Initiation and growth of short cracks in u-notch bend specimens of superalloy IN718 during high temperature low cycle fatigue.

Abstract

Inconel 718 (IN718) is a nickel-based superalloy used extensively for gas turbine discs. Since turbine discs are safety-critical components, considerable effort is expended in determining their safe operating life. This requires a thorough understanding of the fatigue damage which can occur during service. One type of in-service damage is low cycle fatigue cracking in the fir tree root fixtures of discs. To study this, fatigue tests at 600°C were performed on two batches of U-notch bend specimens (Batch A & B), taken from disc forgings with the same composition, heat treatment and microstructure. U-notches were machined by a broaching process similar to that used for fir tree root fixtures. Fractography and metallography were used to study the natural initiation of cracks. Dwells of 1 to 30 seconds at maximum stress were used to investigate previous findings of a beneficial effect on fatigue life under certain conditions. Selected tests were performed on polished notches to measure surface short crack growth rates using replicas. Unstressed thermal exposure experiments were also performed to study primary carbide oxidation, which is implicated in crack initiation. Oxidation of primary carbides readily occurred in unstressed samples and fatigue tests. Localised matrix deformation was observed in the vicinity of oxidised carbides. Theoretical calculations of mismatch strains showed that oxidation of primary carbide particles caused substantial plastic deformation in the surrounding matrix. This was a common cause of crack initiation in Batch A specimens, but was less common in Batch B. In Batch A, short crack growth was approximately constant across a wide range of crack lengths. The cracks observed were straight and transgranular at the surface. Batch B, tested at a higher stress level, exhibited the more typical behaviour of increasing growth rate with increasing crack length, and surface crack paths were more tortuous. A difference between the batches was clearly seen in strain-life results, with Batch B exhibiting longer lifetimes than Batch A under the same experimental conditions. A beneficial effect on life of dwell at maximum stress was observed for Batch A tests. However, no such beneficial effect was observed for Batch B tested at a higher stress level. Differences in crack initiation and propagation between the two batches were investigated. Batch A was found to have a work-hardened deformation layer in the broached notch, while Batch B did not. The effects of the deformation layer and differences in surface finish on observed fatigue behaviour are discussed.

More information

Identifiers

ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

Catalogue record

Export record