Self-toughening mechanism of the ZrO2 scale and the precipitation of Sn during the steam oxidation of ZIRLO™ at 1200 °C

The steam oxidation behavior of ZIRLO™ at 1200 °C was investigated using a thermogravimetric analyzer. The microstructure of the oxide was analyzed using SEM, EBSD, and FIB+TEM in depth. Very long “spaghetti-like” columnar oxide grains were formed owing to the intracrystalline diffusion of O ²⁻ and the high oxidation temperature. These grains extended from the oxide/matrix interface to a thin outer equiaxed grain layer. The special growth mechanism of the columnar grains determines the unique properties of the oxide scale, such that their structural integrity cannot be destroyed. Therefore, the oxidation kinetics followed a parabolic law throughout the oxidation process without breakaway behavior. Observations revealed the presence of twins, slip bands, and subgrains within the ZrO ₂ grains, which were identified as the primary manners of oxide deformation under stress. The residual stress and phase distribution along the thickness of the oxide were investigated using synchrotron radiation X-ray diffraction. The oxide was almost completely transformed into a m phase after oxidation. The compressive stress inside the oxide was relatively low, indicating that the stress was relaxed. Transformation toughening, plastic deformation, densely arranged columnar grains, and the absence of grain boundaries parallel to the direction of compressive stress constitute the self-toughening mechanism of the ZrO ₂ scale. Zr ₅Sn ₃, ZrSn ₂, and β-Nb second phase particles were observed at the ZrO ₂ grain boundaries and inside the ZrO ₂ grains. The outward diffusion of Sn in the liquid state and the peritectic reaction can result in the formation of voids at the edges of the second phase particles.