Assessment of annealing parameter approximations in zirconium alloys

Abstract

Critical to reliable performance of zirconium alloys is an understanding of the effects of processing and service on properties. To obtain this understanding, extensive experimentation is performed, but the challenge then becomes comparing test data from different conditions and using it in extrapolation. To address this, the annealing parameter (AP) has become a popular metric. Various AP formulations have been proposed, enabling data from different times and temperatures to be directly compared. The AP also allows complex multi-step heat-treatments to be related to an isothermal equivalent. This has proved useful in rationalizing the evolution of second phase particles (SPPs) size in Zircaloys and related properties such as corrosion resistance. Implicit in the annealing parameter approach are assumptions about the mechanism controlling microstructural evolution and its path independence. This work critically evaluates the validity of these assumptions through comparison with well controlled predictions from a physical model for SPP evolution. It is shown that once SPP evolution has entered the coarsening regime, AP approaches are valid, but prior to this they are not. It is demostrated that for SPP evolution in Ziraloys, coarsening dominated kinetics are established rapidly and thus for most practical cases the AP approach is appropriate.