Molecular dynamics study of deformation and fracture in a tantalum nano-crystalline thin film

We present results from molecular dynamics simulations of two nano-crystalline tantalum thin films that illuminate the variety of atomic-scale mechanisms of incipient plasticity. Sample 1 contains approximately 500 K atoms and 3 grains, chosen to facilitate study at 10⁵ s⁻¹ strain rate; sample 2 has 4.6 M atoms and 30 grains. The samples are loaded in uniaxial tension at deformation rates of 10⁵–10⁹ s⁻¹, and display phenomena including emission of perfect 1/2〈1 1 1〉-type dislocations and the formation and migration of twin boundaries. It was found that screw dislocation emission is the first deformation mechanism activated at strain rates below 10⁸ s⁻¹. Deformation twins emerge as a deformation mechanism at higher strains, with twins observed to cross grain boundaries as larger strains are reached. At high strain rates atoms are displaced with the characteristic twin vector at a ratio of 3 : 1 (10⁸ s⁻¹) or 4 : 1 (10⁹ s⁻¹) to characteristic dislocation vectors. Fracture is nucleated through a nano-void growth process. Grain boundary sliding does not scale with increasing strain rate. Detailed analysis of nano-scale deformation using these tools enhances our understanding of deformation mechanisms in tantalum. (paper)