Full-stage precipitation during aging of Cu-0.55Cr-0.07Zr alloy for high heat flux fusion reactor technology

Abstract

We have monitored the full-stage precipitate evolution in Cu-0.55Cr-0.07Zr alloy during aging at 480 °C (753 K) up to 14 days (20160 min), using micro-hardness, positron annihilation spectroscopy, 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 potentially to help reduce local coherency strains. 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. At the longest aging time of 14 days, the precipitates maintain a face-centred cubic symmetry, reaching an average size of ∼9 nm at a density of 0.2×1022 m-3. The presence of Zr atoms at the precipitate site and interface, and the disc-like morphology of overaged precipitates, should be considered when assessing the precipitate stability under irradiation and the precipitate/matrix interface as a potential sink for radiation-induced lattice defects.

Keywords

• Cu-base alloys
• heat treatment
• precipitation
• microstructure characterization
• fusion reactor technology