The Study Of Electron Density Distribution And Positron Annihilation Mechanism Of Micro-defects In Fe-9Cr Alloy

As a nuclear physics technology,positron annihilation spectroscopy uses modern nuclear spectroscopy to obtain the microscopic structure,electron momentum distribution,and defect states of materials through the annihilation of electrons and positrons.The distribution of electron density in metal materials,which means the distribution of free electrons after the charge density removes atomic nuclei,is not only determines the physical and chemical properties of the metal,but also affects the positron annihilation behavior in metals.As a matrix of ferrite/martensite steel in fusion reactor structural materials,the microscopic defects of Fe-9Cr alloy is prone to suffer special conditions such as irradiation and high temperature.These atomic-scale defects have a great influence on the properties of the material,so the microscopic defects in Fe-9Cr alloy can be investigated by positron annihilation spectroscopy.In this manuscript,the electron density distribution of different defect types of Fe-9Cr alloy is obtained through the first-principles calculations,and the positron annihilation mechanism is inferred and verified by positron theory calculations.At the same time,the physics annihilation spectroscopy and transmission electron microscopy were used to study the evolution of the micro-defects of Fe-9Cr alloy with different temperature.Based on the positron annihilation mechanism,which is inferred from the theory calculation,the thermal-induced defects evolution of Fe-9Cr alloy was analyzed.The defect types of Fe-9Cr alloy in theoretical calculation are classified into intrinsic defects and extrinsic defects.For intrinsic defects,according to the electron density,positrons are easily trapped by defects without nuclei,and the defect information is reflected by positron annihilation spectroscopy parameters,such as the positron annihilation lifetime of vacancy clusters increased with the increasing of vacancy numbers.The extrinsic defects take He atoms as an example.When the He atoms are in different gap positions and single vacancies of the Fe-9Cr alloy,the S-W parameters on the same line indicate that they are all small space defects.As the increasing of the He atoms,the changes of the slope of the SW parameter curve reflects the change in the defect type,ie,the vacancy group to the He vacancy complex to He bubble,and the excess He atom will cause the positron annihilation lifetime decrease significantly.A small amount of He atom attracts and accumulates after entering the dislocation,and has little effect on the lifetime value.The evolution of thermal defects in Fe-9Cr alloy was investigated experimentally.Combined with the positron calculation and TEM diagram,it is found that there is a large number of dislocation and monovacancy type defects in Fe-9Cr alloy without any pretreatment,and the monovacancy type defects will migrate with the increases of annealing temperature.When the temperature rises from 573 K to 773 K,the dislocation will aggregate and form a complex dislocation network structure.The formation of dislocation network and the accumulation of carbon elements in the dislocation will lead to the growth of unstable open space defect.When the temperature rises from 773 K to 1073 K,the dislocation density decrease from 1.08 × 10 to 3.55 × 10 ,and the dislocation recovered.A bcc-fcc phase transformation occurred in Fe-9Cr alloy beyond 1073 K according to the Fe-9Cr binary alloy phase diagram.In this paper,the study of electron density distribution and positron annihilation mechanism of micro-defects in Fe-9Cr alloys are not only helpful to analyze the behavior of defects in fusion reactor structural materials,but also extend the application of positron annihilation spectroscopy in metal defects.