Date of Graduation

5-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Douglas E. Spearot

Committee Member

Arun Nair

Second Committee Member

Paul Millett

Third Committee Member

Min Zou

Fourth Committee Member

Salvador Barraza-Lopez

Keywords

Atomistic Simulations, Cu, Plastic Deformation, Shock, Applied sciences

Abstract

The objective of this dissertation is to characterize the evolution of plastic deformation mechanisms in single crystal and nanocrystalline Cu models during shock by atomistic simulations. Molecular dynamics (MD) simulations are performed for a range of particle velocities from 0.5 to 1.7 km/s and initial temperatures of 5, 300 and 600 K for single crystal models as well as particle velocities from 1.5 to 3.4 km/s for nanocrystalline models with grain diameters of 6, 11, 16 and 26 nm. For single crystal models, four different shock directions are selected, <100>, <110>, <111> and <321>, and dislocation density behind the shock wave front generally increases with increasing particle velocity for all shock orientations. Plastic relaxation for shock in the <110>, <111> and <321> directions is primarily due to a reduction in the Shockley partial dislocation density. In contrast, plastic relaxation is limited for shock in the <100> orientation. This is partially due to the emergence of sessile stair-rod dislocations with Burgers vectors of 1/3<100> and 1/6<110> due to the reaction of Shockley partial dislocations with twin boundaries and stacking fault intersections. For <100> shock, FCC Cu is uniaxially compressed towards the BCC structure behind the shock wave front; this process is more favorable at higher shock pressures and temperatures. For particle velocities above 0.9 km/s, regions of HCP crystal structure nucleate from uniaxially compressed Cu. Free energy calculations proves that the nucleation and growth of these HCP clusters are an artifact of the embedded-atom interatomic potential. In addition, simulated x-ray diffraction line profiles are created for <100> shock models of single crystal Cu at the Hugoniot state. Generally, peak broadening in the x-ray diffraction line profiles increases with increasing particle velocity. For nanocrystalline models, the compression of the FCC lattice towards the BCC structure is more apparent at particle velocity of 2.4 km/s, and at this particle velocity, the atomic percentage of BCC structure increases with increasing grain size. The observation of BCC structure strongly depends on grain orientation; grains with <100> directions closely aligned with the shock loading direction show a higher percentage of BCC structure.

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