Effect of Boron Concentration on Fatigue Crack Propagation Resistance and Low Cycle Fatigue Properties of INCONEL 718

Abstract

The fatigue crack growth (FCG) rate and low cycle fatigue (LCF) life were determined in Inconel 718 (IN 718) alloy with 12 ppm and 29 ppm boron (B) at room temperature. The results showed that B increased the FCG resistance and improved the LCF life of IN 718. The fatigue crack growth path in the alloy with the lower B concentration was relatively smooth and straight, whereas it was tortuous and rough in the higher B alloy. Transgranular fracture surfaces with some fatigue striations were observed. A large number of secondary cracks appeared on the fracture surface of LCF samples, particularly in the alloy with the lower B concentration. TEM examination of the plastic zone ahead of the crack tip in the FCG samples showed that the plastic deformation seems to be produced and accommodated by dislocation slip between grain boundaries and precipitates, and by the activation of mechanical twins. Precipitate-free deformation bands were also observed in the plastic zone ahead of the crack tip. The LCF deformation microstructures were observed to be regularly spaced arrays of planar deformation bands on the {111} slip planes, consisting of characteristic diamond-shaped structures. A ladderlike microstructure was also observed in local regions in the fatigued IN 718 alloy with the lower B concentration. In the LCF specimens with a higher B concentration, the deformation substructure was composed of relatively uniformly distributed planar slip bands. The mechanisms which explain the effect of B on the fatigue properties of IN 718 are discussed.