UKAEA-CCFE-PR(23)176

Early-stage precipitation behaviour during aging of Cu-0.55Cr-0.07Zr alloy

We have monitored the full-stage precipitate evolution in Cu-0.55Cr-0.07Zr alloy during aging at 480 C using positron annihilation spectroscopy, transmission electron microscopy and atom probe tomography. Cr-rich precipitates form during the early stages of aging, and after 5.5 min the distribution is characterised by a number density of 81023 m-3 and an average precipitate size of 2.5 nm. At that time, Zr segregation is also detected at the precipitate sites. Longer aging times lead to precipitate coarsening, simultaneously with the occurrence of a Zr-rich shell around precipitates. Peak aging of the alloy is attained after 120 min of aging, and overaging induces further precipitate coarsening and the transition from spherical to disc-like morphology in those precipitates enriched in Zr at the interface with the matrix. At the longest aging time of 14 days, the precipitates maintain a face-centred cubic symmetry, reaching an average size of 8.5 nm at a density of 0.2  1022 m-3. The formation of a Zr rich shell around Cr-rich precipitates can help reduce the local coherency strains, and its presence should be taken into account when assessing the precipitate/matrix interface as a potential sink for radiation-induced lattice defects.

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