PNNL

Abstract

Oxidation degrades the performance of several structural alloys, resulting in significant economic cost and safety concerns. Detailed understanding of the underlying dynamic processes of oxidation, including the evolution of metastable suboxide phases, remains elusive in part due to limited experimental observations of the rapid, atomic-level changes at alloy surfaces and buried oxide/metal interfaces. These transient suboxide phases are linked intimately to oxidation kinetics and the integrity of the oxide/metal interface, but are difficult to predict from thermodynamic phase diagrams. In this work, we studied the evolution of oxide phases using complementary scanning transmission electron microscopy and atom probe tomography. Nanoscopic volumes of a zirconium alloy (Zircaloy-4) were oxidized at 300 °C for short periods (1 or 5 minutes) in a controlled gas phase reactor chamber attached directly to the atom probe system. Analyses of the suboxide film morphology, composition, and structure revealed a cubic ZrO suboxide at the oxide/metal interface, a nanoscopic transition region with a graded composition ZrO2-x (where 0