The origin of microtwinning at low strains during low-cycle fatigue of Inconel 718 at room temperature

The low-cycle fatigue (LCF) behaviour of a Ni-Fe base superalloy Inconel 718 was studied at room temperature. Deformation mechanisms operating at different strain amplitudes were established by detailed transmission electron microscopy at various fractions of fatigue life. It was observed that there was an increase in the number of microtwins present in the microstructure with increase in number of cycles, N, at low strains, where very little slip activity was identified. The undeformed specimens contain a number of annealing twins in the microstructure which increased in number with increase in cyclic deformation. Electron microscopy also revealed the existence of grain-boundary ledges in the undeformed specimens. A typical region of the grain-boundary area of the deformed samples showed a front of microtwins in their "nascent" state, advancing from one grain into the other. It was concluded that in the low-strain regime, cyclic deformation was accommodated by forming new microtwin interfaces emerging from grain-boundary ledges present in the undeformed specimens. (C) 1999 Kluwer Academic Publishers.