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

A new method for assessing size distribution of intermetallic particles in Zircaloy has been developed and evaluated. The method is based on the imagi ng of lightly etched surfaces of bulk specimens using secondary electrons i n scanning transmission electron microscopy (STEM). Comparisons with establ ished methods using thin foils in TEM and bulk specimens in SEM have been m ade. It is concluded that the STEM technique covers the entire range of par ticle sizes, whereas TEM tends to overestimate the fraction of small partic les and SEM suffers from insufficient resolution to image the smallest part icles and to resolve overlapping particles. A high accelerating voltage and a high intensity of the primary beam result in a high signal to noise rati o, as a result of which secondary electron imaging in the STEM is ideal for automatic image analysis. The experimental results favour the STEM techniq ue, which gives more reliable measurement of particle size distributions in zirconium alloys. Inter metallic particles in Zircaloy 4 were identified t o be gamma-Zr(Fe, Ci)(2). The chemical composition of this phase was found to be independent of particle size. MST/4263.