Dislocation loops, segregation and hardening induced by high-dose ion irradiation of NbMoTaW and VCrTaW high-entropy alloy coatings on the T91 substrate

NbMoTaW and VCrTaW high-entropy alloys (HEA) coatings with near-equal atomic ratios were deposited onto the T91 substrate by magnetron sputtering. The two HEA coatings and the T91 substrate were irradiated to the peak damage 50 displacements per atom (dpa) and 100 dpa using 2.7 MeV Si2+ at 550 degrees C. X-ray diffraction (XRD), Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and nanoindentation techniques were used to study the changes in grain morphology, microstructure, element distribution and hardness before and after irradiation. The results showed that the grain structure was stable and the coatings were closely bonded to the substrate after irradiation. The elements (Ta and W) near the grain boundary in NbMoTaW coating were depleted before irradiation, but they became homogenized after irradiation. A large number of dislocation loops were observed in both the T91 substrate and HEA coatings after irradiation, the HEA coatings significantly increased the doses required for the full evolution of the dislocation loops. With the increase of damage doses, the dislocation loops size of HEA coatings increased and the number density decreased. The size of the dislocation loops of the two coatings was similar, while the number density of the dislocation loops of NbMoTaW coating was lower than that of VCrTaW coating. Both HEA coatings were significantly hardened after irradiation and the hardening rate of NbMoTaW coating was lower than that of VCrTaW coating. Interestingly, the hardening rate of 50 dpa was higher than 100 dpa for both HEA coatings, which was determined by softening factor and hardening factor in the irradiation process. Based on the defects and hardening rate caused by irradiation, it was preliminarily concluded that NbMoTaW coating had better irradiation resistance, and the above results provide an important reference for the subsequent optimization of lead-based fast reactors structural materials.