Fatigue crack growth behavior of an oxide dispersion strengthened MA 956 alloy

Abstract

Fatigue crack growth behavior of oxide dispersion strengthened ferritic MA 956 alloy was studied at 25 °C and 1000 °C in air at 0.17 Hz. The growth rates were analyzed using the linear elastic parameter ΔK and the elastic-plastic parameter ΔJ. Crack growth, although transgranular at both temperatures, increased by nearly three orders of magnitude with increase in temperature from 25 to 1000 °C. The growth rates were essentially the same in terms of either ΔK or ΔJ parameters indicating that plasticity effects are small even at 1000 °C. Detailed fractographic analysis revealed the presence of ductile striations in the ΔK range of 25 to 40 MPa√m at 25 °C and in a much narrower range at 1000 °C. Presence of voids could be detected at 1000 °C. Using the measured load-displacement hysteresis energies for a unit increment in crack length, crack growth rates were calculated using cumulative damage models and were compared with the experimental data. At 1000 °C the predicted and the experimental values agree within a factor of two and it is concluded that the growth occurs essentially by a damage accumulation process except in a narrow range of ΔK where the plastic blunting process is superimposed, resulting in ductile striations that were observed. At 25 °C the predicted and the experimental value reasonably agree for ΔK values greater than 40 MPa√m, and below this value the two diverge with predicted values being much lower. This divergence is related to occurrence of the plastic blunting process in this ΔK range as confirmed by fractographic evidence. The cumulative damage process at 1000 °C was related to the environmentally assisted void formation at dispersoid-matrix interfaces. At 25 °C the damage is related to the formation of microcracks ahead of the crack tip. These results and interrelation between alloy microstructure and fatigue fracture path are discussed in detail.