Propagation of stress-corrosion cracks in unirradiated zircaloy

Propagation of iodine-induced stress-corrosion cracks in Zircaloy was studied using pre-cracked and internally pressurized cladding tubes. These were recrystallized at different temperatures, to obtain grain sizes between 4 μ m and 10 μ m. No statistically significant difference in propagation rate due to the difference in grain size was observed. If the obtained data, with K I values ranging from 4 to 11 MNm −3 2 , were log-log plotted (d a/d t = CK n I ), as usual, they fell within the scatter-band of data reported earlier. But from this plot it could also be seen that the K I interval can be divided into two separate parts having different d a/d t- K I relations. The transition takes place at a K I value of about 8 MNm −3 2 . The region with lower K I values shows a substantially lower n value than the upper region (2.4 and 9.8 respectively), and earlier reported values ( n = 7 to 10). This transition is in good agreement with a transition from an intergranular to a transgranular propagation mode of the stress-corrosion crack.