Understanding Grain Boundary and Stress Concentration Effects on Strengthening Mechanisms in Nanotwinned Metals

• Main Author: Fang, Qiongjiali
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2020
• Subjects: Materials science; Molecular physics; Nanoscience; Nuclear physics
• ISBN: 9798691232374

The superior strength and large tensile plasticity of nanotwinned (nt) face-centered-cubic metals have been explained by different twin size-dependent dislocation mechanisms and their inherent strengthening/softening effects. Grain boundary (GB) plasticity generally induces softening in nanocrystalline metals; however, our current understanding of the role of GBs in plasticity of nt metals remains limited. Contradicting reports exist in literature on how twin size influences stress concentration at GB – twin boundary (TB) intersections, which facilitates dislocation nucleation. In this thesis, molecular dynamics (MD) simulations and finite element analysis (FEA) were used to study the effects of GB plasticity and stress concentrations, on the mechanical behaviors of four different columnar-grained nt fcc metals. First, the extent of GB plasticity was found to be the same for different TB spacing (TBS). A special GB deformation mechanism, ductile cracking along GB was observed in nt Ni at small TBS, resulting from the higher GB plasticity of nt Ni. In addition, GB plasticity also depends on the strain rate, which contributes to strain rate sensitivity in nt fcc metals. Second, it was found that stress concentration at GB – TB intersections decreases as TBS decreases in both anisotropic elasticity theory and atomistic simulations, which is contrary to past experimental results. Stress concentration at the intersection of two regions with different TBSs could be estimated simply by geometric average of stress concentrations in adjacent regions with homogeneous CTB distributions, suggesting that the paradox in stress concentration between experiments and our simulation may be related to the uneven TB distribution in experiments. Initial nucleation of twinning partials is predicted to occur at lower strain as TB spacing decreases, when the stress concentrations are smaller, because of a transition from stress-controlled to source-controlled dislocation nucleation. Therefore, this thesis shows that it is necessary to tailor the size and distribution of grains and nanotwins to achieve ideal mechanical properties in columnar-grained nt metals.