Void formation in nanocrystalline Cu film during uniaxial relaxation test

Void formation in nanocrystalline Cu thin films with a grain size of 100 nm during uniaxial tensile relaxation experiments is quantitatively studied. Cu thin films with a two-dimensional fiber structure were deposited on heat-resistant polyimide substrates and subject to various subcritical uniform uniaxial tensile strains at an elevated temperature (∼0.3Tm), to observe void formations in nanocrystalline metals with a reduced amount of dislocation-based deformation. Microstructural observations were carried out at several stages of deformation, and the evolutions of void formation in subcritical strain levels are quantitatively discussed. A void formation model is proposed for approximating the nucleation and growth rate of voids. The resulting model shows a reasonable agreement with the observed number density and area fraction of voids for various strain levels and grain sizes. On the basis of the results, the stress and grain size dependences of the void formation process are further discussed.