Theses and Dissertations

Issuing Body

Mississippi State University

Advisor

El Kadiri, Haitham

Committee Member

Tschopp, Mark A.

Committee Member

Tome, Carlod N.

Committee Member

Wang, Jian

Date of Degree

1-1-2014

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

Improving the formability and crashworthiness of wrought magnesium alloys are the two biggest challenges in current magnesium technology. Magnesium is the best material candidate for enabling required improvements in fuel economy of combustion engines and increases in ranges of electric vehicles. In hexagonal closed-packed (HCP) structures, effects of grain size/morphology and crystallographic texture are particularly important. Prior research has established a general understanding of the dependences of strength and strain anisotropy on grain morphology and texture. Unfortunately, deformation, recrystallization, and grain growth strategies that control the microstructures and textures of cubic metals and alloys have not generally worked for HCPs. For example, in Magnesium, the deformation texture induced by primary forming operations (rolling, extrusion, etc.) is not randomized by recrystallization and may strengthen during grain growth. A strong texture reduces formability during secondary forming (stamping, bending, hemming etc.) Thus, the inability to randomize texture has impeded the implementation of magnesium alloys in engineering applications. When rare earth solutes are added to magnesium alloys, distinct new textures are derived. However, rare earth texture derivation remains insufficiently explained. Currently, it is hypothesized that unknown mechanisms of alloy processing are at work, arising from the effects of grain boundary intrinsic defect structures on microstructural evolution. This dissertation is a comprehensive attempt to identify formal methodologies of analyzing the behavior of grain boundaries in magnesium. We focus particularly on twin boundaries and asymmetric tilt grain boundaries using molecular dynamics. We begin by exploring twin nucleation in magnesium single crystals, elucidating effects of heterogeneities on twin nucleation and their relationships with concurrent slip. These efforts highlighted the necessity of imperfections to nucleate {10-12} twins. Subsequent studies encountered the importance of deformation faceting on the high mobility of {10- 12} and stabilization of observed twin mode boundaries. Implementation of interfacial defect theory was necessary to decipher the complex mechanisms observed which govern the development of defects in grain boundaries, disconnection pile-up, facet nucleation, interfacial disclination nucleation, disconnection movements, disconnection transformation across interfacial disclinations, crossaceting, and byproducts of interactions between lattice dislocations and grain boundaries.

URI

https://hdl.handle.net/11668/17685

Comments

Molecular dynamics||Faceting||Interfacial Defects||Dislocations||Twinning

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