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

A Ni-18Cr-16Fe-5Nb-3Mo alloy (a modified alloy 718, referred to herein as 7 18M), in which the y" 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 *;INCONEL is a trademark of INCO Alloys International, Inc., Huntington, WV. heat treate d, aged at 1033 to 1073 K for up to 360 ks, and cold rolled to a reduction of 10 to 15 pet at room temperature. After subsequent annealing heat treatm ents, at the same temperatures used for aging, the morphological and struct ural changes of the y" precipitates were investigated by transmission elect ron microscopic observation using carbon extraction replicas. Although stac king faults were not noticed in the y" precipitates extracted from the aged 718M alloy, stacking faults introduced in the (112)(gamma") 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 lon ger annealing times, necking occurs between the stripes, which eventually s eparate into plural particles. The selected-area electron diffraction (SAED ) patterns of the cold-rolled and annealed particles show that the metastab le y" precipitates with stacking faults are transformed gradually into a st able delta phase. In this article, we discuss the basis of morphological an d structural changes in detail.