Date of Award

5-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Yanfei Gao, Thomas T. Meek, Hairong Qi

Abstract

High entropy alloys (HEAs) are proposed as solid-solution alloys containing five or more principal elements in equimolar or near-equimolar ratios, possessing a single crystal structure rather than several ordered phases. Several studies of HEAs have been performed, with focus on the mechanical behavior and characterization of microstructures. The mechanical behavior and properties of HEAs under various conditions, i.e., strain rates, grain sizes, and temperatures, exhibit great differences, such as strong work hardening, homogeneous macroscopic flow, and excellent compression or tension ductility with obvious serrations at room temperature, and partial or complete dynamic recrystallization at high temperatures. The strong and ductile single-phase body-centered-cubic (BCC) HfNbTaTiZr refractory high-entropy alloy (RHEA) is a potential structural material for high-temperature applications. The present work will focus the mechanical properties and serration behavior in HfNbTaTiZr HEAs, by applying transmission electron microscopy (TEM), atom probe tomography (APT), synchrotron diffraction, and scanning electron microscopy (SEM) to the study of plastic deformation and fatigue behaviors in HEAs under different conditions (covering a wide range of strain rates, temperatures, and tension behaviors), in order to reveal the underlying mechanisms of the plastic deformation for HEAs and to predict the fracture stress. Specifically, an anomaly in strain hardening was observed at elevated temperatures–the strain-hardening exponent decreases expectedly from 77 K to 298 K but reverts to an anomalous ascending trend afterwards. Flow serrations at 673 and 773 K implied the dynamic strain aging (DSA) as an extra strengthening mechanism contributing to the intensified strain hardening at elevated temperatures. The superior fatigue properties during cyclic loading were investigated at room temperature, which present a series of substructures, including dislocation loops, jogs, and dislocation network. The resulting dislocation network was formed by the interaction between dislocations with different Burgers vectors, which can act as the obstacle to dislocation motion to strengthen the fatigue behavior and release the strain energy and stress concentration to improve the resistance to cyclic loading. Moreover, the recrystallization, grain growth and phase transformation of HfNbTaTiZr HEAs were investigated as well in the certain range of temperatures to better understand their grain growth kinetics and phase stability in body centered-cubic (bcc) HEAs, which will be helpful for the materials design and optimization.

Comments

Portions of this document were previously published in journal of Scripta Materialia, Journal of Alloys and Compounds, Journal of Materialia,

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