Influence of hold period on creep–fatigue deformation behaviour of nickel base superalloy

The deformation behaviour and damage mechanisms of the wrought nickel base superalloy EI 698 VD has been investigated in condition of creep–fatigue loading. The alloy was subjected to load controlled isothermal low cycle fatigue at 650°C with superimposed hold periods of constant tensile load simulating creep stress component. The introduced hold periods were in range of 3 min to 10 h. The pure fatigue and pure creep tests were conducted as well. The test were of tension–tension type defined by stress ratio R=0.027. During hold periods a constant stress of σ=740 MPa was maintained leading to pure creep. The tests were conducted until fracture. The influence of hold time was evaluated through the deformation behaviour, fatigue life, and fracture mechanisms participating in failure process. The time to fracture decreased continuously with increasing hold period. Considering the fatigue life achieved after shortening the hold time to 1 min, the life time measured was more than 10 times higher than in case of pure creep. The strain rate dependence achieved within this testing showed inverse dependence with respect to the hold time. The microstructure examination of the fractured specimens, using transmission electron microscopy (TEM), enabled to reveal the participating dislocation mechanisms for combined loading. The fracture analysis of broken up specimens showed that fatigue mechanisms became active at crack nucleation sites and crack propagation in time of testing when hold time was 30 min and less.