Influence of Metallurgical State on Hydride Precipitation and Reorientation in Zircaloy-4

Abstract

Zirconium alloys are often used in safety-critical applications in the nuclear industry, where the ingress of hydrogen and precipitation of hydrides can result in degradation of their mechanical properties. In cladding tubes, the precipitation of hydrides along the radial direction, pose a serious threat to cladding integrity due to their propensity to form brittle pathways through the tube thickness. Extensive radial hydride formation occurs during dry storage whereby changes in temperature and stress favours the reorientation of hydrides during precipitation. The microstructural mechanisms governing hydride precipitation and reorientation within zirconium are not well understood. The aim of this thesis was to correlate microstructural features at varying post deformation heat treatments with the extent of hydride precipitation and reorientation to better understand factors dictating these mechanisms. The influence of metallurgical state is challenging to analyse due to a myriad of factors, such as grain size, dislocation density and texture, dictating its evolution. To address this, microstructural and textural evolution during recrystallisation of deformed Zircaloy-4 was investigated. Microscopy in conjunction with crystal plasticity finite element modelling was utilised to characterise the deformed microstructure prior to annealing. High-temperature in-situ electron back scatter diffraction (EBSD) provided insight into the microstructural evolution of particular texture components during primary recrystallisation and grain growth. Ex-situ EBSD, X-ray diffraction and phase field modelling enabled bulk quantification of the textural, dislocation density and grain size evolution occurring during annealing. This study highlighted that recrystallisation is controlled by the deformed microstructure which dictates the stability and hence extent of recovery and grain growth of different texture components. To quantify the extent of hydride precipitation and reorientation in these microstructures, readily available, standardised methods for hydride characterisation are required. Currently, studies within literature utilise a wide range of analytical techniques and definitions, rendering the data incomparable. To address this issue, an open-source hydride analysis package in Python (HAPPy) was developed and published. A novel Hough transform method was developed to compute the radial hydride fraction from optical micrographs. Additionally, network analysis tools were employed to compute the mean hydride length and visualise hydride connectivity. These methods were tested and validated across multiple literature studies such that the orientation and connectivity of hydrides within microstructures could be quantified. To correlate the microstructural features identified at different metallurgical states to the extent of hydride precipitation and reorientation, cold-worked Zircaloy-4 annealed at temperatures ranging from 485-730 °C were characterised in the annealed, hydrided and reorientated conditions. It was shown that the metallurgical state dictates the morphology, orientation and phase stability of hydrides during precipitation. Further, the extent of reorientation was revealed to be governed more significantly by the hydrogen concentration, temperature and stress at which the thermomechanical loading was conducted rather than the microstructure at different metallurgical states. A model was developed to account for greater hydride dislocation density before when compared to after hydride reorientation. To further understand the effect of microstructure on hydride phase stability, a technique utilising EBSD and dictionary indexing in conjunction with orientation relationship analysis was developed. This enabled the identification and characterisation of d and g hydrides to better understand their order of precipitation and stability within the microstructure upon cooling. This thesis, for the first time, provides a comprehensive

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Pratheek Shanthraj (Supervisor) & Michael Preuss (Supervisor)