Transmission Electron Microscopy Study of Radiation Damage in Potential Nuclear Materials

A study of the radiation response of two classes of prospective materials for future generations of nuclear reactors is presented in this thesis. These materials are highly concentrated alloys { commonly known as High-Entropy Alloys (HEAs) { and the Ti-based Mn+1AXn phase ternary carbides. Ion irradiation in situ within a Transmission Electron Microscope (TEM) was used to investigate the effects of energetic particle irradiation on these materials. This methodology allowed the real-time monitoring of the microstructural evolution of the studied materials whilst under irradiation over a wide variety of dose and temperature conditions of relevance to nuclear technology.

To shed light on the core effects responsible for enhanced radiation resistance in HEAs, such as the sluggish mobility of atomic defects and the superior thermodynamic stability, a quaternary HEA, FeCrMnNi, was selected for investigation. For this purpose, experiments with the FeCrMnNi HEA were directly compared with a conventional nuclear structural material, the austenitic stainless steel grade 348, which is an Fe-based alloy containing Cr, Ni and Mn as major alloying elements. The stainless steel 348 has the same elements as the HEA in solid-solution, but not in equiatomic composition: thus it can be considered as a "low-entropy" version of the FeCrMnNi HEA. It was shown that the sluggish diffusion property played only a minor rule in suppressing the nucleation and growth of He and Xe bubbles under irradiation. However, under heavy ion irradiation, the phase stability of the HEA was observed to be superior to its low-entropy counterpart, the steel, in the temperature range from 298 to 573 K: at higher irradiation temperatures both alloys displayed similar radiation responses. The results suggest (for the alloys investigated in this work) that the relationship between the key high-entropy core effects and superior radiation tolerance of HEAs is limited to low and moderate temperatures.

Following the results with the bulk FeCrMnNi HEA and given the possibility of designing radiation tolerant structural nuclear materials by tuning the elemental composition, High-Entropy Alloy Thin Films (HEATF) within the quaternary metallic system FeCrMnNi were developed through the technique of ion beam sputter-deposition. A complete synthesis and characterisation investigation was firstly performed on Si wafer substrates in order to demonstrate the feasibility of depositing equiatomic metallic thin films within the FeCrMnNi system. Then, these thin films were deposited onto Zircaloy-4TM substrates and their radiation tolerance was assessed under medium-energy, heavy ion irradiation in situ within a TEM. By comparing the radiation response of the HEATF with titanium nitride (a material currently under consideration for coating Zr alloys) using the ion irradiation with in situ TEM technique, it was found that the HEATF possessed superior radiation tolerance and this alloy is thus proposed in this thesis as an alternative to ceramic coatings in the context of the accident tolerant fuels programme.

An extensive study of the neutron and ion irradiation responses of two Ti-based MAX phases is also presented. Firstly, an electron-microscopy post-irradiation study on the Ti3SiC2 and Ti2AlC MAX phases irradiated with neutrons in the High-Flux Isotope Reactor (HFIR) at high temperatures (1273 K) is presented. This study, which was carried out up to 10 dpa, revealed a complex chain of radiation damage effects: from perfect basal dislocation loops to irradiation-induced segregation with formation of secondary phases. The heavy ion irradiation with in situ TEM methodology was utilised to explore possible experimental comparisons between ion and neutron irradiation of these materials. In situ TEM annealing was also performed to investigate the thermal stability of both Ti3SiC2 and Ti2AlC MAX phases at high temperatures and, under the studied conditions, these materials in a form of electron-transparent lamellae were found decompose at temperatures around of 1273 K.

The results obtained with all the materials studied led to the major conclusion that there is a strong connection between the thermodynamics of materials and their radiation tolerance. Due to the possibility of tuning the elemental composition of metallic alloys with the aim of optimising the key core effects of high-entropy systems, the outcomes of this thesis indicate that these metallic alloys can be considered promising candidates for future generations of nuclear reactors operating at moderate temperatures. Ion irradiation with the in situ TEM methodology is thus shown to be fast and efficient for triaging innovative candidate materials for use in nuclear reactors.