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Atomistic Simulations of Deformation Twins in HCP Metals

Mingyu Gong, University of Nebraska - Lincoln

Abstract

This dissertation investigates mechanisms associated with deformation twinning in hexagonal close-packed (HCP) metals with atomistic simulations assisted by topological analysis and experimental characterization. Twinning involves nucleation, propagation and growth. This work focuses on the latter two stages which significantly determines twin morphology and affects materials’ mechanical behavior. A twin, either of annealing/growth or deformation type, is enclosed by twin boundaries (TBs). We study the equilibrium and non-equilibrium TBs and their corresponding boundaries defects associated with the three-dimensional (3D) propagation of {10-12} annealing/growth and deformation twins. Deformation twinning, although exhibiting some similarity to growth/annealing twinning after stress relaxation, following the shear-shuffle mechanism. Propagation and growth of deformation twins are accomplished by nucleation and motion of twinning dislocations/disconnections (TDs). Combining micrographs and atomistic simulations, we concluded that for {11-22} twinning in Ti, 3-layer TD is the elementary TD, 1-layer TD is reassembly TD, and TDs with other heights are combination of 3-layer and 1-layer TDs. Once twinning occurs, dislocation-twin and twin-twin interactions greatly attribute to the hardening behavior. With atomistic simulations, we studied the interaction between basal 〈a〉 dislocations and 3D {10-12} twins. During the interactions, dislocation transformation, dissociation, cross-slip take place, further resulting in twinning/detwinning and evolution of various microstructures such as jogs and stacking faults (SFs). Some of these processes can only be captured by 3D simulations. The microstructures and interaction mechanisms of non-cozone twin-twin junctions have been studied by experimental characterization and atomistic simulations. For type II(a) (T2→T1) interaction, twin-twin boundary (TTB) forms on the obtuse side of the incoming twin, implying that the growth of two twins is favored on the obtuse side while impeded on the acute side. For type II(b) (T3→T1) interaction, incoming twin is obstructed by the encountering twin, and the growth of both twins seems impeded. This work enhances our understanding of twinning mechanisms, further facilitating development of micro/macro-scale predictive models and benefiting HCP metals’ processing optimization and performance improvement.

Subject Area

Materials science

Recommended Citation

Gong, Mingyu, "Atomistic Simulations of Deformation Twins in HCP Metals" (2019). ETD collection for University of Nebraska - Lincoln. AAI27548772.
https://digitalcommons.unl.edu/dissertations/AAI27548772

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