Showing papers on "Volume fraction published in 1999"

Influence of martensite content and morphology on tensile and impact properties of high-martensite dual-phase steels

Abstract: A series of dual-phase (DP) steels containing finely dispersed martensite with different volume fractions of martensite (Vm) were produced by intermediate quenching of a boron- and vanadium-containing microalloyed steel The volume fraction of martensite was varied from 03 to 08 by changing the intercritical annealing temperature The tensile and impact properties of these steels were studied and compared to those of step-quenched steels, which showed banded microstructures The experimental results show that DP steels with finely dispersed microstructures have excellent mechanical properties, including high impact toughness values, with an optimum in properties obtained at ∼055 Vm A further increase in Vm was found to decrease the yield and tensile strengths as well as the impact properties It was shown that models developed on the basis of a rule of mixtures are inadequate in capturing the tensile properties of DP steels with Vm>055 Jaoul-Crussard analyses of the work-hardening behavior of the high-martensite volume fraction DP steels show three distinct stages of plastic deformation

On the relationship of high coercivity and L10 ordered phase in CoPt and FePt thin films

Abstract: The microstructure and the room-temperature hysteretic magnetic properties of sputtered, 10 nm thin films of equiatomic binary alloys of CoPt and FePt were characterized using transmission electron microscopy (TEM) and a superconducting quantum interference device (SQUID) magnetometer. A transformation from an atomically disordered, face-centered-cubic structure to the L10 ordered structure occurred during postdeposition annealing and was characterized using digital analysis of dark-field TEM images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from the SQUID magnetometer. In contrast to the relationship most commonly described in the literature, that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study wa...

Microstructure and mechanical properties of Mo–Mo3Si–Mo5SiB2 silicides

Abstract: Molybdenum boron silicides consisting of 20–50 vol.% of α-Mo and different ratios of the intermetallic phases Mo5SiB2 (‘T2’) and Mo3Si were prepared by arc-melting followed by dropcasting into Cu chill molds. For α-Mo volume fractions of ≈50% sound castings were obtained. For lower α-Mo volume fractions such as 25% macroscopic cracks were often observed. Preliminary oxidation tests verified the expected increase in the oxidation resistance as the B-concentration increases and the α-Mo volume fraction decreases. Also, the formation of glass films was observed. Depending on composition and heat treatment, the room temperature flexure strengths varied usually between 300 and 600 MPa. To some extent, these variations could be rationalized by differences in the microstructures. Annealing for 1 day at 1873 K in vacuum caused distinct microstructural coarsening. Annealing was sometimes accompanied by microcracking in the Mo3Si and the T2 phases. Cooling curves suggest that the liquidus temperature of the T2 phase is above 2400 K. Depending on the composition, final freezing of Mo–Si–B alloys was found to occur at temperatures as low as 2200 K. Care is therefore required during thermomechanical processing to avoid the formation of liquid phases.

The yield stress of concentrated flocculated suspensions of size distributed particles

Abstract: An investigation of shear yield stress is made on well-characterized alumina suspensions of different distributed particle sizes at the vicinity of the particle isoelectric point (IEP) across a wide range of volume fractions. Experimental results are compared with recently developed models [; ] and structural effects on the yield stress are examined. The models predict the magnitude order of the yield stress below a volume fraction of approximately 0.42, suggesting that interparticle forces play a dominant role in determining the network strength in this concentration region. Deviations between experimental results and theoretical predictions are explained in terms of structural effects being controlled by a competition between weak particle–particle linkages and geometric resistance on the network strength. At higher volume fraction, the effect of geometric resistance on the deformation of suspensions becomes more pronounced. A number of models for the yield stress of size distributed suspensions are then proposed. Results suggest that the effect of polydispersity of particles on the yield stress of suspensions can be well characterized by a surface area average diameter and the broad size distributed suspension exhibits a higher yield stress than the narrow size distributed suspension of the same volume average diameter.

Modeling the Phase Behavior of Polymer/Clay Nanocomposites

Abstract: To model the phase behavior of polymer−clay composites, we develop a free energy expression for a mixture of polymers and solid, thin disks. The free energy expression is adopted from the Onsager model for the equilibrium behavior of rigid rods. Thus, our theory takes into account the possible nematic ordering of the disks within the polymer matrix. By minimizing this free energy and calculating the chemical potentials, we construct phase diagrams for the polymer−disk mixtures. The findings provide guidelines for tailoring the polymer molecular weight and the volume fraction of the different components to fabricate thermodynamically stable mixtures with the desired morphology.

Growth of a semiconductor nanoparticle ring during the drying of a suspension droplet

Abstract: The first study on the growth process of an array comprising colloidal semiconductor nanoparticles (quantum dots) is presented. Two types of particles, CdS and CdSe/CdS (core/shell), suspended in pyridine and water, respectively, have been used. A liquid drop containing the particles is placed on a solid substrate, and the solvent is allowed to evaporate in a nitrogen atmosphere. As the result, a ring-shaped multilayer forms at the drop periphery, with the ring width depending on the particle volume fraction. The kinetics of ring growth is described by a theoretical model which accounts for the effect of experimental parameters. The reported study can serve as background for the preparation of more sophisticated and ordered arrays of semiconductor nanoparticles whose optical properties can be utilized in light-emitting and -converting devices.

Compressive response and failure of fiber reinforced unidirectional composites

Abstract: The compressive response of polymer matrix fiber reinforced unidirectional composites (PMC's) is investigated via a combination of experiment and analysis The study accounts for the nonlinear constitutive response of the polymer matrix material and examines the effect of fiber geometric imperfections, fiber mechanical properties and fiber volume fraction on the measured compressive strength and compressive failure mechanismGlass and carbon fiber reinforced unidirectional composite specimens are manufactured in-house with fiber volume fractions ranging over 10∼60 percent Compression test results with these specimens show that carbon fiber composites have lower compressive strengths than glass fiber composites Glass fiber composites demonstrate a splitting failure mode for a range of low fiber volume fractions and a simultaneous splitting/kink banding failure mode for high fiber volume fractions Carbon fiber composites show kink banding throughout the range of fiber volume fractions examined Nonlinear material properties of the matrix, orthotropic material properties of the carbon fiber, initial geometric fiber imperfections and nonuniform fiber volume fraction are all included in an appropriate finite element analysis to explain some of the observed experimental results A new analytical model predictionof the splitting failure mode shows that this failure mode is favorable for glass fiber composites, which is in agreement with test results Furthermore, this modelis able to show the influence of fiber mechanical properties, fiber volume fraction and fiber geometry on the splitting failure mode

Evolution of microstructure and phases in in situ processed Ti–TiB composites containing high volume fractions of TiB whiskers

Abstract: A series of titanium composites, with varying volume fractions of titanium monoboride (TiB) whiskers, were made by mixing various proportions of titanium (Ti) and titanium diboride (TiB2) powders followed by hot pressing. The phases present were identified by x-ray diffraction. Microstructural examination revealed three different types of TiB whisker morphologies: (i) long and needle-shaped TiB whiskers that are isolated and randomly oriented in the Ti matrix at relatively low volume fractions (0.3), (ii) colonies of refined and densely packed TiB whiskers from intermediatevolume (0.55) to high volume (0.73 and 0.86) fractions, and (iii) coarse and elongated TiB particles with a few needle-shaped whiskers at the highest volume fraction (0.92). In all the composites, TiB was found to be the predominant reinforcement. However, in Ti–TiB composites with 0.86 and 0.92 volume fractions of TiB, a significant amount of TiB2 was also present. The relative volume fractions of Ti, TiB, and TiB2 phases were estimated from the integrated intensities of diffraction peaks by the direct comparison method employing the calculated structure factors and Lorentz polarization factors. The composite microstructure, as well as the evolution of different morphologies, of TiB whiskers is discussed.

Anomalous percolation transition in carbon black-epoxy composite materials

Abstract: ac and electrical properties of carbon-black–epoxy composites measured in the frequency range of 100 Hz to 1 MHz are reported. Experimental results are in remarkable disagreement with the predictions of the statistical percolation theory for real and imaginary parts of the conductivity. They both exhibited, in particular, an abrupt variation at a given fraction of carbon black. The evolution of these transport quantities with the volume fraction of carbon black can only be explained considering a phase transition from a dispersed to an agglomerated state of the conducting fillers. This transition has been successfully verified by optical micros-copy of thin polished sections of cured samples. A simple model based on electrostatic repulsion between the carbon-black particles is proposed to interpret such a singular insulator-to-conductor transition. S0163-18299901718-X

Concentration dependence of the low-shear viscosity of polyelectrolyte micro-networks: From hard spheres to soft microgels

Abstract: The low-shear viscosity of polyelectrolyte microgels is studied as a function of concentration, crosslink density and ionic strength. The variation of the viscosity with volume fraction is very similar to that found in hard-sphere colloidal suspensions. In salt-free dispersions, osmotic effects are responsible for the de-swelling of microgels at high polymer concentration. This softening effect modifies the viscosity near the overlap concentration.

Experimental study of the sedimentation of dilute and semi-dilute suspensions of fibres

Abstract: Steady-state velocity and orientation distributions of sedimenting fibres were measured as a function of particle concentration and aspect ratio. Two different regimes of sedimentation were clearly identified. For dilute suspensions, the fibres tend to align in the direction of gravity with occasional flipping and clump together to form packets. In this regime, the vertical mean sedimentation speed is not hindered and can be larger than the Stokes' velocity of an isolated vertical fibre. Its scaling is a complex function of particle volume fraction and aspect ratio. As the concentration is increased, the fibres still tend to orient in the direction of gravity. The mean velocity becomes hindered and scales with particle volume fraction. The velocity fluctuations were found to be large and anisotropic. They were found to increase with increasing volume fraction. A similar substantial anisotropy of the orientation distribution was observed for all particle concentrations and aspect ratios studied.

Effect of Volume Fraction of Constituent Phases on the Stress-Strain Relationship of Dual Phase Steels

Abstract: Ferrite-martensite and ferrite-bainite dual phase steels (DP-steels) were prepared by applying accelerated cooling (AcC) process on a linepipe steel. Their stress-strain relationships were predicted by micromechanics. In the predictions, the stress-strain relationships of the constituent phases whose chemistries were determined by microscopic examinations and some thermodynamic data were used. The effect of volume fraction of the constituent phases on the stress-strain relationships of the DP-steels was also examined. According to the applied model, a simple stress-strain curve can be divided into three stages. As a result of this investigation, work hardening takes place in stage II and at the beginning of stage III. Further, in stage II, the hardening rate is strongly dependent on the volume fraction of the harder phase. In stage III, the hardening rate for each DP-steel is smaller than that in stage II and is related to the difference in tensile strength between the harder and the softer phases. Furthermore the second investigation by means of FEM analysis was carried out in order to evaluate the influence of variation of the volume fraction of the harder phase on the stress-strain behavior of a DP-steel. Tensile tests showed that by increasing the amount of the harder phase (bainite) in the DP-steel, Luders elongation disappears. According to the results obtained by the FEM calculations, the stress-strain behavior is related to the microstructure, such as volume fraction and shape of the grains in the DP-steel.

Mechanically alloyed Zr55Al10Cu30Ni5 metallic glass composites containing nanocrystalline W particles

Abstract: Composites based on the Zr55Al10Cu30Ni5 bulk metallic glass forming alloy, containing up to 17.5 vol % W particles were synthesized by mechanical alloying. Milling produces a metallic glass matrix with a homogeneous dispersion of nanoscale W particles. The composites exhibit almost the same thermal stability and no reduction of the supercooled liquid region compared to the particle-free metallic glass despite some small amount of dissolution of W into the glassy matrix. The viscosity in the supercooled liquid increases with increasing volume fraction of particles. This will be discussed with respect to the contribution of the particles as well as to changes in matrix composition and in the free volume of the material in the framework of the free volume model for viscous flow. Independent of the W content, the samples behave as moderately strong glasses. The viscous flow of the supercooled liquid is used to consolidate dense bulk samples. The Vickers hardness, HV, of the composites increases with increasing volume fraction of particles. It is suggested that both the matrix and the nanocrystalline particles contribute to the overall hardness of the composites.

Ultra Grain Refining and Decomposition of Oxide during Super-heavy Deformation in Oxide Dispersion Ferritic Stainless Steel Powder

Abstract: Mechanical milling using a high energy planetary ball mill was applied to the powder mixtures of iron, chromium and yttria (Y2O3) (Fe-24mass%Cr-0-15mass%Y2O3) to introduce a very large strain into the iron-base matrix, and microstructural changes during mechanical milling were investigated in relation to decomposition behavior of Y2O3 particles Mechanical milling of more than 36 ks was long enough to allow the mechanical alloying of iron and chromium powders After the milling of 36 ks, ultrafine bcc crystalline grains of 10 to 20 nm were formed within Fe-24mass%Cr-15 mass%Y2O3 powder mixture and 15 mass% of Y2O3 particles were almost decomposed The resultant powder mixture markedly hardened to about 1000 Hv The decomposition of Y2O3 particles can be explained as being due to the formation of an amorphous grain boundary layer where yttrium and oxygen atoms are enriched As a result, it is proposed that, for the dissolution of Y2O3, bcc crystalline grains should be refined to a nanometric size to provide a sufficient volume fraction of the grain boundary layer, and that Y2O3 particles should be crushed to several nanometers to produce the driving force for the decomposition of Y2O3 particles

Effect of carbon content on the microstructure and mechanical properties of (Ti, W, Ta, Mo)(C, N)-(Co, Ni) cermets

Abstract: Five model alloys with different carbon content were studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in combination with energy dispersive X-ray analysis (EDX). Vickers hardness (HV) and transverse rupture strength (TRS) were measured, and continuous turning tests were performed. The aim of this work was to relate the variation in carbon content to the resulting microstructures and to the mechanical behaviour of these model alloys. An increased carbon content resulted in decreased concentrations of tungsten and titanium in the binder phase, a lower volume fraction of undissolved Ti(C, N) cores, and a higher volume fraction of heavy (Ti, W, Ta)(C, N) cores. The volume fractions and compositions of the other phases were not much affected by the carbon content. In addition, a coarser carbonitride grain structure was observed in the material with a high carbon content. Hardness was found to decrease and TRS to increase with carbon content, but no obvious trend was found for the wear resistance as a function of carbon content. Several microstructural features interact and influence the final properties. The heavy (Ti, W, Ta)(C, N) cores formed during sintering and a highly solution hardened binder phase seem to be favourable for a high wear resistance.

Nanocrystalline composites with high strength obtained in Zr–Ti–Ni–Cu–Al bulk amorphous alloys

Abstract: Nanocrystalline composites with the grain size less than 10 nm were produced by annealing of Cu-mold cast Zr70−x−yTixNi10Cu20Aly (X=5–7.5 and Y=10–15 at %) bulk amorphous alloys. The nanostructured alloys show increased tensile strength at the volume fraction of nanoparticles less than 30%. The microstructure of the amorphous alloys was found to contain medium range order (MRO) domains, which uniformly distributed in the amorphous matrix. We suggest that MRO domains provide nucleation sites for precipitation of the primary crystals and lead to the formation of nanocrystalline composites.