ORCID Identifier(s)

0000-0003-1265-4194

Graduation Semester and Year

2018

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Haiying Huang

Abstract

Understanding the formation of surface roughness could help better diagnose a metal component’s health and could potentially help in making better microstructure design decisions. Both of these, in turn, contribute to the design of efficient structural components. In this work, the research objective is to understand the relationship between the surface microstructure (i.e., grain orientations) and the surface-height changes in nickel polycrystals undergoing small amounts of tensile plastic deformation. The secondary objective is to determine the relationship between surface height and strain localization. Primarily simulations were used, along with experimental surface roughness observations, to validate the simulation results. Discrete dislocation plasticity (DDP) was used to simulate slip-step-type surface roughness, and the crystal plasticity finite element method (CPFEM) was used to simulate grain-scale surface roughness. Electron backscatter diffraction (EBSD) characterized the microstructure on the surface, and surface white light interferometer (SWLI) measured the surface heights of deformed samples. Randall Kelton, a Ph.D. candidate in Material science and Engineering, performed experiments and I focused primarily on simulations and analysis of experimental data. The first phase of the work has an emphasis on simulating the surface roughness at the scale of the slip steps. Calculation of stress fields associated with dislocations at the surface is essential in the simulation of slip-step formation. So, a novel, efficient way to simulate stress fields of dislocations at the free surface of 3D elastic, anisotropic materials was devised. Subsequently, it was found that DDP can only generate results at a much smaller scale than that of this study’s experimental surface-height measurements. In the second part of this work, the focus shifts to roughness at a greater length scale, the grain scale. First, the effect of grain orientation on its surface height was investigated, ignoring the role of the interaction of neighboring grains in roughness formation. Experimental and simulations have revealed that the plastic hardness of a grain does not determine the grain-surface height as previously thought. It was found that the plastic deformation of the grain’s most stressed slip systems along the direction normal to the surface determines a grain’s surface height. Simulations also indicated that plastic strains of a grain in the loading direction are not be related to its surface height. However, plastic strains in the other two directions can be potentially estimated using surface heights. For many grain orientations, simulating the grain itself does not produce the experimentally observed surface behavior. The neighboring grains were found to be highly influential in determining a grain’s surface height, especially the grains lying underneath the surface up to depths of 3–4 grain diameters. Thus, discrepancies between experiments and simulations are justified. While doing so, it was also found that a few specific grain orientations exhibit the same surface behavior irrespective of the neighboring grains. The studies conducted in this research have led to an understanding of grain-scale surface roughness formation, and as a result, the limitations in predicting surface roughness using surface-grain orientations are that surface-height data can make only a limited diagnosis of strain localization.

Keywords

Surface roughness, Nickel, Grain orientation, Dislocation, FCC, Crystal plasticity, Finite element method

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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