Characterisation of zirconium alloys by identification of intermetallic particles and fully automatic measurement of particle size distribution using secondary electron imaging in scanning transmission electron microscope

Abstract

A new method for assessing size distribution of intermetallic particles in Zircaloy has been developed and evaluated. The method is based on the imaging of lightly etched surfaces of bulk specimens using secondary electrons in scanning transmission electron microscopy (STEM). Comparisons with established methods using thin foils in TEM and bulk specimens in SEM have been made. It is concluded that the STEM technique covers the entire range of particle sizes, whereas TEM tends to overestimate the fraction of small particles and SEM suffers from insufficient resolution to image the smallest particles and to resolve overlapping particles. A high accelerating voltage and a high intensity of the primary beam result in a high signal to noise ratio, as a result of which secondary electron imaging in the STEM is ideal for automatic image analysis. The experimental results favour the STEM technique, which gives more reliable measurement of particle size distributions in zirconium alloys. Intermetallic particles in Zircaloy 4 were identified to be γ-Zr(Fe,Cr)₂. The chemical composition of this phase was found to be independent of particle size.