The evolution of void nucleation inα-A12O3 irradiated by En ≥ 1 MeV neutrons of 3× 1020 cm-2 and post-annealed from 100°C to 1050°C is studied by a positron annihilation lifetime technique. The void n...The evolution of void nucleation inα-A12O3 irradiated by En ≥ 1 MeV neutrons of 3× 1020 cm-2 and post-annealed from 100°C to 1050°C is studied by a positron annihilation lifetime technique. The void nucleation starts at 500℃. The radius of created voids is 0.31 urn and the number of voids increases with increasing annealing temperature from 550℃ to 750℃. Afterwards, the radius of voids increases rapidly and reaches 1.21nm at 1050℃.展开更多
Void nucleation within measured particle fields of an aluminum alloy is investigated to develop a continuum nucleation model which reflects nucleation at the individual particle scale. A nucleation model for heterogen...Void nucleation within measured particle fields of an aluminum alloy is investigated to develop a continuum nucleation model which reflects nucleation at the individual particle scale. A nucleation model for heterogeneous particle distributions is synchronized with the continuum model of Chu and Needleman using the average nucleation strain. The parameters in the continuum model are identified from the particle fields and are evaluated over the range of stress states observed in sheet metal forming. The synchronized continuum nucleation model achieves very good agreement with the nucleation trends for three measured particle fields in uniaxial tension, plane strain, and equal-biaxial tension.展开更多
To discover the microscopic mechanism responsible for cavitation nucleation in pure water,nucleation processes in pure water are simulated using the molecular dynamics method.Cavitation nucleation is generated by unif...To discover the microscopic mechanism responsible for cavitation nucleation in pure water,nucleation processes in pure water are simulated using the molecular dynamics method.Cavitation nucleation is generated by uniformly stretching the system under isothermal conditions,and the formation and development of cavitation nuclei are simulated and discussed at the molecular level.The processes of energy,pressure,and density are analyzed,and the tensile strength of the pure water and the critical volume of the bubble nuclei are investigated.The results show that critical states exist in the process of cavitation nucleation.In the critical state,the energy,density,and pressure of the system change abruptly,and a stable cavitation nucleus is produced if the energy barrier is broken and the critical volume is exceeded.System pressure and water density are the key factors in the generation of cavitation nuclei.When the critical state is surpassed,the liquid is completely ruptured,and the volume of the cavitation nucleus rapidly increases to larger than 100 nm^(3);at this point,the surface tension of the bubble dominates the cavitation nucleus,instead of intermolecular forces.The negative critical pressure for bubble nucleation is-198.6 MPa,the corresponding critical volume is 13.84 nm^(3),and the nucleation rate is 2.42×10^(32)m^(-3)·s^(-1)in pure water at 300 K.Temperature has a significant effect on nucleation:as the temperature rises,nucleation thresholds decrease,and cavitation nucleation occurs earlier.展开更多
According to classical nucleation theory, gas nuclei can generate and grow into a cavitation bubble when the liquid pressure exceeds a threshold. However, classical nucleation theory does not include boundary effects....According to classical nucleation theory, gas nuclei can generate and grow into a cavitation bubble when the liquid pressure exceeds a threshold. However, classical nucleation theory does not include boundary effects. An enclosed spherical liquid cavity surrounded by elastic medium is introduced to model the nucleation process in tissue. Based on the equilibrium pressure relationship of a quasi-static process, the expressions of the threshold and the modified nucleation rate are derived by considering the tissue elasticity. It is shown that the constraint plays an important role in the nucleation process. There is a positive correlation between nucleation threshold pressure and constraint, which can be enhanced by an increasing tissue elasticity and reducing the size of the cavity. Meanwhile, temperature is found to be a key parameter of nucleation process, and cavitation is more likely to occur in confined liquids at temperature T > 100℃. In contrast, less influences are induced by these factors, such as bulk modulus, liquid cavity size, and acoustic frequency. Although these theoretical predictions of the thresholds have been demonstrated by many previous researches, much lower thresholds can be obtained in liquids containing dissolved gases, e.g., the nucleation threshold is about-21 MPa in a liquid of 0.8-nm gas nuclei at room temperature. Moreover, when there is a gas nucleus of 20 nm, the theoretical threshold pressure might be less than1 MPa.展开更多
基金the National Natural Science Foundation (No.19835050) and Nuclear industryScience Foundation (No.H7196BOll6)
文摘The evolution of void nucleation inα-A12O3 irradiated by En ≥ 1 MeV neutrons of 3× 1020 cm-2 and post-annealed from 100°C to 1050°C is studied by a positron annihilation lifetime technique. The void nucleation starts at 500℃. The radius of created voids is 0.31 urn and the number of voids increases with increasing annealing temperature from 550℃ to 750℃. Afterwards, the radius of voids increases rapidly and reaches 1.21nm at 1050℃.
基金supported by the Natural Sciences and Engineering Research Council of Canada(NSERC)the New Brunswick Innovation Foundation (NBIF)the Auto 21 Network of Centers of Excellence
文摘Void nucleation within measured particle fields of an aluminum alloy is investigated to develop a continuum nucleation model which reflects nucleation at the individual particle scale. A nucleation model for heterogeneous particle distributions is synchronized with the continuum model of Chu and Needleman using the average nucleation strain. The parameters in the continuum model are identified from the particle fields and are evaluated over the range of stress states observed in sheet metal forming. The synchronized continuum nucleation model achieves very good agreement with the nucleation trends for three measured particle fields in uniaxial tension, plane strain, and equal-biaxial tension.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51779187 and 51873160)。
文摘To discover the microscopic mechanism responsible for cavitation nucleation in pure water,nucleation processes in pure water are simulated using the molecular dynamics method.Cavitation nucleation is generated by uniformly stretching the system under isothermal conditions,and the formation and development of cavitation nuclei are simulated and discussed at the molecular level.The processes of energy,pressure,and density are analyzed,and the tensile strength of the pure water and the critical volume of the bubble nuclei are investigated.The results show that critical states exist in the process of cavitation nucleation.In the critical state,the energy,density,and pressure of the system change abruptly,and a stable cavitation nucleus is produced if the energy barrier is broken and the critical volume is exceeded.System pressure and water density are the key factors in the generation of cavitation nuclei.When the critical state is surpassed,the liquid is completely ruptured,and the volume of the cavitation nucleus rapidly increases to larger than 100 nm^(3);at this point,the surface tension of the bubble dominates the cavitation nucleus,instead of intermolecular forces.The negative critical pressure for bubble nucleation is-198.6 MPa,the corresponding critical volume is 13.84 nm^(3),and the nucleation rate is 2.42×10^(32)m^(-3)·s^(-1)in pure water at 300 K.Temperature has a significant effect on nucleation:as the temperature rises,nucleation thresholds decrease,and cavitation nucleation occurs earlier.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 11974232 and 11727813)。
文摘According to classical nucleation theory, gas nuclei can generate and grow into a cavitation bubble when the liquid pressure exceeds a threshold. However, classical nucleation theory does not include boundary effects. An enclosed spherical liquid cavity surrounded by elastic medium is introduced to model the nucleation process in tissue. Based on the equilibrium pressure relationship of a quasi-static process, the expressions of the threshold and the modified nucleation rate are derived by considering the tissue elasticity. It is shown that the constraint plays an important role in the nucleation process. There is a positive correlation between nucleation threshold pressure and constraint, which can be enhanced by an increasing tissue elasticity and reducing the size of the cavity. Meanwhile, temperature is found to be a key parameter of nucleation process, and cavitation is more likely to occur in confined liquids at temperature T > 100℃. In contrast, less influences are induced by these factors, such as bulk modulus, liquid cavity size, and acoustic frequency. Although these theoretical predictions of the thresholds have been demonstrated by many previous researches, much lower thresholds can be obtained in liquids containing dissolved gases, e.g., the nucleation threshold is about-21 MPa in a liquid of 0.8-nm gas nuclei at room temperature. Moreover, when there is a gas nucleus of 20 nm, the theoretical threshold pressure might be less than1 MPa.
