Volume Fraction Of Cavities Research Articles

Mechanism Insights in Anticorrosion Performance of Waterborne Epoxy Coatings Reinforced by PEI-Functionalized Boron Nitride Nanosheets.

Functionalized hexagonal boron nitride nanosheets (BNNSs) have arisen as compelling anticorrosive additives, yet the precise mechanism of their corrosion resistance enhancement in coatings remains unclear. Here, polyethylenimine functionalized BNNSs (PEI-BNNSs) with approximately 6-11 layers were prepared through a "one-step" method. Then, the PEI-BNNSs/Waterborne epoxy (WEP) composite coatings were incorporated via the waterborne latex blending method for the anticorrosion of the Q235 substrate. The impedance modulus (|Z|f = 0.01 Hz) of 0.5 wt % PEI-BNNSs/WEP composite coating soaked in 3.5 wt % NaCl solution for 35 days increased by 4 orders of magnitude compared to pure WEP coating, exhibiting exceptional long-term resistance against corrosion. The positron annihilation lifetime spectroscopy and corrosion product analysis demonstrated that the reinforced anticorrosion capabilities are not solely ascribed to the "tortuous path effect" arising from BNNSs impermeability. These mechanisms also encompass the reduction in free volume fraction and radius of the free volume cavities within the composite coating brought about by the PEI molecules. Additionally, the increase in coating adhesion, promoted by PEI, plays an important role in augmenting the barrier properties against corrosive agents. This study provided a full comprehension of the role played by functionalized BNNSs in fortifying the anticorrosion attributes of WEP coatings.

Pore-scale study on the effects of randomly distributed void cavities on the thermal performance of composite phase change materials

Composite phase change materials (PCMs) can increase the overall effective thermal conductivity of latent heat thermal energy storage (LHTES) systems and improve their heat transfer performance. However, owing to density differences caused by solid–liquid phase change, void cavities with low thermal conductivity are generated in small spaces where the volume shrinks, thereby increasing the heat transfer resistance. To analyze the pore-scale effects of void cavities on the thermal performance of composite PCMs, a model without void cavities and six models with randomly distributed void cavities were established in this study. A graphics processing unit (GPU) accelerated multiple-relaxation time lattice Boltzmann method was adopted to implement the phase change process and obtain the evolutions of temperature distribution, average liquid fraction, and energy storage performance. The results indicated that, as the volume fraction of the void cavities increased, the average liquid fraction of the composite PCMs slightly increased, whereas the heat storage significantly decreased. At a Fourier number of 0.1, the heat storage per unit width of the composite PCMs was reduced by 3.81 %, 8.02 %, and 11.22 % when the volume fractions were 5 %, 10 %, and 15 %, respectively, compared to the case without void cavities. Additionally, the composite PCMs with large pore-size void cavities had slightly better heat storage efficiency than those with small pore sizes at the same volume fraction of void cavities. It was also found that the LHTES cavity with right-half void cavities had the minimal influence on the heat storage performance.

Pore-scale investigation on effects of void cavity distribution on melting of composite phase change materials

Composite Phase Change Materials (PCMs) incorporated with metal foams are promising candidates for thermal management for space exploration. However, void cavities are generated as a result of the volume change of PCM during the melting process, introducing a resistance to heat transfer due to the low thermal conductivity of the void. In this work, the effects of void cavity distribution on the conduction-dominated melting of composite PCMs under microgravity conditions are studied by a two-dimensional pore-scale lattice Boltzmann method, in which a microstructural description of the metal foam is experimentally characterized with the help of X-ray micro-Computed Tomography. Two typical distribution patterns of void cavities are analsed and computed performance is compared (1) a near-wall void cavity, and (2) randomly distributed void cavities. The evolutions of temperature distributions and melting interfaces are compared, and the average liquid fraction and energy stored per width are deduced to describe the energy storage performance. Moreover, the influence of the volume fraction of void cavities is investigated by comparing temperature distributions and energy storage performances of composite PCMs with four different volume fractions of void cavities (0%, 3.7%, 7.6%, 15.2%). After introducing void cavities, the energy stored per width is reduced by 5.7%, 12.3% and 20.2% for randomly distributed void cavities when volume fraction of void cavities is 3.7%, 7.6%, and 15.2%, respectively, and reduced by 42.2%, 64.1% and 79.7% for the near-wall void cavity, respectively. This work initiates the study of the effects of void cavity distribution on composite PCMs, which will stimulate work on structural optimization of thermal management systems.

Formation and growth of cavities in tensile deformation of Poly(ε-caprolactone) and its miscible blends

Microstructural development of poly(ε-caprolactone) (PCL) and its miscible blends with poly(vinyl methyl ether) (PVME) and poly(styrene-co-acrylonitrile) (SAN) were investigated as a function of strain in the process of uniaxial stretching using in situ synchrotron ultrasmall- and small-angle X-ray scattering (USAXS and SAXS) techniques. The addition of amorphous polymer SAN gives rise to a distinct reduction in the entanglement density of the amorphous phase of PCL, whereas an introduction of PVME does not affect the entanglement density in the blend. At small strains, the initial cavities in pure PCL are oriented perpendicular to the drawing direction, while the cavities at the onset of their formation are oriented along the extensional direction for the PCL/PVME blend. Nevertheless, no cavity scattering is observed in PCL/SAN blend during the process of tensile deformation. These results confirm that the cavitation is directly linked to the lamellar crystallites with their orientation parallel to the drawing direction at small extension ratios. For the two cavitated samples, the volume fraction of cavities was assessed from the integrated scattering intensity based on a three phase model. A critical stress value, above which the lamellar mosaic blocks begin to be pulled apart and the cavitation just sets in, was found to be nearly constant at around 15 MPa. Furthermore, the dimensions of cavities were evaluated at different deformations via direct cylinder model fitting to the USAXS patterns. The height of cavities is much larger than the long spacing of lamellar stacks even at the early stages of cavity formation, which indicates the cavities emanating from the lamellae into several amorphous phases at small elongations. Analysis of the SAXS patterns shows an independence of lamellar long spacing on the sample composition at large strains, which strongly supports the deformation scheme of stress-induced fragmentation and recrystallization.

Effect of Strain Rate and Temperature on the Tensile Flow Behavior and Microstructure Evolution in Fe-0.3 Pct C-CrMoV Grade Steel

The effect of temperature and strain rate on the tensile flow behavior of Fe-0.3 pct C-CrMoV grade steel was studied over a wide range of strain rates (10−4 to 10−1 s−1) and temperatures (700 °C to 950 °C). The flow curves of the steel showed typical dynamic recovery (DRV)-type characteristics at low temperature, high strain rate, and dynamic recrystallization (DRX) type at high temperature > 775 °C. Stress regimes with stress exponent (n) of 3.6 to 5.5 for low–high stresses were observed. The ‘n’ values at temperatures of 850 °C and 900 °C were found to be > 4, which correspond to dislocation climb as the rate controlling mechanism. At 950 °C, ‘n’ value was found to be < 4, where viscous glide is the rate controlling mechanism. The apparent activation energy (Q) was found to be 320 ± 12 kJ mol−1. Hence, the dominant high-temperature deformation mechanism was identified as high-temperature climb of edge dislocations. The strain rate sensitivity index (m) of the steel was evaluated using jump strain rate tests and cyclic temperature and strain rate jump tests over temperatures of 700 °C to 950 °C and strain rates of 10−4 to 10−3 s−1 . Although, ‘m’ value as high as 0.5 was observed, cavitation resulted in premature failure during deformation resulting in low elongation. The volume fraction of cavities was inversely proportional to the strain rate at all temperatures. The fine-grained microstructure aids grain boundary sliding, and diffusion thereby favors the cavity growth at low strain rates. Microstructures evolved during the high-temperature tensile tests were analyzed and the optimum conditions for hot deformation i.e., hot rolling/hot forming schedules were determined as the temperature range of 850 °C to 950 °C and strain rate range of 10−3 to 10−4 s−1. The flow stress data for the steel were found to follow the universal Dorn sine hyperbolic equation.

Size effects on rhodium nanoparticles related to hydrogen-storage capability.

To unveil the origin of the hydrogen-storage properties of rhodium nanoparticles (Rh NPs), we investigated the electronic and crystal structures of the Rh NPs using various synchrotron based X-ray techniques. Electronic structure studies revealed that the hydrogen-storage capability of Rh NPs could be attributed to their more unoccupied d-DOSs than that of the bulk near the Fermi level. Crystal structure studies indicated that lattice distortion and mean-square displacement increase while coordination number decreases with decreasing particle size and the hydrogen-absorption capability of Rh NPs improves to a greater extent with increased structural disorder in the local structure than with that in the mean structure. The smallest Rh NPs, having the largest structural disorder/increased vacancy spaces and the smallest coordination number, exhibited excellent hydrogen-storage capacity. Finally, from the bond-orientational order analysis, we confirmed that the localized disordering is distributed more over the surface part than the core part and hydrogen can be trapped on the surface part of Rh NPs which increases with a decrease in NP diameter.

Microstructures and Tensile Mechanical Properties of Consolidated Copper

Two consolidated copper samples were prepared by consolidation of a gas atomized powder using two different routes of spark plasma sintering (SPS), respectively. The first route was direct SPS of copper powder at 800 °C for 10 minutes, and the second route was compacting copper powder by die-pressing to produce a powder compact followed by SPS of the powder compact at 800 °C for 10 minutes. The microstructures and tensile mechanical properties and the fracture surfaces of specimens have been studied. It was found that the level of consolidation of the sample made using Route 1 was higher than that of the sample made using Route 2, as reflected by the relatively higher strength and better ductility of the first sample than those of the second sample. The lower consolidation level of the second sample was depicted by the higher volume fraction of cavities in the fracture surfaces of specimens which were formed by separation of the powder particles at the weakly bonded interparticle boundaries. The grain sizes of the first sample were larger than those of the second sample. The reasons for the differences in the levels of consolidation and grain sizes associated with two SPS routes are discussed.

Characterisation and quantification of cavities in 9Cr martensitic steel for power plants

This work focuses on the characterisation of cavities evolution in a P91 steel pipe in three conditions: as received, and after creep at 650°C and 60 MPa for 7000 and 9000 h. A field emission gun scanning electron microscope (FEG-SEM) equipped with focused ion beam (FIB) gun, a conventional scanning electron microscope (SEM) and a light optical microscope (LOM) have been employed for the investigation. This study reveals two types of cavities: the pre-existing cavities, which are rare in this type of heat resistant steels, with a mean diameter of 2·56 μm and the cavities produced during creep with diameters smaller than 0·6 μm. Lath boundaries and precipitates are found to be preferential sites for cavity nucleation. Furthermore, the number density and volume fraction of these small cavities are calculated from 2D measurements and compared to 3D results obtained by FIB serial sectioning.

Effects of strain rate and temperature on hot tensile deformation of severe plastic deformed 6061 aluminum alloy

The hot ductility of severe plastic deformed AA6061 was studied at different temperatures and strain rates. The large processed by ECAP specimens with the dimensions of about 100mm×100mm×14mm were then subjected to cold rolling (CR) in order to fabricate the long Ultra-fine grained sheets. According to microscopic observations and hot tensile tests, the combination of two severe plastic deformations (i.e. ECAP+CR) can affect significantly the refinement of grain/subgrain and the ductility of the studied alloy. The results were then compared with those of as-received and CRed cases. In case of ECAP followed by the cold rolling process, the effect of temperature on the ductility, strain rate sensitivity, activation energy and Zener–Hollomon parameter was higher than strain rate. It can be suggested that the possible mechanism dominated the hot tensile deformation during tensile testing is dynamic recovery and dislocation creep. Although the maximum hot ductility was obtained at 500°C, an increase in the volume fraction of cavities and their distribution led to a decrease in hot ductility and superplasticity of AA6061 alloy processed by SPD .

Microstructures and tensile mechanical properties of Ti–6Al–4V bar/disk fabricated by powder compact extrusion/forging

Ti–6Al–4V (wt%) bar and disk with a density close to 100%, a lamellar structure and oxygen content of 0.44 and 0.51 wt%, respectively, have been made by powder compact extrusion and powder compact forging of HDH Ti–6Al–4V powder respectively. Their microstructure, tensile mechanical properties at different strain rates in the range of 10−4–10−1 s−1 and the fracture surfaces and longitudinal sections of the tensile test specimens have been studied. It was found that the level of consolidation of the PE/Ti–6Al–4V bar and PF/Ti–6Al–4V disk was very high, as reflected by their high strength and good ductility, small untransformed interparticle boundaries (smaller than 3 μm in sizes), and low volume fraction of cavities (<1.5%) formed by separation of powder particles at the untransformed interparticle boundaries in the specimens tensile tested at a high strain rate of 10−2 s−1. The high level of consolidation achieved within a short time through induction heating and extrusion/forging of the Ti–6Al–4V powder compacts suggests that the rate of transforming interparticle boundaries to grain boundaries is very high, and the reasons for this might be formation of new powder particle surfaces through plastic deformation and rapid dissolution of the powder particle surface oxide films. The formation of cavities from the untransformed interparticle boundaries in the PE/Ti–6Al–4V bar and PF/Ti–6Al–4V disk is highly strain rate sensitive with the number and volume fraction of the cavities formed increases significantly with increasing the strain rate.

Separation of nucleation and growth of voids during tensile deformation of a dual phase steel using synchrotron microtomography

The damage evolution in a DP980 dual phase steel is followed in situ by synchrotron microtomography during tensile deformation focusing on the effect that the triaxiality, induced by different sample geometries, exerts on damage formation and damage evolution. The growth of existing voids is separated from the voids nucleated between consecutive deformation steps using three-dimensional image analysis. The experimental results are correlated with those obtained by finite element analysis using a Gurson–Tvergaard–Needleman framework with a Chu and Needleman formulation to introduce the effect of nucleation of cavities. A relatively simple way to determine the nucleation parameters is proposed based on the volume of nucleated voids obtained from the tomographies. The evolution of the total volume fraction of cavities obtained from the calculations shows a good agreement with the experiments for the notched samples and reflects the effect of triaxiality on damage. Contrarily to experiments, the calculated accumulated volume fraction of nucleated voids does not reflect the effect of triaxiality suggesting the necessity to implement this parameter in the nucleation model.

Nanoscale Inhomogeneities in Thermoresponsive Triblock Copolymers

We present continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy data of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in aqueous solutions of poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronic or Poloxamer). TEMPO is notably smaller than the spin probes conventionally used in the context of polymer science and reveals the early emergence of small hydrophobic cavities when PPO strands of several molecules aggregate and collapse upon temperature increase. The occurrence of hydrophobic cavities appears independent of the overall molecular weight of the Pluronics, but clearly depends on the relative PPO/PEO contents. In all the cases studied, the volume fraction of hydrophobic cavities increases in a broad temperature range of ≥40 °C. The appearance of the hydrophobic regions does not seem to be directly correlated to micellization of the polymers. A decrease of the relative PPO amount in the polymers not only hinders collapse of the PPO strands, it can also be described as a site exchange of the spin probes between hydrophobic cavities and the surrounding medium. On the other hand, in cases of high PPO contents, spin probe exchange could not be observed. This suggests that one may potentially control the diffusion of small molecules between the micellar cores and the surrounding medium by adjusting the PEO/PPO ratio of the used Pluronics.

Effect of zirconium(IV) propoxide concentration on the thermophysical properties of hybrid organic-inorganic films

In this work, micrometer-thick organic-inorganic hybrid films are fabricated. A photothermal experiment is designed and conducted to characterize the thermophysical properties of hybrid films, as well as the thermal contact resistance between the film and substrate. The molecular cagelike or nanopores, which can strongly enhance the phonon scattering, are considered to be formed inside the films during fabrication. The first order estimation of the volume fraction of cavities and its effect on thermophysical properties are obtained. The effect of zirconium(IV) propoxide (ZPO) concentration on the thermophysical properties of hybrid films is also studied. The effective (measured) thermal conductivity and thermal effusivity of hybrid films are close to those of polymethyl methacrylate (PMMA) films, and are not significantly affected by the added ZPO, which is used to adjust the optic properties of films. The extracted bulk thermal conductivity of the hybrid films is close or smaller than that of bulk PMMA, and shows certain thermal conductivity reduction by the ZPO addition. The thermal effusivity study indicates that the response of the surface temperature change to an abrupt heat flux across the surface of hybrid films will be similar to that of PMMA films.

Effect of strain rate path on cavitation in superplastic aluminium alloy

The effect of a rapid pre-strain on the cavitation behaviour of commercially processed superplastic (SP) aluminium alloy 5083 sheet material deformed in uniaxial tension has been compared with that for deformation under constant strain rate conditions. It was observed for the two test conditions, that there was continuous nucleation of cavities. There were some differences in the distributions of cavity sizes for similar strains between the two testing conditions. However, these were not considered of significance because when the volume fraction of cavities was plotted against SP strain the two sets of data lay on, or close to, the same exponential curve. Metallographic examination showed that cavities were often associated with intermetallic particles. Analysis of the experimental data led to the conclusion that the growth of cavities was controlled primarily by plastic deformation, although diffusion may play a role in the early stages of growth. Overall, the levels of cavitation were lower than those often reported for other SP aluminium alloys. This is attributed to the low levels of iron and silicon impurities, and hence the absence of large constitutive particles considered to promote cavitation, and to the relatively low strains attainable in the alloy.

Stress-state dependence of cavitation and flow behavior in superplastic aluminum alloys

A detailed and quantitative investigation of the stress-state dependence of superplastic cavitation in fine-grained aluminum alloys has been carried out to develop clear evidentiary support to build future models. Several stress states, such as uniaxial tension, plane-strain tension, plane-strain compression, shear, and equibiaxial tension have been examined. Tests were carried out to large strain in an interrupted manner under a constant effective strain rate (\(\dot \varepsilon \)e) in the range of 10−4 to 10−2 s−1. Measurements of volume fraction, population density, and size distribution of cavities, made by image analysis via optical microscopy, show continuous emergence of new cavities as well as growth of cavities during superplastic straining. The total cavity volume fraction (V) increases exponentially with strain. The cavity growth rate, represented by η (equal to d ln V/dee), as well as the cavity population evolution rate with strain (dNc/dee, where Nc is the cavity number/unit area) are found to increase with normalized mean hydrostatic tensile stress (σm/ σe). An empirical equation for the biaxial forming limit in terms of the principal surface strains (e1 and e2) has been defined for a fixed cavity volume, as given by e1=a Vb − α e2, where a and b are constants determined from e1 values for plane strain (e2=0). The value of b is found to be 0.2 to 0.3, and α is 0.4 to 1.0.

Cavitation in superplastic 7075Al alloys prepared via friction stir processing

The effect of strain rate, temperature, and grain size on the cavitation of superplastically deformed 7075Al alloys prepared by friction stir processing (FSP) was systematically examined. The nucleation of cavities was generally observed to be associated with the grain triple junctions and coarse particles. While new cavities were continuously nucleated during deformation, the cavity density did not increase above a certain strain due to significant cavity coalescence and linkage. The density, size, and volume fraction of cavities increased with increasing initial strain rates from 1×10 −2 to 1×10 −1 s −1. Increasing the temperature from 450 to 510 °C resulted in a decrease in the cavity density. However, the specimen deformed at the optimum superplasticity temperature of 480 °C exhibited the lowest cavity volume fraction and ratio of large-size cavities. The decrease in the grain size from 7.5 to 3.8 μm resulted in a significant decrease in the density, size, and volume fraction of cavities. The growth of cavities was controlled by plasticity, and the cavity growth rate parameter, η, decreased with decreasing grain size and strain rate. FSP aluminum alloy exhibited lower cavity level and higher critical strain compared to a thermo-mechanically processed one.

Ultrasonic Velocity Technique for Nondestructive Quantification of Elastic Moduli Degradation during Creep in Silicon Nitride

The ultrasonic velocity technique was used for nondestructive quantification of creep damage during interrupted tensile creep tests at 1400°C in an advanced silicon nitride to investigate the possibilities of this technique for creep damage monitoring in ceramic components. The longitudinal and shear wave velocities, Poisson's ratio, and Young's, shear, and bulk moduli linearly decreased with strain. Precise density change measurements indicated a linear relationship with a coefficient of proportionality of 0.69 between the volume fraction of cavities and tensile strain. Cavitation was identified as the main creep mechanism in the studied silicon nitride and the reason for ultrasonic velocity and elastic moduli degradation. The measurement of just the longitudinal wave velocity changes was found to be sufficient for quantification of cavitation during creep. The capability of the ultrasonic velocity technique for simple, sensitive, and reliable nondestructive monitoring of creep damage during intermittent creep was demonstrated in silicon nitride.

Effect of Superplastic Deformation on Thermal Expansion Behavior of Tetragonal Zirconia Polycrystals

Three mol% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP) were superplastically deformed under various conditions and microstructural evolution was characterized. Thermal expansion properties of the 3Y-TZP specimens were then measured by a push-rod type dilatometer in a temperature range from 300 K to 1473 K. Experimental results showed that the mean coefficient of linear thermal expansion, α, decreased with an increase in the volume fraction of cavities. The cavity dependence of α value was dependent on temperature and was weakened with an increase in temperature. Changes in the average grain size and grain aspect ratio due to the superplastic deformations were found to have little effect on the thermal expansion property within the present experimental range.

Cavity growth during superplastic flow in an Al–Mg alloy: I. Experimental study

Cavitation caused by superplastic straining of a fine-grained Al–Mg–Mn–Cu alloy under uniaxial tension has been quantitatively evaluated. Tensile tests were conducted at constant true strain rate in a range of 10 −4s −1 to 10 −2s −1 at constant temperatures between 450°C and 550°C. Care was exercised to achieve precision in strain-rate control in these tests since strain-rate and temperature history do affect the extent of cavitation. Measurements of the number and size of cavities were made by using image analysis on tested specimens viewed by optical microscopy and further supported by SEM. With increasing strain, the cavity population density increases, a result seen previously but not studied in detail. Cavity growth was also monitored carefully and found to be due to the plastic deformation of the matrix surrounding the cavity. The total volume fraction of cavities which is the product of the above two components was found to increase exponentially with strain. The dependencies of cavity volume fraction on strain-rate and temperature are not straightforward, however. Based on experimental observations of decohesion between matrix and non-deformable particles, continuous nucleation of new cavities, and data related to plasticity-based growth of cavities, attempts are made to explain these complex effects.

Rapid forming of superplastic aluminium alloy 5083

Decreasing the cycle time significantly for forming the commercially available superplastic aluminium alloy 5083 has been achieved. Forming results and conditions are compared with previous relevant works which are actually scarce. A circular cup having a depthdiameter ratio of 1:2 can be formed in 70 s. This ratio requires flat sheet to be stretched in area by up to three times, which should be large enough when dealing with actual industrial sheet forming. On average, the thickness is decreased by two-thirds; in fact, the thickness distribution is not uniform and the gradient is concentrated at the wall of the cup. The location of minimum thickness in rapid forming is different from that in conventional forming. Disregarding the traditional approach, the pressure-time profile employed in this work was not restricted to yield the so called optimum strain rate, which is usually low. Following the same processing profile, but proceeding in stages of partial forming, a series of progressive forming configurations was obtained in order to analyse the strain rate path leading to the successful rapid forming. For a specimen processed at 500C, the maximum volume fraction of cavities is 4 existing at the location of minimum thickness.