Kazuhito Ohsawa | Last modified date：2021.07.12 |

Graduate School

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##### https://kyushu-u.pure.elsevier.com/en/persons/kazuhito-ohsawa

Reseacher Profiling Tool Kyushu University Pure

Academic Degree

Physics

Country of degree conferring institution (Overseas)

No

Field of Specialization

lattice defects

Total Priod of education and research career in the foreign country

00years02months

Outline Activities

1 Activation Energy for a Dislocation Loop

According to Vineyard’s theory, we estimate the activation energy of a dislocation loop. Our analytic solution shows that the activation energy is represented with elliptic integrals. Besides, plural saddle points with different energies simultaneously appear in the phase space in an appropriate condition. Therefore, the transition process of the dislocation loop can be regarded as a complicated bifurcation problem.

2 Relation between Peierls Stress and Slip System

Peierls stress τp, which is substantial crystal strength for plastic deformation, are widely distributed. For example, covalent crystal have the largest Peierls stress 10^-1, BCC metals' ones are 10^-3G and FCC metals' are 10^-5G, where G is shear modulus. The magnitude of Peierls stress is approximately determined from the crystal structure. Especially important parameter is h/b, where h is the width of slip plane and b is length of Burgers vector. We investigate the relation between the parameter h/b and Peiels stress with numerical simulations and obtained an exponential dependence τp/G = A exp(-h/b C).

3 Lattice Statics Green's Function

Lattice statics Green's function describes displacements of atoms induced by applied external force. Traditional elastic Green's function has a logarithmic divergence at the point where the external force is exerted. On the other hand, the lattice statics Green's function avoids such divergence. So it is very avaiable to calculate dislocation behaviors or crack expansions. Our developed Green's function is adjustable about lattice constant, elastic momulus to actual crystal and keeps the symmetry of stress tensor in the contimuun elastic limit. Besides, we calculate lattice Green' function for semi-infinite half space derived from Dyson's equation.

4 Flexible Boundary Condition for a Moving Dislocation

The displacement field around a dislocation is long-range one which decreases as r^-1. So, in a computer simulation, image force from the boundary often affects the results in a small simulation box. It is necessary to avoid or decrease such boundary effects. For this purpose, we introduce a flexible boundary condition, which changes associated with the state of dislocation core region. The equation of motion for the boundary condition is based on the principle of least action. Applied the boundary condition to a simulation of moving dislocation, we obtained expected results which do not strongly depend on model size.

5. Computer simulation in terms of first-principle calculation

Tungsten is used in fusion reactors. In particular, interaction of hydrogen with vacancy in the tungsten materials.

According to Vineyard’s theory, we estimate the activation energy of a dislocation loop. Our analytic solution shows that the activation energy is represented with elliptic integrals. Besides, plural saddle points with different energies simultaneously appear in the phase space in an appropriate condition. Therefore, the transition process of the dislocation loop can be regarded as a complicated bifurcation problem.

2 Relation between Peierls Stress and Slip System

Peierls stress τp, which is substantial crystal strength for plastic deformation, are widely distributed. For example, covalent crystal have the largest Peierls stress 10^-1, BCC metals' ones are 10^-3G and FCC metals' are 10^-5G, where G is shear modulus. The magnitude of Peierls stress is approximately determined from the crystal structure. Especially important parameter is h/b, where h is the width of slip plane and b is length of Burgers vector. We investigate the relation between the parameter h/b and Peiels stress with numerical simulations and obtained an exponential dependence τp/G = A exp(-h/b C).

3 Lattice Statics Green's Function

Lattice statics Green's function describes displacements of atoms induced by applied external force. Traditional elastic Green's function has a logarithmic divergence at the point where the external force is exerted. On the other hand, the lattice statics Green's function avoids such divergence. So it is very avaiable to calculate dislocation behaviors or crack expansions. Our developed Green's function is adjustable about lattice constant, elastic momulus to actual crystal and keeps the symmetry of stress tensor in the contimuun elastic limit. Besides, we calculate lattice Green' function for semi-infinite half space derived from Dyson's equation.

4 Flexible Boundary Condition for a Moving Dislocation

The displacement field around a dislocation is long-range one which decreases as r^-1. So, in a computer simulation, image force from the boundary often affects the results in a small simulation box. It is necessary to avoid or decrease such boundary effects. For this purpose, we introduce a flexible boundary condition, which changes associated with the state of dislocation core region. The equation of motion for the boundary condition is based on the principle of least action. Applied the boundary condition to a simulation of moving dislocation, we obtained expected results which do not strongly depend on model size.

5. Computer simulation in terms of first-principle calculation

Tungsten is used in fusion reactors. In particular, interaction of hydrogen with vacancy in the tungsten materials.

Research

**Research Interests**

- Stabilization effect of impurities for vacancy-cluster growth in tungsten

keyword : tungsten

2018.04. - Hydrogen absorption properities of depleted uranium intermetallics

keyword : hydrogen absorption, depleted uranium

2013.04. - Study on interaction between BCC metals and hydrogen by first principles calculations

keyword : BCC metals, hydrogen, first principles calculation

2010.04. - Study on radiation damage by first principle simulations

keyword : radiation damage, first principle

2008.04. - Study on stress function for dislocation loops in anisotropic crystals

keyword : stress function, dislocation, anisotropic crystal

2007.01. - Activation energy for dislocation loops

keyword : dislocation、 saddle point、activation energy、Vineyard theory

2003.09～2008.03Activation Energy for a Dislocation Loop. - Flexible boundary condition for a moving dislocation

keyword : Flexible Boundary Condititon

1998.01～2002.03Flexible Boundary Condition for a Moving Dislocation. - Lattice statics Green's function

keyword : elasticity, Green's function

1996.01～2000.01Lattice Statics Green's Function. - Study on the relation between Peierls stress and crystal structure

keyword : Peierls stress、 plasticity

1990.04～1996.03Relation between Peierls Stress and Slip System.

**Academic Activities**

**Papers**

**Presentations**

1. | Stress function for dislocation loops in anisotropic crystals. |

2. | Equilibrium structure and thermal activation of dislocation loops in BCC metals. |

Educational

**Educational Activities**

I belong to a laboratry of interdisciplinary graduate school of engineering sciences, so I study with gradurated students. They and I read scientific textbooks and solve problems as practice.

Social

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