Degree

Doctor of Philosophy (PhD)

Department

Mechanical & Industrial Engineering

Document Type

Dissertation

Abstract

Radiation effects in apatite and high entropy alloy under energetic ions and electrons are studied in this doctoral dissertation to develop advanced crystalline ceramic waste forms and nuclear structural materials. Apatite is proposed as a ceramic waste form for the immobilization of radionuclides, but its performance is strongly affected by the irradiation of the incorporated radionuclides. It is thus important to understand the radiation effects in apatite structure and the underlying physics. Effects of chemical composition, grain size, interfacial structure, as well as radiation conditions on the microstructural evolution, phase transformation and damage mechanisms of apatite under alpha-decay and beta-decay events, simulated by 1 MeV Kr ions and 200 keV electrons respectively, are investigated. Composition effect on silicate apatite shows the better radiation tolerance under higher cerium content. Size effect on hydroxyapatite exhibits the reduction of radiation stability with the decrease of grain size due to excess surface energy in nanoparticles. A further study addresses densified nanocrystalline hydroxyapatite exhibits higher radiation tolerance than the same sized hydroxyapatite nanoparticle as a result of lower interface energy. Effect of radiation conditions on the recrystallization behaviors of pre-amorphized hydroxyapatite is also studied. In-situ TEM observation reveals a rapid recrystallization process and a notable size effect, which smaller sized sample nucleates and fully recrystallizes under lower electron fluence. Recrystallization mechanism is attributed to ionization process as a result of breaking and reforming of dangling bonds. The radiation effect study is further extended to include high entropy alloys intended as structural materials in advanced nuclear reactors. Two types of high entropy alloys are selected as model alloys to investigate the irradiation-induced behaviors under 1 MeV Krions. Study on nanocrystalline AlxCoCrFeNi alloys shows a notable ion-irradiation-induced grain growth, whose mechanisms are attributed to a disorder-driven mechanism for the initial fast increase of grain size and defect-stimulated mechanism for the later slow grain size increase, elucidated by the thermal spike model. Study on HfNbTaTiVZr alloy reveals a crystal-to-amorphous phase transformation with critical amorphization dose of 2 displacements per atom (dpa) at 298 K, while the amorphization is suppressed when the temperature increases to 423 K.

Date

6-29-2018

Committee Chair

Lu, Fengyuan

Available for download on Monday, June 28, 2021

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