Molecular Dynamics Simulation Of Radiation Damage And Interaction With Grain Boundaries In Titanium

Radiation damage is one of the most important origins for failure of structural materials in reactors, the radiation-induced point defects created in the initial stage of the cascade can interact with the inherent defects (e.g. dislocations, grain boundaries, phase boundaries,etc) and irradiated products, such as He, H atoms resulting in the change of the microstructure of the materials which may eventually lead to swelling, harding, amorphization, and embrittlement, primary causes of materials failure. Grain boundaries(GB) as the existing defects in the materials can affect the degree of radaition damage through its interaction with the radiation-induced point defects; titanium and titanium alloy have been considered as a potential cladding materials and structural materials for the next generation of fast reactor and fusion reactor for their excellent properties. In this paper, the displacement cascade occurring in the early stage of irradiation caused by primary knock-on atom (PKA) in hcp titanium single crystal system and the system with grain boundaries have been investigated by molecular dynamics.At first, the classical molecular dynamics method was adjusted accordingly to the study of radiation damage based on the particularity of the process; then the displacement cascade process in hcp titanium single crystal system and in the system with grain boundaries was simulated by the adjusted molecular dynamics method. During all the simulations, the temperature of the system is fixed at100K, the PKA energies are selected ranging from0.5keV to5keV (the corresponding neutron irradiation energy range would be12.5to125keV), and the size of the simulated system is ajusted based on the PKA energy.The specific study is divided into two parts:the study of single crystal system and the system containing grain boundaries. For the single crystal system, after the introduction of the PKA, the displacement cascade process was simulated and its characters, together with the distribution, number and the time evolution of the radiation-induced point defects are investigated to understand the radiation damage process during neutron irradiation; the anisotropy of the hcp titanium in radiation damage is researched by simulating the effect of the PKA direction on the cascade process; and the neutron irradiation of different energy is studied by changing the PKA energy. The system with grain boundaries is built by adding different structures of grain boundaries into the single crystal system. First, the interaction between grain boundaries and the radiation-induced point defects were investigated with different overlap between grain boundaries and radiation damage region by creating PKAs at various distances from a GB. The results indicate that GBs act as sinks of radiation-induced point defects, with an efficiency depending on the degree of overlap between the GB and the cascade damage region; by comparing the results of systems with different structural grain boundaries, we find that the efficiency of grain boundaries "absorbing" the radiation-induced point defects is affected by the matching degree of the grains on its two sides. At last, further research is discussed on the basis of our researches and the conclusions of other researchers on the high-energy grain boundaries.In this paper, by simulating the dynamic process of radiation damage in titanium system at atomic level, the main features of the displacement cascade occurring in the early stage of irradiation and the acting mechanism of grain boundaries as sinks of radiation-induced point defects are investigated. The results we get can make appropriate interpretation about the experimental conclusions that radiation resistance of nano-crystal materials is better than that of the corresponding polycrystalline materials in recent years, and they can provide some theoretical basis and guideline for further development of nuclear materials. In particular, our study has some relevance with that of the dependence of the radiation resistance of materials on the grain size which may obtain some useful guide from our results.