Nucleation and Growth of Intermetallic Precipitates in Zircaloy-2 and Zircaloy-4 and Correlation to Nodular Corrosion Behavior

Abstract

One of the fundamental aspects in the history of the development of zirconium alloys for nuclear applications is the corrosion behavior under in-pile conditions. In boiling-water reactors (BWRs) and pressurized-water reactors (PWRs) the zirconium alloys Zircaloy-2 and Zircaloy-4 are the most commonly used materials, permitting attainment of a very high level of integrity and reliability. Nevertheless, efforts are required to optimize these well-established alloys with regard to their resistance to nodular corrosion, where improvements will give long-term advantages in fuel integrity and fuel economy. Phenomenological studies allow correlation of the nodular corrosion behavior with the morphological appearance of precipitated intermetallic phases in the microstructures of Zry-2 and Zry-4. To understand the fundamental processes of precipitation, particle nucleation-and-growth studies were made with Zry-2 and Zry-4 in different fabrication dimensions and with variations in β-quenching rates followed by isothermal and isochronical heat treatments. The microstructural characteristics of the precipitates were investigated by optical and transmission-electron microscopy. The macroscopic behavior was studied by electrical-resistivity measurements and hardness measurements. The nodular-corrosion susceptibility was determined by weight-gain and nodule distribution measurements after a 500°C laboratory-autoclave test. Precipitation of the intermetallic phases (Zry-4: hexagonal (Fe, Cr)2 Zr; Zry-2: hexagonal (Fe, Cr)2 Zr and Zr2 (Fe, Ni)) occurs at the β/α transition. Particle size and particle distribution depend on the β-quenching rate. Rapid quenching results in complete supersaturation. The isothermal and isochronical aging heat treatments are used for evaluation of the thermodynamic parameters for nucleation and growth of the precipitates. Nucleation and growth of the precipitates lead to characteristic hardness effects and an increase in electrical resistance, which are superimposed on matrix recovery. The nodular corrosion effects in the 500°C test can be correlated with the size and distribution of the precipitates. This correlation results in a limit to the particle size for desired nodular corrosion behavior in the final product. A strong influence of cold work on particle growth appears to exist and is expected to cause a significant increase in the particle growth velocities determined in this study.