EFFECT OF STRAIN-RATE ON THE HIGH-TEMPERATURE LOW-CYCLE FATIGUE PROPERTIES OF A NIMONIC PE-16 SUPERALLOY

Strain-rate effects on the low-cycle fatigue (LCF) behavior of a NIMON IC PE-16 superalloy have been evaluated in the temperature range of 52 3 to 923 K. Total-strain-controlled fatigue tests were per-formed at a strain amplitude of +/-0.6 pct on samples possessing two different pr ior microstructures: microstructure A, in the solution-annealed condit ion (free of gamma' and carbides); and microstructure B, in a double-a ged condition with gamma' of 18-nm diameter and M23C6 carbides. The cy clic stress response behavior of the alloy was found to depend on the prior microstructure, testing temperature, and strain rate. A softenin g regime was found to be associated with shearing of ordered gamma' th at were either formed during testing or present in the prior microstru cture. Various manifestations of dynamic strain aging (DSA) included n egative strain rate-stress response, serrations on the stress-strain h ysteresis loops, and increased work-hardening rate. The calculated act ivation energy matched well with that for self-diffusion of Al and Ti in the matrix. Fatigue life increased with an increase in strain rate from 3 x 10(-5) to 3 x 10(-3) s-1, but decreased with further increase s in strain rate. At 723 and 823 K and low strain rates, DSA influence d the deformation and fracture behavior of the alloy. Dynamic strain a ging increased the strain localization in planar slip bands, and impin gement of these bands caused internal grain-boundary cracks and reduce d fatigue life. However, at 923 K and low strain rates, fatigue crack initiation and propagation were accelerated by high-temperature oxidat ion, and the reduced fatigue life was attributed to oxidation-fatigue interaction. Fatigue life was maximum at the intermediate strain rates , where strain localization was lower. Strain localization as a functi on of strain rate and temperature was quantified by optical and scanni ng electron microscopy and correlated with fatigue life.