The Influence of Environment Purity on Gaseous Iodine Embrittlement of Zircaloy-2 and High Purity Zirconium

The influences of environment purity on gaseous iodine embrittlement of high purity zirconium were investigated and compared to data obtained for Zircaloy-2. Iodide zirconium was embrittled at 350 C by constant extension rate tensile loading in highly purified and low constant partial pressure (40 Pa) iodine gas. Cracking was preceded by significant plastic deformation. Crack initiation probably involved sample pitting and subsequent cracking at the bottoms of pits. Brittle crack propagation proceeded by mixed intergranular separation and transgranular cleavage processes, compared to microvoid nucleation, growth, and coalescence typical of ductile fracture in argon. Purified and oxygenated (6 kPa) iodine gas also embrittled zirconium; however, the mechanism for crack initiation was influenced by oxygen additions. Sample pitting was eliminated, and crack initiation required local plastic strain concentrated at microstructural discontinuities such as grain boundaries. Similar findings were obtained for Zircaloy-2, however, the alloy was more severely embrittled in both purified and oxygenated iodine than pure zirconium. From these observations, it was concluded that water vapor, hydrogen, and volatile metal iodide contaminants, which were eliminated from the purified iodine test gas, were not prerequisites for embrittlement. Similarly, oxygenated iodine embrittlement was produced without zirconium tetraiodide formation. Lastly, it was inferred that the mechanism for iodine embrittlement was not totally dependent on either impurity or alloying elements or second phase particles in Zircaloy.