On Cr₂N precipitation mechanisms in high-nitrogen austenite

Abstract

Precipitation mechanisms in a high-nitrogen (0.87 wt%) fully austenitic 25Cr–5Ni (wt%) steel have been studied after ageing at 900°C and 960°C. Ageing induced discontinuous precipitation of Cr₂N from grain boundaries, discontinuous precipitation at twin fronts, continuous precipitation throughout the matrix, continuous precipitation in association with ferrite plate formation and also grain boundary precipitation in the form of both discrete precipitates and films. The observed morphologies at the onset of discontinuous precipitation from grain boundaries suggest a Tu–Turnbull initiation mechanism. The orientation relation between Cr₂N and γ in well-developed lamellae is not strictly (111)γ//(0001)Cr₂N and γ//Cr₂N. The fact that the process shows some orientation tolerance, together with the lamellae irregular and curved interfaces, indicates that minimization of the system interfacial energy does not play a key role during growth. In this precipitation mechanism, twinning inside growing γ lamellae is associated with abrupt local changes in the growth direction. In the case of cooperative growth at incoherent twin fronts, the irregular habit surfaces of Cr₂N point to a Fournelle–Clark initiation mechanism. At early precipitation stages, the interlamellar spacing for cooperative growth at twin fronts is similar to the one observed for cooperative growth from grain boundaries (∼100 nm). It is proposed that the low growth efficiency observed at twin fronts results from lower diffusivity conditions at twin fronts as compared with grain boundaries.