Discontinuous γ′–precipitation in directionally solidified IN 738 LC alloy

Abstract

An investigation is reported on the formation of the discontinuous mode of precipitation of γ′–phase in directionally solidified (DS) IN 738 LC alloy. The major mode of γ′–precipitation is continuous, but small proportions of discontinuous γ–γ′ cells form at relatively slow cooling rates during directional solidification. The cells contain rod–like γ′–particles showing dendrite–shaped growth instabilities. From detailed observations on DS ingots and an ingot quenched during DS processing, it is deduced that discontinuous γ′ –precipitation originates by migration of grain boundaries activated by the impingement of colonies of the rod–like type of γ –γ′eutectic. Annealing in the range l050–1120°C leads to the formation of discontinuous γ′–regions of a coarsened type and a change to a serrated interface morphology on the rods.

MST/252