Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on INCONEL 718 (IN718) containing 12, 29, 60, and 100 ppm B at 650 °C. The results showed that all the alloys experienced a relatively short period of initial cyclic hardening at low strain amplitudes, followed by a regime of saturation or slightly continuous cyclic softening. The initial cyclic hardening phase decreased with increasing strain amplitudes, and disappeared at the high strain amplitudes. A serrated flow was observed in the plastic regions of cyclic stress-strain hysteresis loops. The saturated cyclic stress amplitude at a given strain amplitude was highest for the alloy with 60 ppm B, and lowest for the alloy with 12 ppm B. The LCF lifetime increased with increasing B concentration up to 60 ppm, and then decreased as the B content increased from 60 to 100 ppm. Fractographic analysis suggested that the fracture mode changed from intergranular to transgranular cracking as the B concentration increased. The characteristic deformation microstructures produced by LCF tests at 650 °C, examined via transmission electron microscopy, were regularly spaced arrays of planar deformation bands on {111} slip planes in all four alloys. A ladderlike structure was observed in some local regions in the alloy with 12 ppm B. Heavily deformed planar deformation bands were observed in the fatigued specimens with 100 ppm B. The mechanism of improvement in the LCF life of IN718 due to B addition is discussed.
Biaxial fatigue behavior of hydrided in the as-cold-worked (CW) and recrystallized (RZ) conditions under in-phase (IP) and out-of-phase (OP) cyclic loading was investigated. The CW Zr-4 showed cyclic softening followed by a saturation stage during biaxial cyclic loading. Additional cyclic softening was displayed in CW Zr-4 under OP loading with the phase lag of 30 and 60 deg. The additional softening level decreased as the phase lag increased. On the other hand, RZ Zr-4 showed cyclic hardening followed by a saturation stage, and additional cyclic hardening was obtained under OP loading. The additional hardening arose as the phase lag increased. Observation of the fracture surface showed that the biaxial fatigue failure of the CW Zr-4 under OP loading was controlled by crack initiation and propagation through the hydrides, while the nucleation and coalescence of microvoids were dominant in the failure of CW Zr-4 under IP loading and RZ Zr-4 under both IP and OP loading. The typical deformation substructure in CW Zr-4 specimens was composed of dislocation tangles together with parallel dislocation lines under IP and OP loading. Whereas the parallel dislocation lines were formed by prismatic slip for RZ Zr-4 under IP loading and OP loading with the lower phase lag, they developed into dislocation networks, loops, and debris as the phase lag increased under OP loading. The additional cyclic softening for CW Zr-4 was due to the relief of the anisotropic hardening mechanisms when the loading mode changed from IP to OP. The additional cyclic hardening of RZ Zr-4 under OP loading is attributed to an increase in the interaction between the primary dislocations and other dislocations from different slip systems.
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