4.05.4.5.1 Introduction

Historically, the hardening observed in low Cu steels has been considered to arise from matrix defects. MD arises from the clustering of irradiation-induced point defects to form either vacancy or interstitial clusters, and/or solute-defect complexes (see, e. g., Odette and Lucas53). In addition, various solutes may diffuse to these clusters giving rise to complex defect-solute configurations. A critical factor in attempting to develop insight is that there is no one technique that allows a direct characterization of matrix defects. Insight has been obtained from indi­rect studies using techniques such as positron annihi­lation, where the nature of matrix defects can be inferred only after data analysis or modeling.

Within these constraints, there are two aspects which should be discussed. First, the nature of MD, in particular whether it is vacancy or interstitial clus­ters that give rise to the observed hardening, and, second, the evolution of MD clusters at different fluxes and increasing fluence. The discussion also needs to include an assessment of whether MD is dependent on the presence of other solutes and whether MD can be treated independently of CEC formation. It should be noted that in generating insight into MD, studies of model alloys (e. g., Fe-Cu alloys) have been particularly important, including ion and electron irradiation studies. This is in contrast to the study of Cu precipitation where the majority of information has arisen from studies of Cu-containing neutron-irradiated steels.