Some observations on grain boundary shearing during creep

McLean’s technique was employed to determine the effect of temperature on the contribution of grain boundary shearing to the total creep strain in pure aluminum over the range of 610° to 747°K. The contribution of grain boundary shearing at the same stress was found to depend upon a temperature-compensated time, te^?ΔH/RT, where the activation energy ΔH was practically identical with that for total high temperature creep.     

The efficiency of bridges in a crack

Bridges are cross connections in a crack that contract its faces behind the crack tip. They appear because of the inhomogeneity of the material or differences in the kinematics of fracture. We obtained a general solution to the nonlinear singular integral equation that relates the crack opening h(x) to the stress caused by bridges p(x) and determined the stress-intensity factor F(B) (degree of unloading of the crack tip due to the presence of bridges) depending on the dimensionless stiffness of bridges B. As the measure of the efficiency of bridges, we suggest the ratio of the stress for crack start in a matrix without bridges σ_m to that in the presence of bridges σ_∞. When bridges with a strength σ_u occupy a fraction f of the fracture area, their efficiency Y(ζ, B) depends on the degree of reinforcement ζ = fσ_u/σ_m and on their stiffness B. The Y(ζ, B) dependences have been obtained in an explicit form for the limiting cases of brittle and fully ductile bridges.     

Preparation and high-frequency soft magnetic property of FeCo-based thin films

A series of FeCo-based thin films were prepared by magnetron sputtering without applying an induced magnetic field.The microstructure,electrical properties,magnetic properties and thermal stability of FeCo,FeCoSiN monolayer thin film and[FeCoSiN/SiN_x]_n multilayer thin film were investigated systematically.When FeCo thin film was doped with Si and N,the resistivity and soft magnetic properties of the obtained FeCoSiN thin film can be improved effectively.The coercivity(H_c),resistivity(ρ) and ferromagnetic resonance frequency(f_r) can be further optimized for the[FeCoSiN/SiN_x]_n multilayer thin film.When the thickness of FeCoSiN layer and SiN_x layer is maintained at 7 and 2 nm,the H_c,p and f_r for[FeCoSiN/SiN_x]_n multilayer thin film are 225 A·m~(-1)392 μΩ·cm~(-1) and 4.29 GHz,respectively.In addition,the low coercivity of easy axis(H_(ce) ≈ 506 A·m~(-1)) of[FeCoSiN/SiN_x]_n multilayer thin film can be maintained after annealing at 300 ℃ in air for 2 h.     

Embrittlement and conditions of the optimization of magnetic properties in the amorphous alloy Co<Subscript>69</Subscript>Fe<Subscript>3.7</Subscript>Cr<Subscript>3.8</Subscript>Si<Subscript>12.5</Subscript>B<Subscript>11</Subscript> in the absence of a viscous–brittle transition

The influence of the holding time upon annealing on the temperature of the viscous–brittle transition (temperature of embrittlement) T_f in a cobalt-based amorphous alloy of the composition Co₆₉Fe_3.7Cr_3.8Si_12.5B₁₁ with a very low saturation magnetostriction λ_s (<10^–7) has been studied. It has been established that the dependence of the embrittlement temperature T_f on the of time of holding t_a can be described by an Arrhenius equation and that the embrittlement at the annealing temperatures above and below 300°C is described by different kinetic parameters. In the alloy under study, irrespective of the holding time, embrittlement occurs in a very narrow range of annealing temperatures, which does not exceed 5 K. Based on the experimental data on the evolution of the hysteresis magnetic properties upon the isochronous annealings and upon the isothermal holding, the regime of heat treatment that ensures a very high (about 50000) magnitude of the permeability µ₅ (H = 5 mOe, f = 1 kHz) without the transition of the alloy into a brittle state has been determined.     

Electrical Conductivity and Dielectric Permittivity of γ-Irradiated Nanocomposites Based on Ultrahigh-Molecular-Weight Polyethylene Filled with α-SiO<Subscript>2</Subscript>

UHMWPE/SiO₂ composites were obtained from a homogeneous mixture of ultrahigh-molecular- weight polyethylene (UHMWPE) and silicon dioxide (α-SiO₂) by hot pressing. The temperature dependence (20–170°C) of their conductivities σdс prior to and after γ irradiation (D = 200 kGy), the effect of absorbed dose on the σ_dс value (dose dependence), the behavior of the function logσ_dc = f(T) under heating–cooling conditions, and the frequency dependences (25–106 Hz) of the real (ε') and imaginary (ε'') parts of the complex dielectric constant were studied. The logσ_dc = f(T) dependence was shown to have in both cases a complex pattern: “breaks” due to phase transitions were observed. With an increase in the concentration of α-SiO₂, the ε' and ε'' (tanδ) values in the matrix increased and the frequency dependences of these values corresponded to an exponential rule.     

Self-Diffusion in Magnesium Single Crystals

Radioactive Mg²⁸ has been used to study the rate of self-diffusion in oriented single crystals of magnesium in the temperature range 468° to 635°C. The diffusion coefficients parallel and perpendicular to the c-axis are: On = 1.0 exp (—32,200/KTJ cm² per sec and D₁ = 1.5 exp (—32,500/RT) cm² per sec. The ratio D₁/D₁₁ was found to vary from 1.13 at 468° to 1.24 at 575°C. Assuming a vacancy mechanism, an explanation of this anisotropy of diffusion follows from a consideration of the difference in the saddle points for diffusion in and out of the basal plane.     

Inverse magnetocaloric effect in the uniaxial paramagnet with non-Kramers ions

It has been shown that, in a uniaxial paramagnet with non-Kramers ions with a spin of S = 1 and single-ion anisotropy of the easy-plane type (DS _Z ² ), there is a low-field (μ₀ H ≤ D) and low-temperature (k _B T < 0.68D) region in which the isothermal magnetization along the hard direction H||OZ increases the magnetic entropy by ΔS _M (T, ΔH = H _f - H _i > 0) > 0 and the adiabatic magnetization along the same direction reduces the sample temperature by ΔT _ad(T, ΔH > 0) < 0 (inverse magnetocaloric effect (MCE)). The main features of the inverse MCE in uniaxial paramagnets with large spins (S = 2, 3, …) of the non-Kramers ions have been discussed.     

Size dependence of the melting temperature of metallic nanoclusters from the viewpoint of the thermodynamic theory of similarity

The generalized Thomson formula T_m = T_m^(∞)(1-δ)R for the melting point of small objects T_m has been analyzed from the viewpoint of the thermodynamic theory of similarity, where R is the radius of the particle and T_m^(∞) is the melting point of the corresponding large crystal. According to this formula, the parameter δ corresponds to the value of the radius of the T_m(R^-1) particle obtained by the linear extrapolation of the dependence to the melting point of the particle equal to 0 K. It has been shown that δ = αδ₀, where α is the factor of the asphericity of the particle (shape factor). In turn, the redefined characteristic length δ₀ is expressed through the interphase tension σ_sl at the boundary of the crystal with its own melt, the specific volume of the solid phase v_s and the macroscopic value of the heat of fusion λ_∞:δ₀ = 2σ_slv_s/λ_∞. If we go from the reduced radius of the particle R/δ to the redefined reduced radius R/r₁ or R/d, where r₁ is the radius of the first coordination shell and d ≈r₁ is the effective atomic diameter, then the simplex δ/r₁ or δ/d will play the role of the characteristic criterion of thermodynamic similarity. At a given value of α, this role will be played by the simplex Estimates of the parameters δ₀ and δ₀/d have been carried out for ten metals with different lattice types. It has been shown that the values of the characteristic length δ₀ are close to 1 nm and that the simplex δ₀/d is close to unity. In turn, the calculated values of the parameter δ agree on the order of magnitude with existing experimental data.     

Deformation mechanisms in Cr20Ni80 alloy at elevated temperatures

The mechanisms of plastic deformation of Cr20Ni80 nichrome with an initial grain size of 80 μm were studied in the temperature range 600–950°C and the strain-rate range 1.5 × 10^?6?5 × 10^?2s^?1. Nichrome is shown to exhibit anomalously high values of stress exponent n and a high deformation activation energy Q. These unusual properties were found to be caused by “threshold” stresses below which deformation does not occur. An analysis of the deformation behavior with allowance for threshold stresses reveals the regions of hot, warm, and cold deformation in nichrome. At normalized strain rates \(\dot \varepsilon \) kT/D _{1 Gb} < 10^?8, the true values of n and Q are ～4 and 285 ± 30 kJ/mol, respectively. In the normalized-strain range 10^?8?10^?4 n ～ 6 and the deformation activation energy decreases to 175 ± 30 kJ/mol. This change in the deformation-behavior characteristics is explained by the transition from high-temperature dislocation climb, which is controlled by lattice self-diffusion, to low-temperature dislocation climb, which is controlled by pipe diffusion, as the temperature decreases. At \(\dot \varepsilon \) kT/D _{1 Gb} = 10^?4, a power law break-down takes place and an exponential law (which describes the deformation behavior in the range of cold deformation) becomes operative.     

Cohesive mechanism of the FeCr/Ni Interface: A first-principles study

Based on previous experimental results, a series of FeCr/Ni interface models have been constructed and analyzed using a first-principles pseudopotential plane-wave method. Several parameters, such as the ideal work of separation (W), formation enthalpy (ΔH), cohesive energy (ΔE), and electronic structure were calculated in order to analyze the bonding performance and adhesion mechanisms of elements along an FeCr/Ni interface. The largest ideal work of separation was obtained for the Fe(100)/Ni(100) interface, which implies that this interface model presented the most stable structure among a series of crystal interface indices, e.g., (100), (110), and (111). With Cr doping, the W of the FeCr(100)/Ni(100) interface was increased by 101.571 mJ/m². The corresponding ΔH and ΔE values also indicated that the FeCr(100)/Ni(100) interface model was strengthened by doping with chromium. Furthermore, the overlap population ratio, R _LBOP (R _LBOP= 1.04), of FeCr(100)/Ni(100) was smaller than that of Fe(100)/Ni(100) (R _LBOP = 1.35), which implies that the toughness of the Fe(100)/Ni(100) interface can be improved by the presence of chromium impurities. Moreover, electronic structure analysis provided an understanding of the mechanical performance of the various Fe(Cr)/Ni interface models. Thus, our findings open a potential avenue for the comprehensive study of composite material designs.     

Quantum chemical modeling of the adsorption of chloride ion and water molecule on group 1B metals

Quantum chemical modeling (at DFT-B3LYP level) of the adsorption of Cl^? and H₂O particles on top and into hollow sites of defect-free low index faces of copper, silver, and gold simulated by n-atomic clusters with n = 9 to 17 is carried out. The validity of simulations is confirmed by the comparison to the available literature data on the work function of the metals and the calculated gas-phase adsorption of chlorine. Relative effects of the chemical (metal nature), macrostructural (crystal face), and coordination (adsorption site) factors on the parameters of the chemisorption bond and molecular characteristics of the adsorbate and adsorbent, namely, the E _ads ads adsorption energy at % = 0K, R (Me^? Cl^?) and R(Me-O) adsorption bond lengths, Q effective charges of chlorine atom and water molecule, O-H distance and ∠HOH angle in H₂O molecule, deviation of the H₂O dipole moment vector from normal orientation to the cluster surface, and E _HOMO energies of the highest occupied molecular orbital of Me _n , [Me _n Cl]^?, Me _n H₂O, and [Me _n ClH₂O]^? clusters, are considered. The inner hydration shell of Cl^? is shown to involve six water molecules, the most stable configuration of (H₂O)₆ cluster being prism-like. An electron density shift from chlorine and water molecule to the metal cluster is found to accompany the adsorption and be more pronounced in the case of anion. The character of differences in the hydrophilicity of the group 1B metals and their diverse crystal faces is discussed. The role of hydration effects in the chemisorption of chloride ion on copper, silver, and gold is analyzed in terms of the continuum, molecular, and combined molecular-continuum models.     

Superconducting cellular MgB<Subscript>2</Subscript>

A new composite superconducting material in the form of a cellular structure consisting of large Mg grains surrounded by thin MgB₂ layers has been prepared. The superconducting properties of such a cellular structure were found to depend on the thickness of the superconducting layer d _s. As d _s decreases from ～30 to 1 μm, the critical temperature decreases by more than 10 K. The derivative of the upper critical field with respect to the temperature (dH _c2/dT) near T _c increases from ～0.2 T/K at high d _s to ～0.35 T/K at d _s ～6 μm. The critical current density of the cellular samples calculated for the area of the superconductor section is above 10⁵ A/cm² (T = 4.2 K and H = 1 T).     

A Study of the Room Temperature Fatigue Properties of Molybdenum

The powder metallurgy and arc-cast types of wrought molybdenum stock were studied in rotating beam fatigue. Endurance ratios of unnotched specimens after 5×10⁷ cycles were found to be 0.74 and 0.81, respectively. Fatigue notch factors (K_f) of 1.36 and 2.05, respectively, were obtained on V-notched specimens having a theoretical stress-concentration factor K_t = 3.1.     

A Study of Tensile Flow and Work-Hardening Behavior of Alloy 617

The simple power relationship σ?=?Κε _p ⁿ satisfactorily expresses the tensile flow behavior of many metals and alloys in their uniform plastic strain regime. However, many FCC materials with low stacking fault energy have opposed such power law relationship. Alloy 617, an age-hardenable Ni-based superalloy is also observed not to obey the simple power law relationship neither in its solution-treated nor in its aged conditions. Various flow relationships were used to obtain the best fit for the tensile data, and different relationships were identified for the different aged conditions. The work-hardening rate (θ) demonstrates three distinct regions for all aged conditions, and there is an obvious change in the trend of θ versus σ. In the initial portion, θ decreases rapidly followed by a gradual increase in the second stage and again a decrease in its third stage is perceived in the Alloy 617. These three-stage characteristics are attributed to a commonly known precipitate, γ′: Ni₃(Ti, Al) which evolves during aging treatment and well recognized under transmission electron microscopy (TEM) observation. TEM results also reveal a slight degree of coarsening in γ′ over aging. The tensile flow and the work-hardening behavior are well correlated with other microstructural evolution during the aging treatments.     

Cross flow around a cylinder at periodic disturbances in the boundary layer by electrolysis bubbles

Particularities of dynamic flow control in an open-loop system around a cylinder with a boundary layer by means of periodic generation of microbubbles are presented. Experiments were carried out in the range of Reynolds numbers of Re = (0.3?1.2) × 10⁵. It is shown that this method provides very similar results as in the case of continuous generation of microbubbles; however, the energy expenditure is two times less. The effect can be explained by the delayed response of the boundary layer separation on the disturbance cutoff. Lock-in modes of the natural vortex shedding frequency f _N and the frequency of forced oscillations f _E, as well as superposition of these frequencies, are registered. Maximal values of the amplitude of the drag and lift oscillations are registered in the range of low frequencies (f _E/f _N < 0.07).     

Electrical Conductivity of Nanocomposites Based on Low-Density Polyethylene and Cu<Subscript>2</Subscript>S Nanoparticles

The temperature dependences of the electrical conductivity of nanocomposites based on low-density polyethylene (LDPE) and Cu₂S nanoparticles are studied. It is shown that, starting from a certain temperature, the temperature dependence of the electrical conductivity is described by the following Arrhenius equation: σ = σ₀exp(–E/kT); the logσ = f(10³/T) dependence has several linear portions with different activation energy values. The observed behavior of the logρ = f(1/T) dependence of the LDPE/Cu₂S nanocomposites suggests that an increase in temperature is accompanied by an increase in the mobility of the structural units of the polymer matrix in the bulk and on the surface of the sample. The polymer–filler interfacial interactions decrease the electrical resistance of the boundary layer and thereby lead to a decrease in the activation energy of the charge carriers and an increase in the electrical conductivity of the nanocomposite.     

Effect of nitrogen on sigma formation in Cr-Ni steels at 1200°F (650°C)

The addition of nitrogen (0.10 to 0.20 pct) to Fe-Cr-Ni alloys of simulated commercial purity results in a real displacement of the σ phase boundaries to higher chromium contents. The effect is small for the (γ + σ)/γ boundary, but is pronounced for the (γ + α + σ)/(γ + α) boundary. Although there is an indication of an exceptionally large shift of the σ boundaries to higher chromium contents, especially in steels with nitrogen over 0.2 pct, the major portion of this apparent shift results from the fact that carbide and nitride precipitation cause “chromium impoverishment” of the matrices. The effect of combined additions of nitrogen and silicon to the Fe-Cr-Ni phase diagram is demonstrated also. Nitrogen can nullify the effect of about 1 pct Si in shifting the (γ + σ)/γ phase boundary to lower values of chromium at all nickel levels from 8 to 20 pct. Nitrogen can nullify this σ-forming effect of about 2 pct Si at the 8 pct Ni level, but not at the 20 pct Ni level. The alloys studied were in both the cast and the wrought conditions. There are indications that the σ phase forms more slowly in the cast alloys than in the wrought alloys if both are in the completely austenitic state. The presence of δ ferrite in the cast alloys accelerates the formation of σ. Cold working increases the rate of σ formation in both cast and wrought alloys.     

Effect of the structure and thickness of layers on the magnetic and magnetoresistive properties of <Emphasis Type="Italic">Py</Emphasis>/Co/Cu/Co nanocrystalline films

Py/Co₁/Cu/Co₂ films (Py is the Ni₆₃Co₂₅Fe₁₂ ternary permalloy) were produced by magnetron sputtering on glass substrates. The thicknesses of layers are d _Cu = 0.7–4 nm, \(d_{Co_2 } \) = 2–12 nm, and d _Py = 2–12 nm. The polar diagrams of the films with d _Cu = 1.2nm demonstrate a biaxial anisotropy, whereas the films with 2.0 ≤ d _Cu ≤ 3.0 nm are characterized by uniaxial anisotropy. The first maximum in the Δρ/ρ = f(d _Cu) curve is due to an antiferromagnetic coupling between the Py/Co₁ and Co₂ layers; the second maximum is due to the independent magnetization reversal of the Py/Co₁ soft magnetic bilayer and Co₂ hard magnetic layer. The magnetoresistance measured along the EA exhibits an almost twofold increase with increasing thickness of ferromagnetic layers. The coercive force was shown to decrease with increasing thickness of the Co₂ layer, whereas the magnetic anisotropy field is unchanged. The coercive force and induced magnetic anisotropy were estimated theoretically taking into account changes in the mean-square amplitude and roughness period with increasing thickness of the Co₂ layer. The results obtained agree with experimental data.     

Specific Features of the Frequency Variations in Magnetic Losses in Fe–3%Si {110} Crystals in Rotating Magnetic Fields

The frequency dependence of the total and eddy-current magnetic losses P_tot and P_edd for rotary magnetization reversal has been studied on an Fe–3%Si single crystal with a thickness of d = 0.10 mm within a range of frequencies f = 20–200 Hz and induction amplitudes B_m = 0.25–1.90 T. Some new specific features in the behavior of magnetic losses have been revealed. In particular, the induction that corresponds to the maximal magnetic losses in the curve P_tot = P(B_m) has been established to monotonically decrease with increasing frequency of magnetic field rotation. The specific features of the frequency variation of magnetic losses were discussed based on the observed dynamics for the domain structure of the specimen.     

Magnetotransport of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> single crystals in two regimes of variable–range hopping conduction

The resistivity, ρ(T), and the magnetoresistance (MR) of Cu₂ZnSnS₄ (CZTS) single crystals are investigated at temperatures T = 2–300 K in pulsed magnetic fields of B up to 20 T. The Mott variable–range hopping (VRH) conductivity over localized states of the defect acceptor band is observed between T ~ 50–150 K. The Shklovskii–Efros (SE) VRH conduction over the states of the Coulomb gap is found below T ~ 3–4 K. The positive MR is observed at all temperatures and magnetic fields, its value decreasing with T. In the Mott VRH conduction region, MR follows the law ln ρ(B) ∝ B ² up to the highest applied fields. The joint analysis of the resistivity and MR data in this region has yielded values of the localization radius as well as a set of important microscopic parameters, including the mobility threshold in the acceptor band, the values of the density of localized states near the Fermi level and the critical concentration of the metal–insulator transition. In the SE region, the MR law above is observed only in much smaller fields, transformed into those of lnρ(B) ∝ B ^2/3 or ∝ B ^3/4 when B increases. Such transformation, accompanied by a strong increase of the localization radius, give evidence for an important role of scattering and interference phenomena in the VRH conduction at low temperatures.