Guanze He

Refractory high-entropy alloys (RHEAs) are emerging materials with potential for use under extreme conditions. As a newly developed material system, a comprehensive understanding of their long-term stability under potential service temperatures remains to be established. This study examined a titanium-vanadium-niobium-tantalum alloy, a promising RHEA known for its superior high-temperature strength and room-temperature ductility. Using a combination of advanced analytical microscopies, Calculation of Phase Diagrams (CALPHAD) software, and nanoindentation, we investigated the evolution of its microstructure and mechanical properties upon aging at 700°C. Trace interstitials such as oxygen and nitrogen, initially contributing to solid solution strengthening, promote phase segregation during thermal aging. As a result of the depletion of solute interstitials within the metal matrix, a progressive softening is observed in the alloy as a function of aging time. This study, therefore, underscores the need for a better control of impurities in future development and application of RHEAs.

Complementary characterization techniques were used to investigate two commercial Zr-Nb alloys exposed in reactor to understand how the corrosion process is affected by changes in the oxidation state of niobium. Electron energy-loss spectroscopy analysis was carried out to better understand the oxidation state of niobium in the β phase found in dual-phase Zr-2.5Nb and partially recrystallized Zr-2.5Nb alloys irradiated to different damage levels. The different rates of oxidation of niobium in different materials (or samples) are attributed to the manner in which the microstructure of the β phase develops when incorporated into the oxide. Transmission electron microscopy and atom probe tomography were used to show that most of the original β-Zr phase found in the as-received dual-phase Zr-2.5Nb has decomposed to form β-Nb precipitates at 1.9 dpa and 25.2 dpa, whereas energy-dispersive X-ray spectroscopy results show the β-Zr phase found in partially recrystallized Zr-2.5Nb has not decomposed after three cycles in reactor. The possible cause of these different behaviors of the β-Zr phase is discussed. The rate at which niobium in the β phase is oxidized and released into the surrounding oxide controls the aliovalent niobium composition in solid solution and contributes to the charge-balancing effect. These results can help to explain the measured reduced oxidation rate. Furthermore, the oxidation state of niobium in the β phase is compared with results from a different study by spatially resolved X-ray absorption near-edge spectroscopy on the oxidation state of niobium in solid solution in low-tin ZIRLO irradiated for three cycles in reactor that shows a similar “delayed oxidation” phenomenon. All of these results are combined to discuss the overall effect of niobium on the in-reactor corrosion rate of the Zr-Nb alloys.

Bragg coherent X-ray diffraction imaging (BCDI) allows the 3D measurement of lattice strain along the scattering vector for specific microcrystals. If at least three linearly independent reflections are measured, the 3D variation of the full lattice strain tensor within the microcrystal can be recovered. However, this requires knowledge of the crystal orientation, which is typically attained via estimates based on crystal geometry or synchrotron microbeam Laue diffraction measurements. Presented here is an alternative method to determine the crystal orientation for BCDI measurements using electron backscatter diffraction (EBSD) to align Fe–Ni and Co–Fe alloy microcrystals on three different substrates. The orientation matrix is calculated from EBSD Euler angles and compared with the orientation determined using microbeam Laue diffraction. The average angular mismatch between the orientation matrices is less than ∼6°, which is reasonable for the search for Bragg reflections. The use of an orientation matrix derived from EBSD is demonstrated to align and measure five reflections for a single Fe–Ni microcrystal via multi-reflection BCDI. Using this data set, a refined strain field computation based on the gradient of the complex exponential of the phase is developed. This approach is shown to increase accuracy, especially in the presence of dislocations. The results demonstrate the feasibility of using EBSD to pre-align BCDI samples and the application of more efficient approaches to determine the full lattice strain tensor with greater accuracy.

Superconducting windings will be necessary in future fusion reactors to generate the strong magnetic fields needed to confine the plasma, and these superconducting materials will inevitably be exposed to neutron damage. It is known that this exposure results in the creation of isolated damage cascades, but the presence of these defects alone is not sufficient to explain the degradation of macroscopic superconducting properties and a quantitative method is needed to assess the subtle lattice damage in between the clusters.

We have studied REBCO‐coated conductors irradiated with neutrons to a cumulative dose of 3.3 × 10²² n/m² that show a degradation of both T_c and J_c values, and use HRTEM analysis to show that this irradiation introduces ∼10 nm amorphous collision cascades. In addition, we introduce a new method for the analysis of these images to quantify the degree of lattice disorder in the apparently perfect matrix between these cascades. This method utilises Fast Fourier and Discrete Cosine Transformations of a statistically relevant number of HRTEM images of pristine, neutron‐irradiated and amorphous samples and extracts the degree of randomness in terms of entropy values. Our results show that these entropy values in both mid‐frequency band FFT and DCT domains correlate with the expected level of lattice damage, with the pristine samples having the lowest and the fully amorphous regions the highest entropy values.  Our methodology allows us to quantify ‘invisible’ lattice damage to and correlate these values to the degradation of superconducting properties, and also has relevance for a wider range of applications in the field of electron microscopy where small changes in lattice perfection need to be measured.

We have used both in situ radiation damage techniques and direct observations of ex-reactor materials to study radiation damage mechanisms in a range of zirconium-niobium (Zr-Nb) alloys with different initial microstructures. The aim has been to determine the relative stability of the different phases present under in-service conditions, including oxides and second phase particles (SPPs), and how damage to these phases alters the chemistry of the surrounding alloy matrix. A monoclinic-to-cubic transformation of the bulk oxide is observed by in situ ion irradiation experiments, followed by irradiation-induced grain growth. The possibility of radiation-induced stabilization of this cubic phase thus needs to be considered as an additional process that can occur in the regions of oxides exposed to high fluxes in service and may further affect the corrosion rates. In situ studies of β-Nb and Laves phase SPPs under ion irradiation showed that they behaved differently as a function of ion fluence and irradiation temperatures. The β-Nb SPPs show good stability under both ion and neutron irradiation to high damage levels and over a wide temperature range. The formation in flux, by a combination of irradiation-enhanced oxygen diffusion and the direct effects of radiation, of oxides that are both less well textured and with a more disrupted grain structure will also contribute to different corrosion rates in reactor. Finally, high-resolution energy-dispersive X-ray and atom probe tomography analysis were used to study changes to both SPP and matrix chemistry as result of radiation damage.