Microstructure of Oxide Films Formed on Zircaloy-4

The microstructure of oxide films formed on Zircaloy-4 was investigated by HRTEM and HRSEM after autoclave corrosion tests performed at 360 ℃/18.6 MPa in deionized water for 395 days.The microstructural evolution of the oxide films was analyzed by comparing the microstructure at different depths in the oxide layer.The crystal structure of the oxide films is complicated.Monoclinic (m),tetragonal (t),cubic (c) and amorphous phases were detected in the oxide layer near the metal matrix,and the coherent relationships between m/t and m/c were identified as (001)m//(110)t,(010)m//(1-10)t:and(001)m//(002)c,(010)m//(020)c.The defects consisting of vacancies and interstitials were produced during the oxide growth.The diffusion,annihilation and condensation of vacancies and interstitials occurred under the action of stress,temperature and time.The vacancies absorbed by grain boundaries formed pores to weaken the bonding strength between grains and to result in the microstructural evolution.Based on the formation of pores along the grain boundaries,the development of micro-cracks in the oxide films will lead to the loss of the protective characteristic,thus the phenomenon of corrosion transition appears and the corrosion rate is accelerated.The microstructural evolution of the oxide films is an inevitable process.The effects of different factors on the microstructural evolution of the oxide are discussed.Finding and controlling the factors that could retard the microstructural evolution are the effective method to improve the corrosion resistance of zirconium alloys.