Oxidation assisted intergranular cracking in 718 Nickel Superalloy: On the mechanism of dynamic embrittlement

Abstract

Nickel Superalloy 718 was specially designed for high temperatures. However, due to an embrittlement phenomenon, its application is limited up to 650 °C. This work presents a contribution to the understanding of the embrittlement mechanism focusing on the crack propagation kinetics responsible for the brittle fracture. This manifestation is known as OAIC (Oxidation Assisted Intergranular Cracking), although the phenomenology resulting in such degradation is not well understood. The approach to the problem was accomplished in the solubilized condition by mechanical testing, fractography and activation energy calculation. Generally, high temperature dissociation of NbC in the presence of oxygen is pointed as the source of niobium that ultimately lead to fracture by Nb2O5 formation at grain boundaries. However, metallographic analysis of fractured samples between 600 and 925 °C at different strain rates indicated otherwise. Samples tested at 850 °C showed minimum ductility and fast intergranular crack propagation, at a short time incompatible with carbide dissociation and Nb2O5 formation. Furthermore, there is no evidence of sufficient NbC along grain boundaries to supply elemental niobium to the formation of the Nb2O5 continuous film. Besides, onset to OAIC as a function of temperature and strain rate led to a 223 kJ/mol activation energy, which can be associated to niobium self diffusion within the matrix. Considering that the dynamic aging phenomenon and the γ″ precipitation occur concomitantly with the deformation, the results suggest that niobium segregation together with preferential γ″ precipitation at grain boundary and the consequent stress state build up should play also a decisive role in the embrittlement.