The influence of crack and interface oxidation on dwell fatigue crack propagation behavior of the nickel-base superalloy ATI A718Plus.

The crack propagation properties are an important and often limiting factor for the commercial application of superalloys in aerospace applications where the oxidation ahead of the crack tip plays an important role. Previous research on the Ni-base superalloy A718Plus revealed that the orientation and the volume fraction of the high temperature grain boundary phases δ and η have a major influence on the dwell fatigue crack propagation rate at 650 °C in ambient air but not in vacuum. In this work, the effect of internal oxidation at the η/δ–matrix interface and its effect on dwell fatigue crack propagation and mechanical properties are examined by advanced microscopic methods and micro-cantilever testing. During crack propagation, the crack tip is exposed to air where a Nb-rich oxide layer forms and embrittles the η/δ–matrix interface causing the cracks to deflect and propagate along the oxidized interfaces. Micro-cantilever tests on the oxidized interface show that these oxide layers also significantly reduce the local strength and fracture toughness of the material. This proves that interfacial oxide layers are the underlying reason for the reduction of the dwell fatigue crack growth resistance at 650 °C, particularly in microstructures whose η/δ–matrix interfaces are oriented parallel to the crack growth direction. [ABSTRACT FROM AUTHOR]