Structure, mechanical and thermodynamic stability of vacancy clusters in Cu

The atomic structure, mechanical and thermodynamic stability of vacancy clusters in Cu are studied by atomistic simulations. The most stable atomic configuration of small vacancy clusters is determined. The mechanical stability of the vacancy clusters is examined by applying uniaxial and volumetric tensile strains to the system. The yield stress and yield strain of the system are significantly reduced compared with the perfect lattice. The dependence of vacancy formation and binding energy as a function of strain is explored and can be understood from the liquid-drop model. We find that the formation energy of the vacancy clusters decreases monotonically as a function of the uniaxial strain, while the formation energy increases first then decreases under the volumetric tensile strain. The thermodynamic stability of the vacancy clusters is analyzed by calculating the Helmholtz free binding energy and the total probability of dissociation of the vacancy clusters at 300 and 900 K under uniaxial and volumetric strains. We find that although most of the vacancy clusters appear to be thermodynamically stable, some of the intermediate sized clusters have a high probability of dissociation into smaller clusters.