LOW-CYCLE FATIGUE DEFORMATION-BEHAVIOR OF A NIMONIC PE-16 SUPERALLOY IN A DOUBLE-AGED MICROSTRUCTURAL CONDITION

The cyclic stress response behaviour of a gamma' hardened Nimonic PE-1 6 superalloy in a prior-aged microstructural condition showed hardenin g, softening and saturation regime's when tested at 823 K at a strain rate of 3 x 10(-3) s(.)(-1) Tests were performed at strain amplitudes in the range +/-0.25 to +/-1.0% in a servohydraulic system in total ax ial strain control mode. Transmission electron microscopy of samples f atigue tested to various fractions of life, representative of the thre e regimes, indicated that the dislocations get accumulated in slip ban ds; softening was caused by repeated shearing of gamma' by planar disl ocations which led to a gradual reduction in their projected size in t he slip plane. Finally precipitates were completely offset in the slip plane and in the saturation regime dislocations were piled up against the slip band intersections. An analysis of the work hardening behavi our of the alloy in the tensile portions of the successive hysteresis loops was performed using Estrin-Mecking's models. Linear variation of theta sigma with sigma(2) and agreement between the experimental and computed stress-strain curves indicated the applicability of the modif ied Estrin-Mecking model. From the dislocation mean free path computed from the model as a function of successive cycles, it is suggested th at the reduction in the gamma' precipitate size due to repeated sheari ng causes cyclic softening. Further it is proposed that slip band inte rsections adas impediments to the dislocation motion in the saturation regime. The mean slip band spacing computed was found to agree well w ith the measured values.