Environment-Sensitive Fatigue Crack Nucleation in Nickel-Based Superalloys.

摘要

The type of thermo-mechanical processing, often referred to as 'grain-boundary engineering,' involves repeated steps of cold work followed by annealing for re-crystallization. This has been shown in the past to result in an increase of special grain boundaries in FCC alloys such as stainless steels, meaning grain boundaries having a low value of the Sigma parameter, which is the inverse of the concentration of coincident sites on the coincident-site lattice across the boundary. Such boundaries have been shown to improve the resistance of alloys to intergranular stress corrosion cracking and creep rupture, for example. The investigation was based on the idea that dynamic embrittlement represents the percolation of intergranular decohesion along the grain boundaries and that in IN718 this is caused by stress-driven diffusive penetration oxygen along boundaries having a high oxygen diffusivity. Thus, the starting hypothesis was that a decrease in the concentration of random boundaries, i.e., those with comparatively large free volume, would retard dynamic embrittlement This kind of experiment had never been done before in this area or on such a complex alloy as IN718. The experiments entailed not only the processing, but also the analysis of the grain-boundary-character distribution using orientation-imaging microscopy, before carrying out the mechanical testing at 650t in an environmentally controlled chamber. The results showed that this processing substantially retarded the cracking, and this now will lead the way toward possible improvements in the serviceability of this important alloy in the field of gas turbines and aircraft engines.