Crystal Structure of Hydride Platelets in Hot Rolled Zircaloy-2, Characterized with Synchrotron X-Ray Diffraction, S/TEM, and EELS

In the current study, transmission synchrotron X-ray diffraction, scanning/transmission electron microscopy, and electron energy loss spectroscopy were utilized to characterize the crystal structure and strain state evolution of zirconium (Zr) hydride precipitates in hot rolled Zircaloy-2 containing ~ 200 wt-ppm hydrogen. Hydride precipitates were formed in two different cooling regimes: furnace cooled (FC) and water quenched (WQ). Following the cooling procedure, samples were aged at 200°C for 15 days to evaluate the stability of the hydride phases with time. Considerably different behaviors were observed between FC and WQ conditions. In the FC condition, hydride platelets formed with a face centered cubic structure with a contracted unit cell compared to that of the standard strain-free δ hydride. After aging for 15 days at 200°C, residual strains were partially relieved and no transformation to a new hydride phase was observed. In the WQ condition, synchrotron X-ray diffraction showed there was a coexistence of three hydride populations: γ hydride with a minute volume fraction, a slightly strained δ hydride prevalently observed in Zr grains with basal poles aligned close to the normal direction, and a third δ' hydride population that was a severely strained δ hydride with an apparent tetragonality of a = 4.709 Å and c = 4.783 Å. This severely strained phase was prevalently observed in Zr grains with basal poles aligned close to rolling/transverse directions. Nano-beam electron diffraction analysis revealed that, unlike γ hydride, δ` is not an ordered phase. Aging at 200°C for 15 days resulted in the complete disappearance of the γ phase and commencement of a gradual relaxation of δ` to δ.