Low cycle fatigue behavior of AlSi10Mg(Cu) alloy at high temperature

Low cycle fatigue (LCF) behavior of AlSi10Mg(Cu) alloy was investigated at 100 °C and 250 °C over a range of strain amplitude from 0.25% to 0.40%. Cycle stress response, hysteresis loops, fatigue life, as well as post-fatigue microstructure were studied in detail. Except 0.25%, the alloy performed cycle hardening behavior at 100 °C and softening behavior at 250 °C at any given strain amplitude, which corresponded well with the observed phenomena in hysteresis loops. At any given temperature, cyclic stress increases while fatigue life decreases with the increasing strain amplitude. The fatigue life of the studied alloy at 100 °C was higher than that at 250 °C under the same strain amplitude. Fractography indicated that most fatigue crack initiated from casting defects. The fatigue crack propagation was mainly characterized by fatigue striations and tear ridges in any conditions. Section inspection shows that the number of cracked eutectic Si increases with decreasing the temperature or increasing strain amplitude. Post-mortem transmission electron microscopy (TEM) investigations indicated that cyclic hardening behavior at 100 °C was due to the pinning effect of coherent precipitates (β″-Mg₂Si and GP zones). Most of tiny coherency precipitates and clusters transformed to large semi-coherency Q′ and β′-Mg₂Si precipitates after cyclic loading at 250 °C, leading to the sharp decrease of pinning number and dislocation density, which is a key reason for the cyclic softening. Another factor is that more dislocations can be activated and dislocation climb is easier at higher temperature.