The effect of nitrogenation on the giant magnetoresistance (GMR) and microstructure of (Ni0.8Fe0.2)20Ag80 thin films prepared by d.c. reactive magnetron sputtering has been investigated on a superconducting quantum interference device (SQUID) at 4.2K and at room temperature. Also, the phase constitution and microstructure of these thin films have been studied via transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The magnetoresistance (MR) ratio of (Ni0.8Fe0.2)20Ag80 was found to be 25% at 4.2K and 5.4% at room temperature. When nitrogen was introduced during sputtering and keeping the other processing parameters the same, the change in the MR ratio of such prepared specimens was one half of the pure counterpart at room temperature. However, the saturation field was markedly lowered from 5T to about 1T and at the same time the differential sensitivity improved markedly. The MR ratio became larger with increasing annealing temperature. XPS results show that the core electron levels of Ag 3d3/2, Ag 3d5/2 and Ni 2p3/2 of the nitrogenated specimens shift to slightly higher energy levels on nitrogenation. At the same time, an additional shoulder on the satellite Ni 2p3/2 peak shows up around 861eV. Such XPS observations indicate nitride formation during sputtering under nitrogen atmosphere. The nitrides and microstructural changes seem to have caused the GMR and saturation field to change.
The effect of various milling parameters such as, milling intensity, ball:powder weight ratio and number of balls on the glass forming ability of an elemental blend of composition Ti50Ni50 has been studied by mechanical alloying. In order to understand the results, all the milling parameters have been converted into two energy parameters, namely, impact energy of the ball and the total energy of milling. In a milling map of these two parameters, the conditions for amorphous phase formation have been isolated. A similar exercise has been carried out for Ti50Cu50 as a function of milling time at two milling intensities. The results indicate that a minimum impact energy of the ball and a minimum total energy are essential for amorphization by mechanical alloying.
Internal stresses in planar random alumina fibre reinforced aluminium with a range of fibre volume fractions have been studied theoretically and in tensile tests and cyclic Bauschinger experiments at room temperature and 77 K. The Eshelby S tensor for a planar random array of fibres is calculated, which allows the mean field model to be used to predict the internal stresses. The conventional Orowan-Wilson method of analysing cyclic Bauschinger experiments is modified, enabling the plastically and thermally induced matrix mean stresses to be separated. This analysis is applied to experimental results and the plastic mean stress and the initial magnitude of the thermal mean stress in the matrix are measured. The results for the thermal matrix mean stress are compared with measurements from monotonic flow curves and generally good agreement is observed. The measured thermal matrix mean stress in these composites is approximately independent of fibre volume fraction.
The superplastic properties of several mechanically-alloyed materials. including IN9021, IN9052, IN905XL and 15 vol.% SiCp/IN9021, have been characterized. The tensile elongation to failure, strain rate sensitivity and activation energy are found to be dependent upon the testing temperature. Specifically, marked changes in these properties are closely related to incipient melting points in the materials. These observations confirm the suggestion that the presence of a small amount of liquid phase at interfaces and grain boundaries not only enhances the strain rate for superplasticity, but also has a strong influence on the deformation mechanisms. When the amount of liquid phase increases, as a result of increasing temperature, the expected observation is noted, i.e. the presence of the liquid phase degrades the material properties. A model is proposed to explain the experimental observations.
Residual thermal stresses in graphite/aluminum fiber-reinforced metal—matrix composites is studied using three-dimensional finite element modeling and assuming the anisotropic thermal expansion coefficient of graphite fiber. The finite element results are compared with the two-cylinder model results of Vedula et al. and the experimental results of Tsai et al. During cooling from the processing temperature, the anisotropy of the graphite fiber results in the increase of the temperature range over which the elastic deformation of the matrix occurs. The effect of fiber volume fraction on the distribution of hoop and radial stresses at the fiber—matrix interface is also investigated. In the square and hexagonal array arrangements of fibers, the possible sites for the crack initiation are the points where the fiber—matrix interface intersects the lines joining the fiber center to its first nearest neighbors.
A constitutive description of creep curves is derived from the time evolution of the internal stress in creep determined by means of strain transient dip test technique and from the kinetic equation between this stress and the instantaneous creep rate. Several constitutive equations have been tested on creep curves of aluminium. It is shown that fitting of the curves and its statistical judgement cannot be used as the only criterion for an investigation of the evolution and role of the internal stress in creep. Such procedure must be completed by another independent procedure. As an appropriate procedure, the measurement of the internal stress in the steady state creep has been successfully applied.
An internal stress has been introduced by the blocking of grain boundary sliding at 548 K by small coherent γ-Fe particles on a small angle boundary in bicrystals of a Cu-Fe alloy. The martensitic transformation in larger γ-Fe particles is more easily induced by the internal stress during cooling to 77 K. The effect of plastic strain on the transformation is completely retarded. Some specific martensite variants are formed preferentially and the results can be well explained by postulating that the internal stress aids the lattice shear deformation involved in the f.c.c. to b.c.c. lattice change.
Polymer composites with 3D woven graphite fiber reinforcement (3D interlock weaves) have been tested in compression-compression fatigue under load control. As under monotonic loading, the principal mechanism of failure is kink band formation in the primary load bearing tows. Observations of kink bands and microcracking in sectioned specimens suggest that fatigue progresses by the accumulation of damage to the resin within individual tows. It is conjectured that resin damage leads to failure by lowering the critical stress for kink band formation on a single cycle. If resin damage is assumed to accumulate at a rate proportional to some power of the local axial shear stress in a misaligned tow, then a simple formula follows for the cycles to kink band formation. Under load control, only a few kink bands are required for specimen failure. Then the formula is also the basis for estimates of fatigue life. Fatigue life data and measured misalignment angles, which determine the local axial shear stress, support the fatigue model.
The concentration dependencies of the interdiffusion coefficient, ($) over tilde D(c), have been determined with the aid of electron probe microanalysis at 11 temperatures above and below the temperature T-ord of the Al-Ll(2) (Cu3Au) ordering transition. At temperatures T > T-ord there are minima on these dependencies near the Cu3Au composition. Slightly below T-ord the ($) over tilde D(c) dependencies are monotonous. At T < (T-ord - 20 K) maxima appear on ($) over tilde D(c) which grow with decreasing temperature. Such a behaviour is explained by the concentration dependence of the thermodynamic factor Phi. Dependencies Phi (c, T) have been calculated within the framework of the tetrahedron approximation of the cluster variation method. The calculated dependencies are in good agreement with the experimental data.
The residual stresses generated by Vickers indentation in brittle materials and their changes due to annealing and surface removal were studied in 4 mol% yttria partially stabilized zirconia (4Y-PSZ). Three experimental methods to gain information about the residual stress field were applied: (i) crack profile measurements based on serial sectioning, (ii) controlled crack propagation in post indentation bending tests and (iii) double indentation tests with smaller secondary indents loccated around a larger primary impression. Three zones of different residual stress behavior are deduced from the experiments. Beneath the impression a crack free spherical zone of high hydrostatic streses exists. This “core” zone is followed by a transition regime where indentation cracks develop but still experience hydrostatic stresses. Finally, in an outward third zone, the crack contour is entirely governed by the tensile residual stress intensity (elastically deformed region). Annealing and surface removal reduce this crack driving stress intensity. The specific changes of the residual stresses due to the post indentation treatments are described and discussed in detail for the three zones.
The effect of large-scale yielding at warm prestressing (WPS) is investigated, and it is found that the small-scale yielding solutions can completely represent the actual toughness obtained after WPS in a wide range of WPS level. Both Je and Jw can be regarded as the parameter to explain the WPS phenomenon.
A new method for solving arbitrary-shaped inhomogeneity problems has been developed. The approach combines boundary integral equations, based on Betti's princple, with a sequence of cutting, straining, and welding procedures to numerically acquire stress and strain distributions at an inhomogencity with an arbitrary shape. Physically, the strain misfit is considered as the driving force to cause the stress concentration when a system comprised of an infinite domain and an inhomogeneity is subjected to a far-field stress. A general discussion of the effect of the inhomogeneity shape on fracture initiation has been included.
Based on the random walk of point defect in concentrated alloys, the kinetics of the annihilation of point defects in a f.c.c. alloy is studied by computer simulation. The effect of the degree of order is especially considered by using the pair model for describing the atomistic interaction. The simulations cover nearly the whole temperature range in which short range order or short range clustering exists. Due to the special treatments, the escape probability for various trap structures (0D or 1D and with different reaction radius) can be determined as a function of the annealing time. Further evaluation yields the trapping time of point defects for broad variation of relevant parameters including the composition of alloy and the mobility ratio of the components. Empirical functions are developed for expressing these dependences.Compared to the random alloy simplification reported in the preceding paper [1], there is no change in the essential form of the trapping kinetics. The relation between the activation energy for the annihilation of point defects and that for the self-diffusion of atoms in real alloys is revealed, which yields the background for studying the diffusion properties through the investigation of point defect annihilation.
We demonstrate that the seemingly complex stacking sequence changes observed in topologically close-packed phases can be interpreted in terms of the ground state phases derived from a simple one-dimensional stacking model involving only a few interplanar interactions. We also show that the energetics of creating various possible stacking defects in such phases can be assessed from this model.
In many industrial applications, like high precision force measurements or nanopositioning, the elastic and dimensional stabilities of materials are required at a nanometric scale. Therefore, some aspects of room temperature creep of commercial Al-Zn and Al-Cu alloys have been studied. The anelastic creep was measured at room temperature by means of a high resolution laser interferometer. The irreversible component of the deformation was quantified by measuring the viscoelastic after-effect. The anelastic relaxation of Al-Zn has been studied as a function of annealing time and temperature. The results are correlated with the size and the density of precipitates and the extension of the precipitate free zone. The influence of a surface treatment (sandblasting, electropolishing, Cu-galvanizing, chroming, Zn-plating) on anclastic relaxation has also been considered and explained in terms of the anelastic and elastic properties of both the surface layer and the bulk.
The crystallization of minory amorphous constituents in liquid phase sintered ceramics, for example in Si3N4, is usually accompanied by a volume change. The resulting mismatch between crystallizing second phase inclusions and the surrounding matrix of the primary phase leads to the formation of transformation stresses. The strain energy stored in the stress field reduces the thermodynamic driving force of crystallization. The coupling of crystallization, stress formation and relaxation is modelled. The extended duration of the crystallization process due to an intermediate stress induced decrease of the crystallization rate is assessed. The properties of amorphous grain boundary films are discussed with respect to stress relaxation and creep resistance at high temperatures.
A micromechanical model was developed to study the influence of reinforcement fracture on the tensile strength of discontinuously-reinforced metal-matrix composites. The analyses were carried out within the framework of the shear lag model, which provides simple expressions for the average stresses acting on the reinforcement as a function of the matrix strength and of the reinforcement aspect ratio. The reinforcement strength was assumed to follow the Weibull statistics, and in this way, the fraction of intact and broken reinforcements can be obtained for any combination of matrix and reinforcement properties. The overall composite strength was then calculated by assuming that broken reinforcements do not contribute to the composite strengthening. The model was employed to study the influence of various parameters, such as matrix and reinforcement strength, and reinforcement aspect ratio and size, on the strength of discontinuously-reinforced metal-matrix composites. Finally, the model predictions were compared with experimental results on several high strength Al alloys reinforced with SiC particulates.
A micromechanical model is derived to simulate the nucleation and evolution of damage in a material containing second phase particles, which can exhibit fracture or decohesion. The damaged particle is replaced by an equivalent one having anisotropic properties, whose interaction with the matrix is solved by means of the Eshelby method applied in incremental elastoplasticity. Two bounds are determined with this method. The results show typically a strong dependence upon the loading path. Quantitative measurements are made to characterize the damage existing in the material prior and during testing. Comparisons are made between theoretical and experimental results, and show that the latter fall in between the bounds determined in the model. It is also shown that using a single fitting parameter in the model allows to reproduce all experimental results.
