We report a computational analysis of the atomistic mechanisms of relaxation of biaxially applied tensile strains over a range of strain levels up to 17% in free-standing ultrathin metallic films with the film plane oriented normal to the [111] crystallographic direction. The analysis is based on molecular-dynamics simulations using slab supercells that contain millions of atoms to model single-crystalline thin films without and with cylindrical voids oriented normal to the film plane and penetrating through the film thickness. At high levels of applied strain (>8%), a strain relaxation regime other than the ductile void growth is revealed that gives rise to a practically uniform distribution of dislocations in the film and subsequent formation of nanometer-scale face-centered-cubic crystalline domains, i.e., a single-to-polycrystalline structural transition. It is demonstrated that in this strain relaxation regime, void growth is inhibited as the dislocations emitted from the void surface are pinned by their interaction with the simultaneously generated network of defects in the nanocrystalline material.

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