Plastic Strain Accommodation in Crystalline-Amorphous Nanolaminates containing Columnar Nanograins Quantified through Continuum Deformation Metrics

Nanocrystalline metals with grain sizes less than 100 nm have become widely accepted as a technologically important class of materials owing particularly to their remarkable strength under ambient conditions.  However, due to the lack of strain accommodation processes available as dislocations impinge upon grain boundaries under an applied stress, nanovoids form at these boundaries and their coalescence considerably limits ductility.  By adding amorphous layers into a nanocrystalline metal to form a crystalline-amorphous nanolaminate, vast improvements in ductility have been achieved while retaining the exceptional strength characteristic of the nanocrystalline state. A number of previous studies have demonstrated a unique strain accommodation process where dislocations are absorbed at the amorphous-crystalline interfaces (ACIs).  In this study, new columnar nanolaminate simulation cells were constructed that incorporated nanoscale grain boundaries and ACIs in the same structure to more closely represent experimental materials.  Molecular dynamics simulations of uniaxial tensile deformation were conducted on these structures with particular focus on the role of stress state in the deformation physics.  An atomic slip vector characterization was employed to uncover the coupling between shear transformation zone (STZ) dynamics and dislocation plasticity, and the distribution of plastic strain among the each of the deformation mechanisms was quantified using a Green strain tensor analysis.  Initially, plastic strain was accommodated within the amorphous layers with STZ activity preferentially located directly adjacent to the ACIs.  Relative to nanolaminates free of grain boundaries, this enhanced slip in the columnar nanolaminates was further biased to amorphous atoms near the intersection of ACIs with grain boundary planes.  Lattice dislocations, often involving leading partials and stacking faults, were first emitted from these regions of locally high shear strain, and the addition of grain boundaries significantly reduced the stresses required for global yielding via dislocation plasticity. The emission of trailing partials wasn’t biased to the grain boundaries, but rather depended on the stress state at the ACIs.  Full dislocation formation followed emission of these trailing partials, which represented the dominant strain accommodation process in the columnar nanolaminates.  Absorption of these dislocations at ACIs suppressed strain accommodation at the grain boundaries, which eliminated the formation of deleterious stress concentrations that ultimately lead to grain boundary microcracking in nanocrystalline metals.