Zr-4合金氧化膜的显微组织研究

Microstructure of Oxide Films Formed on Zircaloy-4

Zr-4合金样品在高压釜中经过360℃/18.6 MPa高温去离子水腐蚀395天后,用扫描电镜观察了氧化膜的断口形貌,用高分辨透射电镜观察了氧化膜不同深度处的显微组织和晶体结构,研究了氧化膜的显微组织在腐蚀过程中的演化过程。氧化膜的晶体结构非常复杂,在靠近金属基体处除了稳定的单斜(m)晶体结构外,还存在非晶、立方(c)和四方(t)等亚稳相,晶体中还生成了很多缺陷。m/t和m/c晶体之间存在共格关系,满足(001)m//(110)t,(010)m//(1-10)t;以及(001)m//(002)c,(010)m//(020)c的取向关系。氧化膜中的晶体缺陷在应力、温度和时间作用下会发生扩散,并在氧化锆晶界上凝聚生成孔隙,弱化了晶粒之间的结合力并引起显微组织的演化。在内应力的作用下,孔隙之间的扩展和连结发展成微裂纹,氧化膜变得比较疏松,导致了耐腐蚀性能的变化和腐蚀加速。讨论了各种影响显微组织演化因素之间的关系,氧化膜显微组织的演化过程是不可避免的,寻找并控制能够延缓显微组织演化过程的因素是提高锆合金耐腐蚀性能的有效途径。

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 rnicrostructural 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.