ABSTRACT
Systematic analysis of the surface morphology, crystalline phase, chemical composition and elemental distribution along depth for nitrogen-doped niobium was carried out using different methods of characterization, including Scanning Electron Microscopy (SEM), Atomic-Force Microscopy (AFM), Grazing Incidence X-ray Diffraction (GIXRD), Rutherford Backscattering Spectrometry (RBS) and layer-by-layer X-ray Photoelectron Spectroscopy (XPS) analysis. The results showed that, after nitrogen doping, the surface was covered by densely distributed trigonal precipitates with an average crystallite size of 32 ± 8 nm, in line with the calculation result (29.9 nm) of nitrogen-enrichedß-Nb2N from GIXRD, demonstrating the phase composition of trigonal precipitates. The depth analysis through RBS and XPS indicated thatß-Nb2N was dominant in the topmost 9.7 nm and extended to a depth of 575 nm, with gradually decreased content. In addition, the successive change along depth in the naturally oxidized states of niobium after nitrogen doping, was revealed. It was interesting to find that the oxygen diffusion depth could be moderately enhanced by the nitridation process. These results established the near-surface phase composition of nitrided niobium, which is of great significance in evaluating the effect of nitrogen doping and further understanding the Q improvement of the superconducting radio frequency cavities.
ABSTRACT
The oxidation behavior of body-centered cubic (bcc) structure Cr20Mn17Fe18Ta23W22 refractory high-entropy alloy (RHEA) and the microdefects induced by hydrogen ions before and after oxidation were investigated. The results revealed that compared with oxidizing Cr20Mn17Fe18Ta23W22 at 800 °C (6.7 °C/min) for 4 h (ST3, Ar:O2 = 3:1), the heating procedure of oxidizing Cr20Mn17Fe18Ta23W22 at 300 °C (6 °C/min) for 2 h and then increased to 800 °C (5 °C/min) for 4 h is more conducive to the production of oxides without spalling on the surface, i.e., HT1 (Ar:O2 = 1:1), HT2 (Ar:O2 = 2:1) and HT3 (Ar:O2 = 3:1) samples. The oxidation of Cr20Mn17Fe18Ta23W22 RHEA is mainly controlled by the diffusion of cations instead of affinities with O. Additionally, HT1 and HT3 samples irradiated with a fluence of 3.9 × 1022 cm-2 hydrogen ions (60 eV) were found to have a better hydrogen irradiation resistance than Cr20Mn17Fe18Ta23W22 RHEA. The microdefects in irradiated Cr20Mn17Fe18Ta23W22 mainly existed as hydrogen bubbles, hydrogen-vacancy (H-V) complexes and vacancy/vacancy clusters. The microdefects in irradiated HT3 were mainly vacancies and H-V complexes, while the microdefects in irradiated HT1 mainly existed as vacancies and vacancy clusters, as large amounts of hydrogen were consumed to react with oxides on the HT1 surface. The oxides on the surface of the HT3 sample were more stable than those on HT1 under hydrogen irradiation.