Structure of nanoscale copper precipitates in neutron-irradiated Fe-Cu-C alloys

The structure of copper nanoclusters/precipitates formed under neutron irradiation in Fe-0.3 wt.% Cu-C alloy is studied by the internal friction and positron annihilation experiments of postirradiation annealed alloys. The appearance of a carbon-relaxation peak during the first recovery stage at about 723 K reveals the fact that complex carbon-vacancy-copper clusters have formed during the irradiation-mediated copper precipitation process. The stability of Cu-C-vacancy clusters is confirmed by ab initio calculations. The existence of a structural phase transition of copper precipitates is observed in the alloys that are annealed at temperatures between the first and second recovery stages, which indicates that the dissociation of vacancies and carbon atoms from the vacancy-copper-carbon clusters is accompanied by the crystallization of Cu precipitates.