Effect of boron on the low-cycle fatigue behavior and deformation structure of INCONEL 718 at 650 °C

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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.