The discontinuous precipitation of M₂₃C₆ in Nimonic 80A

Abstract

An important distinction is made between the discontinuous precipitation Of M₂₃C₆ carbide (where M = metal) in a nickel-base superalloy compared with those in other systems. The reaction is considered pseudo-discontinuous since carbon must diffuse to the precipitate from the general matrix. The additional factor imposed by the partial carbon diffusion controlled growth is utilized to uniquely define the kinetics of the discontinuous precipitation. This paper details the theory involved end then compares the computed results with extensive measurements made in a transmission electron microscope of precipitates in Nimonic 80A. The results show the feasibility of the participation of carbon-diffusion control in growth. They also confirm the high dependence of discontinuous precipitation on grain boundary nucleation and on the direction of growth in the receding grain. The theoretical analysis is used to indicate how discontinuous precipitation may be avoided by heat treatment and compositional control.