Effect of surface nanocrystallization on fatigue crack initiation and propagation behavior in pure Zr

Through an innovative fatigue test design, the initiation and propagation of fatigue cracks were observed with a microscope in real time, and the effects of surface nanocrystallization on the fatigue properties of pure Zr were systematically studied in the work. The results showed that the fatigue limit of pure Zr treated by ultrasonic shot peening (USSP) for 45 min was up to 11.2% higher than that of the original pure Zr. Both the initiation life and propagation life of fatigue cracks could be increased after USSP treatment. When the cyclic stress was set as 225 MPa, the initiation life and propagation life of fatigue cracks in the 45-min USSP-treated pure Zr were 3.1 times and 1.48 times those of the original samples, respectively. The lower the cyclic stress was, the more obvious the improvement in the initiation life of the fatigue cracks. This improvement in initiation life was the main reason for the promotion in the overall fatigue life of the 115SP-treated samples. The results of the fatigue fracture analysis showed that a large number of secondary cracks and dimples appeared in both the original and the 115SP-treated samples. However, the fatigue bands in the original pure Zr were disorderly, whereas those in the 115SP-treated pure Zr were parallel to each other and perpendicular to the direction of crack propagation.