Effects of microstructure and temperature on fatigue behavior of E319-T7 cast aluminum alloy in very long life cycles

The effects of microstructure and temperature on the fatigue behavior of a commercial Al–Si–Cu alloy used in automotive engine components were investigated for lifetimes as long as 109 cycles using ultrasonic fatigue instrumentation operating at 20 kHz. The primary finding of this study is that the influence of microstructure on the cyclic properties is greater than the influence of the testing temperature. Fractographic studies indicated that most fatigue cracks initiate from microshrinkage pores located at or very near to the specimen surface, while a much smaller number of cracks initiate from twin boundaries. Increasing test temperature resulted in a modest decrease in endurance limit by about 12% from 20 to 150 °C, while a significant decrease in endurance limit by about 23% was observed from 150 to 250 °C at high number of cycles. Using fatigue data developed in this study, a statistical model, the random fatigue-limit model (RFL), was evaluated for its utility in estimating fatigue behavior in the gigacycle regime.