Fracture mechanisms during intergranular hold time fatigue crack growth in Inconel 718 superalloy

Paper i proceeding, 2013

Ni-base superalloy IN718 is known to display time-dependent intergranular crack growth under dwell time mechanical loading at high temperature under atmospheric conditions. Oxygen has been pointed out as a cause of the intergranular damage causing embrittled crack growth during both cyclic and hold time loading. Investigation of the mechanisms responsible for the embrittlement should not only focus on the effect of environment but also on the combined action of fatigue, creep, temperature and time. In this work material from experiments with fatigue crack growth in combination with hold times of different length at different temperatures has been investigated. Fractographic studies and metallographic cross sections of fatigued specimens has been subjected to careful analysis using ECCI-imaging in order to shed light on the fracture mechanisms. The results show that the damage is caused by the influence of a combination of environment and severe local damage manifested as a transformation of the microstructure into sub cells, micro twins and recrystallised areas close to the crack tip. The damage mechanism is thus influenced by a combination of oxidation and severe local plastic deformation.