Influence of dwell time on the creep–fatigue behavior of a directionally solidified Ni-based superalloy DZ445 at 850 °C

Abstract

Total strain-controlled creep-fatigue tests were performed for a directionally solidified Ni-based superalloy at 850 °C in air. Dwell times of 0, 1, 2, 3, 5, and 8 min were introduced at the tensile peak for each cycle. The results demonstrated that the fatigue life firstly decreased and then slightly increased with increasing dwell time. The fatigue life showed a minimum at a dwell time of 3 min. The initial softening rate was faster than the subsequent softening rate when dwell times of 1 and 2 min were used compared to the case with no dwell time. A compressive mean stress was produced by stress relaxation in the tensile dwell period. The reduction in the fatigue life was caused by a mixture of fatigue and creep deformation behaviors. However, the initial softening rate increased and a small amount of subsequent hardening occurred when 5 and 8 min of dwell times were applied. The higher compressive mean stress and a slight increase in the fatigue life for these samples of 5 and 8 min dwell times were attributed to a mixture of creep and plastic deformation. The change in the fatigue life with increasing dwell time was explained on the basis of the mechanical response, deformation mechanism, and fracture mechanism of the DZ445 superalloy.