Oxygen-Induced Intergranular Decohesion in IN718

Grain boundary microstructure and fatigue crack growth in Allvac 718Plus superalloy

“…Ma and Cheng [14] have reported similar effects for Alloy 783, as have Pfaendtner and McMahon [5] for Alloy 718. However, these reports have contradicting interpretations concerning the mechanism for the occurrence of these zones.…”

“…This model, suggesting partial cracking along preferential grain boundaries (explained in more detail in McMahons comments [15] on the work by Ma and Cheng), would allow an explanation of the mixed mode crack propagation observed for the cyclic regions in the Standard condition. Whether these mixed zones were created ahead of the main crack tip could not be discerned in the present study as the samples were fully fractured (not allowing for the same fractographic methods as in [5]). In the Clean condition no correlations could be made to uncracked ligaments as here also 10 Hz cyclic loading exhibits fully intergranular fracture.…”

“…However, these reports have contradicting interpretations concerning the mechanism for the occurrence of these zones. While Ma and Cheng [14] ascribe this to SAGBO, Pfaendtner and McMahon [5] point out DE as the responsible mechanim. In the model presented for DE the damaged zone is said to consist of some embrittled grain boundaries ahead of the main crack, mixed with uncracked ligaments.…”

“…In the Clean condition no correlations could be made to uncracked ligaments as here also 10 Hz cyclic loading exhibits fully intergranular fracture. It is worth considering that the temperatures used in the referred work [5,14] were up to 650 • C while in the current work 704 • C was used. For both SAGBO and DE thermal activation has been shown to be of great importance and also creep may play a greater role at this higher temperature.…”

“…trapezoidal periods of static tensile loading introduced into cyclic loading sequences. Two main damage mechanisms are often considered to occur during holdtimes; Dynamic Embrittlement (DE) [5,6] and Strain Assisted Grain Boundary Oxidation (SAGBO) [4]. Both have similar precursors for occurring, however the principal difference is that while fracture by DE occurs through decohesion of grain boundaries, fracture by SAGBO occurs in oxidized and thus embrittled grain boundaries.…”