Though Alloy 718 is strengthened by γ″ precipitation, the nucleation of δ phase is used to stabilize grain size and also to improve its high temperature stress rupture properties. Large body of work has been reported in the literature on the precipitation of the γ″ phase. However, hardly very few reports are available on the evolution of δ particles in Alloy 718. The present paper reports the results of a detailed microstructural investigation carried out using Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) on the evolution of δ microstructure. Based on the SEM observations, three different mechanisms have been proposed for the generation of intersecting δ microstructure. In addition to the stacking faults in the matrix and γ″ particles acting as nuclei for δ phase formation, a new mechanism has been observed where the faults generated on the conjugate habit plane by dissociation of misfit dislocation on the interface of a δ variant acted as nuclei for another δ variant precipitation.

No abstract

Alloy 718 has been reported to undergo serrated yielding (SY) phenomenon over the temperature regime that coincides with its useful service temperature range. The SY is reported to reduce ductility in several alloy systems. The present paper reports the results of detailed investigation carried out on SY phenomenon in Alloy 718 in three different microstructural conditions viz. solution annealed (ST), γ′′ precipitated (STA) and δ precipitated (DELTA). Based on the estimated activation energy for appearance and disappearance of serrations as well as the nature of serrations, two regimes of dynamic strain ageing (DSA) have been identified -one below 525° C controlled by diffusion of interstitial carbon and the other above 525° C controlled by diffusion of substitutional niobium atom. The initial microstructure appeared to strongly influence the temperature regime of serrated flow. The mechanism for the delayed disappearance of serrations in ST specimens has been identified from TEM examination of deformed specimens.