Precipitation behavior of sigma phase and its influence on mechanical properties of a Ni-Fe based alloy

Precipitation behavior of sigma (sigma) phase of a Ni-Fe based alloy, considered as boiler materials in advanced ultra-supercritical (A-USC) coal-fired power plants, was investigated during thermal exposure at 650-800 degrees C for up to 40000 h with and without applied stress. Moreover, its influence on mechanical properties was clarified. During thermal exposure without applied stress, the plate-like sigma phase precipitates directly in the grain, enriches in Ni, Fe, Cr and Mo and has tetragonal structure. Stress promotes the precipitation of sigma phase. The incubation stage, rapid growth stage and near-equilibrium can be clearly distinguished during the precipitation process. At the incubation stage, the most visible change is the increase of sigma density, but a pronounced increase appears regarding the density and size of sigma phase at the rapid growth stage. The incubation and rapid growth stages of sigma phase are controlled by interface and the preferential growth is in two directions, but it is related to diffusional-controlled growth at near-equilibrium stage. sigma phase slightly reduces the tensile strength because of the crack initiation at gamma/sigma interface, but has no obvious influence on ductility. sigma phase drastically decreases stress rupture life, but improve ductility by depleting strengthening elements in gamma matrix. (C) 2019 Elsevier B.V. All rights reserved.