Hysteresis energy based low cycle fatigue properties analysis in extruded Al-7Zn-2Mg-1.5Cu-0.2Sc-0.1Zr alloy at low temperature

The influences of low-temperature on low cycle fatigue (LCF) behaviors and fracture mechanisms of extruded Al-7Zn-2Mg-1.5Cu-0.2Sc-0.1Zr alloy with T6 state were investigated by strain-controlled LCF tests. The results show that the tensile strength increases, elongation and LCF life decreases with decreasing the temperature. The cyclic stress response behaviors of alloy are stable at various temperatures of room temperature (RT), 0 °C, −20 °C and −40 °C. And cyclic hysteresis energy densities are also stable after a short period of fluctuations at different conditions. In consideration of the different temperatures, the fatigue life evaluation model based on hysteresis energy is adopted. And the relationship between temperature and LCF life is derived from the parameters (Wo and β) of hysteresis energy model. The increase of hysteretic energy will lead to greater plastic damage, which is the main reason for the decrease of fatigue life, and the corresponding value of Wo significantly decreases. In addition, the fatigue crack initiates from the free surface of specimens and there exist a mass of fatigue striations in crack propagation region. The secondary cracks appear at low temperature, and the corresponding value of β slightly increases.