Effects of temperature on the ion-induced bending of germanium and silicon nanowires

Nanowires can be manipulated using an ion beam via a phenomenon known as ion-induced bending (IIB). While the mechanisms behind IIB are still the subject of debate, accumulation of point defects or amorphisation are often cited as possible driving mechanisms. Previous results in the literature on IIB of Ge and Si nanowires have shown that after irradiation the aligned nanowires are fully amorphous. Experiments were recently reported in which crystalline seeds were preserved in otherwise-amorphous ion-beam-bent Si nanowires which then facilitated solid-phase epitaxial growth (SPEG) during subsequent annealing. However, the ion-induced alignment of the nanowires was lost during the SPEG. In this work, in situ ion irradiations in a transmission electron microscope at 400°C and 500°C were performed on Ge and Si nanowires, respectively, to supress amorphisation and the build-up of point defects. Both the Ge and Si nanowires were found to bend during irradiation thus drawing into question the role of mechanisms based on damage accumulation under such conditions. These experiments demonstrate for the first time a simple way of realigning single-crystal Ge and Si nanowires via IIB whilst preserving their crystal structure

Perspectives on Novel Refractory Amorphous High-Entropy Alloys in Extreme Environments

Two new refractory amorphous high-entropy alloys (RAHEAs) within the W--Ta--Cr--V and W--Ta--Cr--V--Hf systems were herein synthesized using magnetron-sputtering and tested under high-temperature annealing and displacing irradiation using \textit{in situ} Transmission Electron Microscopy. While the WTaCrV RAHEA was found to be unstable under such tests, additions of Hf in this system composing a new quinary WTaCrVHf RAHEA was found to be a route to achieve stability both under annealing and irradiation. A new effect of nanoprecipitate reassembling observed to take place within the WTaCrVHf RAHEA under irradiation indicates that a duplex microstructure composed of an amorphous matrix with crystalline nanometer-sized precipitates enhances the radiation response of the system. It is demonstrated that tunable chemical complexity arises as a new alloy design strategy to foster the use of novel RAHEAs within extreme environments. New perspectives for the alloy design and application of chemically-complex amorphous metallic alloys in extreme environments are presented with focus on their thermodynamic phase stability when subjected to high-temperature annealing and displacing irradiation

Preliminary assessment of the irradiation behaviour of the FeCrMnNi High-Entropy Alloy for nuclear applications

In the search for new nuclear materials with improved radiation tolerance and behavior, the high-entropy alloys (HEAs) have arisen as new candidates for structural components in nuclear reactors due to their suspected superior stability under irradiation. The metallurgical definition of HEAs is any alloy with multiple elements, five or more all in equiatomic compositions. The basic principle is the high mixing entropy of its solid solution lowers the Gibbs free energy giving a strong enhancement of the microstructural stability at low and high temperatures. The objective of this project is to assess the irradiation behaviour of the FeCrMnNi HEA system in order to investigate whether the high entropy effect is responsible for a microstructure with better radiation resistance compared to conventional alloys. In this work transmission electron microscopy (TEM) with in-situ ion irradiation has been used at the MIAMI-1 facility at the University of Huddersfield, UK: a 100 kV ion accelerator coupled with a JEOL JEM-2000FX TEM. This methodology allows the evolution of the HEA microstructure to be studied on the nanoscale during the ion irradiation

Preliminary assessment of the irradiation behaviour of the FeCrMnNi High-Entropy Alloy for nuclear applications

In the search for new nuclear materials with improved radiation tolerance and behavior, the high-entropy alloys (HEAs) have arisen as new candidates for structural components in nuclear reactors due to their suspected superior stability under irradiation. The metallurgical definition of HEAs is any alloy with multiple elements, five or more all in equiatomic compositions. The basic principle is the high mixing entropy of its solid solution lowers the Gibbs free energy giving a strong enhancement of the microstructural stability at low and high temperatures. The objective of this project is to assess the irradiation behaviour of the FeCrMnNi HEA system in order to investigate whether the high entropy effect is responsible for a microstructure with better radiation resistance compared to conventional alloys. In this work transmission electron microscopy (TEM) with in-situ ion irradiation has been used at the MIAMI-1 facility at the University of Huddersfield, UK: a 100 kV ion accelerator coupled with a JEOL JEM-2000FX TEM. This methodology allows the evolution of the HEA microstructure to be studied on the nanoscale during the ion irradiation

A contamination-free electron-transparent metallic sample preparation method for MEMS experiments with in situ S/TEM

Microelectromechanical systems (MEMS) are currently supporting ground-breaking basic research in materials science and metallurgy as they allow in situ experiments on materials at the nanoscale within electron-microscopes in a wide variety of different conditions such as extreme materials dynamics under ultrafast heating and quenching rates as well as in complex electro-chemical environments. Electron-transparent sample preparation for MEMS e-chips remains a challenge for this technology as the existing methodologies can introduce contaminants, thus disrupting the experiments and the analysis of results. Herein we introduce a methodology for simple and fast electron-transparent sample preparation for MEMS e-chips without significant contamination. The quality of the samples as well as their performance during a MEMS e-chip experiment in situ within an electron-microscope are evaluated during a heat treatment of a crossover AlMgZn(Cu) alloy.Comment: Preprint submitted to Microscopy and Microanalysi

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