Atomic scale simulations of ductile failure micromechanisms in nanocrystalline Cu at high strain rates

The micromechanisms related to ductile failure during dynamic loading of nanocrystalline Cu are investigated in a series of large-scale molecular-dynamics (MD) simulations. Void nucleation, growth, and coalescence are studied for a nanocrystalline Cu system with an average grain size of 6 nm under conditions of uniaxial tensile strain and triaxial tensile strain at a strain rate of 10⁸ s⁻¹. The MD simulations of deformation of the nanocrystalline system under conditions of triaxial tensile stress show random nucleation of voids at grain boundaries and/or triple point junctions. The initial shape of the voids is nonspherical due to growth of the voids along the grain boundaries. Void growth is observed to occur by the creation of a shell of disordered atoms around the voids and not by nucleation of dislocations from the void surface. Void coalescence occurs by the shearing of the disordered regions in between the voids. The nucleation and growth of voids result in the relaxation of tensile stresses, after which growth of the voids is slower. The slower growth is accompanied by recrystallization of the surrounding disordered regions resulting in near-spherical shapes of the voids.