The coordination environment dominated clustering behavior of vacancy and rhenium in molybdenum

Refractory molybdenum-rhenium (Mo-Re) alloys are considered as the best core structural materials in the space nuclear reactor. Irradiation will induce obvious microstructure damage and formation of Re-rich precipitates, but the underlying mechanism remains unclear due to the complicated interaction between Re and defects. In this work, by using first-principles and molecular static calculations, we obtain the thermodynamically stable configurations and binding energies of vacancy and vacancy-Re clusters in bulk Mo. A general expression of the vacancy binding energy for a stable vacancy cluster is proposed, which is determined by the numbers of the formed 1st and 2nd nearest neighboring vacancy-vacancy pairs. A scenario for the clustering of vacancy and Re is also revealed. With the presence of solute Re in Mo, the vacancy clustering initiates when the vacancy number reaches a certain value. Prior to the vacancy aggregation, the newly-added vacancy combines with the present Re and other vacancies, due to the strong attraction of Re-vacancy. It is noteworthy that the formed vacancy-Re and vacancy-Re-vacancy pairs contribute to the vacancy binding energies in Mo-Re system. Especially for the formed V₂-Re₁ complex, it could not only indicate the initiation of vacancy clustering, but also be the nucleus for further Re segregation towards the vacancy cluster. Our results provide fundamental insight to the irradiation-induced formation of Re-rich clusters in Mo-Re system, from the aspect of the atomic interaction between Re and vacancy.