Microstructural analysis of hydride precipitation in zirconium alloys
Creators: name: El Chamaa, Said
Publisher: name: Imperial College London
Year: 2020

description: Zirconium alloy cladding and structural materials are central to the structural integrity of nuclear fuel pins as well as dry storage of spent fuel rods. However, zirconium alloys are susceptible to hydrogen pick-up in the reactor environment and its operation lifetime can be severely compromised by a hydride embrittlement mechanism. In this project, the effects of microstructurally local stress raisers on hydride precipitation, in Zr-4 and commercially pure zirconium, such as second phase particles (SPPs) and grain boundaries (GBs) were studied in unloaded samples and ones containing long range stress concentration features (SCFs) such as loaded notches. In unloaded Zr-4 and commercially pure Zr, quantifi cation of hydride precipitates as a function of grain boundary density and SPP content, show that for smaller grain specimens, there was a higher proportion of inter-granular hydrides, relative to intra-granular hydrides, due to the higher GB density. But, as the grains were grown in size to produce blocky-alpha microstructures, the proportion of intra-granular hydrides increased; this effect was slightly more observed for Zr-4 specimens with a higher SPP content than in commercially pure Zr. Results also show that not all GBs are favourable sites for hydride precipitation, an observation attributed to the different energy at GBs between grains of different crystallographic orientations. Further studies relating to the microstructural effects on hydride precipitation, were then performed on as-received and blocky-alpha Zr-4 microstructures after undergoing three thermomechanical cycles at a maximum temperature of 350°C and at loads ranging between 2 and 8 MPam^1/2. Thermomechanical cycling of Zr-4 'V' (1 mm notches with 45° angle and 50 μm root radius) and 'U' (1 mm notches with 700 μm root radius) notched specimens showed that the microstructure interacts with stresses at notch regions to produce a higher hydride number density and hydride area fraction than in the specimen bulk. Furthermore, results show that blocky-alpha Zircaloy-4 specimens showed no hydride reorientation at all applied loads and the increase in hydride number density from the bulk towards the notches in blocky-alpha Zr-4, was found to be lower than that in as-received Zr-4 specimens. Hydride quanti fication results can be used to parameterise predictive hydride embrittlement numerical and analytical models with microstructural data that relate to hydride precipitation sites around different notch acuities. Finally, a method of incorporating microstructural effects into a finite element model was introduced via a computational framework that incorporates atomic scale anisotropy through elastic dipole tensors. A crystal plasticity polycrystalline fi nite element model was then employed to provide computational insights into the effects of plasticity and polycrystallinity on the steady state hydrogen distribution profiles in blocky-alpha microstructures. Results show that the plastic zone and the effective plastic strain in blocky-alpha grains are highly dependent on the crystallography of the notch tip grain. Plasticity was found to reduce the hydrostatic stress and deplete hydrogen concentration in hard crystals ahead of 'V' and `U' notch geometries. Grain boundaries were also found to have an effect on hydrostatic stress distribution that in-turn effects hydrogen concentration pro les and therefore hydride precipitation. Results between model and experimental blocky-alpha micrograph were found to be in a good agreement, as hydride precipitates can be seen at GBs of large hydrostatic stress from the model and vice versa. A key outcome of the project will be to quantify and qualify hydride precipitation DHC models that can be used to support industrial safety assessments.