Extremely-low-cycle fatigue behaviors of Cu and Cu-Al alloys: Damage mechanisms and life prediction

The extremely-low-cycle fatigue (ELCF) behaviors of pure Cu and Cu-Al alloys are comprehensively studied following the cyclic push-pull loading tests with extremely high strain amplitudes (up to ±9.5%). Compared with the common low-cycle fatigue (LCF) region, several unique features in the ELCF regime can be noticed, including the deviations of fatigue life from the Coffin-Manson law, the non-negligible proportion occupied by the cyclic hardening stage of the whole fatigue life, special microstructures formed by cyclic loading containing deformation twins, shear bands and ultra-fine grains and the transformation of fatigue cracking modes. All these characteristics indicate the existence of special interior fatigue damage mechanisms of ELCF. To help discover the new damage mechanisms under ELCF, a model of fatigue life prediction with a hysteresis energy-based criterion is proposed. Based on the analysis of the experimental and modeling results, two intrinsic factors determining the ELCF properties were concluded: the capacity of ELCF damage, and the defusing and dispersion ability of the external mechanical work. The former can be evaluated by a parameter of the model called the intrinsic fatigue toughness W0, which is related to the microstructure evolution condition, the cyclic hardening ability, the deformation homogeneity and possibly the static toughness. The latter can be represented by the damage transition exponent β, which can be enhanced by improving the planarity, reversibility and uniformity of plastic deformation, reflecting the decline in the degree of surface damage and the dispersion of fatigue cracks. For Cu-Al alloys with increasing Al content, cooperation between an increasing damage capacity and a decreasing damage accumulation rate leads to a comprehensive improvement in the ELCF properties.