Investigation of Oxide Layers Formed Earlier on Zircaloy-4 Specimens Corroded at 300 ℃ in Deionized Water

Abstract:

Thin specimens in Φ3 mm for transmission electron microscopy observation were prepared using Zircaloy-4 plate. Corrosion tests of these thin specimens were conducted in the autoclave at 300 ℃/8 MPa in deionized water for short time exposure. The oxide layers formed earlier on Zircaloy-4 specimens have been investigated using optical microscopy, scanning probe microscopy and electron microscopy. The results show that the different thickness of oxide layers formed on metal grains with different orietations was in different colours. The oxide layers were mainly composed of columnar grains with m-ZrO2, but a small amount of t-ZrO2 and c-ZrO2 could be also detected. In addition, a kind of zirconia having BCC structure with a=0.88 nm was observed. Semi-coherent relationships between α-Zr matrix and the different crystal structure of zirconia with certain orietation relationship were observed：(10-11)α-Zr//(020)m-ZrO2//(002)t-ZrO2//(020)c-ZrO2. Therefore compressive deformation of 3%-7% in different directions for different crystal structure of zirconia was produced. A lot of dislocations were observed in oxide crystals and the strain in the area around the dislocations was about -4.8% to 3.5% obtained by geometric phase analysis.

Key words:Zircaloy-4; corrosion resistance; oxide layer; crystal structure; crystallographic orientation relationships