Atomistic Study of Fracture and Deformation Mechanisms in Nanotwinned FCC Metals
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Nanotwinned metals have opened up exciting avenues for the design of high-strength, high-ductility materials owing to the extraordinary properties of twin boundaries. This dissertation presents insights into the deformation mechanisms governing the high temperature response and fracture behavior of nanotwinned face-centered-cubic (fcc) metals using molecular dynamics simulations. The aim of our atomistic modeling is to elucidate the role of coherent twin boundaries (CTB) in the interaction with dislocations (thus mediating strength and hardening) and in inhibiting crack propagation (thus contributing to toughness). Our simulations reveal an intriguing transition in the behavior of CTBs at higher temperatures as the deformation mechanism changes from shear-coupled normal motion to deformation twinning, an occurrence that has not been reported before in fcc metals. This anomalous response of twin boundaries at high temperatures is studied for different fcc metals and analyzed based on the energetics of the competing mechanisms. Our simulations of pre-existing cracks along CTBs reveal that CTBs in nanotwinned structures exhibit alternating intrinsic brittleness and intrinsic ductility. This is a startling consequence of the directional anisotropy of an atomically sharp crack along a twin boundary that favors cleavage in one direction and dislocation emission from the crack tip in the opposite direction owing to the effect of the crystallographic orientations in the adjoining twins. These results shed light on the previously held notion that twin boundaries are inherently brittle, and can also explain the brittle versus ductile behavior of CTBs reported in recent literature. We also investigate the effect of twin boundary spacing, and sample thickness on the crack-propagation in twinned nanopillars. The simulations show that CTBs serve as effective barriers for dislocation motion and restrict the plasticity in the vicinity of the crack tip. We finally extend our study of crack propagation to polycrystalline nanotwinned structures. We observe multiple mechanisms such as dislocation-twin interactions, twin migration, and dislocation nucleation from grain boundaries that govern the ductile response of a pre-existing crack. The findings reported in this dissertation demonstrate remarkable properties of twin boundaries and open further avenues for the design of novel nanotwinned structures for next-generation structural applications.