Low-cycle fatigue behavior of INCONEL 718 superalloy with different concentrations of boron at room temperature |
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Authors: | L Xiao M C Chaturvedi D L Chen |
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Affiliation: | (1) the Department of Mechanical and Manufacturing Engineering, University of Manitoba, R3T 2N2 Winnipeg, MB, Canada;(2) the Department of Mechanical, Aerospace and Industrial Engineering, Ryerson University, M5B 2K3 Toronto, ON, Canada |
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Abstract: | Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on INCONEL 718 superalloy containing 12, 29, 60, and 100
ppm boron (B) at room temperature (RT). The results showed that all four of these alloys experienced a relatively short period
of initial cyclic hardening, followed by a regime of softening to fracture at higher cyclic strain amplitudes (Δɛ
t
/2≥0.8 pct). As the cyclic strain amplitude decreased to Δɛ
t
/2≤0.6 pct, a continuous cyclic softening occurred without the initial cyclic hardening, and a nearly stable cyclic stress
amplitude was observed at Δɛ
t
/2=0.4 pct. At the same total cyclic strain amplitude, the cyclic saturation stress amplitude among the four alloys was highest
in the alloy with 60 ppm B and lowest in the alloy with 29 ppm B. The fatigue lifetime of the alloy at RT was found to be
enhanced by an increase in B concentration from 12 to 29 ppm. However, the improvement in fatigue lifetime was moderate when
the B concentration exceeded 29 ppm B. A linear relationship between the fatigue life and cyclic total strain amplitude was
observed, while a “two-slope” relationship between the fatigue life and cyclic plastic strain amplitude was observed with
an inflection point at about Δɛ
p
/2=0.40 pct. The fractographic analyses suggested that fatigue cracks initiated from specimen surfaces, and transgranular
fracture, with well-developed fatigue striations, was the predominant fracture mode. The number of secondary cracks was higher
in the alloys with 12 and 100 ppm B than in the alloys with 29 and 60 ppm B. Transmission electron microscopy (TEM) examination
revealed that typical deformation microstructures consisted of a regularly spaced array of planar deformation bands on {111}
slip planes in all four alloys. Plastic deformation was observed to be concentrated in localized regions in the fatigued alloy
with 12 ppm B. In all of the alloys, γ″ precipitate particles were observed to be sheared, and continued cyclic deformation reduced their size. The observed cyclic
deformation softening was associated with the reduction in the size of γ″ precipitate particles. The effect of B concentration on the cyclic deformation mechanism and fatigue lifetime of IN 718 was
discussed. |
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Keywords: | |
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