An attempt to correlate the change of the swept area (∆A/b2) by mobile dislocations with the mechanism of the fatigue crack propagation rate (da/dN) in environmental hydrogen: Application in Inconel 690

The thermally activated dislocation motion due to plastic deformation in Inconel 690 has been carried out by performing tensile tests in air and in environmental hydrogen using the strain relaxation technique. The results show that the variation of - δ(ΔA/b2) or ln(VH/VAir) with εp depends on the nickel content in the alloy and the microstructure, and generally increases with increasing Ni% for low value of εp (tensile test). The maximum increase in the rate change was observed for the austenized and aged microstructure. On the other hand, fatigue crack propagation rate (da/dN) has been measured in the same environments. An attempt has been carried out to correlate the rate change - δ(ΔA/b2) or ln(VH/VAir) with the variation of (da/dN)H against (da/dN)Air (the variation of Ω) and has shown that for the austenized and aged microstructure, Ω is higher than those for stress relieved or only aged at 700 °C. These results are similar to those obtained by the thermal activation parameter - δ (∆A/b2) or the ratio ln(VH/VAir). The maximum values of - δ (∆A/b2) and Ω are obtained when the density of precipitates on the grain boundaries is higher than that in the matrix. In this case, intergranular fractures with fin slip lines features on the grain surfaces were observed. Finally, in the presence of environmental hydrogen, about the fatigue crack propagation rate, two relationships were established and expressed as: 1) in stages I and II: (Δεp/2) = Θ ΔK + Γ and 2) only in stage II: (da/dN)II = cII (Δεp / 2) λ, where λ ≈ m = 2, the power factor in Paris' law.