Ken-ichi Ebihara

Mobile effect of hydrogen on intergranular decohesion of iron: first-principles calculations

Atomistic mechanisms of hydrogen-induced cracking along a bcc Fe Σ3(111) symmetrical tilt grain boundary (GB) have been studied by first-principles calculations. The mobile and immobile effects of hydrogen on the GB decohesion are analyzed by calculating the dependence of hydrogen segregation energy on the coverage relevant to the repulsive interaction among segregated hydrogen atoms at the GB and on its fracture surfaces, together with generalizing McLeans formula. It was found that the segregation of combined mobile and immobile hydrogen atoms from the bulk and/or GB on the fracture surfaces causes much stronger reduction (70–80%) in the GB cohesive energy. It can occur even at a very low bulk hydrogen content of about 10−9 atomic fraction during slow cracking. This is in contrast to only 10–20% decohesion induced by immobile hydrogen at much higher hydrogen content during fast cracking. The mobile effect of hydrogen, giving rise to a profound reduction in the GB cohesive energy, is a key factor controlling the mechanism of hydrogen-induced GB cracking.

Atomic scale HAADF-STEM study of η′ and η 1 phases in peak-aged Al–Zn–Mg alloys

The microstructures of precipitates in Al–Zn–Mg alloys in peak-aged condition have been studied using scanning transmission electron microscope. The same thermo-mechanical treatment was applied in all alloys. Investigation of peak-aged samples revealed that the most commonly found phases were η′ and η1 with their respective habit planes on {111}Al and {100}Al. η′ phases under [110]Al were analyzed and compared with η′ structure models. Furthermore, a close inspection of η1 phase as the second most found precipitate revealed that it incorporates an anti-phase resembling boundary, not observed in other orientation relationships that precipitates create with Al matrix, in addition, differences in matrix-precipitate interfaces between η′/η2 and η1 phases were noticed. This paper addresses the first part to the analysis of η′ phase. Next part is extended to the analysis of the η1 phase.

Surface of Dense Phase in Lattice-Gas Fluid with Long-Range Interaction

A lattice gas with long-range interaction can simulate phase separation in the system consisting of one kind of component particle like the liquid-vapor theory of van der Waals. The generated phases are distinguished from each other by their particle density. In lattice-gas fluid with long-range interaction, the phase with high density can be observed in the phase with low density like the droplet in vapor. In this paper, the surface of the droplet in lattice-gas fluid with the long-range interaction is determined from the local density and its position is compared with that of Gibbss dividing surface. The inside region and the outside region of the droplet are defined on the basis of the mean free path in each region. The surface tension is calculated through Laplaces formula using the droplet radius and the pressures in both regions. It is shown that the surface thickness becomes 4r where r is the distance of the long-range interaction.

Numerical Simulation of Irradiation-Induced Grain-Boundary Phosphorous Segregation in Reactor Pressure Vessel Steels Using Rate Theory Model With First-Principles Calculations

The experimental results on neutron-irradiated reactor pressure vessel (RPV) steels have revealed grain boundary segregation of phosphorous (P) due to neutron irradiation, which may lead to intergranular fracture. Because of the lack of experimental database, however, the dependence of the segregation on variables such as dose, dose-rate, and temperature is not clear. Here, we incorporate the parameters determined by first-principles calculations into the rate theory model which was developed for bcc lattice on the basis of the fcc lattice model proposed by Murphy and Perks [1], and apply it to the simulation of irradiation-induced P segregation in bcc iron. We evaluate the grain boundary P coverage and discuss its dependence on dose-rate and irradiation temperature by comparing our results with previously reported results and experimental data. As results, we find that dose-rate does not affect the grain boundary P coverage within the range of our simulation condition and that the dependence on irradiation temperature differs remarkably from the previous results.Copyright

LATTICE BOLTZMANN SIMULATION OF THE INTERFACIAL GROWTH OF THE HORIZONTAL STRATIFIED TWO-PHASE FLOW

The interfacial growth of the horizontal stratified two-phase flow, which is caused by the velocity difference between two phases, is simulated by the lattice Boltzmann scheme for two-phase fluid. In order to verify the validity of the interface generated by the lattice Boltzmann method, the critical velocity difference that cause the interfacial instability and growth is measured in 2D and 3D for several wave numbers. The measurement is nearly agreement with the analysis of the Kelvin-Helmholtz instability theory.

Multiscale thermodynamic analysis on hydrogen-induced intergranular cracking in an alloy steel with segregated solutes

Abstract A multiscale analysis has been conducted on hydrogen-induced intergranular cracking at ambient temperature in medium strength (840 MPa) Ni-Cr steel with antimony, tin, and phosphorous segregation. Combining first-principles calculations and fracture mechanics experiments, a multiscale relationship between threshold stress intensity factor (Kth) and cohesive energy of grain boundary (the ideal work of interfacial separation, 2γint) was revealed. The Kth was found to decrease rapidly under a certain threshold of 2γint, where the 2γint decreases mainly by mobile hydrogen segregation on fracture surfaces. This segregation is considered to arise during formation of the fracture surfaces under thermodynamic equilibrium in slow crack growth. The resulting strong decohesion probably makes it difficult to emit dislocations at the microcrack tip region, leading to a large reduction in stress intensity factor. Our analysis based on this mobile hydrogen decohesion demonstrates that the Kth decreases dramatically within a low and narrow range of hydrogen content in iron lattice in high-strength steels.

Influence of Hydrodynamic Interaction on Jet Breakup and Fragmentation Behavior

Mitigative measures against a Core Disruptive Accident (CDA) are important from the viewpoints of safety of a Fast Breeder Reactor (FBR). If a CDA occurs, Post Accident Heat Removal (PAHR) must be surely achieved. In the PAHR, molten materials are likely to be injected into the coolant like a jet and they must satisfy two requests simultaneously: fast ejection and stable cooling after quenched. In order to estimate the quench behavior of the molten jet, it is important to understand how the jet breaks up.The objective of this study is to clarify that the influence of hydrodynamic interaction between a jet and the surrounding fluid on jet breakup. Previous works have clarified that one cause of the jet breakup is provoked by fragmentation at the side of a jet. However, there are few detailed results describing the correlation between jet breakup and hydrodynamic interaction at the leading-edge region of a jet. Additionally, air entrainment with a jet is always observed in our past experiments using simulants, but its influence has not been discussed yet.In this study, jet injection experiments in liquid-liquid system were conducted for investigating the interaction a jet and an ambient fluid, and the effect of air entrainment on jet breakup behavior. Both simulant core materials and coolants were transparent liquids for visualization. The stored simulant core material was injected into a tank filled with the simulant coolant. In order to realize the condition without air entrainment, the air remaining within the nozzle was removed using a syringe. The jet breakup behavior was observed with a high speed video camera. A normal backlight system and a Laser Induced Fluorescence (LIF) system were employed for visualization. The inner velocity distribution of a jet was measured by Particle Image Velocimetry (PIV).As a result, in the experiments without air entrainment the jet breakup lengths were described by Epstein’s equation. In addition, a pair of vortices was observed at the leading-edge region. The vortices were generated at the leading edge and the leading edge rolled up by the vortices returned toward a jet core. Thus, it was very likely that the vortices at the leading edge region promoted jet breakup.Copyright

Parallel computation of two-phase flows using the immiscible lattice gas

Publisher Summary Two-phase flow phenomena are complicated and difficult to simulate numerically since two-phase flows have interfaces of phases. Numerical techniques for simulating two-phase flows with interfaces have recently progressed significantly. Two types of numerical methods have been developed and applied: a continuous fluid approach, in which partial differential equations describing fluid motion are solved, and a discrete particle approach, in which motions of fluid particles or molecules are calculated. It is necessary in a continuous fluid approach to model the coalescence and disruption of the interface. In this chapter, the two-dimensional two-phase flow simulation code based on the immiscible lattice gas, which is one of the discrete methods using particles to simulate two-phase flows is developed and parallelized using the MPI library. Parallel computations are performed on a workstation cluster to study the rising bubble in a static fluid, the phase separation in a Couette flow, and the mixing of two phases in a cavity flow. The interfacial area concentration is evaluated numerically, and its dependencies on the average density, wall speed, and region size are discussed.

Lattice Boltzmann simulation of solution chemistry for crevice corrosion

We apply the two-dimensional lattice Boltzmann method (2D LBM) to the simulation of solution chemistry for crevice corrosion. The 2D distributions of pH and electrical potential are obtained by the numerical simulation. The critical value of pH which brings about the rapid crevice corrosion and the incubation period until the solution reaches this pH are estimated from the simulation results. It is found that the estimated pH and incubation period are in nearly agreement with experiment.