Effects of cold rolling and annealing on the structure of gamma double prime precipitates in a Ni-18Cr-16Fe-5Nb-3Mo alloy

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 30A; no. 8; pp. 1923 - 1931
• Main Authors: Kusabiraki, Kiyoshi, Tsutsumi, Tomoya, Saji, Shigeoki
• Format: Journal Article
• Language: English
• Published: 01.08.1999
• ISSN: 1073-5623

A Ni-18Cr-16Fe-5Nb-3Mo alloy (a modified alloy 718, referred to herein as 718M), in which the gamma double prime phase is the only precipitation-strengthening phase, was made by substituting most of the aluminum and titanium in INCONEL 718 with niobium. The specimens of alloy 718M were solid-solution heat treated, aged at 1033 to 1073 K for up to 360 ks, and cold rolled to a reduction of 10 to 15 pct at room temperature. After subsequent annealing heat treatments, at the same temperatures used for aging, the morphological and structural changes of the gamma double prime precipitates were investigated by transmission electron microscopic observation using carbon extraction replicas. Although stacking faults were not noticed in the gamma double prime precipitates extracted from the aged 718M alloy, stacking faults introduced in the 112 planes by cold rolling of the samples were confirmed. The stripelike contrast due to the stacking faults becomes less clear with increasing annealing time. With longer annealing times, necking occurs between the stripes, which eventually separate into plural particles. The selected area electron diffraction (SAED) patterns of the cold-rolled and annealed particles show that the metastable gamma double prime precipitates with stacking faults are transformed gradually into a stable delta phase. In this article, we discuss the basis of morphological and structural changes in detail. (Author)