Study On The Mechanism Of Hydride Precipitation In Zirconium Alloy By Elastoplastic Phase Field Method

Nuclear power,as a clean energy source,has the advantages of high efficiency,stability,and low carbon emissions,making it one of the important choices for China to promote the transformation of clean energy.Safety is a top priority in the development of nuclear power,and zirconium alloy is often chosen as the material for nuclear fuel cladding tubes,serving as the second line of defense for nuclear power plants.Due to the influence of the service environment of zircon cladding tubes,water side corrosion occurs,and some of the hydrogen generated will enter the zirconium to form hydrides.The initial hydrogen content,grain boundary energy,and other factors can affect the morphology of hydride precipitation in zirconium,and different morphologies can cause varying degrees of damage to the mechanical properties of zirconium alloys.Therefore,in-depth understanding of the mechanism of hydride precipitation in zirconium and reducing the harm of hydride is one of the important ways to ensure nuclear safety.In this paper,the elastic-plastic Phase field models is derived based on the solid phase transformation theory and the plastic increment theory of Khachaturyan and Shatalov,and the grain boundary energy is introduced into the model,so that the model can describe the precipitation process of hydride in single crystals and polycrystals.Firstly,the microstructure and performance evolution of precipitated hydrides in single crystal and polycrystalline zirconium alloys under different initial hydrogen contents were studied,and the effects of initial hydrogen content,hydride structure,and applied stress on the morphology of hydrides were analyzed.The research results indicate that the initial hydrogen content has a greater impact on the morphology of hydrides than the effects of hydride structure,applied stress,and grain boundaries.When the hydrogen concentration is low,the influence of hydride structure,applied stress,and grain boundaries on hydride morphology is significant.As the hydrogen content increases,the effect weakens.At the same time,the precipitation of hydrides in twin and polycrystalline zirconium under different grain boundary energies was studied,and the composition of grain boundary hydrides and the growth patterns of different types of grain boundary hydrides were explored.Research has found that in the process of random nucleation,hydrides are prone to nucleation and growth at grain boundaries,and the hydrides at grain boundaries are composed of three variants stacked together.In the process of fixed point nucleation,when the grain boundary energy exceeds a certain threshold,nucleation is induced by intragranular hydrides at the grain boundary.The higher the grain boundary energy,the more nucleation points there are.Under non initial nucleation conditions,hydrides at grain boundaries can be formed by the growth of intragranular hydrides across grain boundaries,by the merger of intragranular hydrides and hydrides induced at grain boundaries,and by the growth of hydrides induced at grain boundaries.Finally,it was explained that the growth patterns of three types of grain boundary hydrides may be related to the orientation and relative position of grain boundaries and hydride nuclei.This study shows that hydrogen content is the decisive factor affecting the precipitation of hydrides in zirconium alloys.Therefore,developing low hydrogen absorption zirconium alloys is an important approach to develop new cladding materials.Furthermore,the characteristics of hydrides that are easy to precipitate at grain boundaries and the influence of external stress can be utilized to regulate the distribution of hydrides in zirconium,thereby reducing the harm of hydrides and ensuring nuclear safety.