Helium ion irradiation enhanced precipitation and the impact on cavity formation in a HfNbZrTi refractory high entropy alloy

Refractory high entropy alloys have gained increasing research attention on their irradiation response for the potential applications in nuclear engineering. Considering the metastable nature of their single-phase solid solution structures, the phase stability under irradiation, especially at elevated temperatures, should be a critical concern during the long-term service. In the present study, a typical refractory high entropy alloy with a single-phase body-centered cubic (BCC) structure, HfNbZrTi, is irradiated with 1.5 MeV He ions under 700 °C to various fluences from 5 × 10¹⁵ to 1 × 10¹⁷ cm⁻². Significant irradiation-enhanced precipitation with Hf and Zr enrichment is observed. Although most extended precipitates with tens of nanometers in size are with a hexagonal close-packed (HCP) structure, different structures, including HCP, face-centered cubic (FCC), and the close-packed structures containing highly mixed stacking sequences, are observed in the small precipitates with a few nanometers in size formed at the early-stage precipitation process, suggesting various precipitation paths. By carefully examining the interfaces between the matrix and precipitates, the precipitation mechanism is discussed with similar but slightly different orientation relationships from the Burgers path. Such irradiation-enhanced precipitation process has a strong impact on the cavity formation in this alloy. The cavities are dominantly formed through the heterogeneous nucleation and growth mechanisms, accompanied by the precipitation process. The cavities with low densities but large sizes are observed to be highly clustered with the precipitates. These findings not only unveil the detailed precipitation behavior in a model refractory high entropy alloy under irradiation, but also highlight the critical role that the phase stability plays on the irradiation damage of this novel alloy system.