[Paper Review] Nanoscale characterisation of hydrides and secondary phase particles in Zircaloy-4

Wenjun Lu, Paraskevas Kontis+5 more|arXiv (Cornell University)|Sep 22, 2021

Nuclear Materials and Properties39 references4 citations

TL;DR

This study employs cryo-FIB, STEM, EDS, EELS, and atom probe tomography to investigate nanoscale hydrides and secondary phase particles in Zircaloy-4. It reveals up to 6 at% hydrogen in solid solution, identifies embryonic hydrides with only 17 at% hydrogen (below ξ-phase stoichiometry), and demonstrates solute redistribution ahead of hydride interfaces, advancing understanding of hydrogen behavior in nuclear fuel cladding materials.

ABSTRACT

The interaction of hydrogen and metals continues to be industrially relevant and is a critical part of creating and supporting a safety case for nuclear reactor operation. In the present work, we explore hydrogen storage and hydride formation in a zirconium alloy. We characterise the structure and interfaces of fine scale hydrides using scanning transmission electron microscopy (STEM) including energy dispersive spectroscopy (EDS/EDX), electron energy loss spectroscopy (EELS), and high-resolution STEM. Chemical characterisation is supported further with atom probe tomography (APT). Samples were prepared with cryo-focussed ion beam machining (cryo-FIB) and contain hydrides in α-Zr matrix and hydrides associated with one FeCrZr secondary phase particle (SPP). Major findings include characterisation of different interface planes based upon the size of the hydrides and chemical redistribution of solute ahead of the hydride-metal interface. We also find significant (up to 6 at%) hydrogen retained in solution within the zirconium matrix and show a hydride with only 17 at% hydrogen, which is well below that of a ξ-phase stoichiometry suggesting it is an embryonic hydride. These findings help us understand the distribution of hydrogen and the nanoscale morphology of hydrides, which may influence the lifetime of zirconium-based nuclear fuel cladding.

Motivation & Objective

• To understand hydrogen distribution and hydride morphology at the nanoscale in Zircaloy-4, a key nuclear fuel cladding material.
• To characterize the structure and chemical composition of hydrides and secondary phase particles (SPPs) in the α-Zr matrix.
• To investigate interfacial phenomena, including solute redistribution ahead of hydride-metal interfaces.
• To determine the extent of hydrogen retained in solid solution within the zirconium matrix.
• To identify and characterize embryonic hydrides with sub-stoichiometric hydrogen content.

Proposed method

• Cryo-focussed ion beam (cryo-FIB) was used to prepare site-specific, high-quality TEM lamellae from Zircaloy-4 samples containing hydrides and SPPs.
• Scanning transmission electron microscopy (STEM) with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) enabled nanoscale chemical and structural analysis.
• High-resolution STEM provided atomic-scale imaging of hydride-matrix and hydride-SPP interfaces.
• Atom probe tomography (APT) was employed to quantify hydrogen and solute element distributions with near-atomic resolution.
• Hydride morphology and interface planes were correlated with hydride size and local chemical environment.
• Chemical analysis focused on hydrogen content, solute segregation (e.g., Fe, Cr, Zr), and interfacial diffusion profiles.

Experimental results

Research questions

• RQ1 What is the extent of hydrogen retained in solid solution within the α-Zr matrix of Zircaloy-4?
• RQ2 How do the crystallographic orientation and size of hydrides influence their interfacial structure and chemistry?
• RQ3 What is the chemical composition of hydrides associated with FeCrZr secondary phase particles (SPPs)?
• RQ4 Can embryonic hydrides with sub-stoichiometric hydrogen content be identified and characterized?
• RQ5 How does solute element distribution evolve ahead of the hydride-metal interface?

Key findings

• Up to 6 at% hydrogen was found in solid solution within the zirconium matrix, indicating significant hydrogen solubility.
• A hydride with only 17 at% hydrogen was identified, well below the stoichiometric ξ-phase (typically ~25 at%), indicating an embryonic hydride.
• Chemical redistribution of solute elements (e.g., Fe, Cr, Zr) was observed ahead of the hydride-metal interface, suggesting dynamic interfacial processes.
• Different interface planes were characterized based on hydride size and morphology, revealing size-dependent interfacial structure.
• Hydrides were found to nucleate and grow at FeCrZr secondary phase particles, with distinct interfacial chemistry compared to hydrides in the α-Zr matrix.
• Atom probe tomography confirmed the presence of hydrogen-rich regions with non-stoichiometric composition, supporting the existence of metastable or early-stage hydride phases.