It has been recognised for many years that copper plays a significant role in the embrittlement process of reactor pressure vessel (RPV) steels subjected to neutron irradiation. In developing fundamental understanding, and mechanistic models to predict embrittlement, it is important to identify the microscopic mechanisms underlying this phenomena. The purpose of this paper is to discuss recent results obtained from thermal ageing experiments on model alloys doped with copper and nitrogen. These experiments were designed to characterise the material prior to receiving irradiated samples encompassing a wide range of flux, fluence and temperature conditions.

Results will be presented from a microstructural and microchemical examination using transmission electron microscopy (TEM), field emission gun scanning transmission electron microscopy (FEGSTEM) and positron annihilation (PA). Precipitation kinetics have been followed and comparison made between microstructure and measured hardness changes. In alloys containing both titanium and nitrogen, a second form of barrier hardening centre is observed. The insight these studies give into the observed macroscopic hardness increase after irradiation or thermal ageing will be discussed. In particular the implications of a potential further mechanism for embrittlement in commercial steels will be considered.