible to control whether the deposition results in largely intact clusters on the substrate or in a spreading of the clusters. This separation into hard and soft clusters can be related to the relative cohesive energy of the crystalline materials.     

Energy of planar defects in the Ni<Subscript>3</Subscript>Al phase: Theory and experiment

The Ni₃Al phase with an L1₂ surperstructure exhibits a clearly pronounced positive temperature dependence of the yield stress. The nature of this phenomenon is still not clearly understood. The temperature dependence of the yield stress under consideration is determined, in many respects, by the structure of glide superdislocations and planar defects of these superdislocations involved in the L1₂ superstructure. A critical analysis is made of the current state of the art in the solution of the problem regarding the energy of antiphase boundaries in planes of an octahedron and a cube, as well as the energy of stacking faults in the Ni₃Al phase.     

Experimental Study and Analysis of Absorption Spectra of Ni<Superscript>2+</Superscript> Ions in Nickel Orthoborate Ni<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>2</Subscript>

The results of studies of the absorption spectra of nickel orthoborate Ni₃(BO₃)₂ in the range of electronic d–d-transitions are reported. The obtained data are analyzed in the framework of the crystal field theory. The Ni²⁺ ions are located in two crystallographically nonequivalent positions 2a and 4f with point symmetry groups C_2h and C₂, respectively, surrounded by six oxygen ions forming deformed octahedra. The absorption spectra exhibit three intense bands corresponding to spin-resolved transitions from the ground state of nickel ion ³A_2g (³F) to the sublevels of the ³T_2g (³F), ³T_1g (³F) and ³T_1g (³P) triplets split by the spinorbit interaction and the rhombic component of the crystal field. At temperatures below 100 K, the spectra exhibit a thin structure, in which phonon-free lines can be distinguished. Comparison of the calculated frequencies of the zero-phonon transitions with the experimental data allows estimating parameters of the crystal field acting on the nickel ions in the 2a- and 4f-positions, as well as the parameters of electrostatic interaction between the 3d electrons and spin-orbit interaction constants.     

Role of the third component in the high-temperature hardening of Ni<Subscript>3</Subscript>Al phase

The high-temperature solid solution hardening of the ternary Ni₃Al + Me phase (Me is a metal) is analyzed. The hardening resulting from the alloying by refractory elements (Nb, Ta, Hf) is considered in detail. This phenomenon is established to have a multifactorial nature.     

Effect of plasma spraying regimes on structure and properties of Ni<Subscript>3</Subscript>Al coatings

The structure and tribological properties of coatings made of PN85YU15 powder were studied. The coatings were deposited on the mild steel blanks by the technology of air-plasma spraying using a unit of annular input and gas-dynamic powder focusing. Efficiency of heating and acceleration of powder particles was studied preliminarily. Measurement results on temperature and velocity distributions of particles at a certain spraying distance by the method of spectral pyrometry and time-of-flight method are presented. The effect of plasmatorch arc current and amount of propane-butane in the plasma flow on the structure and properties of coatings is analyzed in this paper. It is determined that the phase composition of coatings and initial powder is the same: the main phase is Ni₃Al compound; moreover, the structure contains Ni₅Al₃ phase. It is shown that an increase in the amount of propane-butane increases coatings porosity. The densest coatings (5.77%) were obtained at the plasmatorch arc current of 200 A with the reduced amount of propane-butane. The coatings obtained at the minimal arc current of 100 A with an increased amount of propane-butane are characterized by maximal porosity (20.38%). The results of tribological testing of the coatings under the conditions of sliding friction with a lubricant by the disc-plane scheme are presented. From the standpoint of obtaining the densest coatings with high performance, the optimal regimes of plasma spraying of PN85YU15 powder are the current from 140 A to 200 and using the air and propane-butane mixture only as the shielding gas (anode curtain).     

Initial stages in the growth of {111}-faceted CoCr<Subscript>2</Subscript>O<Subscript>4</Subscript> clusters: mechanisms and strained nanometric pyramids

The epitaxial growth of ferrimagnetic CoCr₂O₄ films on MgAl₂O₄(001) results in self-organized {111}-faceted pyramids and hut clusters having base dimensions that can reach above a micron in size. This paper reports the morphology and structural characterization of the initial stages of film growth. Three distinctive stages are identified as the growth proceeds: in the initial stage the surface is homogeneous, in the second one nanometric strained pyramids appear, and in the third one there are relaxed pyramids and hut clusters that continue growing up to a micronic size. The islands maintain the {111}-faceted surfaces and the spatial order along 110 directions during the process. We discuss the mechanisms that are the origin of first the coherent pyramids and later the spectacular growth of part of the islands. Besides illustrating the formation process of the final structures, the results presented here demonstrate that arrays of strained {111}-faceted CoCr₂O₄ pyramids with nanometric dimensions can be fabricated. PACS 81.07.-b; 68.65.-k     

A molecular dynamics study on melting point and specific heat of Ni<Subscript>3</Subscript>Al alloy

Using the Embedding Atom Method (EAM) for highly undercooled Ni₃Al alloy, the melting point and the specific heat were studied by a molecular dynamics simulation. The simulation of melting point was carried out by means of the sandwich method and the NVE ensemble method, and the results show a good agreement, whereas are larger than the experimental value of 1663 K. This difference is attributed to the influence of surface melting on experimental results, which causes the smaller measurements compared with the thermodynamic melting point. The simulated specific heat of Ni₃Al alloy weakly and linearly increases with the increase of undercooling in the temperature range from 800 K to 2000 K. Supported by the National Natural Science Foundation of China (Grant No. 50395101)     

The creep behavior of single-crystalline Ni<Subscript>3</Subscript>Ge alloys

The creep of single-crystalline Ni₃Ge alloys subjected to compression along the strain axis [001] has been investigated. The resulting creep curves have revealed low and high secondary-creep rates. Inverse hightemperature creep behavior has been observed.     

The evolution of electronic configuration and magnetic characterization of Fe<Subscript>9</Subscript>Ni<Subscript>1</Subscript>, Fe<Subscript>8</Subscript>Ni<Subscript>2</Subscript> alloy in theoretical calculation

To analyze the origin of the magnetic enhancement of Fe-Ni alloy, the electronicconfigurations and magnetic properties were investigated using density functional theorybased on the first-principle. The supercell (5 × 1 × 1) of Fe,Fe₉Ni₁ and Fe₈Ni₂ were constructed. Thedefect formation energy, band structure, density of states and electron density differencewere calculated. The results showed that Ni doping changed the electronic configuration ofFe atoms, resulting in the enhancement of spin polarization of Fe and the larger Bohrmagnetic moment in Fe-Ni alloys (Fe₉Ni₁). The results showed thatthe charge transfer and the atomic spacing between Fe atoms and the dopant Ni atoms playedan important role in determination of magnetic moment. The value of Fe supercell(5 × 1 × 1), Fe₉Ni₁ and Fe₈Ni₂ were 23.14,23.34 and 22.61μ _B, respectively.     

Mechanical alloying of Ni<Subscript>80</Subscript>Ta<Subscript>20</Subscript> and Ni<Subscript>80</Subscript>Nb<Subscript>20</Subscript>. Crystallization of the amorphous phase

The sequence and crystallization kinetics of the amorphous phase have been analyzed by differential scanning calorimetry and X-ray diffraction for mechanochemically activated Ni₈₀Ta₂₀ and Ni₈₀Nb₂₀ samples. The formation of equilibrium products from the amorphous phase occurs through the formation of metastable products of the A3 type. The kinetic parameters (activation energy and reaction order) and thermodynamic characteristics of this process are determined. Crystallization of the amorphous phase for the Ni₈₀Ta₂₀ and Ni₈₀Nb₂₀ systems occurs due to the growth of existing nuclei through the polymorphic and eutectic mechanisms, respectively. The thermal effects of the synthesis of equilibrium products from a mechanochemically activated mixture of components are identified.     

Superconducting weak bonds at grain boundaries in MgB<Subscript>2</Subscript>

The possibility of preparing bicrystalline Josephson junctions and bolometers based on superconducting MgB₂ on specially prepared bicrystalline MgO substrates is investigated. Microbridges 0.85–6.00 μm in width, intersecting the bicrystalline interface, are formed in epitaxial bicrystalline MgB₂ films grown on these substrates. It is found that annealing of bicrystalline samples in oxygen leads to a systematic decrease in the critical current, an increase in the temperature width of the superconducting transition region, and to an improvement of the current-voltage (IV) characteristic, which becomes close in shape to the IV characteristic of a Josephson junction. The response of such a junction to radiation at a frequency of 110 GHz with an amplitude attaining 0.5 mV is measured.     

Large successive magnetocaloric effects around room temperature in Ni<Subscript>50</Subscript>Mn<Subscript>34</Subscript>In<Subscript>15</Subscript>Al alloy

Two successive magnetocaloric effects consisting of inverse magnetocaloric effect around martensitic transition and negative magnetocaloric effect around magnetic transition of austenitic phase have been observed in Ni₅₀Mn₃₄In₁₅Al alloy. Large inverse magnetic entropy change ΔS_m ( ~ 21.3 J kg^?1 K^?1), small thermal and magnetic hysteresis of martensitic transition give rise of large net refrigerant capacity ( ~ 152.3 J kg^?1) under a magnetic field of 50 kOe, which is comparable with that ( ~ 157.9 J kg^?1) of second-order transition. The large combined magnetocaloric effects make the Ni₅₀Mn₃₄In₁₅Al alloy as a promising candidate material for room temperature magnetic refrigeration.     

The temperature dependence of the magnetoelastic characteristics of cores for force sensors utilizing Fe<Subscript>70</Subscript>Ni<Subscript>8</Subscript>Si<Subscript>10</Subscript>B<Subscript>12</Subscript> amorphous alloy

This paper presents the results of investigation on the influence of temperature on magnetoelastic characteristics of the two ring-shaped cores, made of Fe₇₀Ni₈Si₁₀B₁₂ amorphous alloy. The cores were annealed for 1 h at 350 and 400°C, respectively. The compressive force F was applied perpendicular to the direction of the magnetizing field H in the sample. Special cylindrical backing enables application of the uniform compressive stress σ to the wound ring sample. A resistive furnace heated the experimental set-up. Results presented in the paper indicate a significant influence of the temperature on the magnetoelastic characteristics of Fe₇₀Ni₈Si₁₀B₁₂ amorphous alloy. Information about the magnetoelastic characteristics of this material may be useful in the magnetoelastic sensor development. Also this will create new possibilities in the development of physical model of magnetoelastic effect.