Positron Annihilation Study of High-Temperature Oxidation Behavior of Zr–1Nb Alloy

Abstract

The oxidation behavior of Zr–1Nb alloys exposed at 873 and 973 K in air was investigated by positron annihilation lifetime spectroscopy together with mass gain, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Mass-gain results showed that during the oxidation process, a transition of the oxidation rate occurred. The transition times of the specimens oxidized at 873 and 973 K were 30 and 6 h, respectively. In the pre-transition stage, the mass-gain curves obeyed the subparabolic law (n = 2.3), while at the post-transition stage, the mass-gain curves obeyed the linear law. The positron lifetime measurements indicated that in pre-transition stage, the formed oxide scale mainly consisted of a compact layer that only contained small-size vacancy defects. The accumulation of these vacancy defects together with the high compressive stress might cause the breakaway of the oxide layer. During the post-transition stage, the thickness of the porous oxide layer with more and larger-size defects such as voids and pores increased rapidly as increasing the oxidation time. These large-size defects, together with the cracks produced during the transition from protective to breakaway-type oxide, increased the oxygen absorption rate and accelerated the diffusion of oxygen. The formation of cracks in the porous layer was confirmed by SEM examinations.