Effect of ultrasonic surface rolling process on the microstructure and corrosion behavior of zirconium alloy in high-temperature water condition

Ultrasonic surface rolling processing (USRP) was carried out at zirconium (Zr) alloy with different rolling speeds to prepare gradient nanostructured layers, and the corrosion behavior of USRP treated Zr alloy was studied under 360 °C/18.6 MPa water condition. The surface morphology, microstructure, phase composition, and element distribution of USRP treated Zr alloy after corrosion was investigated. Results show that the oxide layer on Zr alloy comprises of t-ZrO 2, m-ZrO 2, and metal Zr. The thickness of oxide layer Zr alloy is reduced after USRP treatment, indicating that the gradient nanostructure can effectively improve corrosion resistance. This process is attributed to that the gradient nanostructure layer is helpful for the nucleation of equiaxed t-ZrO 2 at the oxide/metal (O/M) interface with small grain size. The high compressive stress induced by USRP treatment is beneficial for the maintenance of t-ZrO 2 phase and inhabiting the phase transformation from t-ZrO 2 to m-ZrO 2, which reduces the micro-cracks and defects due to volume expansion. The oxide layer on USRP treated Zr alloy with high content of t-ZrO 2 phase presents a dense structure and prevents oxygen anions from diffusing to Zr alloy.