Date of Award

5-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

William J. Weber

Committee Members

Yanwen Zhang, Steven J. Zinkle, Claudia J. Rawn

Abstract

The energy dissipation pathways of electronic and nuclear energy loss (Se and Sn, respectively) during ion irradiations are investigated to evaluate their influence on the damage evolution in Ti3SiC2 [titanium silicon carbide] MAX phase. Interest in the unusual properties of Mn+1AXn [MAX] phase ceramics for high radiation environments has led to numerous neutron and ion irradiation studies; however, an understanding of the response to energy deposition from high-energy ions is necessary for a comparison between the two irradiation environments. Two novel experiments were developed to compare variation in energy dissipation at constant damage dose: 1) a series of high energy self-ion irradiations at room temperature, 500°C [degrees Celsius] and 880°C, and 2) a sequential irradiation study. Grazing incidence x-ray diffraction and transmission electron microscopy were utilized to characterize structural variation as a function of depth, and integral to this evaluation was the novel adaptation of a method to measure fine changes in lattice strain in hexagonal polycrystals. The results reveal a threshold in Se, above which the inelastic energy dissipation is a major contributor to the damage evolution in the system, exhibited by increased lattice strain and the formation of a FCC [face centered cubic] phase. High temperature in situ irradiation conditions appear to mitigate resulting damage effects, leading to more uniform lattice strain and significant reduction in FCC phase formation. Further, separating the energy loss pathways, the Sn elastic collision cascades are demonstrated to produce Ti and Si anti-site point defects and eventually a structural shift to the FCC phase embedded in the hexagonal structure. While Se above the threshold adds heat and additional point defects, through a thermal spike and its subsequent shockwaves, there is competition between the dissipation pathways, defined by their ratio. When Se/Sn is low, the formation of the FCC-type phase is enhanced by the additional defects and energy in the system, but as Se/Sn increases, in-cascade annealing by the thermal spike, while insufficient to anneal preexisting point defects, suppresses the formation of the FCC phase, thus drastically increasing the relative lattice strain in the hexagonal structure and eventually saturating it.

Orcid ID

http://orcid.org/0000-0002-9336-5556

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