Effects Of Strain On Structural Evolution Of Vacancy Clusters In Copper

Structural materials in nuclear fission and fusion systems are often exposed to applied stresses, which will have a serious impact on the structure and properties of the materials. Thus, there is a distinct need for the information related to the relative stability and structural evolution of vacancy clusters under applied strain.To develop a more detailed picture of the effects of strain on the relative stability and structural evolution of vacancy clusters, molecular dynamics simulation has been performed in copper in the paper. First, under zero strain, the binding energies for different cluster types, including linear, planar, and body types, have been calculated using the EAM, FS, and 2NN MEAM potentials. All these potentials give a reasonable description of vacancy clustering. Consequently, a vacancy cluster tends to grow by a combination of the tetrahedral and octahedral cluster. Then, the effects of [001] uniaxial strain on the energetics, stable structures, and structural evolution of vacancy clusters with different orientation characteristics have been studied. The dependence of binding energies as functions of strain for different cluster types shows complicated behavior. The binding energies of both linear and planar clusters monotonously vary with the strain from-10% to 10%, while those of body clusters decrease with increasing both tensile and compressive strain. According to the variation of the binding energies, it has been suggested that the linear and planar clusters tend to align parallel (perpendicular) to the strain axis under tensile (compressive) strain. Moreover, both the{001} planar cluster and body cluster become the dominant types when the clusters grow under high strain. Then, a mechanism that the local structure around a vacancy cluster tends to approach the ideal lattice structure without defects and strain has been applied to explain the effects of the uniaxial strain on the relative stability of the vacancy clusters. This tendency is closely tied to the level of the atomic relaxation which can be measured by the average atomic displacement of the nearest-neighbor atoms surrounding the vacancy cluster. Finally, the effects of volumetric strain have also been studied, and the structural evolution process of vacancy clusters have also been investigated by dynamic simulation. It is found that a vacancy cluster tends to form stacking fault tetrahedron (SFT) under volumetric compression, while tends to grow by a combination of the tetrahedral and octahedral clusters under volumetric tension. Dynamic simulation results show that under volumetric compression, void transforms to SFT, and under volumetric tension, SFT transforms to void, while under cyclic strain loading, there is a reversible transformation between void and SFT.