Effect of volume fraction of gradient nanograined layer on high-cycle fatigue behavior of Cu

Fatigue behaviors of two Cu samples with different volume fractions of gradient nanograined (GNG) surface layer, prepared by surface mechanical grinding treatment, are investigated under stress-controlled high-cycle fatigue tests. With increasing the volume fraction of GNG layer from 4.3% to 13%, the fatigue endurance limit (at 107 cycles) of GNG Cu is enhanced from 88 to 98 MPa, much higher than that of coarse grained counterpart. Large volume fraction of GNG layer effectively postpones the extension of abnormal grain coarsening from a deeper subsurface layer to the topmost surface and thereby retards the initiation of surface fatigue crack.