Cavity Growth Rate Research Articles

The application of self-healing concept in Ni superalloys: Theoretical design and experimental validation

The self-healing mechanism has been newly applied in Ni superalloys to extend the creep lifetime during service. Quantitative selection criteria are proposed to screen the potential self-healing elements in Ni alloying system. Ti atoms and η-Ni3Ti precipitates are thus selected as the candidate healing atoms and the healing agent, respectively. The ideal composition interval for Ti to achieve the effective healing effects is computationally calculated as 0.076–0.1 mol fraction. On this basis, SHNi (self-healing Ni alloy) has been developed and compared with nSHNi (no self-healing Ni alloy). The creep lifetimes of SHNi alloys are observed 8–10 times higher than that of nSHNi alloys. Microstructure characterizations have shown that η-Ni3Ti healing agents in SHNi alloys can form site-specifically at the cavity sites during service. Meanwhile, the creep cavities in SHNi alloys present much smaller average sizes during creep, and the cavity growth rates are generally one magnitude lower compared to that of nSHNi alloys. The excellent creep properties of SHNi alloys can be well ascribed to their creep cavitation resistance brought by the self-healing behavior. The effectiveness of self-healing mechanism in improving the creep properties has proposed a new strategy in developing novel Ni superalloys.

Effect of Ce on creep damage behavior of 316LN austenitic stainless steel at 700 °C

The effect of rare earth element Ce on creep damage behavior of 316LN austenitic stainless steel (316LN) at 700 °C was systematically investigated under stress ranging from 125 MPa to 200 MPa. The results show that the addition of 0.032 wt% Ce remarkably improves the rupture life of 316LN steel under various stress. Ce addition in 316LN steel decreases the nucleation stress, stabilizes stress concentrations at particles, and notably raises the cavity nucleation rate. Moreover, Ce addition suppresses cavity growth during creep, thus the cavity growth rate significantly decreases and the creep rupture ductility increases under high stress over 175 MPa. However, the anomalous growth rate was engendered in some cavities by Ce addition in 316LN steel under low stress below 150 MPa. Consequently, Ce addition in 316LN steel reduces the creep rupture ductility and changes the rupture mode from trans-granular dimple fracture under high stress to mixed fracture under low stress.

The breakage of original crystallites determined small strain cavitation in syndiotactic polypropylene during step-cycle tensile deformation

The step-cycle tensile experiments were carried out on syndiotactic polypropylene to explore the crystallization and deformation temperature dependency of cavitation via the wide and small angle scattering X-ray techniques. The development of cavitation process exhibited a dependency on the original crystallites. The cavities appeared around the yield point could be healed after releasing the tensile force before a specific strain, while the ones occurred beyond this point are permanent. This phenomenon was in accord with the process of the recoverable lamellar thickness within one stretching and relaxation cycle. After reaching the critical point of fragmentation and recrystallization of lamellae, the growth rate of cavities started to decrease due to the destruction of the original crystallites. Moreover, the crystallites formed at higher crystallization temperature could be preserved to a larger strain and thus more intense cavitation could be expected. The high deformation temperature could weaken the intensity of cavitation, then, the weakest cavitation phenomenon was observed in the sample crystallized at low temperature but drawn at high temperature.

Prolonged creep lifetime of ferritic self-healing steels achieved by offline healing treatment

For the ferritic self-healing (FSH) steel, it remains challenging to solve the low healing efficiency problem, due to the mismatch between the formation kinetics of creep cavities and the precipitation kinetics of healing agents. In this research, an offline healing treatment has been proposed to stimulate the formation of the Laves phase precipitates as self-healing agents to retard creep cavity growth at elevated temperatures. The corresponding treatment parameters were determined by the precipitation behaviors of Laves phases on the external free surfaces and internal cavity surfaces of FSH steels. Results have shown that the creep lifetime of the offline healing treated samples can be extended more than 30% compared to that of non-healed sample, while the creep rates can be reduced by one order of magnitude. The formation kinetics of Laves phases and creep cavities have been quantitatively analyzed. It was found that after the offline healing treatments, the average cavity sizes of the samples decreased significantly, while the growth rates of creep cavities rather than the nucleation kinetics are compensated by the healing phenomena of Laves phase formation. The relationship between the extended lifetime and filling ratios of creep cavities by the healing agents was established as well. The demonstrated potential of offline healing treatments provides a new strategy to cultivate the self-healing capabilities of heat resistant steels.

Cavitation behavior during tensile deformation at room temperature in fine-grained pure magnesium

Cavitation behavior associated with grain boundary sliding at room temperature was investigated using fine-grained magnesium (Mg) specimens produced by extrusion or high-pressure torsion. Regardless of the process method, flow stress in both of Mg specimens depended on the tensile strain rate, and the strain rate sensitivities (m-values) were determined to be approximately 0.3 at low strain rates. Deformed microstructural observations revealed that many cavities were formed at grain boundaries and grain boundary triple junctions, even during the plastic deformation at room temperature. The size and density of such cavities increased with the progression of deformation. The majority of cavities were controlled by plasticity, and the cavity growth rate was similar to those typically observed in superplastic Mg alloys subjected to elevated temperature tensile tests. In addition, initial basal texture was unlikely to affect these cavitation behaviors, e.g., cavity growth rate.

Regression Models Utilization to the Underground Temperature Determination at Coal Energy Conversion

The underground coal gasification represents a technology capable of obtaining synthetic coal gas from hard-reached coal deposits and coal beds with tectonic faults. This technology is also less expensive than conventional coal mining. The cavity is formed in the coal seam by converting coal to synthetic gas during the underground coal gasification process. The cavity growth rate and the gasification queue’s moving velocity are affected by controllable variables, i.e., the operation pressure, the gasification agent, and the laboratory coal seam geometry. These variables can be continuously measured by standard measuring devices and techniques as opposed to the underground temperature. This paper researches the possibility of the regression models utilization for temperature data prediction for this reason. Several regression models were proposed that were differed in their structures, i.e., the number and type of selected controllable variables as independent variables. The goal was to find such a regression model structure, where the underground temperature is predicted with the greatest possible accuracy. The regression model structures’ proposal was realized on data obtained from two laboratory measurements realized in the ex situ reactor. The obtained temperature data can be used for visualization of the cavity growth in the gasified coal seam.

The Role of Co-injected Helium on Swelling and Cavity Evolution at High Damage Levels in Ferritic-Martensitic Steels

The influence of helium co-injection at rates from 0 to 4 appm He/dpa on swelling in ferritic-martensitic alloys T91 and HT9 was explored. Irradiations with 5.0 MeV Fe++ ions and degraded He++ ions were performed at 445°C up to damage levels of 150 dpa and helium co-injection rates of 0, 0.02, 0.2 and 4 appm He/dpa in T91, and at 460°C to a damage level of 188 dpa and helium co-injection rates of 0, 0.06 and 4 appm He/dpa in HT9. Helium was observed to enhance cavity nucleation at low damage levels, resulting in the maximum swelling at the highest helium co-injection rate. As the damage level was increased, the helium content at which swelling was maximized shifted to lower helium concentrations, ultimately resulting in the highest swelling occurring with zero helium by 150 dpa. This behavior was due to the helium-stabilized bubble microstructure that increased the cavity sink strength and reduced both cavity growth rate and swelling relative to the helium-free condition.

Electrostrictive Cavitation in Water Induced by a SnO2 Nanoparticle.

Cavitation phenomenon in dielectric fluids has been a recent topic of interest in theory and experiment. We study a dielectric fluid–nanoparticle system subjected to an external electric field using molecular dynamics simulations. Electric fields ranging from 0.042 to 0.25 V/Å are applied to a water and tin dioxide system. Cavitation is observed in simulations with both SPC/E water and the hydrogen bonding polarizable model. The cavitation onset time displays a stretched exponential relaxation response with respect to the applied electric field with an exponent β = 0.423 ± 0.08. This is in accordance with the exact theoretical value for systems with long-ranged forces. Cavity growth rates are divided into two phases, a spherical growth phase and a cylindrical one. Both are reported as a function of the applied electric field. The structure of the electric field is analyzed both spatially and temporally.

A 0-dimensional cavity growth submodel for use in reactor models of underground coal gasification

Modelling of the underground coal gasification process is dependent upon a range of sub-models. One of the most important is the calculation of the cavity growth rate as a function of various operating conditions and coal properties. While detailed 1-dimensional models of coal block gasification are available, it is not easy to couple them directly with reactor models, which aim to simulate the complete process. In this paper, a 0-dimensional cavity growth sub-model is presented. The model is based on the concept of a surface reaction and incorporates physics to account for moisture evaporation, water influx, coal pyrolysis, coal thermo-mechanical fragmentation and the build up of an ash layer on the char. The model is validated using measurements from laboratory experiments on coal cores and coal blocks. A comparison of calculated results from several UCG field trials shows that the model can provide good estimates of cavity growth rate for reasonable input parameters. Finally, simulation results of cavity growth in the combustion and gasification zones as a function of the bulk gas temperature, gas pressure, water influx rate, ash layer thickness and coal fragmentation behaviour are presented.

In situ nanotomography study of creep cavities in Al-3.6-Cu alloy

Cavity growth during high temperature deformation has been extensively studied theoretically, but experimental investigation of nucleation and early stages of growth has been limited. In this work, in situ nanotomography investigation of cavity nucleation and growth during high temperature deformation (7.9 MPa, 698 K), has been carried out in Al −3.6 wt% Cu alloy. This model alloy allows controlled generation of second phase particles to promote cavity nucleation and has hence been chosen for the study. Fast 3D imaging using synchrotron X-Rays with 100 nm pixel size and scan time of 7 s has been done to track nucleation and volumetric growth of individual cavities during deformation. Using this, change in shape of cavities with straining has been studied. Also, the change of volumetric cavity growth rate vs equivalent radius of individual cavities has been compared to existing models of cavity growth by diffusion and plasticity. It was seen that several pre-existing porosities were present in the alloy, while very few cavity nucleations were observed. The experimental data of growth rate matched well with the studied models and it was concluded that cavities initially grew by diffusion, while the growth mechanism changed to plasticity near failure.

Cavitation Growth Phenomena in Pure-Sliding Grease EHD Contacts

This article describes experimental and theoretical studies on the cavitation phenomena in the grease lubrication film under pure sliding elastohydrodynamic contact. In situ observation tests using the optical interferometry technique were conducted, and the growth of cavitation was captured using a high-speed camera. The results showed that the cavity grew in two stages, which was similar to the behavior in the base oil, and that the cavity growth rate in the initial stage was higher than that in the second stage. In the initial stage, the cavity growth time in the grease was longer than that in the base oil, and the cavity length after the growth depended on the base oil viscosity. It was also found in the test using diurea grease that small cavities were formed by the lumps of thickener. The cavity growth in the initial stage was discussed by numerical simulation of pressure distribution based on a simple rheological model.

Modelling the formation and self-healing of creep damage in iron-based alloys

A self-consistent model is applied to predict the creep cavity growth and strain rates in metals from the perspective of self-healing. In this model, the creep cavity growth rate is intricately linked to the strain rate. The self-healing process causes precipitates to grow inside creep cavities. Due to the Kirkendall effect, a diffusional flux of vacancies is induced in the direction away from the creep cavity during this selective self-healing precipitation. This process impedes the creep cavity growth. The critical stress for self-healing can be derived, and an analysis is made of the efficiency of self-healing elements in binary Fe–Cu, Fe–Au, Fe–Mo, and Fe–W alloys. Fe–Au is found to be the most efficient self-healing alloy. Fe–Mo and Fe–W alloys provide good alternatives that have the potential to be employed at high temperatures.

Superplasticity of a dual-phase-dominated Mg-Li-Al-Zn-Sr alloy processed by multidirectional forging and rolling

The microstructures, mechanical properties, deformation mechanism and cavitation growth of Mg-10.2Li-2.1Al-2.23Zn-0.2Sr alloy subjected to multidirectional forging and rolling (MDFR) were studied to examine the deformability of the Mg-Li alloy. X-ray diffraction (XRD) results confirm the existence of α (Mg) and β (Li) phases and Mg17Al12, Al4Sr and LiMgAl2 intermetallic compounds. Studies of the microstructures reveal that a thin banded-grained microstructure with a grain size less than 3.75µm is obtained via MDFR and annealing at 523K for 1h. There were significant grain refinements. The maximum elongation to failure was 712.1%, and this was obtained in the current alloy at 623K with a strain rate of 1.67 × 10−3s−1. The strain rate sensitivity exponent and the activation energy for deformation were estimated to be 0.884 and 91.9kJ/mol, respectively. This indicates that the rate-controlling deformation mechanism under the aforementioned condition is grain boundary sliding controlled by lattice diffusion. Experimental cavity observations show the existence of cavity interlinkage or stringers. A new plasticity-controlled cavity growth rate equation considering cavity interlinkage was established, and a cavity growth diagram was constructed. The diagram prediction is consistent with the experimental results. In addition, fractographs show that the intergranular fracture is a ductile fracture mechanism. At room temperature, the ultimate tensile strength of 242MPa and the elongation of 23.59% were obtained in the present alloy.

Excellent high strain rate superplasticity of Al-Mg-Sc-Zr alloy sheet produced by an improved asymmetrical rolling process

A fine-grained Al-6.10%Mg-0.30%Mn −0.25%Sc-0.1%Zr (wt. %) alloy with an average grain size of ∼1.10 μm was subjected to an improved asymmetrical rolling (ASR) process. Superplastic behavior of the asymmetrical rolled alloy was investigated at 450–500 °C and strain rate range of 5 × 10−3 s−1 to 2.5 × 10−1 s−1. It is indicated that the asymmetrical rolled alloy exhibited excellent superplasticity (elongations of >1000%) at high strain rates ranging from 1 × 10−2 s−1 to 2.5 × 10−1 s−1, and maximum elongation of ∼3170% was obtained at 500 °C and a high strain rate of 5 × 10−2 s−1. The microstructural results showed that the shear components and the β-fiber rolling texture gradually transferred into a random texture, and LABs progressively changed to HABs during deformation. Such a microstructure can accelerate the cooperative grain boundary sliding leading to a higher superplasticity. Further, superior superplastic behavior of the asymmetrical rolled alloy can be ascribed to the fine (sub)grains which may slow down the rate of cavity growth and the stable coherent Al3(Sc1-xZrx) nano-particles that can ensure a good stability of the fine-grained structure during superplastic deformation. Analyses on superplastic data and microstructure characteristic revealed that grain boundary sliding might be the main deformation mechanism during deformation.

Analysis of Void Growth During Superplastic Deformation of Commercial AL5083 Alloy

Superplastic alloys and metals possess the ability to undergo large uniform strains prior to failure. A number of materials are subject to the cavitation during superplastic deformation. Cavitation usually leads to either the undesirable post-forming characteristics or to the premature tensile failure. It is also apparent that the cavities can preexist in the form of cracks and decohered interfaces, which develop during thermo-mechanical processing necessary to produce the superplastic microstructures. Evidently, extensive cavitation imposes significant limitations on their commercial application. The material constitutive equation constants of commercial AL5083 alloy contain strength coefficient, and strain rate sensitivity index is determined by superplastic bulge forming tests for 400, 450, 500, 550 °C. By comparing the results of a deformed sample, good accordance between experimental and FEM results is observed. Using the calculated values for C and m parameters, the effect of material properties such as the cavity growth rate, strain rate sensitivity index, strain hardening exponent and number of intentionally preexisted voids on specimens voids growth subject to the tensile deformation and to the biaxial deformation has been determined numerically. The tensile tests have been simulated by the FEM software ABAQUS v6.9 using commercial aluminum 5083 alloy that presents superplastic properties at temperatures in the range 400–550 °C. The simulations are implemented at 400, 450, 500 and 550 °C. The results of the numerical prediction obtained are in good agreement with the results of the experiments done by some other authors.

Spectral content of cloud cavitation about a sphere

The physics and spectral content of cloud cavitation about a sphere are investigated in a variable-pressure water tunnel using dynamic surface pressure measurement and high-speed imaging. Experiments are conducted using a polyvinyl chloride sphere at a Reynolds number of $1.5\times 10^{6}$ with cavitation numbers, $\unicode[STIX]{x1D70E}$, ranging from inception to supercavitation. Three distinct shedding regimes are identified: a uni-modal regime for $\unicode[STIX]{x1D70E}>0.9$ and two bi-modal regimes for $0.9>\unicode[STIX]{x1D70E}>0.675$ and $0.675>\unicode[STIX]{x1D70E}>0.3$. For small cavity lengths ($\unicode[STIX]{x1D70E}>0.9$), Kelvin–Helmholtz instability and transition to turbulence in the overlying separated boundary layer form the basis for cavity breakup and coherent vortex formation. At greater lengths ($\unicode[STIX]{x1D70E}<0.9$), larger-scale shedding ensues, driven by coupled re-entrant jet formation and shockwave propagation. Strong adverse pressure gradients about the sphere lead to accumulation and radial growth of re-entrant flow, initiating breakup, from which, in every case, a condensation shockwave propagates upstream causing cavity collapse. When the shedding is most energetic, shockwave propagation upstream may cause large-scale leading edge extinction. The bi-modal response is due to cavity shedding being either axisymmetric or asymmetric. The two bi-modal regimes correspond to $\unicode[STIX]{x1D70E}$ ranges where the cavity and re-entrant jet either remain attached or become detached from the sphere. There is a distinct frequency offset at transition between regimes in both shedding modes. Despite the greater cavity lengths at lower $\unicode[STIX]{x1D70E}$ values, the second bi-modal regime initially exhibits shorter shedding periods due to increased cavity growth rates. The second regime persists until supercavitation develops for $\unicode[STIX]{x1D70E}<0.3$.

Effect of β/B2 phase on cavitation behavior during superplastic deformation of TiAl alloys

Fine-grained Ti-(42.5–43.5)Al-8Nb-0.2W-0.2B-(0–0.1)Y (in at.%) alloys with two different phase constituents (i.e., (α2+γ) alloy and (β/B2+γ) alloy) have been prepared by means of thermo-mechanical processing. Tensile tests were performed at 1000 °C under an initial strain-rate of 10−4 s−1 to evaluate the superplastic properties, and it was found that both alloys exhibited impressive superplasticity so that the elongations reached ∼400%. Texture measurements preliminarily revealed that intragranular deformation and the subsequent dynamic recrystallization was the predominant superplastic deformation mechanism in (β/B2+γ) alloy, while grain boundary sliding was responsible for the large elongation of (α2+γ) alloy. Cavities caused by superplastic deformation have been quantitatively evaluated by using SEM and X-ray tomography to reveal the effect of β/B2 phase on cavitation behavior. It was found that in (α2+γ) alloy the cavities preferentially nucleated in the triple-junctions of interphase boundaries, whereas most of the small cavities in (β/B2+γ) alloy were located along the β/γ interfaces. Quantitative analysis revealed that the alloys had comparable cavity number density after deformation, but the (β/B2+γ) alloy was characterized by larger cavity sizes, and thus by higher cavitation volume fraction and coalescence parameter in comparison with that of (α2+γ) alloy. Moreover, the large cavities in (β/B2+γ) alloy preferentially elongated along the direction parallel to the tensile axis, whereas the large cavities were more equiaxed in (α2+γ) alloy. The different cavity nucleation–growth–coalescence behavior of the two alloys was clarified based on the divergence in superplastic deformation mechanisms. Besides, theoretical evaluations were performed and revealed that the cavity growth rate of γ-TiAl was much higher than ordinary Al alloys under similar deformation conditions. This was believed to be the main reason for the relatively lower superplastic elongations of TiAl-based intermetallics.

Creep cavity growth models for austenitic stainless steels

Diffusion controlled cavity growth models tend to exaggerate the growth rate. For this reason it is essential to take into account the restrictions caused by creep rate of the surrounding material, so called constrained growth. This has the consequence that the stress that the cavities are exposed to is reduced in comparison to the applied creep stress. Previous constrained growth models have been based on linear viscoplasticity. To avoid this limitation a new model for constrained growth has been formulated. Part of the work is based on a FEM study of expanding cavities in a creeping material. Compared with the previous constrained cavity growth models, the modified one gives lower reduced stresses and thereby lower cavity growth rates. By using recently developed cavity nucleation models, the modified creep cavity growth model can predict the cavity growth behaviour quantitatively for different types of austenitic stainless steels, such as 18Cr10Ni, 17Cr12NiNb and 17Cr12NiTi.

A process model for underground coal gasification – Part-I: Cavity growth

In underground coal gasification (UCG), a cavity is formed in the coal seam due to consumption of coal. The irregular-shaped cavity consists of a spalled-rubble on the cavity floor, a cavity roof and a void zone between the two. Depending on the cavity growth pattern, UCG process can be divided into two distinct phases. In phase-I, coal/char near injection well gets consumed and cavity grows in a vertical direction and hits the overburden. Phase-II starts thereafter, in which the cavity grows in the horizontal direction toward the production well. This paper presents an unsteady-state model for gas production during phase-I for a coal under consideration for UCG. The non-ideal flow patterns in the cavity are determined using computational fluid dynamics (CFD). The CFD results and residence time distribution (RTD) studies show that the complex UCG cavity can be reduced to a computationally less time consuming compartment model consisting of a radial plug flow reactor (PFR) followed by a continuous stirred tank reactor (CSTR). The developed compartment model incorporates reaction kinetics, heat-transfer, mass-transfer, diffusional limitations and thermo-mechanical failure effects for the coal of interest. The model is tested on a lab scale UCG; it can predict the location of reaction and drying fronts, profiles of solid and gas compositions, exit gas calorific value and cavity growth rates. Further, the model predictions show an excellent match with the cavity growth rate and exit gas quality observed during laboratory-scale UCG-like experiments on the coal of interest. Therefore, the model can potentially be used to determine feasibility of UCG for any other coal for the known kinetics and spalling parameters.

Prediction of cavity growth rate during underground coal gasification using multiple regression analysis

During underground coal gasification (UCG), whereby coal is converted to syngas in situ, a cavity is formed in the coal seam. The cavity growth rate (CGR) or the moving rate of the gasification face is affected by controllable (operation pressure, gasification time, geometry of UCG panel) and uncontrollable (coal seam properties) factors. The CGR is usually predicted by mathematical models and laboratory experiments, which are time consuming, cumbersome and expensive. In this paper, a new simple model for CGR is developed using non-linear regression analysis, based on data from 11 UCG field trials. The empirical model compares satisfactorily with Perkins model and can reliably predict CGR.