  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
strain to the system. The yield stress and yield strain of the system are
significantly reduced comparing to the prefect 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 Gibbs free binding energy and the total
probability of dissociation of the vacancy clusters at 300 K 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 immediate sized
clusters have high probability of dissociation into smaller clusters.
