High temperature cyclic deformation of a directionally solidified Ni-base superalloy

The high temperature low cycle fatigue behaviour of a directionally solidified Ni-base superalloy hardened by about 65% volume fraction of gamma'-precipitates was investigated in order to determine the fatigue hfe parameters for longitudinal (L) and longitudinal transverse (LT) grain orientations. The fatigue resistance was compared with that of two oxide dispersion strengthened (ODS) Ni-base superalloys with a similar elongated grain structure. The fatigue life of the alloy can be adequately predicted by Basquin and Coffin-Manson empirical relationships and the fatigue ductility parameters in these relationships show a similar trend with the tensile ductility properties. The studied alloy exhibits a fairly stable cyclic stress response, with only a slight stress softening. Fatigue crack initiation occurs mainly at shrinkage pores on the surface or sub-surface of the specimens. The crack growth direction is predominantly perpendicular to the applied load. The fracture mode in the LT-direction is transgranular and fatigue life is shorter by a factor of about six compared to the L-direction. The fatigue life of the alloy is longer than that of the ODS Ni-base superalloys with which it is compared.