HIGH-TEMPERATURE CYCLIC DEFORMATION OF A DIRECTIONALLY SOLIDIFIED NI-BASE SUPERALLOY

The high temperature low cycle fatigue behaviour of a directionally so lidified Ni-base superalloy hardened by about 65% volume fraction of g amma'-precipitates was investigated in order to determine the fatigue hfe parameters for longitudinal (L) and longitudinal transverse (LT) g rain orientations. The fatigue resistance was compared with that of tw o oxide dispersion strengthened (ODS) Ni-base superalloys with a simil ar elongated grain structure. The fatigue life of the alloy can be ade quately predicted by Basquin and Coffin-Manson empirical relationships and the fatigue ductility parameters in these relationships show a si milar trend with the tensile ductility properties. The studied alloy e xhibits a fairly stable cyclic stress response, with only a slight str ess softening. Fatigue crack initiation occurs mainly at shrinkage por es on the surface or sub-surface of the specimens. The crack growth di rection is predominantly perpendicular to the applied load. The fractu re mode in the LT-direction is transgranular and fatigue life is short er by a factor of about six compared to the L-direction. The fatigue l ife of the alloy is longer than that of the ODS Ni-base superalloys wi th which it is compared.