Ageing characteristics of furnace cooled eutectoid Zn-Al based alloy

Phase transformations and microstructural changes of a furnace cooled eutectoid Zn-Al based alloy were studied during ageing at 100 and 170°C using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Three phase transformations occurred in the furnace cooled eutectoid Zn-Al alloy. The metastable _FC phase decomposed during isothermal ageing. The four-phase transformation, + T + followed the discontinuous decomposition of the _FC phase. Typical morphologies of the decomposition of the _FC and phases were observed in scanning electron microscopy. Decomposition of Al-rich phase was observed during the prolonged ageing by transmission electron microscopy. The different types of decomposition of the different metastable phases dominated at different stages of ageing.     

Fatigue crack growth behaviour of tungsten carbide-cobalt hardmetals

Fatigue crack propagation studies have been carried out on a range of WC-Co hardmetals of varying cobalt content and grain size using a constant-stress intensity factor double torsion test specimen geometry. Results have confirmed the marked influence of mean stress (throughK _max), which is interpreted in terms of static modes of fracture occurring in conjunction with a true fatigue process, the existence of which can be rationalized through the absence of any frequency effect. Dramatic increases in fatigue crack growth rate are found asK _max approaches that value of stress intensity factor ( 0.9K_IC) for which static crack growth under monotonic load (or static fatigue) occurs in these materials. Lower crack growth rates, however, produce fractographic features indistinguishable from those resulting from fast fracture. These observations, and the important effect of increasing mean free path of the cobalt binder in reducing fatigue crack growth rate, can reasonably be explained through a consideration of the mechanism of fatigue crack advance through ligament rupture of the cobalt binder at the tip of a propagating crack.     

Characterization of a δ/γ duplex stainless steel

A duplex stainless steel was investigated in both as-received sheet and after annealing at temperatures ranging from 850 to 1100°C. The sheet presents a deformation texture in both phases, austenite and ferrite, induced by cold rolling. Microstructure in the as-received material consists of island-like austenitic grains in a ferrite matrix. These austenitic grains are elongated with an average size of 6, 20 and 40 m along the normal (ND), transversal (TD) and rolling direction (RD). Quantitative texture measurements demonstrated that texture components are distributed mainly along the -fiber (ND ‹100›) and -fiber (RD ‹110›) for the ferrite and the -fiber (ND ‹110›) for the austenite. After recrystallization, a decrease in the intensity of the mean fibers and an increase in the minor components was observed in both, ferrite and austenite. Therefore, a similar texture was reached in both phases after annealing at 1050°C. Microstructural characterization after annealing at temperatures above 850°C showed that the elongated austenitic grains transform in colonies of equiaxic grains of about 10–15 m in size. These colonies are surrounded by a ferritic matrix at annealing temperatures above 1000°C or by a laminar microstructure at temperatures below 950°C. This laminar microstructure includes sigma phase and austenite formed from delta ferrite, and untransformed delta ferrite.     

Effect of strain rate on sigma formation in ferrite-austenite stainless steel at high temperatures

Sigma precipitation in an Fe-19Cr-5Ni-2.7Mo ferrite -austenite duplex stainless steel has been investigated during and after uniaxial compression at 900° C. At this temperature ferrite and austenite phases are present in equivolume proportions (1:1 ). It was found that sigma precipitation was enhanced by hot working and the volume proportion of sigma decreased as the straining rate increased. Sigma nucleated at ferrite-austenite interfaces and grew into ferrite grains by the eutectoid reaction + . Transmission electron microscopy revealed that large sigma particles enhanced recrystallization of the austenite phase in duplex structures at high temperature.     

Size-strength effects in sapphire and silicon nitride whiskers at 20° C

Tensile tests at 20° C have been carried out on seventy-three sapphire whiskers and on seventeen silicon nitride whiskers. The sapphire whiskers were of 0001, 1¯120, 10¯10, and 10¯11 orientations, while the silicon nitride whiskers were 0001, 11¯20, and 10¯13. Tensile strengths were in the range 45 to 1500 kg/mm², and deformation was found to be purely elastic. The tensile strength data have been analysed and fitted to empirical equations describing the effect of size on strength for different orientations. These empirical equations have been used to deduce possible fracture nucleation mechanisms. It is concluded that, in the case of 0001 sapphire whiskers, fracture nucleation may be due to dislocation pile-ups or interactions, while in the other cases a Griffith flaw mechanism is probably applicable.     

Precipitate strengthening mechanisms in magnesium zinc alloy single crystals

The mechanical behaviour of Mg-5.1 wt % Zn alloy single crystals was studied in the 4.2 to 300° K temperature range. Quenched crystals have activation energies and volumes best associated with the cutting of small clusters of Zn atoms by dislocations. Fully hardened crystals contain fine ₁ and occasional ₂ precipitates with an average ₁ interparticle spacing of 330 to 660 Å. Strengthening in these crystals is mainly ascribed to the cutting of ₁ particles by dislocations. In the overaged condition ₁, ₂ and equilibrium particles are present and lead to a considerable temperature-dependence unusual for an overaged condition. Analysis of this temperature-dependence suggests that below 77° K the relatively easy cutting of ₁ particles by dislocations takes place in addition to the cutting of ₂ and particles. Above 77° K the difficult cutting of ₂ and particles alone controls the deformation, ₁ being more easily cut with the aid of thermal fluctuations.     

Interfacial and mechanical properties of environment-friendly “green” composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin

Physical and tensile properties of pineapple fibers were characterized. Tensile properties of pineapple fibers, like most natural fibers, showed a large variation. The average interfacial shear strength between the pineapple fiber and poly(hydroxybutyrate-co-valerate) (PHBV) was 8.23 MPa as measured by the microbond technique. Scanning electron microscopy (SEM) photomicrographs of the microbond specimens revealed an adhesive failure of the interface. Fully degradable and environment-friendly green composites were prepared by combining pineapple fibers and PHBV with 20 and 30% weight content of fibers placed in a 0°/90°/0° fiber arrangement. Tensile and flexural properties of these green composites were compared with different types of wood specimens. Even though tensile and flexural strength and moduli of these green composites were lower than those of some wood specimens tested in grain direction, they were significantly higher than those of wood specimens tested in perpendicular to grain direction. Compared to PHBV virgin resin, both tensile and flexural strength and moduli of these green composites were significantly higher. SEM photomicrographs of the fracture surface of the green composites, in tensile mode, showed partial fiber pull-out indicating weak bonding between the fiber and the matrix.     

Thermal Conductivity of the Four-Leg Spin-Ladder System La2Cu2O5 Single Crystal

We have investigated the magnetic susceptibility, , and the thermal conductivity, , in magnetic fields for the four-leg spin-ladder system La₂Cu₂O₅ single crystal. The in a magnetic field parallel to the ladder exhibits a kink at 130 K in correspondence to the magnetic ordering. The along the ladder exhibits a peak at 25 K and a shoulder at 14 K, which are probably related to the thermal conductivity due to magnons, _magnon, and that due to phonons, _phonon, respectively. The perpendicular to the ladder, on the other hand, exhibits only one broad peak related to _phonon. The observed large anisotropy of has been explained based upon the anisotropy of _magnon.     

Nonisothermal film flow of a magnetizable liquid

The problem of the flow of a nonisothermal magnetizable liquid with a free surface in a nonuniform magnetic field is formulated and investigated theoretically by considering a specific example.Notation H magnetic field intensity - M magnetization - _o magnetic permeability of vacuum - I current (r, , z), cylindrical coordinates - (, gz) coordinates of free surface - R radius of current-carrying conductor - p pressure - v axial component of velocity - viscosity - R₁, R₂ principal radii of curvature of surface - surface tension - Q flow rate of liquid - G characteristic value of gradient of magnetic field intensity - density - g acceleration due to gravity Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 37, No. 5, pp. 881–885, November, 1979.     

Distribution of α-AlFeSi and β-AlFeSi particles in surface layer of AA6063 alloy billets after heat treatment

We developed the EPMA mapping method of small -AlFeSi(Al_8.3Fe₂Si) and -AlFeSi(Al_8.9Fe₂Si₂) particles in the billets of Al-Mg-Si alloys such as AA6063 alloys. To discriminate between -AlFeSi and -AlFeSi particles we used the relative X-ray intensities of Fe/Si ratio, the I _Fe/I _Si ratio, instead of the Fe/Si mass ratio. To obtain the I _Fe/I _Si ratio, we used a Monte Carlo method. In this study, using this method the mapping of -AlFeSi and -AlFeSi particles in the surface layer of AA6063 billets after the heat treatment (for 2 h at 580°C) was done. Namely, the distribution of -AlFeSi and -AlFeSi particles of zones from the billet surface to a depth of 800 m was measured. Results showed the zone from the surface to a depth of 200 m was occupied mainly by -AlFeSi particles and the zone from a depth of 200 m toward the center was occupied mainly by -AlFeSi particles. From these results, it was found that if we remove zones from the surface to a depth of 200 m, we can remove the majority of the -AlFeSi particles, and thus improve the quality of anodizing performance of Al-Mg-Si alloys extrusions.     

Phase decomposition of liquid-quenched β-type Ti-Cr alloys

Phase decomposition behaviour of liquid-quenched (bcc) type Ti-Cr alloys was investigated by means of transmission electron microscopy and hardness measurements. It was found that decomposition of to ₁ (Ti-rich, bcc) + ₂ (Ti-lean, bcc) takes place in the intermediate composition range of the Ti-Cr system. This experimental result proves the theoretical prediction made by Menon and Aaronson, but the observed ₁ + ₂ two-phase field expands towards higher temperatures than the predicted binodal line. The coherent ₁ + ₂ two-phase state exhibits the so-called 100 modulated structure and it was concluded that the formation of such a structure is a result of spinodal decomposition of the -phase. We obtained time-temperature-transformation (TTT) diagrams of -type Ti-30, 40 and 50 at % Cr alloys. A typical sequence of structural change is coherent ₁ + ₂ incoherent ₁ + ₂ incoherent ₁ + ₂ + grain boundary precipitates stable state of + TiCr₂ or + TiCr₂. Not all the states in the above sequence appear, depending on alloy composition, liquid-quenching rate and ageing temperature.     

Universal simulation of degenerating homogeneous turbulence

The problem of universal simulation of the dynamics of a turbulent velocity field (universal in the sense of arbitrary values of the Reynolds turbulence number) is treated on the basis of the moment model in the second approximation.Notation ¯q² _i ² double the kinetic turbulence energy - _u ² =5v¯q²/_u Taylor turbulence scale squared - _u=v1/x_k)²> kinetic-energy dissipation function - N_Re,=¯q²_u / Reynolds turbulence number Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 42, No. 1, pp. 46–52, January, 1982.     

Where Are We in HTCS?

Address given at the closing of the conference on Major Trends in Superconductivity in the New Millennium, MTSC 2000 and the opening of the Symposium on Itinerant and Localized States in HTCS SILS in HTCS.     

Theoretical estimation of the effect of interfacial energy on the mechanical strength of spinodally decomposed alloys

New interfaces are produced on the slip plane when a crystal with continuous composition fluctuation arising from spinodal decomposition is deformed by slip. In this work, the energy of such interfaces is evaluated for both modulated and mottled structures, and their effects on slip behaviour are discussed. It is concluded that the contribution of this interfacial energy is large enough to account for the age-hardening concomitant with spinodal decomposition.     

New results on the Fermi surface of β brass type alloys

De Haas-van Alphen frequencies not previously reported have been measured for -PdIn and -CuZn. The results for PdIn suggest a reinterpretation of some of the previous results and offer an explanation of a previously unexplained set of de Haas-van Alphen frequencies in PdIn and AgZn. A recent nonlocal pseudo-potential model by Taylor can account for the experimental results reported here as well as the earlier data.National Research Council of Canada Postdoctorate Fellow.     

Radiation of internal waves during vertical motion of a body through a nonuniform liquid

Energy losses to radiation of internal waves during the vertical motion of a point dipole in two-dimensional and three-dimensional cases are computed.Notation _o(z), p_o(z) density and pressure of the ground state - z vertical coordinate - v, p, perturbed velocity, pressure, and density - H(d 1n _o/dz)^–1 characteristic length scale for stratification - N=(gH^–1–g²c _o ^–2 )^1/2 Weisel-Brent frequency - g acceleration of gravity - c_o speed of sound - vertical component of the perturbed velocity - V vector operator - k wave vector - frequency - d vector surface element - W magnitude of the energy losses - (t), (r) (x)(y)(z) Dirac functions - v_o velocity of motion of the source of perturbations - d dipole moment of the doublet - _o,l length dimension parameters - _o intensity of the source Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 619–623, October, 1980.     

Influence of Multiple‐Scattering Processes on the Laser Probing of Biological Tissues

Theoretical aspects of multiplescattering processes in laser probing of biological tissues have been considered. The method of digital dynamic specklephotography has been described. The results of experimental studies of the nearsurface blood flow and stressedstrained states in a pinstructure–toothroot model are presented.     

Effect of annealing upon decomposition of electrodeposited iron-zinc alloy coatings

As-deposited electrodeposited iron-zinc alloy coatings containing phase, decompose upon heating through a sequence of metastable phases. The h c p phase transforms to b c c G, or -like phase via a rapid diffusional phase transformation in the vicinity of 150 °C. For bulk iron contents of 8–13 wt%, transforms to 100% G phase. The G phase subsequently transforms at 240 °C to phase, which in turn transforms to or ₁ phase near 300 °C by depletion of iron from the surrounding matrix. The decomposition process may be driven by supersaturation of with iron.     

Flux motion in anisotropic type-II superconductors nearH c 2 with arbitrary vortex orientation

Flux motion in anisotropic type-II superconductors near H_{c 2} is studied in the framework of the time-dependent Ginzburg-Landau (TDGL) theory for the case that the average flux densityB is oriented in an arbitrary direction relative to the principal axes of the sample. The linearized TDGL equation for a uniformly translating order parameter is solved and expressions for all elements of the flux-flow resistivity tensor _ij (including the off-diagonal Hall elements) are obtained. The diagonal elements _ii show the angular scaling property that _ii(B) = _ii(B/H_{c 2}(, ø)), whereas the Hall elements _ij (i j) have additional angular dependences that are not contained in H_c2. For the case that the normal state Hall elements _ij ⁽ⁿ⁾ B _k with i j k, the ratios _ij/ _ij ⁽ⁿ⁾ are functions of B/H_c2 (, ) only.     

Deformation approach to the evaluation of the period of initiation and growth of fatigue macrocracks

We performed experimental investigation of the opening displacements of the contours of stress concentrators (notches and cracks) for various amplitudes of cyclic loading. On the basis of experimental results, we propose a new deformation parameter _t which is a function of the notch (crack) tip opening displacement , namely, _t /(+d*), where is the radius of the tip of the notch andd* is the characteristic size of the prefracture zone. It is shown that this parameter uniquely determines the number of cyclesN _l to the initiation of a fatigue macrocrack independently of the geometry of the specimens and stress concentrators in elastic and elastoplastic materials, i.e., the dependence of _t onN ₁ is a characteristic of the material. It is experimentally demonstrated that this dependence enables one to quantitatively describe the process of fatigue fracture both in the stage of initiation of macrocracks and their propagation.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 5, pp. 7–21, September – October, 1995.