PSI - Issue 71

Tapan K. Sawarn et al. / Procedia Structural Integrity 71 (2025) 263–270

268

1200°C

Prior β -Zr

α -Zr(O)

Fig. 4: Typical evolved microstructures from near and opposite to burst location at different temperatures.

The prior β -Zr layer thickness averaged over the clad circumference and oxygen in it were observed to be in the range of 273 – 344 µm and 0.16- 0.66 wt% respectively. Average hydrogen at the fracture location was found to be in the range of 28 – 932 wppm. An abrupt load drop in the linear portion of the curve (Fig. 2) corresponds to oxidation temperature of 1200 °C indicating a gross brittle failure of the specimen. The oxygen and hydrogen content at the fracture location of this specimen were 0.59 wt% and 95 wppm respectively. It is well established that oxygen and hydrogen degrade the mechanical properties of th e prior β -Zr phase. Within the prior β Zr phase, a ductile -brittle transition was observed (in the impact tests carried out at room temperature), for a critical oxygen content of around 0.4 wt% (Brachet et al., 2008). This is consistent with the observations of Kim et al. on Zircaloy-4 tubes oxidized under steam at 900 – 1250 °C, quenched in water and tested in ring compression or 3-point bending (Kim et al., 2007). In addition, when the material is brittle, for an oxygen content greater than 0.4 wt% at room temperature, the results highlight a more or less linear decreasing evolution of the maximum tensile strength and the elastic limit with the increasing oxygen content (Stern, 2007). Stern et al., 2008; observed a brittle cleavage fracture in th e prior β -Zr microstructure for bulk oxygen contents greater than or equal to 0.5 wt. %. In the literature, very little work on the properties of zirconium alloys containing both oxygen and hydrogen were reported. Cabrera Salcedo, 2012; observed a ductile-brittle transition of Zircaloy-4 containing 400 wppm hydrogen and 0.25 wt% oxygen at room temperature using uniaxial tensile test. Hydrogen is a β stabiliser, means it lowers the α+β Zr to β – Zr phase transformation temperature. The indirect effect of hydrogen is, it increases the oxygen solubility in the β -Zr phase and thereby making it brittle (Yamanaka et. al. 1989). However, in order to understand the influence of hydrogen along with oxygen in the present study, more brittle failure data need to be generated. It was observed that maximum bending moment and offset displacement of cladding decrease with decreasing prior β -Zr layer thickness and increasing oxygen concentration in it. In other words, it can be said that with increasing prior β -Zr to oxygen ratio maximum bending moment and offset displacement both increase as shown in Figs. 5 and 6, respectively.