Oxygen Induced High Temperature Crack Growth in Ni-base Superalloys

Doktorsavhandling, 2011

Oxygen induced crack growth is a serious concern for the performance of Ni-base superalloys. These alloys are used in gas-turbine engines where they are exposed to high temperatures, mechanical loads and oxygen-rich environments. Although these alloys are often considered well resistant to degradation, it has been shown that oxygen may cause time-dependent intergranular crack growth. After decades of research, consensus has still not been reached regarding the detailed effect(s) of oxygen at the crack tip. The present work aims to contribute to better understanding of the detrimental effect of oxygen on high temperature crack growth in Ni-base superalloys. Studies were performed on the wrought alloys Alloy 718 and Allvac 718Plus, which were subjected to high temperature mechanical testing. High-resolution analysis methods, such as atom probe tomography, transmission electron microscopy and NanoSIMS, were used to study the crack tip chemistry and microstructure. It is shown that the dominant effect of oxygen is the repeated formation and fracture of a closed, approximately *1 m long intergranular oxide ahead of the open crack tip. The oxide is some tens of nanometres thick and is symmetrically layered with a Ni-rich oxide nearthe prior (now oxidized) grain boundary and an inner Cr-rich oxide near the grains. The Ni-rich oxide is defect-rich and allows high oxygen partial pressures at the tip of closed the oxide. The Cr-rich oxide surrounds the crack and is also present below the Ni-rich oxide in the direction of crack growth and acts as a protective barrier for further oxidation. Oxygen enrichment of the Cr-depleted/Ni-enriched matrix below the Cr-rich oxide was observed and may contribute to the cracking process. Oxidation of Ni3(Nb,Al) gamma-prime precipitates were found to cause local regions of stoichiometric NiO in the otherwise layered oxide. The applied mechanical load was found not only to cause fracture of the closed oxide but also to increase diffusivity at the crack tip. Moreover, the load generates dislocations along which preferential matrix oxidation may occur beyond the protective Cr-rich oxide. Grain-boundary delta-phase was found to lower the susceptibility for intergranular fracture. Uncracked ligaments in the crack wake are believed to have a benefcial effect on hindering crack growth, mainly by bearing much of the applied mechanical load. The ligaments were however found to be possible sources of artefacts when using direct current potential drop method for crack growth monitoring.