Volume Fraction Of Fine Particles Research Articles

Microstructure and mechanical properties of V–Cr–Zr alloy with carbide and oxide strengthening

A comparative study of the effectiveness of carbide and oxide types of strengthening of V–Cr–Zr alloy was carried out by means of a comprehensive certification of structural-phase state parameters and measuring the mechanical properties characteristics. It has been shown that the use of chemical-heat treatment contributes to a significant increase in the thermal stability of the microstructure and mechanical properties of V–Cr–Zr alloy in comparison with carbide strengthening under the conditions of thermomechanical treatment. A controlled increase in the volume fraction of fine particles based on ZrO2, along with an increase in the concentration of oxygen in the solid solution, leads to a decrease in the rate of oxides coagulation and an increase in the thermal stability of high disperse heterophase structure. These effects contribute to the retention of high defect structural states with nonzero values of crystal lattice curvature even after high-temperature (0.67 Tmelt) anneals. The high efficiency of dispersion and substructural strengthening is a consequence of blocking dislocation slip by fine particles stabilized by oxygen in a solid solution.

The Theoretical Packing Density Model (TPDM): A versatile model for estimating the minimum void ratio of binary soil mixtures.

Understanding the behavior of sand-fines mixtures and assessing their shear strength or permeability require knowledge of the influence of fine particles content on the minimum void ratio. The latter is often evaluated using empirical methods or models using parameters without physical significance. Validated for both ordered and disordered packings and for concrete mixtures, the Theoretical Packing Density Model (TPDM) is tested for predicting the minimum void ratio on binary sand-fines mixtures, whatever the fines content. It requires only two parameters: the critical cavity size ratio x0 from which the loosening effect occurs and the compaction index K describing the packing efficiency. Previously collected data from 30 binary soil mixtures including river, lake, mountain and crushed sands representing 285 individual specimens for which minimum void ratios are available, were used. The coefficient of determination provided by the TPDM is equal to 0.968 on average, and greater than 0.99 for 30% of the mixtures. Thanks to the compaction index K, the mathematical model is therefore capable to capture the non-linear behavior of the void ratio variation with respect to the solid volume fraction of fine particles. Finally, a method is proposed to approximate the value of the critical cavity size ratio x0 to be used as a function of the combination of the shapes of coarse and fine particles.

Study on Two-Phase Fluid-Solid Coupling Characteristics in Saturated Zone of Subgrade Considering the Effects of Fine Particles Migration

The fluid seepage in saturated zone of subgrade promotes the migration of fine particles in the filler, resulting in the change of pore structure and morphology of the filler and the deformation of solid skeleton, which affects the fluid seepage characteristics. Repeatedly, the muddy interlayer, mud pumping, and other diseases are finally formed. Based on the theory of two-phase seepage, the theory of porous media seepage, and the principle of effective stress in porous media, a two-phase fluid-solid coupling mathematical model in saturated zone of subgrade considering the effects of fine particles migration is established. The mathematical model is numerically calculated with the software COMSOL Multiphysics®. The two-phase seepage characteristics and the deformation characteristics of the solid skeleton in saturated zone of the subgrade are studied. The research results show that the volume fraction of fine particles first increases then decreases and finally becomes stable with the increase of time, due to the continuous erosion and migration of fine particles in saturated zone of the subgrade. The volume fraction of fine particles for the upper part of the subgrade is larger than that for the lower part of the subgrade. The porosity, the velocity of fluid, the velocity of fine particles, and the permeability show a trend of increasing first and then stabilizing with time; the pore water pressure has no significant changes with time. The vertical displacement increases first and then decreases slightly with the increase of time, and finally tends to be stable. For the filler with a larger initial volume fraction of fine particles, the maximum value of the volume fraction of fine particles caused by fluid seepage is larger, and the time required to reach the maximum value is shorter. It can be concluded that the volume fraction of fine particles in the subgrade filler should be minimized on the premise that the filler gradation meets the requirements of the specification in actual engineering.

Investigation of Nanometer- and Submicron-Size Dispersed Phases in Titanium Pseudo- β -Alloys using Scattering and Diffraction of Neutrons, Electrons, and X-Ray Radiation

The size and volume fraction of fine particles of secondary β -phase formed during hardening heat treatment of pseudo-β-titanium alloy VT22 are determined using low-angle scattering of neutrons, x-ray diffractometry, electron back-scatter diffraction, and transmission microscopy combined with x-ray spectral microanalysis. Based on comprehensive use of experimental data obtained by methods of different restriction levels analysis is conducted for the size, shape, elemental composition, and volume content of fine particles obtained after different versions of hardening heat treatment. Activation of β -phase decomposition energy due to low-temperature aging is determined.

ВПЛИВ ВУГЛИЦЮ НА КОРОЗІЙНУ ПОВЕДІНКУ ТЕРМІЧНО ЗМІЦНЕННОГО ПРОКАТУ

In this work, we studied the corrosion behavior of hot-rolled and heat-hardened rolled products made of carbon steel with different carbon content in order to select the optimal conditions for its heat treatment to obtain a satisfactory combination of strength and corrosion resistance during the development of processes of general corrosion with hydrogen depolarization. The studies were carried out on samples of carbon steel (0.14-0.8% C) in hot-rolled and heat-hardened states. It has been established that the corrosion rate (K) in specimens made of steels in the quenched state is lower than in the hot-rolled state; however, even in this state, with an increase in the carbon content within the studied limits, it increases 40 times. An increase in the corrosion rate with an increase in the carbon content in hot-rolled steel is explained by an increase in the amount of pearlite and an increase in the length of the ferite-cementite interphase boundaries. It was determined that in quenched hardened steel in the carbon content range from 0.14 to 0.80 % C, an increase in the corrosion rate is apparently due to an increase in microstresses (accumulation of excess internal energy) associated with an increase in the tetragonality of the martensite lattice and the appearance of concentration inhomogeneity in the solid solution. It was found that during the course of corrosion with hydrogen depolarization, the dependences of the general corrosion rate of thermally hardened structural steels on the tempering temperatures are described by extreme dependences with a maximum corrosivity in the temperature range of 350-500 oC, while, with an increase in the carbon content in steel, rental increases. It was determined that at the tempering temperature corresponding to the maximum K, the corrosion rate is 1,5-3,6 times higher than in the quenched state, and 1,4-2,6 times higher than in hot-rolled, and with an increase in the carbon content this difference is growing. An increase in the rate of corrosion with hydrogen depolarization in the tempering temperature range of 350-450 oC is apparently associated with the process of carbide formation and the appearance of coherent stresses during the release of dispersed particles. An increase in corrosion losses in this interval with an increase in carbon content is due to an increase in the volume fraction of fine particles of the carbide phase and an increase in the level of elastic microdistortions of the crystal lattice (Δa/a).

From Wellbore Instability and Grain Mixing to Injectivity Reduction

Wellbore instability has been registered and expected as one of the risks for successful delivery of horizontal injectors with open-hole stand-alone screens in depleted reservoirs. Special drill-in fluids and breaker fluids were designed to ensure stability of these wells prior to screen installations. Borehole collapse was expected in most cases after the wellbore pressure dropped to hydrostatic at the end of the drilling process. Unlike open-hole producers, the loose particles (or grains) trapped in the annulus of the open-hole injectors will likely undergo constant mixing and resorting during each injection and shut-in cycles. In this study, we examined the impacts of grain mixing on the reduction of porosity and permeability in the annulus using a simple binary mixing model. Laboratory experiments based on this hypothesis have also been conducted. Our preliminary results suggest that if the fines are 10 to 20 times smaller than the larger particles, the presence of a few percent in volume fraction of fine particles can reduce permeability by an order of magnitude. The predicted reductions are consistent with the observed equivalent skin increases at some of our injectors during the first few years of operations. After multiple stimulations to remove fines and residual drilling additives, we successfully re-established injectivities to the initial states.

Features of formation of nanocrystalline state in internal- oxidized V-Cr-Zr-W and V-Mo-Zr system alloys during deformation by torsion under pressure

The results of investigation of features of nanostructural state formed during deformation by torsion under pressure in high-strength vanadium V-Cr-Zr-W and V-Mo-Zr systems alloys are presented. It was found that after deformation at number of revolutions N = 1, samples are characterized by high anisotropy of defect and grain structure. Inside grains, limited by high-angle boundaries, the formation of two-level structure states was revealed: fragmentation of the above grains on nanofragments from 5 to 20 nm in size with a dipole nature of low-angle misorientations and high (hundreds of degrees per micron) elastic curvature of crystal lattice. Formation of the above structural states leads to a 3-fold increase in microhardness values. Further increase in deformation degree leads to fracture of samples of vanadium alloy V-Mo-Zr with a high volumetric content of fine-disperse oxide phase. At the same time V-Cr-Zr-W-system alloy with a lower concentration of Zr and, as a result, a lower volume fraction of fine particles remains ductile.

Titanium compacts with controllable porosity by slip casting of binary powder mixtures

Titanium compacts with tailored porosity and pore size were fabricated by slip casting of binary powder mixtures, followed by debinding and vacuum sintering. Two different particles sizes (avg. 14μm & avg. 56μm) of pure titanium hydride–dehydride powder were used in six volume ratios (0:100, 20:80, 40:60, 60:40, 80:20 & 100:0). The packing density and sintering behaviour of the titanium compacts were characterised, in terms of porosity, pore size analysis, tensile properties and gas permeability. A theoretical model predicted that the green density reaches a maximum when the volume fraction of fine particles is 0.35. It was found that although experimental results showed similar behaviour there was no well-defined maximum green density. The sintered compacts showed that an increase in the volume fraction of fine powder particles reduced the porosity, permeability level and pore size, and increased the tensile properties. The relationship between permeability and porosity level was non-linear and this was caused by the differences in pore diameters in the compacts. The capillary tube model was used to discuss the relationship between the permeability, pore diameter and porosity. Using this information, a graded porosity compact was designed and fabricated by slip casting.

Uniformity of Grain Coarsening in Submicron Grained Al-Sc Alloy Containing Local Variations in Texture

The effect of fine particles on the uniformity of grain coarsening in a submicron grained Al-Sc alloy containing significant local variations in texture has been investigated using high resolution EBSD. The alloy was processed by severe plastic deformation and low temperature ageing to generate a fine-grained (0.8 µm diameter) microstructure containing either a dispersion of nanosized Al3Sc particles or a particle-free matrix. The initial processing generated a uniform grain size distribution, but the distribution of grain orientations was inhomogeneous with the microstructure containing colonies of grains consisting predominantly of either HAGBs or LAGBs with the latter possessing orientation gradients of up to 10 o/µm. Despite the marked differences in boundary character between these regions, the alloy undergoes slow and uniform grain coarsening during annealing at temperatures up to 500 oC with no marked change in the grain size distribution, boundary distribution and texture. A model of grain coarsening that takes into account the influence of fine particles on the kinetics of grain growth within an orientation gradient is outlined. The model predicts that a large volume fraction of fine particles (large f/r-value) tends to homogenize the overall rate of grain coarsening despite the presence of orientation gradients in the microstructure.

Influence of fine particles on grain coarsening within an orientation gradient

A simple model of grain coarsening is presented that takes into account the influence of both an orientation gradient within the microstructure and a dispersion of fine particles on the kinetics of grain coarsening. The model is used to illustrate the substantial influence of fine particles on grain coarsening whereby, in a particle-free system, grains within a large orientation gradient grow rapidly whereas a system containing a large volume fraction of fine particles results in very limited coarsening, despite the presence of the orientation gradient. The model is used in conjunction with the analysis of grain coarsening in some aluminium alloys where deformation was found to generate a range of microstructural inhomogeneities such as either deformation bands in hot deformed {100}〈001〉 Al–0.05%Si single crystals or a microstructure consisting of colonies of either low angle or high angle grain boundaries in a severely strained particle-containing Al–0.2%Sc alloy. The simulations using the model are found to be in broad agreement with the experimental observations of grain coarsening in these alloy systems.

Thermal expansion coefficient and wear performance of aluminium/SiC composites with bimodal particle distributions

High volume fraction metal matrix composites were produced by infiltration of liquid aluminium into preforms made by mixing and packing SiC particulates having two different average diameters (170 and 16 μm). The maximum particle volume fraction (0.74) was attained for a mixture containing 67% of coarse particles. The variation of particle volume fraction with percentage of coarse particles can be reasonably well understood in terms of a simple model. Experimental results for the threshold pressure indicate that it is mainly determined by the local volume fraction of fine particles, a result which is shown to be compatible with the model used to estimate the particle volume fraction. On the other hand, the coefficient of thermal expansion (CTE) is mainly determined by the total particle volume fraction of the composite. Wear performance was evaluated through sliding wear of the composite against a static alumina ball. The results indicate that, in this case, the key parameter is the coarse particle content of the composite.

Dispersoid Formation and Stability in Alloys

Analysis of dispersoid formation and stability indicates that in-situ formation of an adequate volume fraction of fine particles can be thermodynamically incompatible with stability against high-temperature coarsening. The theory of particle coarsening is extended to include the effects of grain boundaries and dislocations. Theoretical analysis predicts that particle dragging by migrating grain boundaries combined with enhanced coarsening by grain boundary diffusion can give denuded regions near grain boundaries. These predictions of enhanced coarsening and particle dragging are in accord with experimental observations on α-Ti and Ti3Al based alloys.