Characterizing and Modelling Precipitation in Zirconium Alloys

Abstract

Zirconium alloys (Zr) are used in the nuclear industry in components present in light- and heavy-water reactors cores. Zr alloys experience corrosion, which is thought to be minimised by adding alloying elements such as iron (Fe), chromium (Cr), and nickel (Ni). This leads to the formation of second phase particles (SPPs): Zr(Fe,Cr)2 and Zr2(Fe,Ni). SPP size, number density, and distribution are thought to be affected by the thermomechanical processing applied to Zr alloys. Therefore, it is important to understand how processing affects these SPP characteristics, which in turn is thought to affect Zr alloy corrosion performance. Such characteristics can be predicted by modelling SPP kinetics. This has the potential to replace expensive experimental procedures when determining SPP characteristics for optimal Zr alloy performance. In this study, SPPs in Zircaloy-2 and HiFiTM – a novel high-Fe alloy – fuel cladding material were analysed using different characterization techniques to determine SPP characteristics and technique limitations. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) directly counted the number of both SPP types. SEM imaging produced reliable particle size distributions, enabling a large number of particles to be counted to give good statistics, although resolution is limited. STEM imaging instead has a higher resolution and enables detailed analysis of SPP chemistry, though fewer SPPs can be counted in a reasonable time-frame. Differential scanning calorimetry (DSC) and thermoelectric power (TEP) are capable of tracking SPP kinetics when the alloys are heat-treated. SPP dissolution temperatures are identified using DSC although their endothermic peaks cannot be separated and thus it is not suitable for discriminating between SPP phases. Changes in solute concentration with further heat treatments were ascertained using TEP, providing a measurement of the aggregate SPP volume fraction, and was used to rapidly determine the onset time for important microstructural events, such as the start of coarsening dominated kinetics. The evolution of SPP characteristics throughout processing was determined using SEM and STEM. SPPs are located on grain boundaries and dislocations during the β-quench stage where SPP nucleation and growth are present. These regimes are complete during the hot work stage with SPP coarsening being dominant. The highest SPP volume fraction is obtained at this stage with subsequent cold pilger and intermediate temperature anneal stages having a similar SPP volume fraction. The hot work stage deforms the microstructure where SPPs are present within the grain interior and on grain boundaries. Cold pilgering decreases the mean SPP size and increases SPP number density as larger SPPs break up. Annealing is dominated by SPP coarsening where the mean SPP size increases and SPP number density decreases. A physical kinetics model, based on classical nucleation, growth and coarsening theories, has been developed to capture the evolution of SPP characteristics when subject to thermal exposure. Model calibration is based on the TEP data and mean SPP size obtained at certain heat treatments. This model, calibrated on data from Zircaloy-2, has been applied directly to predict SPP kinetics in HiFi, demonstrating a good predictive capability. In addition, this model has been applied to a thermal history used in the production of Zr cladding, enabling the dominant SPP evolution process to be determined throughout processing and confirming that coarsening is the main operating mechanism for all annealing stages.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Joseph Robson (Supervisor) & Michael Preuss (Supervisor)