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.