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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.

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Alloy 718 samples under two initial microstructural conditions, viz., solution annealed to form only phase (ST) and aged to precipitate only particles (DELTA), were deformed in tension till fracture in the temperatures range from 200°C -700°C. From the comparison of the evolved microstructure of deformed and undeformed specimens that have been subjected to similar thermal history, deformation induced precipitation could be identified. Deformation in the range of 550°C to 650°C promoted the precipitation of and phases in both structures. In case of DELTA alloy, the precipitation was found to precede the phase precipitation while no such preference for precipitation could be identified in ST specimens. This difference in the precipitation behaviour and the sequence of precipitation has been explained on the basis of the relative concentration of solutes in the matrix of the starting microstructures of ST and DELTA specimens.

A new theory of superalloy creep is presented. It is argued that ' particles in superalloys do not play a rate-controlling role during creep. They are hypothesized to act as composite reinforcements regardless of their size. The rate-controlling role is ascribed to dislocation related obstacles such as nodes, jogs or kinks.The basic constitutive law based on this premise is shown to be capable of representing the time to strain data of a wide range of superalloys. Equations for primary and tertiary creep are developed based on the composite concept and are shown to predict the creep behavior of DS-GTD111 TM a superalloy commonly used in gas turbines. The new approach is also shown to be capable of explaining a wide range of observations reported in the literature. The implications of these findings in terms of the effect of microstructure on creep behavior are discussed.

This paper outlines a microstructure-based model relating gamma prime microstructure and grain size of Ni-base alloys to their creep behavior. The ability of the model to explain creep of multiple superalloys with a single equation and parameter set is demonstrated. The only parameters that are changed from alloy to alloy are related to the gamma prime characteristics and grain size. This model also allows prediction of creep performance as a function of heat treatment and explains some apparently contradictory data from the literature.

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.