Study of the coupled phenomena involved in the oxidation assisted intergranular cracking of Ni based superalloys

Abstract
High temperature fatigue and creep-fatigue crack growth tests were carried out at 650°C on a Ni-base superalloy. During the tests, mechanical as well as chemical perturbations were used in order to determine the critical scale of interactions and the relevant mechanical and chemical local components of the damaging process in relation with the alloy microstructure. In this study a P/M Ni base superalloy (alloy N18) with two different microstructures and creep properties was used. The results obtained can be summarised as follows (i) Intergranular embrittlement occurs under specific local coupling conditions i.e. high tensile stresses at the crack tip in phase with a simultaneously growing cationic oxide. Defects injection (dislocations, vacancies) related to interface reactions occurring in the oxidation process are likely involved as a major component of the embrittlement process. Thus, when these two local mechanical and chemical loading conditions are acting out of phase no embrittlement is observed (ii) Under creep-fatigue loading conditions, an unloading of several percent from the maximum load applied at the beginning of the holding time period can inhibit embrittlement process. Therefore, by varying the unloading amplitude, it is possible to estimate the size of the interaction zone. In our case, the range of interaction is of the order of the grain size (iii)The cracking resistance depends on the creep properties. When the relaxation rate at the crack tip increases the cracking resistance increases. Finally, conclusions can be drawn from this study in terms of physical basis for modelling fatigue crack growth processes and microstructural modifications which might improve the cracking resistance of these alloys at intermediate temperatures.