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Date of Award

2020

Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering (PhD)

Administrative Home Department

Department of Materials Science and Engineering

Advisor 1

Paul G. Sanders

Committee Member 1

Stephen A. Hackney

Committee Member 2

Douglas J. Swenson

Committee Member 3

Erik G. Herbert

Committee Member 4

Gregory M. Odegard

Abstract

Al-Sc-Zr alloys have drawn increasing attention in the last several decades due to their strengthening and coarsening resistance. In this study, solute concentrations of Sc and Zr were increased beyond their equilibrium solubilities without primary precipitate formation using melt-spinning. The melt-spun ribbon was metallurgically bonded into bulk shape using extrusion. With the proper aging treatment, the mechanical properties of the supersaturated melt-spun ribbon and extruded rod were found to be significantly higher than a baseline dilute alloy. Increased mechanical properties include microhardness, tensile strength at ambient-temperature, and compressive strength and threshold stress at and elevated-temperature. These increases were related to the larger precipitate radius and volume fractions found in the melt-spun alloys.

Scanning transmission electron microscopy (STEM) was used to investigate the microstructural changes in the supersaturated alloys that led to the mechanical property improvements. The precipitate size and number density, precipitate volume fraction, grain size and dislocation density in the melt-spun ribbon and extruded rod were quantified in the STEM images. The elemental compositions and distributions in the precipitate and matrix were measured using the energy-dispersive X-ray spectroscopy (EDS) and elemental mapping in STEM mode.

The precipitate strengthening, threshold stress and precipitate coarsening behavior in the supersaturated melt-spun ribbon and extruded rod were modeled using the measured microstructures. The predictions of precipitate strengthening and threshold stress agreed with the observed experimental results. The precipitate coarsening rates were found to be accelerated by the supersaturated solute additions and retarded by the increasing precipitate volume fraction during overaging.

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