PRECIPITATE EVOLUTION IN THE ZIRCALOY-4 OXIDE LAYER

A study by transmission electron microscopy has been made of the incorporation and oxidation of intermetallic precipitates Zr(Cr,Fe)2 into the uniform oxide layer of Zircaloy-4. Oxide thicknesses of 1, 4 and 14 mum were studied. The main results are: (i) most of the precipitates are incorporated unoxidized into the oxide layer and are later oxidized. (ii) iron concentration evolves significantly during oxidation, especially before the transition, generally segregating to the precipitate-matrix interface, occasionally precipitating as metallic iron, before being dissolved in the matrix. In order to understand the behaviour of fuel cladding in a reactor environment, the same approach was then realized on ion irradiated samples. In this case, the studied oxide thickness was 1 mum. The Zr(Fe,Cr)2 precipitates made amorphous by ion irradiation appear to behave as the original intermetallic precipitates. However, the iron redissolution seems to be delayed in the ion irradiated precipitates. The behavior of the precipitates during oxidation is discussed in the light of previous work and possible consequences for thee oxidation process are considered.