Fatigue life improvement and grain growth of gradient nanostructured industrial zirconium during high cycle fatigue

Abstract The effect of surface gradient nanostructure on the fatigue life of commercial pure (CP) Zr was investigated. Four point bending fatigue tests indicated that the fatigue limit of CP Zr with surface gradient nanostructure was increased by about 28.3 % compared with the original sample (annealed state). The microstructure evolution at different fatigue loading stages was characterized. The high strength of surface gradient nanostructure could increase the crack initiation resistance. Furthermore, electron back scattered diffraction (EBSD) analysis demonstrated that the surface nanocrystals grew and rotated gradually during the fatigue loading, which was beneficial to reducing stress concentration, inhibit fatigue crack initiation, and prolong crack initiation life. The stored distortion energy of CP Zr calculated before and after fatigue indicated that the stored distortion energy decreased dramatically during cyclic loading, which provided the driving force for grain growth. Besides, the growth of nanocrystals consumed the mechanical energy produced by the applied load to a certain extent, thus, slowing down the accumulation of fatigue damage. The coarse grains at the interior could deform plastically and reduce the crack growth rate. In addition, the compressive residual stress caused by USSP treatment reduced the local effective stress and the driving force of crack growth.