Nucleation of Cu-vacancy and Ni-vacancy clusters in bcc-Fe


Nucleation of Cu-vacancy and Ni-vacancy clusters in bcc-Fe

Al-Motasem, A. T.; Posselt, M.; Bergner, F.

Experimental investigations revealed that both the impurity Cu and the alloying element Ni may contribute to hardening and embrittlement of reactor pressure vessel (RPV) steels during the irradiation by fast neutrons. The irradiation-induced supersaturation of vacancies and self-interstitials amplifies the diffusion of the foreign atoms in bcc-Fe and causes the formation of nanosized Cu- or Ni-rich clusters which act as obstacles to dislocation motion within the grains of the polycrystalline matrix. The concentration of Cu in RPV steels is typically higher than its solid solubility and, therefore, irradiation-enhanced formation of Cu-rich precipitates is observed. Measurements showed that these clusters may not only consist of pure Cu but also include vacancies [1]. On the other hand, the Ni concentration is typically below its solubility limit. That means, any formation of Ni-rich clusters as found in [2] for neutron-irradiated binary Fe-Ni alloys is essentially irradiation-induced. Obviously, these clusters must contain additional species in order to be stable. Small-angle neutron scattering analysis [2] indicated that vacancies could be the other constituent.
In the present work atomistic computer simulations using the ternary Fe-Cu-Ni interatomic potential by Bonny et al. [3] are employed to investigate the thermodynamics of Cu-vacancy and Ni-vacancy precipitates in bcc-Fe. The nucleation free energy of the clusters is determined by the energy and the entropy change due to precipitation using isolated (diluted) Cu and Ni atoms as well as vacancies as the reference. In agreement with indications from measurements the nanoclusters are assumed to have the bcc structure of the iron matrix. The binding energy of the most stable cluster configurations is calculated by simulated annealing within the framework of on-lattice Metropolis Monte Carlo simulations and by subsequent relaxation using off-lattice molecular dynamics calculations.
[1] Q. Xu, T. Yoshiie, K. Sato, Phys. Rev, B 73 (2006) 134115.
[2] F. Bergner, A. Ulbricht, M. Hernandez-Mayoral, P. K. Pranzas, J. Nucl. Mater. 374 (2008) 334.
[3] G. Bonny, R. C. Pasianot, N. Castin, L. Malerba, Phil. Mag. 89 (2009) 3531.

Keywords: atomic simulation; cu-v clusters; formation energy; binding energy

  • Lecture (Conference)
    the 10th International Conference on Computer Simulation of Radiation Effects in Solids, 19.-23.07.2010, Krakow, Poland

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