Fenômeno de fratura intergranular assistida por oxidação em superligas de níquel 718

Abstract

Nickel Alloy 718 has been used for decades in nuclear and aerospace industries due to its good combination of mechanical and oxidation resistance. However, approximately between 650 and 850ºC, a phenomenon referred to OAIC (Oxidation Assisted Intergranular Cracking) in the literature occurs and it embrittles the alloy, adding a limitation to its application. Despite the numerous studies on this topic, there is no consistent explanation regarding the mechanisms involved nor their dependency on thermodynamic variables or loading conditions. Because of this, we have characterized OAIC by hot tensile testing at different strain rates and scanning and transmission electron microscopies, with the alloy in the solutionized condition. In this study, the phenomenon had different onset temperatures for each deformation rate, indicated by intergranular cracks on the fractured surfaces, between 650 and 900ºC. Oxygen availability was considered, but it was not shown to be crucial to embrittlement. The results suggest that the decomposition of NbC and consequent Nb2O5 formation do not play a decisive role in the mechanism for failure nor agree with its kinetics. On the other hand, we found some evidence that the Nb segregation and dynamic precipitation of γ” on the grain boundaries, together with a stress triaxiality on the region, are associated with fast crack nucleation and propagation.