Doctoral Dissertations

Date of Award

5-2015

Degree Type

Thesis

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Takeshi Egami

Committee Members

Peter K. Liaw, James R. Morris, David Keffer, Brian Wirth

Abstract

The development of the generation IV reactors calls for radiation resistant materials. This thesis proposes that the newly developed single phase solid solution of high-entropy alloys (HEAs) can be such candidates. HEAs can undergo the crystalline to amorphous to crystalline (C-A-C) transitions under radiation. The radiation induced amorphous structure is a highly radiation resistant medium as shown by previous studies, and it further transforms to crystalline phases without much structural defects. In this thesis, by reviewing the formation rules of solid solutions and amorphous metallic glasses, it is suggested that the atomic size plays a key role affecting the C-A-C transitions and the radiation behaviors in HEAs. Experimental local structure studies on a model ternary ZrNbHf HEA using neutron scattering demonstrate that atomic size difference by 9% induces large enough lattice distortions to merge the second nearest neighbor atoms into the first nearest neighbor shells. The lattice distortion is supposed to assist the C-A transition. The atomic size effect on the radiation induced C-A transitions in binary alloys are studies using molecular dynamics simulations. Results show that the glass formation range is related to the atomic size ratio and solute concentrations upon irradiation and a universal atomic level strain characterizes the radiation process. The results on the cascade process show that the cascade morphology, cascade annealing, number of displaced atoms, cascade peak time and surviving defect numbers are affected by the atomic size mismatch in the solid solutions due to various lattice distortion and instability. It is possible to have the C-A transition in the cascade region by tuning the atomic size ratio. The results on recrystallization show that the recrystallization rate and the volume ratio of the supercooled glass phase and the crystal phase are also affected by the atomic size mismatch and temperature. The experimental and simulation results confirm the possibility of C-A-C transition in HEAs. This thesis aims at providing fundamental predictions and guidelines for alloy designs of HEAs for radiation materials.

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