Grain morphology and crystal structure of pre-transition oxides formed on Zircaloy-4

•Grain morphology and crystal structure of pre-transition ZrO2 oxides were studied.•Tetragonal equiaxed ZrO2 grains are embedded in monoclinic columnar ZrO2 grains.•The orientation relationship between monoclinic ZrO2 and Zr was identified.•The formation mechanism of monoclinic ZrO2 texture on Zr substrate is proposed.

Grain morphology and crystal structure of pre-transition oxides formed on a Zircaloy-4 alloy were investigated. The results show that monoclinic columnar grains align tightly in inner oxides, whereas porous monoclinic equiaxed grains exist in outer oxides. Small-sized tetragonal equiaxed grains are embedded in monoclinic columnar grains, with tetragonal phase content declining from the inner to the outer oxides. The crystallographic orientation relationship of (1 1 1)m//(1 0 1¯ 0)α-Zr was identified, which explains well the formation of monoclinic texture on the zirconium substrate. This work advances the understanding of corrosion properties before transition of oxidation of zirconium alloys.