High Particle Volume Fractions Research Articles

タングステン焼結合金の高温強度

Tensile tests of sintered tungsten alloys at elevated temperatures were carried out in air and vacuum atmospheres in order to study the temperature dependency of mechanical properties. Liquid-phase-sintered 93.0W-4.9Ni-2.1Fe and 97.2W-2.0Ni-0.8Fe (by wt%) alloy specimens were prepared. The 0.2% proof stress and tensile strength of the sintered tungsten alloys, which decreased with increasing temperature, were higher than those of conventional high temperature structural steels. From the fractographic observations, cleavage fracture of tungsten grains was dominant in the range from room temperature to 500°C, and the area fraction of decohesion of tungsten-tungsten boundary increased in the temperature range above 600°C. For the material with a high volume fraction of tungsten particles and with direct contact of tungsten particles, lower strength was observed at elevated temperatures in air compared with the material with low tungsten content and Ni-Fe layer around tungsten particles, which may result from oxidation through the boundary of tungsten-tungsten particles.

Abrasion resistance of in situ Fe-TiC composites

Metal matrix composites containing a high volume fraction of carbide, nitride, boride, and/or oxide particles are frequently the materials of choice for applications which require high wear resistance. It is the very hard second phase particles which imbue the metal matrix composite with its superior wear resistance. For example, additions of titanium carbide (TiC), one of the hardest of the carbides with a Vickers hardness of 19.6--31.4 GPa, can be used to improve the abrasion resistance of iron alloys. In the present study in situ metal matrix composites, containing between 23 and 31 volume percent carbides, were produced from Fe-Ti-C and Fe-Cr-Ti-C melts, and their abrasion resistance was compared with that of unreinforced Fe and a white cast iron using a high-stress pin abrasion test.

Study of the damage mechanisms in an OSPREY™ Al alloy-SiC p composite by scanning electron microscope in situ tensile tests

Damage mechanisms in a 7049 Al alloy + 15% SiC p metal matrix composite were studied qualitatively and quantitatively by in situ tensile tests in a scanning electron microscope with gold microgrids deposited onto the surface of the specimen. The first damage mechanisms were found to be rupture of the most elongated particles and, in smaller proportion, decohesion of the particle-matrix interface. A high aspect ratio, large size and low local volume fraction of particles appeared to increase the cracking probability. An Eshelby iterative method modified to account for the elastoplastic behaviour of the matrix was used to calculate the stress field induced by the thermomechanical treatment and mechanical loading of the composite. Knowledge of the statistical characteristics of the damaged particles permitted estimation of the critical stress for the two observed damage initiation mechanisms. In the case of particle cracking this stress depends on the particle size.

Large deformation and fracture behaviour of gels

When gels are used in practice, their large-deformation and fracture characteristics are mostly far more relevant than small-deformation characteristics. In this paper fracture behaviour is discussed of various types of gels, viz. polymer and particle gels, the latter with fairly low and very high volume fraction of particles. First, a general introduction is given on theoretical aspects of fracture mechanics of gels, which involves an essential extension of classical fracture theories. The relationship between large-deformation and fracture behaviour of a gel and its structure proves to be far more complicated than for small-deformation properties. The main reasons for this difference are: (i) the much more important effect of relatively large inhomogeneities on fracture properties and (ii) some very different causes for the strain rate dependence. Not only are the average distance between cross-links and the average stiffness of the strands connecting them of importance, but also the distribution of these parameters. Moreover, inhomogeneities, be it defects (of µm to mm scale) or weak regions (e.g. in composite gels) may have an overriding effect on the fracture properties. To understand the strain rate dependence, one should consider the energies involved as a function of the deformation rate and distinguish between the amount elastically stored during deformation, the amount dissipated due to viscous flow or due to friction processes and the net fracture energy. Moreover crack initiation and fast ‘spontaneous’ crack growth (crack propagation) have to be distinguished. The factors mentioned cause large deformation and fracture properties to be much more strongly dependent on the physical structure of a gel than are the small deformation properties.

Direct as against indirect dispersion work hardening

Abstract In a polycrystal consisting of a matrix and a uniform dispersion of particles or bubbles with a grain size determined by critical Zener pinning it is known that the yield stress is controlled not so much by direct particle—dislocation interactions as by indirect particle-grain-boundary-dislocation interactions. After yield, dislocations accumulate in the matrix, at hard particles (but not at bubbles) and at grain boundaries. These accumulations cause work hardening. In the case of hard particles the particle (direct) work hardening and the grain-boundary (indirect) work hardening are equal. In the case of bubbles the direct work hardening is approximately zero. At low strains, therefore, indirect hardening dominates the flow stress. Only at high strains is the matrix work hardening significant and in strong alloys, with a high volume fraction of small particles, it is never significant

Mechanical behavior of residually stressed composites with ductile and brittle constituents

The mechanical behavior of particulate reinforced composites is studied when residual stresses are present. Brittle matrix composites (BMCs) with ductile inclusions are compared with metal matrix composites (MMCs) with elastic inclusions. Cell models are used to perform finite-element calculations for different volume fractions of uniformly distributed particles with spherical or cylindrical shape. The stresses of the BMCs are shown to follow a bilinear curve with a localized transition region separating linear parts with similar slopes. BMCs are much stiffer than MMCs and cylindrical-shaped particles lead to a higher composite stiffness than spherical particles. Thus, only in the presence of a high volume fraction of cylindrical particles can residual stresses be easily monitored in the stress-strain curve of BMCs. Additionally, hardening is found to affect BMCs to a much lesser degree than MMCs. The influence of particle shape on initial yield stress is elaborated. It is demonstrated that for metal-ceramic composites the contiguity of the ductile phase is necessary to obtain an elastic-plastic mechanical response in agreement with experiments.

Some rheological properties of bimodal sized particulates

The packing of binary mixtures of spheres is a function of the diameter ratio and composition. In this study we have investigated the elastic modulus, dynamic viscosity and osmotic pressure of monodispersed polystyrene latex particles of diameter 204 nm and 1400 nm and their mixtures. The diameter ratio was therefore fixed as 0.15 and composition and total particle volume fraction were the main variables. At high particle volume fractions, <0.6, the extrapolated elastic modulus exhibited a minimum when the ratio of small to large particles was 0.2. In addition the osmotic pressure data at volume fractions of 0.6 and 0.65 also showed a minimum at a ratio of 0.2. The highest maximum packing fraction is expected if the large particles are arranged randomly packed and small particles fill the voids of a large particle system so that the whole system has a minimum voids volume. For the current particle size ratio, as may be deduced from crystallographic arguments, this occurs at a volume ratio of around 20%. For the more dilute systems it was found that the elastic modulus and osmotic pressures decreased as the fraction of small particles in the system decreased. This is essentially a consequence of the mean particle spacing decreasing as the number of small particles in the system decreases.

Attractive double layer forces from hard-core correlations

The osmotic pressure in an electric double layer is studied by means of Monte Carlo simulations. The net osmotic pressure is calculated as the difference between the internal and external pressures calculated at the same chemical potential. This can be obtained with sufficient accuracy using the Widom insertion particle technique in the canonical ensemble. The same technique can also be applied in the calculation of different pressure contributions. We find that the net osmotic pressure is always repulsive in a system with monovalent co- and counterions, which is in agreement with the Poisson–Boltzmann equation as well as with previous simulation studies. In asymmetric salts, when the coion is multivalent we find that the external pressure exceeds the internal pressure resulting in a net attractive interaction. This attraction appears at high particle density or volume fraction in the double layer, for example, at short separations, high salt concentration, and/or high surface charge density. The origin of the attraction seems mainly to be the hard-core interactions, which the ions experience in a dense double layer, which is manifested in a high chemical potential. The corresponding bulk solution at this chemical potential has a very high salt concentration and hence also a high osmotic pressure.

Ductile fracture of mechanically alloyed iron-yttria alloys

The influence of Y203 particle content on the tensile fracture of mechanically alloyed iron has been studied for a series of dispersion-strengthened alloys containing up to 10 vol pct particles. When compared to the behavior of spheroidized steels, the present results indicate that at comparable volume fractions of particles, the Fe-Y203 alloys exhibit a much decreased tensile ductility. Observations of microscopic damage indicate that this is a consequence of rapid void nucleation at small strains, limited void growth, and enhanced void linking, especially at high particle contents. Analysis of these observations suggests (a) a surprisingly high oxide particle-matrix interfacial bond strength, (b) an influence of rapid strain hardening at small strains in creating high flow stresses, which assist void initiation, (c) enhanced void nucleation at high volume fractions of particles due to neighboring particles and voids, and (d) an accelerated void-linking process at high volume fractions of particles when interparticle spacing approaches particle/ void size.

Role of second-phase particles in the oxidation of Al-Li alloys

Oxidation behavior of Al-Li alloys, containing a high volume fraction of Ferich second-phase particles, was studied at 530°C and 200°C. Morphological studies showed enhanced growth of Li-rich oxides in the vicinity of the insoluble second-phase particles. Microanalytical depth profiling of the oxide layer with a high resolution scanning ion microprobe indicated rapid diffusion and subsequent oxidation of Li at the free surface. Examination of the alloy surface after removal of the oxide layer suggested that the initial oxide growth occurred in the lateral direction. Secondary ion image depth profiling of the alloy surface after oxide removal revealed Li segregation to the alloy/second-phase interface, supplying Li for accelerated oxidation. A microstructural model of the oxidation process in this case is presented.

Characterization of the hot-working behaviour of a P/M Al-20Si-7.5Ni-3Cu-1Mg alloy by hot torsion

Hot torsion tests were performed to investigate the mechanical and microstructural responses of a quinary Al-20Si-7.5 Ni-3Cu-1Mg (wt %) alloy, consolidated from a rapidly solidified powder, to deformation at varying temperatures and strain rates. It was found that, under most of the deformation conditions applied, stress-strain curves were characterized by distinct stress peaks, which are usually absent from the curves shown by conventional aluminium alloys. Temperature and strain rate strongly influenced the stress and ductility of the material. Their combined influence on the peak stress has been expressed with a hyperbolic sine equation. The material also exhibited an extraordinarily high strain rate sensitivity,m, and a largem value variation with temperature. A relatively high value of activation energy for deformation was determined, which clearly reflects additional thermal barriers to metal flow, arising from a high volume fraction of multi-phase particles dispersed in the material. Additionally, the microstructure developed in the course of deformation was examined, which showed evidence of the co-operation of dynamic recovery and recrystallization. The initiation of local dynamic recrystallization is a result of a low level of dynamic recovery achievable in the material, which is again different from conventional aluminium alloys.

Compressive failure of silica-filled epoxy resins: influence of matrix strength, interfacial bond strength and porosity

Composites containing a high proportion of hard inert filler particles in a cold-curing polymer resin are relatively inexpensive materials, easily cast to shape and with mechanical strength and durability suitable for use as flooring and grouting compounds. They are also used.in the mineral-handling and mining industries as abrasion-resistant materials, to line plant handling or processing rocks, coal or ores, either dry or in slurry form [1]. Materials of this type are also useful to simulate some of the properties of natural sedimentary rocks, such as sandstones and limestones, for experimental purposes. For example, in studying the abrasive wear of steels against natural sandstone and limestone [2] it is not possible to obtain reproducible samples of rock with independently varied values of interparticle bond strength, porosity and binder strength. Silica-filled epoxy resins, however, can be made which behave in many respects like sandstones, with similar contents of silica particles, values of compressive strength and with the potential for controlled variation of their microstructure and properties. Previous work has shown that the addition of hard filler particles to an epoxide resin generally leads to increased elastic modulus and compressive strength, although the tensile strength may be reduced [3]. Compressive strength is a particularly important property for some applications, including the simulation of abrasive natural rocks, but only limited information is available about the effect on the compressive strength of composites with high proportions of particulate filler of the properties of the matrix material itself, the porosity of the composite or the strength of the bonding between the filler particles and the matrix. A systematic investigation was therefore made of the effects of these variables on the uniaxial compressive strengths of composites containing a constant high volume fraction (72%) of silica sand particles in an epoxy resin matrix.

Effect of nonadsorbing polymers on the rheology of a concentrated nonaqueous dispersion

The rheology of a poly(methyl methacrylate) (PMMA) latex stabilized with poly(12-hydroxystearic acid) and dispersed in dodecane at a volume fraction of 0.42 was observed to deviate from near Newtonian behavior upon the addition of polyisobutylene (PIB) at high and low molecular weights, 380,000 and 2,000, respectively. The high molecular weight PIB gave the dispersion a predicted depletion-flocculated behavior, with observable phase separation and non-Newtonian shear thinning rheology, as a consequence of the additional attractive particle interaction component. Added low molecular weight PIB, however, gave comparatively much stronger non-Newtonian behavior with no observable phase separation. This behavior was observed in a region of polymer molecular weights and size comparable to that of the steric layer. The results are consistent with a steric-elastic repulsion arising from the compression of the nonadsorbing polymer chains at high particle volume fraction and not with an attractive term due to depletion flocculation.

On the validation of a strengthening concept

Louat formulated a strengthening theory in which the four conditions specified above could be met in two phase materials in which the minor phase forms the matrix. Additionally, thermal stability is insured if the two phases are immiscible. It follows from Louat's theory that material with superior strength can be obtained by embedding a high volume fraction (greater than 50%) of ultrafine particles in a ductile matrix. In contrast to conventional materials, these particle reinforced materials are predicted to retain much of their strength even when the matrix melts. In addition, the theory predicts that the strength of such materials increases with decreasing particle size. The results presented in this paper refer to the case of iron or copper particles embedded in a lead matrix

Sedimentation and sediment flow on inclined surfaces

The steady sedimentation of a suspension over an inclined surface is analysed by considering the combined effects of settling hindrance, bulk motion and particle resuspension. The coupled momentum and mass balances suggest that a thin high-density sediment layer will form over the inclined surface, reminiscent of the thin thermal boundary layers in the classical problem of natural convection. It is shown that for a given value of the particle volume fraction in the unsettled suspension, a steady flow of the sediment can be maintained only if the angle of inclination exceeds a minimum value. The analysis further predicts the existence of a sharp discontinuity in the particle volume fraction across the suspension–sediment interface along which the bulk velocity has a local maximum. High particle volume fractions within the sediment are predicted when the unsettled suspension is either very dilute or very concentrated. This leads to the formation of relatively large sediment-layer thicknesses which reflect the fact that a large body force is required in these two limiting cases to overcome the viscous resistance to flow near the inclined boundary.

Preparation and properties of nonaqueous model dispersions of chemically modified, charged silica spheres

We describe the preparation and properties of a novel type of stable dispersion in weakly polar organic solvents of monodisperse, charged silica spheres coated with 3-methacryloxypropyltrimethoxysilane. An important feature of this silica is that it can be optically matched up to high particle volume fractions. Colloidal crystallization is observed in these dispersions as well as the formation of a glass-like phase. We conclude that this silica provides a very appropriate model dispersion for light-scattering studies on concentrated systems of charged particles.

On the threshold stress for dislocation creep in particle strengthened alloys

Abstract Current models describing dislocations by-passing particles by climb do not satisfactorily account for the creep deformation of alloys containing high particle volume fractions such as nickel-base superalloys. An alternative approach involving cooperative climb over groups of particles leads to a resistance to deformation, rather than a threshold stress, that is insensitive to the particle size but which depends strongly on applied stress. Detailed calculations have been carried out for the nickel-base superalloy IN738LC and these are in close agreement with measurements of the friction stress. The theoretical conclusions are generally compatible with the creep behaviour of nickel-base superalloys.

Dynamic behavior of silica dispersions studied near the optical matching point

Photon correlation spectroscopy experiments on silica particles dispersed in cyclohexane over a large concentration range and near the optical matching point have been explained by the occurrence of two different relaxation modes: one mode due to the collective diffusion of the particles and the other mode due to the self-diffusion. This is in agreement with the theoretical considerations of Weissman and Pusey. In the diluted region, information has been obtained about the behavior of the self-diffusion coefficient at zero wave vector (K) as a function of the particle volume fraction (φ). We found that the following relation exists: Ds(K = 0) = D0(1 + k′sφ), where D0 is the diffusion coefficient at infinite dilution and the constant ks′ has a value of −2.7±0.5. The concentration dependent behavior of the collective and self-diffusion coefficient has further been compared with the results of sedimentation and viscosity measurements. At high particle concentrations we observed that the relation between collective diffusion and sedimentation by means of the generalized Einstein relation has been confirmed qualitatively. The self-diffusion coefficient appeared to be inversely proportional to the viscosity of the solution. The sedimentation experiments have been found to be in agreement with the theories of Reed and Anderson over the whole concentration range, while at high particle volume fractions the theory of Lekkerkerker seemed to reasonably fit. Finally, we observed at φ≳0.40 that numerically, the values of the collective and self-friction coefficient are nearly the same.

Microstructural development and evolution in liquid-phase sintered Fe-Cu alloys

An experimental study relating the scale and contiguity of liquid-phase sintered Fe-Cu alloys to sintering conditions has been conducted over a broad range of solid-phase volume fraction. It is found that the solid-phase contiguity attains a steady-state value at fairly short sintering times and that contiguity increases with increasing particle volume fraction, but is essentially independent of sintering temperature. Both the continuity and scale of microstructure are discussed in terms of concurrent particle coalescence and Ostwald ripening. It is found that values of the probability of particle coalescence after contact required to explain the contiguities observed are in reasonable agreement with theoretical predictions. However, comparison of observed contiguities with those predicted by recent studies is found to be unsatisfactory since the latter do not predict the steady-state contiguities observed. On the other hand, microstructural observations and measurements of coarsening-rate constants as a function of particle volume fraction indicate clearly that particle coalescence contributes significantly to the coarsening process at higher particle volume fractions.

Cobalt-chromium coatings by electrodeposition: Review and initial experimental studies

The attraction of electrodepositing composite coatings is discussed in relation to sprayed coatings. The present state of the development of composite electrodeposited coatings is reviewed and the main limitations, such as the bulk hardness of the matrix, are discussed. The feasibility of developing a generation of wear resistant cobalt-chromium composite coatings and electroforms is examined in relation to the factors which would govern such a development. Cobalt-chromium co-deposition is reviewed and the results related to the requirements for composite co-deposition. Initial experimental results are reported on cobalt-chromium deposition from a fluoborate bath and the dimethylformamide-water bath. It is shown that the rate controlling factor is the inability to sustain co-deposition and thus achieve the requisite thickness for coatings and electroforms. It is suggested, on the basis of the results available, that predictable cobalt-chromium composites could be developed by the 1980's based on chromium solid solutions to achieve the hardness, and containing a high volume fraction of particles.