An investigation on notch embrittlement of a Ni-base superalloy

Abstract

Superalloy 718, which is strengthened by {gamma}{prime} and {gamma}{double_prime} precipitates, is one of the most widely used nickel based superalloys for high temperature applications. The alloy also contains incoherent delta phase and MC type of carbides, and in rare cases the Laves phase and minor phases like borides. The performance of this alloy depends on the relative amount of all these phases. Due to its prominent overall mechanical properties up to 7650 C and fairly good formability, the alloy has found extensive usage in gas turbine disc applications. In these applications, the notch stress rupture performance is extremely important. In service, high temperature components are often subjected to a complex stress system which may vary from a uniaxial to a multi-axial stress state. Circumferentially notched round bars have been used to investigate the creep rupture behavior of other superalloys. The experimental results obtained in Inconel X-750 and Nimonic 80A have shown that the creep damage mechanisms were strongly influenced by the state of stress imposed by the notch. Several studies on superalloy 718 at high temperature showed variations in notch rupture life, depending on the size, morphology and distribution of delta phase. This paper reports the investigation carried out tomore » determine the factors responsible for inconsistent notch rupture behavior in superalloy 718.« less