Materials World Network: Kinetics of Hydride Precipitation near a Crack Tip in Zirconium

This is a collaborative research effort between Penn State University, Queen's University in Canada and CNEA (National Atomic Energy Commission) in Argentina to perform in-situ synchrotron radiation diffraction experiments and thermodynamic and mechanical modeling to elucidate the kinetics of hydrogen migration and hydride precipitation in zirconium alloys. The research examines the kinetics of hydride precipitation under high temperatures and stresses, such as can occur near a crack tip. This hydride precipitation and the associated crack advancement are at the root of the cracking process known as delayed hydride cracking (DHC). In the experiments proposed, compact tension (CT) specimens of hydrided Zircaloy with a fatigue pre-crack is subjected to temperature and stress while being examined using microbeam synchrotron radiation at the Advanced Photon Source in Argonne National Laboratory. The precipitation of the hydrides at the crack tip is directly monitored in-situ by monitoring the diffraction signal of hydride precipitates forming at the crack tip. The crack growth behavior of the specimen is monitored simultaneously using potential drop and acoustic emission. Thermodynamic and mechanical modeling is performed to assist in the interpretation the results.

This research provides unique data on the kinetics of hydride precipitation at a crack tip, and as such, helps identify and quantify the mechanisms and operational limits for delayed hydride cracking. The unique aspect of this research project is that the measurements are performed in situ, and with great spatial resolution, near the crack tip so that these localized processes can be directly monitored. The participants from the US, Canada and Argentina bring in complementary research capabilities. The successful completion of this research project will result in increased knowledge about the fundamental mechanisms governing the precipitation of hydrides at crack tips under a stress field in hydride-forming metals such as Zr- and Ti-base alloys. The development of such in-situ techniques for directly monitoring of processes occurring at crack tips may be applicable to many other phenomena, which lack detailed local data for deriving mechanistic understanding. A greater understanding of the phenomena underlying delayed hydride cracking achieved by this data will have an impact on the design and operation of components for nuclear power plants, and it will also lead to enhancement of the research infrastructure by creating new partnerships. The involvement of doctoral students and young postdoctoral researchers will result in the education of young scientists in this important field.