Fatigue Crack Initiation in Alloy 718 at 650�C

Abstract

Fatigue crack initiation life of Alloy 718 specimens, subjected to conventional and modified heat treatments, was determined using ASTM E606 small cylindrical specimens and an AGARD designed double edge notch (DEN) specimen in load control at 650°C. Crack initiation lives were monitored for the DEN specimens using a direct current potential drop technique while the ASTM E606 specimens were cycled to rupture. The DEN specimens were tested in two surface conditions. In one set of specimens the notch was lathe cut and in the other set of specimens the notch was electron discharge machined. It is shown that the notch surface condition can dominate the crack initiation life in Alloy 718 at 650°C. It is also shown that the modified heat treatment forms a notch sensitive microstructure which results in a loss in high cycle fatigue life of two orders of magnitude. However, under high strain low cycle fatigue conditions, normally encountered in aircraft gas turbine engine discs, the loss in fatigue life is less severe. Superalloys 718,625 and Various Derivatives Edited by Edward A. Lmia The Minerals, Metals & Materials Society, 1993