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Low cycle fatigue (LCF) behavior of an Al–Cu–Mg–Si forging alloy was investigated at room temperature. Isothermal aging (T6) and non-isothermal aging (NIA) treatments were applied to produce different microstructures. Under the same strain amplitude, the LCF life of NIA specimen exceeded that of T6 specimen. Besides, NIA specimens exhibited cyclic stability behavior in contrast to cyclic hardening behavior of T6 specimens. The different LCF behaviors could be rationalized by different deformation mechanisms under cyclic straining. T6 specimens with non-shearable θ′-phases underwent Orowan bypassing mechanism leading to the cyclic hardening behavior. While NIA specimens with shearable and non-shearable θ′-phases suffered from both particle shearing and Orowan bypassing mechanisms resulting in the cyclic stability behavior. Non-shearable θ′-phases bypassed by dislocations introduced more stress concentration, while shearable θ′-phases cut through by dislocations caused less stress concentration. Therefore, fatigue life of NIA specimen is longer than that of T6 specimen.
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•Non-isothermal aging treatment is performed to improve low cycle fatigue performance of an Al-Cu-Mg-Si forging alloy.•Mixed precipitation characteristic manifested as shearable and non-shearable θ' phases is obtained by non-isothermal aging.•Planar slip associated with particle shearing mechanism contributes to the high fatigue crack propagation resistance.