The effect of Ni on the microstructural evolution of high Cu reactor pressure vessel steel welds after thermal ageing for up to 100,000 h

To understand the contribution of the long term thermal ageing to Reactor Pressure Vessel embrittlement a series of weld materials containing systematic variations of Ni has been thermally aged for times up to 100,000 h at 330 °C and 365 °C. Microstructural characterisation using Atom Probe Tomography was performed. Thermal ageing produced a high number density of nano-scale Cu-enriched clusters. Ni has a strong influence on Cu precipitation, enhancing the homogeneous nucleation of Cu clusters. The nanometre size Cu clusters have a Ni-Mn-Si rich interface which was found to be wider in steels with higher bulk Ni content. This interface reduces the interfacial energy of the Cu-enriched clusters through a combination of the minimising of unfavourable Fe-Cu bonds and reduction in lattice strain. Matrix Cu levels after ageing for 90,000-100,000 h were found to be around 0.06-0.08 at%, close to the expected solubility limits for Cu in Fe.