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Grain growth and phase stability of a nanocrystalline face-centered cubic (fcc) Ni0.2Fe0.2Co0.2Cr0.2Cu0.2 high-entropy alloy (HEA), either thermally-or irradiation-induced, are investigated through in-situ and post-irradiation transmission electron microscopy (TEM) characterization. Synchrotron and lab X-ray diffraction measurements are carried out to determine the microstructural evolution and phase stability with improved statistics. Under in-situ TEM observation, the fcc structure is stable at 300 C with a small amount of grain growth from 15.8 to ~ 20 nm being observed after 1800 s. At 500 C, however, some abnormal growth activities are observed after 1400 s, and secondary phases are formed. Under 3 MeV Ni room temperature ion irradiation up to an extreme dose of nearly 600 displacements per atom, the fcc phase is stable and the average grain size increases from 15.6 to 25.2 nm. Grain growth mechanisms driven by grain rotation, grain boundary curvature, and disorder are discussed.

Cr2AlC MAX phases were deposited using magnetron sputtering. The synthesis was performed via layer-by-layer deposition from elemental targets onto Si wafer and polished Inconel® 718 superalloy substrates at 650 K and 853 K. Transmission Electron Microscopy (TEM) characterisation showed that the thin films had a thickness of about 0.8 and 1.2 m for Si and Inconel® substrates, respectively, and a MAX phase crystalline structure. Depositions onto Inconel substrate was performed in order to measure film mechanical properties. The films have hardness at around 15 GPa, reduced Young's modulus at around 260 GPa, do not delaminate and showed characteristic ductile behaviour during nanoscratching. Ion irradiations with in situ TEM were performed with 320 keV Xe + ions up to fluence 1×10 16 ions•cm-2 at 300 K and 623 K. At 300 K the Cr2AlC started to amorphise at around 0.3 dpa. At displacement levels above 3.3 dpa all crystalline structure was almost completely lost. Conversely, irradiations at 623 K showed no recordable amorphisation up to 90 dpa. It is discussed that the presence of many grain boundaries and low defect recombination energy barriers are responsible for high radiation hardness of Cr2AlC MAX phase at 623 K. The thin film Cr2AlC MAX phases have mechanical and radiation stability which makes them a candidate for fuel rod coating as Accident Tolerant Fuels (ATF) material for the next generation of nuclear reactors.

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Concentrated solid solution alloys (CSAs) -including high entropy alloys (HEAs)are known for their remarkable mechanical and corrosion resistances with superior tolerance against the deleterious effect of irradiation exposure when compared with pure metals and dilute alloys. To date, however, the mechanisms responsible for such improvements are still unclear and remain a subject of investigation. The present work reports in situ Transmission Electron Microscopy (TEM) study under simultaneous ion irradiation of the face-centred cubic (FCC) FeCrMnNi quaternary HEA, comparing with a non-equiatomic Fe-based alloy, the AISI-348 austenitic stainless steel that has Cr, Ni and Mn as alloying elements. The alloys were irradiated under the same conditions, with 6 keV He + and 134 keV Xe + ions at 298 K up to 1.7×10 17 ions•cm −2 (4 displacements per atom, dpa) and 2.7×10 15 ions•cm −2 (4 dpa), respectively. The nucleation of inert gas bubbles was tracked upon post-irradiation extended annealing up to 673 K. He and Xe bubbles were observed to grow at a rate slightly slower in the equiatomic alloy. Trends from the bubble size analyses show that the nucleation and growth of inert gas bubbles are suppressed or delayed in some conditions in the nearly equiatomic alloy.

Coating nuclear fuel cladding alloys with hard thin films has been considered as an innovative solution to increase the safety of nuclear reactors, in particular during a of loss-of-coolant accident (LOCA). In this context, and due to its suitable mechanical properties and high corrosion resistance, titanium nitride thin films have been proposed as candidate coatings for zirconium alloys in new accident tolerant fuels for light water reactors. Although the properties of TiN hard coatings are known to be adequate for such applications, the understanding of how the exposure to energetic particle irradiation changes the microstructure and properties of these thin films is still not fully understood. Herein, we report on heavy ion irradiation in situ within a Transmission Electron Microscopy of magnetronsputtered TiN thin films. The coatings were irradiated with 134 keV Xe + ions at 473K to a fluence of 6.7×10 15 ions•cm −2 corresponding to 6.2 displacements-per-atom where significative microstructural alterations have been observed. Post-irradiation analytic characterisation with Energy Filtered TEM and Energy Dispersive X-ray spectroscopy carried out in a Scanning Transmission Electron Microscope indicates that TiN thin films are subjected to Radiation Induced Segregation. Additionally, the nucleation and growth of Xe bubbles appears to play a major role in the dissociation of the TiN thin film.