Twinning in nanocrystalline materials: an atomistic scale modelling

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Zahiri, Amir Hassan

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2022

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Dissertation

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Twinning is one of the main mechanisms of plastic deformation in materials. Studies have also demonstrated that twinning can enhance the strength and ductility of nanocrystalline structures due to interactions between twins and gliding dislocations at twin boundaries. As a result, a rigorous understanding of twinning nucleation, growth, and behavior is crucial for developing strategies to enhance the mechanical and physical properties of the materials. This dissertation focuses on the twinning behavior and nucleation process through the phase transformation in fcc, bcc, and hcp materials, which is revealed by molecular dynamics (MD) simulations. The first chapter introduces the twining and twinning formation process in nanocrystalline materials. In the second chapter, we report the strong strain hardening in the nanocrystalline Cu, which is caused by the synergistic effect of restricted twin boundary migration, constant nucleation and impedance of dislocations, and abundant dislocation reactions in fivefold twin networks. Chapter three demonstrates the formation of {11-22} twins with the second undistorted plane of {11-26} through the twin-twin interaction of two non-co-zone extension twins. In the next chapter, we report a non-conventional {11-22} twinning in α-titanium through reversible α→ω→α martensitic phase transformations, which occur through both shear and shuffle mechanisms. This study shows the critical role of the Omega phase in the nucleation and growth of {11-22} twin. In chapter five, we illustrate that four types of transformation twins—{11-22}, {11-21}, {10-11}, and {10-12} twins—can be formed through the ω to α martensitic phase transformation. This work advances the understanding of plastic deformation in ω-Ti and unveils the essential role of the metastable ω-phase in the formation of transformation twins. In the last work, we integrate atomistic simulations with theoretical calculations to investigate the effect of mechanical loading on the martensite microstructure. The calculations of deformation gradients and transformation strains reveal that the {10-11} transformation twins and {10-12} transformation twins are favored by opposite loading directions. For the first time, both {10-11}, and {10-12} transformation twins are found to correspond to the well-established deformation twinning modes in hcp metals. Furthermore, the initial {112} twin in the bcc phase is transformed into {11-22}, and {11-21} twins after the phase transformation. The results reveal the critical role of mechanical loading in the formation of the specific transformation twinning, which could propose a novel approach to engineering twin microstructure using designed thermo-mechanical processing.

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