Effect of pre-fatigue deformation on thickness-dependent tensile behavior of coarse-grained pure aluminum sheets

To explore the size effect on the mechanical properties of the coarse-grained materials and the relevant influence of sub-structures, the coarse-grained pure aluminum sheets with the thickness spanning from 0.2 to 2.0 mm are selected as experimental materials, on which the pre-fatigue deformation (pre-cycle is equal to 5% of fatigue life Nf) is exerted to alter internal sub-structures. The results show that the uniform strain ε of the annealed sheets almost linearly decreases with the reduction of thickness, and the ultimate tensile strength σUTS first slowly declines, then quickly drops as the thickness is below 0.5 mm, but the yield strength σYS does not exhibit an obvious thickness effect. The pre-fatigue deformation not only significantly improves the plasticity, but also decreases the critical thickness tc (below which the decrease in σUTS suddenly becomes notable) from 0.5 mm of the annealed state to 0.3 mm, meanwhile, a higher σUTS is maintained at the thickness ranging from 0.3 to 2.0 mm. For the annealed sheets, the cracks are initiated at grain boundaries (GBs), and with decreasing thickness, the appearance of a lot of straight slip lines leads to the occurrence of single slip separation failure in thinner sheets. The pre-fatigue deformation not only increases the uniform deformation degree in grain interiors, but also promotes the formation of more ill-developed cells in sub-grains and wall-cells and the development of the sub-grains in thinner sheets, so that all the pre-fatigued sheets exhibit ductile failure and a higher σUTS is obtained at the thickness between 0.3 and 1.0 mm.