THEORY OF TRANSITIONS IN DOSE DEPENDENCE OF RADIATION EFFECTS IN STRUCTURAL ALLOYS

During irradiation of structural materials many processes take place simultaneously. These range from the elementary production of point defects, point defect clusters, and transmutation products to more complex processes of large scale cavity and precipitate formation, and alterations in macroscopic properties. Behavior that can be defined as transient or steady-state with respect to iffadiation dose can often be recognized within each process. Transitions from one type of dose dependence to another can be particularly important. These transitions may signal changes in governing physical processes or they may result in changes in the performance of the material in technological applications. Transitions in dose dependence of point defect concentrations, helium buildup and related transient phenomena in swelling, irradiation creep, and embrittlement are discussed, with particular reference to austenitic Fe-Ni-Cr alloys. Special attention is given to the concept, derivation, and usefulness of irradiation variable shifts based on a requirement of invariance in point defect absorption. It is found that the doses at which transitions in dose dependence take place for swelling and creep for example, can vary by many orders of magnitude, depending upon irradiation conditions and material. Transitions for different phenomena can also be widely separated in dose. Under certain conditions, transient behavior can be the dominant feature in microstructure and property changes.