Cyclic damage behavior of Sanicro 25 alloy at 700 °C: Dispersed damage and concentrated damage

The damage evolution behavior of a novel Sanicro 25 alloy was investigated based on low-cycle fatigue tests at 700 degrees C. The results show that at relatively low total strain amplitude (Delta epsilon(f)/2), the dispersed damage was dominant, which led to prolonged fatigue life. However, at relatively high Delta epsilon(f)/2, the concentrated damage was dominant, which led to reduced fatigue life. Based on the damage characteristics, a modified Coffin-Manson equation that accounts for microporosity (f) was derived. The curved surface of plastic strain amplitude (Delta epsilon(p)/2) - f - number of reversals to failure (2N(f)) shifts upward with increasing fatigue ductility exponent c. However, this curved surface shifts downward when the cyclic strength coefficient K' increases. A new model of microcrack initiation considering the precipitates at the grain boundary was derived. The Gibbs free energy change of crack nucleation (Delta G) decreased with decreasing Delta epsilon(t)/2. Finally, based on the prediction model of energy-based fatigue life, the theoretical formulas of the fatigue toughness W-f and the fatigue cracking exponent beta were derived. W(f )increases and g decreases with increasing Delta epsilon(p)/2.