Biaxial fatigue property enhancement of gradient ultra-fine-grained Zircaloy-4 prepared by surface mechanical rolling treatment

Surface mechanical rolling treatment (SMRT) was applied to modify the microstructure and biaxial fatigue properties of Zircaloy-4 tubes. Gradient ultra-fine microstructural layers of ~ 500 μm in-depth from the outside surface were fabricated on rods. Residual compressive stresses with gradient distribution were simultaneously produced. Biaxial tension–torsion fatigue test was conducted to assess the effect of modified microstructures and residual stress on the fatigue lifetime and deformed substructure of Zircaloy-4. The results show that the SMRT Zircaloy-4 alloy possesses more than double biaxial fatigue lifetime than that of the as-received Zircaloy-4 without SMRT at a tension–torsion stress ratio of 1.0. Microstructural examination reveals that the SMRT Zircaloy-4 alloy shows sequential deformation characteristics at different layers across the tubular wall during biaxial fatigue. The fatigued dislocation configuration changes from dislocation cells within the depth of 200 μm, to bent lamellar structures at the depth of ~ 300 μm, and embryonic dislocation cells at the depth of ~ 500 μm. The enhancement of biaxial fatigue life in the SMRT Zircaloy-4 can be attributed to the combined effects of the gradient ultra-fine structure and the gradient residual compressive stress in the vicinity of surface suppress the fatigue crack initiation and propagation.