Monotonic and cyclic deformation structure in Inconel 718: An experimental and modelling study

To understand the cyclic softening and the crack initiation and propagation observed in Inconel 718 during low-cycle fatigue tests, the surface morphology and deformation microstructure, produced under different loading conditions, have been examined by scanning and transmission electron microscopy. The morphology and microstructure produced during room temperature low-cycle fatigue tests were compared using pure tension, pure torsion and combined-proportional torsion and tension tests.; The fracture mode under the different loading conditions was transgranular. In the early stages of the life, near surface, semi-elliptical, part-through the thickness cracks, which had nucleated at inclusions, propagated in a stepped and faceted manner along intense deformation bands. Late in the fatigue life, the fracture mode changed to ductile microvoid coalescence.; The deformation microstructure produced by each loading condition was identical. It consisted of a regularly spaced array of deformation bands. Within the deformation bands dense tangles of dislocations were observed but there was no evidence of the {dollar}gammaspprime{dollar} and {dollar}gammasp{lcub}primeprime{rcub}{dollar} precipitates that contribute to the strength of this material.; Additional experiments were performed to understand the development of the regularly spaced array of deformation bands. The configuration was a result of monotonic deformation. Inside the bands the dislocations were in an inverted pile-up configuration upon a single slip plane. The dislocations were nucleated from grain-boundary sources.; A computer model based on a single-ended dynamic dislocation pile-up model was constructed where the dislocation velocity was drag-controlled. The slip band configuration can be reasonably simulated by properly modelling the important mechanisms. These included a periodic glide resistance due to the precipitates, the effect of dislocation debris left on the slip planes and a reasonable dislocation source-strength distribution.; The dislocations in the model were distributed uniformly along the bands rather than in an "ideal" pile-up configuration. Inhomogeneous deformation in the bands can be understood as a constitutive instability. New bands were generated when new sources became operative as a result of hardening.; The absence of "ideal" pile-ups indicated that the precipitates prevent the formation of internal stress-concentrations. This suggested that fatigue damage evolved in proportion to accumulated plastic strain. The accumulated plastic strain did correlate within a factor of about 2 with a critical crack size of 1.0 mm. (Abstract shortened with permission of author.)...