Ageing characteristics of cast Zn-Al based alloy (ZnAl7Cu3)

Microstructure and ageing characteristics of a cast Zn-Al based alloy (ZnAl7Cu3) were studied using X-ray diffraction, electron scanning microscopy and back-scattered diffraction techniques. Two stages of phase transformation, i.e., decomposition of zinc rich phase and four phase transformation, + T + , were detected during ageing at 150°C. Electron back-scattered diffraction technique was applied in distinguishing both zinc rich and phases.     

The flux-line lattice in high-Tc superconductors

The flux-line lattice in type-II superconductors has unusual nonlocal elastic properties which make it soft for short wavelengths of distortion. This softening is particularly pronounced in the highly anisotropic high-T _c superconductors (HTSC) where it leads to large thermal fluctuations and to thermally activated depinning of the Abrikosov vortices. Numerous transitions are predicted for these layered HTSC when the temperatureT, magnetic inductionB, or current densityJ are changed. In particular, the flux lines are now chains of two-dimensional (2D) pancake vortices which may evaporate by thermal fluctuations or may depin individually. At sufficiently highT, ohmic resistivity(T, B) is observed down toJ 0. This indicates that the flux lines are in a liquid state with no shear stiffness and with small depinning energy or that the 2D vortices can move independently. At lowerT, (T, B, J) is nonlinear since the pinning energy of an elastic vortex lattice or vortex glass increases with decreasingJ as predicted by theories of collective pinning and by vortex glass scaling.     

Heat Exchange on the Front Surface of a Blunt Body in a High‐Speed Flow Containing Low‐Inertia Particles

The method and results of an experimental investigation of the heat exchange on the front surface of a sphere in a supersonic flow containing particles of diameter d = 0.12–2.4 m and hollow particles of d = 160 m are presented. It has been revealed that in the case where even very small particles of d = 0.15 m are contained in an undisturbed flow and their concentration is low (of the order of a percent), the heat flux in the region of the critical point of the model markedly increases. A comparison of the experimental data with the data of calculations of the heat exchange in the region of the forward critical point which are based on the theory of a twophase laminar boundary layer has been made. A glow near the surface of the model was observed in the experiments conducted in total darkness.     

Deformation and recrystallization textures of copper single crystals and bicrystals

Recrystallization textures in rolled copper specimens have been investigated so as to elucidate the mechanisms of cube texture formation. The specimens are single crystals with the orientations corresponding to the main components of the rolled textures, such as {112}111,{110}112, etc. and bicrystals consisting of such oriented crystals. The cube texture was not observed in any single crystal specimens, but observed in only two bicrystal specimens with {112}/111{100}001 and {110}112/{100}001 orientations. The formation of cube texture seems to require the existence of a cube oriented region in the deformed state, and the favourable oriented matrix to allow the growth of such nuclei.     

The nucleation of polyethylene dendrites from solution

This paper is a study of nucleation mechanisms in the formation of polyethylene dendrites from dilute solutions. Polyethylene dendrites may be either of regular morphology or of irregular morphology (hedgehog dendrites). By varying the temperatures of dissolution and crystallisation, as well as the general impurity level of the system, it was possible to alter systematically the relative ratio of regular to hedgehog dendrites. It was found that regular dendrites could be nucleated through a self-seeded process, as well as through homogeneous nucleation. The irregular hedgehog dendrites were observed to be nucleated heterogeneously. The degree of super-cooling necessary for dilute solution homogeneous nucleation was established (30° C).     

An analysis of the mechanics of solid phase extrusion of polymers

An analysis is presented for the mechanics of the hydrostatic extrusion of polymers in the solid phase through a conical die. The analysis starts with the lower bound solution proposed by Hoffman and Sachs and includes the effects of strain, strain rate and pressure on the deformation behaviour. It is proposed that this involves knowledge of the tensile stress-strain-strain rate relationships for each polymer, and it is shown how such information for polyethylene and polyoxymethylene can be used to explain the observed behaviour of these materials in the solid-phase extrusion process.List of Symbols die cone semi-angle - normal stress coefficient of tensile flow stress - d ₀ initial diameter of billet - d _f die exit diameter - axial strain rate (plug flow) - _red shear strain (redundant strain) incurred on crossing die entry or exit boundary - _N In R _N=2ln (d ₀/d _f) nominal true strain in extrusion - _{f red}+ _N - L cot - normal stress coefficient of friction at die/billet interface - P experimental extrusion pressure=total work done per unit volume of material - P _F work done per unit volume against billet-die friction - P _I ideal deformation work done per unit volume of material - P _R total redundant work done per unit volume - P _W=P–P _I extra work required to overcome friction, pressure and redundant strain effects - r ₀ initial radius of billet - r _f die exit radius - r material radius at a point in the deformation zone - R _N=(r ₀/r _f)² nominal extrusion ratio - R=(r ₀/r)² extrusion ratio at a point in the deformation zone - ₀() axial tensile flow stress - _f() process flow stress path, related to die strain and strain rate fields - _h tensile haul-off stress - _x, _y die stresses in deformation zone - ₁, ₂ shear yield stress of material at die entry and exit boundaries, respectively - v _x axial velocity - v _f extrudate velocity at die exit     

Use of Asymptotic Methods for Calculation of a Turbulent Boundary Layer

Using the method of joined asymptotic expansions and the twolayer scheme of incompressiblefluid flow in a turbulent twodimensional boundary layer, we have derived relations for calculating the coefficient of turbulent friction and the distribution of the Reynolds stress over the cross sections of the layer in the first asymptotic approximation. It is shown that in the zone of a defect on bodies having a relatively large disturbancesurface curvature the velocities should be separated into vortex velocities, which are due to the coherent structures, and potential velocities, caused by the transverse pressure gradient. From the available experimental data on the structure of the flow we inferred that the redundantvelocity profile obtained in the limiting asymptotic approximation, which, in this case, as in the case of selfsimilar (equilibrium) regime of flow, is locally dependent on only the Clauser parameter, is universal.     

A method of reducing the severity of the thermal stress state in blades of gas turbine engines by selecting the optimum distribution of the thickness of a ceramic thermal barrier coating

On the basis of a complex solution of the problems of thermal conductivity and thermal elasticity for cooled turbine blades with ceramic thermal barrier coatings, the authors propose a method of selecting local coating thicknesses which make it possible to reduce the level of temperature and thermal stresses in the blade in both non-steady and steady operating conditions. The method is based on using LCB _T-search for multiparametric problems.The work was carried out taking into account the requirements of the project Justification of the possibility of using new high-temperature materials, including those with coatings, for increasing the economic parameters of aviation and ship engines of the program 5.2.3 Increasing the reliability, service life and preventing catastrophic failures of transport gas turbine engines of the state committee of the Ukraine for the problems of science and technology.Translated from Problemy Prochnosti, No. 1, pp. 62–67, January, 1994.     

Mixed convection flow of micropolar fluid over an isothermal plate with variable spin gradient viscosity

Summary A steady two-dimensional mixed convection flow of viscous incompressible micropolar fluid past an isothermal horizotal heated plate with uniform free stream and variable spin-gradient viscosity is considered. With appropriate transformations the boundary layer equations are transformed into nonsimilar equations appropriate for three distinct regimes, namely, the forced convection regime, the free convection regime and the mixed convection regime. Solutions of the governing equations for these regimes are obtained by an implicit finite difference scheme developed for the present problem. Results are obtained for the pertinent parameters, such as the buoyancy parameter, in the range of 0 to 10 and the vortex viscosity parameters, =0.0, 1.0, 3.0, 5.0 and 10.0 for fluid with Prandtl number Pr=0.7 and are presented in terms of local shear-stress and the local rate of heat transfer. Effects of these parameters are also shown graphically on the velocity, temperature and the couple stress distributions. From the present analysis, it is observed that both the momentum boundary layer and the thermal boundary layer increase due to an increase in the vortex viscosity of the fluid.List of symbols f, F, dimensionless stream function for forced convection free convection and mixed convection, respectively - g acceleration due to gravity - Gr_x local Grashof number - j micro-inertia density - m ₂₃ distribution of couple stress - N microrotation component normal to (x, y)-plane - p pressure of the fluid - q dimensionless rate of heat transfer - Re_x local Reynolds number - T temperature of the fluid in the boundary layer - T temperature of the ambient fluid - T temperature at the surface - u, v thex andy-components of the velocity field - U free stream velocity - x, y axis in direction along and normal to the plate Greek thermal diffusivity - coefficient of volume expansion - vortex viscosity parameter - stream function - , , nondimensional similarity variables - buoyancy parameter (=Gr _x Re _x ^/5/2 ) - vortex viscosity - density of the fluid - v kinematic coefficient of viscosity - spin-gradient viscosity - stream function - dimensionless skin-friction - fluid viscosity     

Numerical Analysis of the Effect of Viscosity on the Vortex Dynamics in Laminar Separated Flow Past a Dimple on a Plane with Allowance for Its Asymmetry

Based on numerical solution of the Navier–Stokes threedimensional stationary equations by a factorized finitevolume method, the influence of physical viscosity on selforganizing jetvortex structures in a dimple on a plane immersed in a laminar flow is analyzed with allowance for the asymmetry of the dimple shape.     

Superfluid Spin-down with Random Unpinning of the Vortices

The so-called creeping motion of the pinned vortices in a rotating superfluid involves random unpinning and vortex motion as two physically separate processes. We argue that such a creeping motion of the vortices need not be (biased) in the direction of an existing radial Magnus force, nor should a constant microscopic radial velocity be assigned to the vortex motion, in contradiction with the basic assumptions of the vortex creep model. We point out internal inconsistencies in the predictions of this model which arise due to this unjustified foundation that ignores the role of the actual torque on the superfluid. The proper spin-down rate of a pinned superfluid is then calculated and turns out to be much less than that suggested in the vortex creep model, hence being of even less observational significance for its possible application in explaining the post-glitch relaxations of the radio pulsars.     

Precipitation hardening in Cu 1.81 wt % Be 0.28 wt % Co

The structure of the precipitation hardening alloy Cu 1.81 wt % Be 0.28 wt % Co has been studied as a function of ageing temperature and time, by transmission electron microscopy. The continuous precipitation sequence found is: supersaturated solid solution G.P. zones .The G.P. zone is an ordered platelet precipitate, which is coherent on {100} matrix planes and is nucleated in very high densities (>10²⁴ m^–3). The coherency stress fields, due to the misfit of the G.P. zone and matrix, overlap to produce a net matrix contrast along {110} 10, and give the characteristic tweed structure, which can be described by the kinematical theory of diffraction. The semi-coherent intermediate precipitate is nucleated by the G.P. zones and the transformation is characterized from the changes in the arrowhead structure produced in the electron diffraction patterns. No transformation of to the equilibrium precipitate is found for the ageing times investigated.     

The Interplay of Electronic and Nuclear Magnetism in PtFe x at Milli-, Micro-, and Nanokelvin Temperatures

We have measured ac susceptibility, nuclear magnetic resonance, and nuclear heat capacity of two PtFe _x samples with concentrations of magnetic impurities x = 11 ppm and 41 ppm at magnetic fields (0 ± 0.05) mTB248 mT. The susceptibility data have been measured at temperatures of 0.3 KT100 mK, no hint for nuclear magnetic ordering could be detected to a temperature of 0.3 K. The nuclear heat capacity data taken at 1.4 KT10 mK show enhanced values which scale with x at low polarization. This effect is described by a model assuming an internal magnetic field caused by the impurities. No indication for nuclear magnetic ordering could be detected to 1.4 K. The nuclear magnetic resonance experiments have been performed on these samples at 0.8 KT0.5 mK and 2.5 mTB22.8 mT as well as on three other samples with x = 5, 10, 31 ppm in a different setup at 40 KT0.5 mK and at 5.4 mTB200 mT. Spin-lattice and effective spin-spin relaxation times ₁and ₂ ^* of ¹⁹⁵ Pt strongly depend on x and on the external magnetic field. No temperature dependence of ₁and ₂ ^* could be detected and the NMR data, too, give no hint for nuclear magnetic ordering to 0.8 K.     

A finite volume technique to simulate the flow of a viscoelastic fluid

Hydrodynamically developing flow of Oldroyd B fluid in the planar die entrance region has been investigated numerically using SIMPLER algorithm in a non-uniform staggered grid system. It has been shown that for constant values of the Reynolds number, the entrance length increases as the Weissenberg number increases. For small Reynolds number flows the center line velocity distribution exhibit overshoot near the inlet, which seems to be related to the occurrence of numerical breakdown at small values of the limiting Weissenberg number than those for large Reynolds number flows. The distributions of the first normal stress difference display clearly the development of the flow characteristics from extensional flow to shear flow.List of symbols D rate of strain tensor - L slit halfheight - P pressure, indeterminate part of the Cauchy stress tensor - R the Reynolds number - t time - U average velocity in the slit - u velocity vector - u,v velocity components - W the Weissenberg number based on the difference between stress relaxation time and retardation time - W ₁ the Weissenberg number based on stress relaxation time - x,y rectangular Cartesian coordinates - ratio of retardation time to stress relaxation time - zero-shear-rate viscosity, ₁ + ₂ - ₁ non-Newtonian contribution to - ₂ Newtonian contribution to - ₁ stress relaxation time - ₂ retardation time - density - (, , ) xx, yy and xy components of ₁, respectively - determinate part of the Cauchy stress tensor - ₁ non-Newtonian contribution to - ₂ Newtonian contribution to      

Morphological and crystallographic features of γ-γ′ Ni-Al and Ni-Al-Ti two-phase bicrystals

Several- Ni-Al and Ni-Al-Ti two-phase bicrystals were made by the solid-state diffusion couple method. Each couple consisted of a-phase single crystal and a pure-Ni polycrystal, and was annealed at 1473 K in an Ar gas atmosphere. Single crystal layers of-phase with uniform thickness always grow into the parent-phase single crystals. The resultant/ interface has no voids or facets regardless of the orientation of interface and the chemical composition of the-phase. Porosity formation due to the Kirkendall effect is observed in the diffused region. Concentration profiles exhibit nearly constant gradients in-phase. The orientation relationship between both phases is found to be 001//001, that is, the-phase grows epitaxially along the crystal orientation of-phase.     

Synthesis of the mineral geikielite MgTiO3

The preparation of pure geikielite, MgTiO₃, is carried out by precipitation with NaOH solution at pH 12 from solutions Ti⁴⁺/Mg²⁺ in molar ratio 1/1 M and calcination of the precipitate at 600°C in a dynamic atmosphere: heating rate 300°Ch^–1.At room temperature and a 1 KHz of frequency the dielectric constant () and the electric resistivity () were measured. The values were = 1 5.0 and = 1.1×10¹⁰ cm^–1.     

Numerical Analysis of the Jet‐Vortex Pattern of Flow in a Rectangular Trench

Numerical simulation of the vortex structure of threedimensional laminar flow in a rectangular trench of square cross section has been carried out on the basis of the finitevolume solution of steadystate Navier–Stokes equations.     

Magnetic field dependence of the density of states of YBa2Cu3O6.95: Implications for the vortex structure

The total specific heat of YBa₂Cu₃O_6.95 single crystals includes contributions from phonons and spin-1/2 particles, as well as electronic contributions. The electronic specific heat is described by a quadratic term T² in zero field and a linear term [(0)+(H)]T which is increased when a magnetic field H is applied perpendicular to the CuO₂ planes. In agreement with d-wave superconductivity, we find that _n/T_c and (H)_n(H/H_c2)^1/2, where _n is the coefficient of the normal-state linear term. The H^1/2 dependence of the density of states at the Fermi level was predicted by G. Volovik for lines of nodes in the gap: the quasiparticles which contribute to this density of states are close to the nodes in momentum space and are located outside the vortex core.     

Stability of metastable phases and microstructures in the ageing process of Al–Mg–Si ternary alloys

Precipitation behaviour of Al–Mg–Si alloys, with balanced (Mg/Si=2), excess silicon (Mg/Si<2) and excess magnesium (Mg/Si>2) compositions, were studied by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and Vickers hardness tests. Four significant exothermic peaks were observed in DSC curves which were attributed to metastable clusters, , and stable phases. The peaks corresponding to and were formed closely in the DSC curves but showed different behaviour in isothermal annealing. The additional peak verifying the precipitation of phases, which has recently been proposed by some workers, was not detected. Transmission electron microscope observations and Vickers microhardness tests showed that precipitates played a major role in improving the hardness, but not precipitates. © 1998 Chapman & Hall     

The temperature of an adiabatic surface at a turbulent boundary layer

A calculation of the temperature decrease of an adiabatic surface at a supersonic turbulent boundary layer is conducted. It is shown that the temperature decrease is a consequence of the appearance of a vortex chain in the flow near the walls. Comparison of calculated data with experimental gives qualitative agreement.Notation V incident flow velocity - V_v vortex velocity - v local velocity - u induced velocity - T thermodynamic temperature - T_w, T recovery temperature and undisturbed flow temperature - L length of depression - h₀ depth of depression - h distance from vortex center to wall - b relative vortex velocity - l _v vortex spacing - r recovery coefficient - R₀ recovery coefficient on smooth surface - c_p gas heat capacity at constant pressure - n vortex passage frequency - Re Reynolds number - M Mach number - velocity potential - time - vortex intensity Translated from Ihzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 5, pp. 903–908, May, 1971.