Theses and Dissertations

Author

Deepesh Giri

Issuing Body

Mississippi State University

Advisor

Barrett, Christopher

Committee Member

El Kadiri, Haitham

Committee Member

Rhee, Hongjoo

Committee Member

Hammi, Youssef

Date of Degree

8-1-2020

Original embargo terms

Visible to MSU only for 1 year

Document Type

Dissertation - Open Access

Major

Mechanical Engineering

Degree Name

Doctor of Philosophy

College

James Worth Bagley College of Engineering

Department

Department of Mechanical Engineering

Abstract

Molecular Dynamics (MD) and Molecular Statics (MS) simulations have always proven to be powerful tools to study material behavior at the lowest length scale. However, one of the greatest challenges in material modelling is the effective upscaling of relevant material properties from atomic scale to continuum scale. One such challenge is how best to capture the role of twinning in higher scale plasticity. Capturing twin nucleation in fullield crystal plasticity is a long-standing problem in materials science. The challenge resides mainly in the biased regional lattice transformation associated with twin formation in defiance of its obedience to a threshold stress law which could be fulfilled in regions where twinning is deferred. Hence, determining a favorable site for nucleation of a twin variant remains a daunting task. This dissertation is an attempt to understand twin nucleation in hexagonal close packed (HCP) metals using a technique called Nudged Elastic Band (NEB) method. NEB calculations are performed between two stable configurations and use a number of intermediate images to describe the transition. Results of calculations demonstrate that the role of stress and atomic structure in twin nucleation could be understood in terms of the minimum energy path, energy barrier, and relaxed energy. This method allows to gauge the transformations that the lattice could experience when evolving from a given thermodynamic state to a final state of a predefined twin embryo under various boundary conditions. This allows direct comparisons between various cases with respect to twin nucleation and hence provides a measure of the material plasticity. Such results are crucial in higher scale modelling of the material.

URI

https://hdl.handle.net/11668/18441

Sponsorship

Army Research Laboratory under Cooperative Agreement Number W911NF-15-2-0025.

Comments

Twinning||NEB||Energy||Stress||Vacancies

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