Cyclic deformation and fatigue behavior of 7075-T651 Al alloy with a gradient structure

Highlights: • SMRT specimens show a more significant ratcheting effect under high stress than that of BM. • Under tension–compression symmetrical stress, both BM and SMRT specimens show obvious asymmetry. • Fatigue strength of 7075 Al alloys is increased significantly by the SMRT from LCF to HCF regimes. • Both BM and SMRT specimens present failure mode transition as stress decreases. • The strain-fatigue life containing total, plastic and elastic strain amplitude is evaluated. Fully reversed stress-controlled tension–compression fatigue experiments (R = −1) were conducted to study the cyclic deformation and fatigue behavior of 7075-T651 aluminum alloys with and without a surface mechanical rolling treatment (SMRT). The strain amplitude, ratcheting effect, and asymmetric behavior of base metal (BM) and SMRT specimens were analyzed. SMRT specimens showed more remarkable ratcheting effects and tension–compression asymmetry under high stresses. Fatigue tests showed that the fatigue strengths of 7075 Al alloys were increased significantly by the SMRT. Both BM and SMRT specimens presented a failure-mode transition from the intrusions and extrusions failure modes to the Al₁₈Ti₂Mg₃ constituent phase. The hysteresis loop was utilized to obtain the plastic-strain amplitude. Furthermore, according to the cyclic-deformation behavior combined with the Manson–Coffin law, strain–life evaluation of BM and SMRT specimens under the stress-control is given.