Factors influencing the notch rupture life of superalloy 718

Abstract

The superalloy grade 718 is used extensively for aeroengine applications. Since the fabricated structures operate at high temperatures and engine-designer has also to reckon with stress concentrations, notched stress rupture testing is an important acceptance criterion. The specification stipulates a minimum life of 23 hours and minimum elongation of 4%, when testing is done on specimens with a combined notched and unnotched test section at 922 K and stress level of 690 MPa. Solutionising of 718 is carried out in the temperature range — 1311 to 1339 K or 1200 to 1283 K. Solutionising in the upper range produces notch brittleness; for aeroengine applications requiring good notch toughness, the lower range is recommended. For indigenous production of this superalloy for aeroengine application, solutionising was carried out at 1253 K.

Morphology, volume fraction and distribution of delta phase in the microstructure importantly influence the notch rupture life of 718. Microstructure may be acceptable or unacceptable, based on the nature of presence of δ phase. The hot working practice adopted for indigenous production of this material always resulted in an acceptable microstructure after solution treatment. Presence of Laves phase in the microstructure can adversely affect the notch rupture life. Indigenous production of this material included adequate high temperature soaking treatments to completely dissolve the Laves phase. In spite of the foregoing, there were instances when the alloy showed unacceptably low levels of notch rupture life. The presentation shall comprehensively review the subject of notch rupture life of Superalloy 718.