Fatigue of a nickel base superalloy with bimodal grain size

Abstract

Room temperature fatigue tests in the form of four point bending were performed at a frequency of 20 Hz and a load ratio R= +0·1 on electropolished Waspaloy specimens taken from a forged turbine disc. Samples, which had a partially recrystallised microstructure with a bimodal grain size, were removed from the outer rim of the disc. The S-N curve was similar to that reported for a fully recrystallised structure with a coarse grain size, from the same turbine disc. The dominant crack initiation sites were found to be inclusions with a subsequent stage I crack growth generally along slip bands. Other crack initiation sites observed included slip bands, annealing twins, and grain boundaries. Fatigue deformation occurred by the propagation of planar slip bands. Slip band cracks formed more frequently in coarse grains than expected from their volume fraction. Fluctuations in short crack growth rate were observed and were associated with microstructural features such as grain boundaries and twin boundaries, both of which acted as barriers in the early stages of crack growth. The short crack growth rate versus stress intensity range graph was similar to those from uniform fine grained Waspaloy.MST/3413