INCONEL 718 is a commercial, precipitation-hardenable nickel iron-base superalloy, strengthened primarily by precipitates of the metastable [gamma][double prime] phase which is based on the composition Ni[sub 3]Nb and has an ordered body-centered tetragonal (DO[sub 22]) structure. An earlier investigation on the deformation mechanisms in this alloy has shown that [gamma][double prime] precipitates are sheared by the passage of true crystallographic twins (which do not destroy the ordered atomic arrangements within precipitate crystals) when the [gamma][double prime] particle size is larger than about 10 nm. In a situation where the precipitate size is smaller than about 10 nm, the deformation occurs by the passage of quadruplets or pairs of dislocations which shear the [gamma][double prime] particles. The change in the deformation mechanism from dislocation cutting to twinning with increasing [gamma][double prime] particle size is reflected in a drop in the value of the Croussard-Jaoul work-hardening exponent (from 0.8 to 0.5) which has been attributed to the easy propagation of deformation twins within the particles. The present communication reports the results of a study carried out on the characterization of the deformation microstructure of underaged INCONEL 718. An attempt has been made to explain the work-hardening behavior of the alloy in themore » light of transmission electron microscopic (TEM) observations on its deformation microstructures in the underaged condition.« less