Vol Fraction Research Articles

Study of the Residual Stress in Cold-Rolled 7075 Al-SiC Whisker-Reinforced Composites by X-Ray and Neutron Diffraction

Many surface treatment techniques are used to improve the mechanical behavior of metal matrix composites (MMC) under dynamic loading. Mill machining, shot-peening, hammering, and cold-rolling are the four techniques commonly used. In this paper, we study the effect of the surface treatment with the cold-rolled technique on the residual stresses in an aluminum alloy 7075 reinforced with 27% vol fraction SiC whiskers. The techniques used to measure the residual stresses are X-ray and neutron diffraction methods. Neutron diffraction is a nondestructive technique that allows one to evaluate the residual stresses in both the Al matrix and the SiC whiskers. The X-ray technique was used to evaluate the residual stresses at the surface of the composite material. The results show us that the cold-rolled treatment induced a steep stress gradient near the surface. The comparison of using the two different measuring techniques will be discussed.

Melt-Processed Ni3Al matrix composites reinforced with TiC particles

TiC-reinforced Ni3Al metal matrix composites have been investigated. The composites were prepared by in situ precipitation of either 0.01, 0.05, or 0.1 vol fraction TiC in molten Ni3Al. These precipitates are thermodynamically stable in the Ni3Al at 1000 °C up to 50 hours. Although almost ideal precipitate distribution was achieved at low volume fraction of TiC, at higher volume fraction, there was a tendency toward agglomeration. Room and hightemperature mechanical testing showed significant improvement in modulus, yield strength, and ultimate tensile strength, while still maintaining a 2.5 pct elongation at 0.1 vol. fraction TiC and 500 °C. The density of precipitates (number/m3) is strongly dependent on the fraction of TiC, on temperature, and on the holding time at that temperature. The critical nucleation temperature for these precipitates was determined to be at 1475 °C. Experimentally evaluated growth rate constants indicate a diffusion-controlled coarsening mechanism along with coalescence and multiparticle interaction at higher volume fractions of TiC. This is in accordance with either the Lifshitz Slyozov Encounter Modified (LSEM) model or with the Voorhees and Glicksman (VG) model.

Conductivity percolation in polyiodide/polymer complexes

Variable-temperature four-probe conductivity measurements and Raman spectroscopy were investigated for iodine in poly(propylene oxide) (PPO) and NaI[sub 3] in PPO. The Raman spectra indicate the presence of both triiodide and polyiodide species in samples of I[sub 2]-doped PPO. The conductivity of these PPO/I[sub 2] samples increased with increasing I[sub 2] concentration and reached a plateau at approximately 12 vol % iodine. Raman spectra at 20 [degrees]C indicate that, at concentrations less than 23 vol% I[sup [minus][sub 3]], the dominant species is the triiodide. Polymer salt complexes with varying amounts of I[sup [minus][sub 3]] appear to display a conductivity threshold near T[sub g], at 0.2 vol fraction of triiodide. 6 figs., 1 tab.

Red blood cell mechanics and capillary blood rheology

Blood contains a high vol fraction of erythrocytes (red blood cells), which strongly influence its flow properties. Much is known about the mechanical properties of red cells, providing a basis for understanding and predicting the rheological behavior of blood in terms of the behavior of individual red cells. This review describes quantitative theoretical models that relate red cell mechanics to flow properties of blood in capillaries. Red cells often flow in single file in capillaries, and rheological parameters can then be estimated by analyzing the motion and deformation of an individual red cell and the surrounding plasma in a capillary. The analysis may be simplified by using lubrication theory to approximate the plasma flow in the narrow gaps between the cells and the vessels walls. If red cell shapes are assumed to be axisymmetric, apparent viscosities are predicted that agree with determinations in glass capillaries. Red cells flowing in microvessels typically assume nonaxisymmetric shapes, with cyclic "tank-treading" motion of the membrane around the interior. Several analyses have been carried out that take these effects into account. These analyses indicate that nonaxisymmetry and tank-treading do not significantly influence the flow resistance in single-file or two-file flow.

Relation of the transition-state structure for the water-catalyzed hydrolysis of 1-acetylimidazolium ion to solvent hydrophobicity: proton inventories in water-acetonitrile mixtures

The transition-state structure for the water-catalyzed hydrolysis of 1-acetylimidazolium ion has been probed in solvent systems which may mimic the hydrophobic nature of an enzyme's active site. The kinetic solvent deuterium isotope effects, k/sub H/sub 2/O//k/sub D/sub 2/O/, are 2.58, 2.49, and 2.10 in water, in 0.5 vol fraction of acetonitrile in water and 0.9 vol fraction of acetonitrile in water, respectively. The proton inventory investigations suggest all three solvent systems entertain a transition-state structure composed of a catalytic proton bridge between the reorganizing substrate and a water molecule acting as a general-base catalyst. A compression of the transition-state structure in the solvent system containing the largest amount of acetonitrile is suggested to be responsible for the diminished kinetic solvent deuterium isotope effect. The reaction has been shown to be second order with respect to water.

Crystallisation in films of polar vinyl polymers Part III: Kinetics of the crystal growth in polyvinyl alcohol films prepared by evaporation of concentrated aqueous solutions

This paper describes studies on the kinetics of crystallisation of polyvinyl alcohol films, prepared by the controlled slow evaporation of concentrated aqueous solutions; the initial polymer volume fraction (Vp)0 was 0.2 and the initial evaporation rates (Re)0 were varied from −0.0003 to 0.0150 vol fraction hr−1. The main crystal growth started after prolonged induction periods(t) at polymer contents\(\bar V_p \) =0.66 to 0.82, well beyond the gelation (nucleation) composition. The growth rates reached maximum values (Rg max) at polymer contents (Vp)max = 0.82 to 0.92 and then decreased again to very low values in the viscous systems at the glass transition composition. Growth rates at any polymer content were expressed approximately by the relation, $$R_g = 4R_g \max U(1 - U),$$ , where $$U = \frac{{(V_p - \bar V_p )}}{{2[(V_p )\max - \bar V_p ]}}.$$ (Vp)max andRgmax values did not vary markedly over the fifty-fold range of evaporation rates. (Vp)max values increased andRg max values decreased with rise in crystallisation temperature.

ChemInform Abstract: EFFECT OF MICROSTRUCTURE ON STRENGTH OF SI3N4‐SIC COMPOSITE SYSTEM

AbstractDer Einfluß der Mikrostruktur auf die die Festigkeit bestimmenden Faktoren wie Bruchbildungsenergie, Elastizitätsmodul und Rißgröße des Verbundsystems Si3N4‐ SiC werden untersucht.

Effect of Microstructure on Strength of Si3N4‐SiC Composite System

The Si3N4‐SiC composite system was investigated to better understand the effect of microstructure on the strength‐controlling factors, i.e. fracture energy, elastic modulus, and crack size. Silicon carbide dispersions with average particle sizes of 5, 9, and 32 μm were used to form 3 composite series within this system, each containing 0.10, 0.20, 0.30, and 0.40 vol fraction of the dispersed phase. These composites were fabricated by hot‐pressing. Fracture energy and strength values were measured for each composite. A linear relation between the elastic modulus of the two phases was assumed. The crack size was calculated for each composite using the appropriate property values. The strength behavior of the 9‐ and 32‐μm series was controlled by the crack size, which, in turn, was controlled by the particle size and volume fraction of the SiC phase. Particle size and volume fraction did not affect the crack size of the 5‐μm series, in which strength was controlled by both fracture energy and elastic modulus. Strengths measured at 1400°C and thermal conductivity measurements indicate that several of these composites are promising as high‐temperature structural materials.