On Cr2N precipitation mechanisms in high nitrogen austenite

Patrícia Almeida Carvalho (1) , Izabel Fernanda Machado (2) , Guillermo Solorzano (3) , Angelo Fernando Padilha (2)

1 Departamento de Engenharia de Materiais
2 Mechanical Systems
3 Materials Science

Precipitation mechanisms in a high-nitrogen (0.87 wt.%) fully austenitic 25Cr-5Ni (wt.%) steel have been studied after ageing at 900oC and 960şC. 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 and Turnbull initiation mechanism. The orientation relation between Cr2N and γ in well-developed lamellae is not strictly (111)γ//(0001)Cr2N and γ[1-10]//[1-100]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 γlamellae 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 and Clark type of 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 (~100nm). 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.

Mots clés: Physical Sciences

Patrícia Almeida Carvalho, Izabel Fernanda Machado, Guillermo Solorzano, Angelo Fernando Padilha. On Cr2N precipitation mechanisms in high nitrogen austenite. Philosophical Magazine, 2008, 88 (02), pp.229-242. ⟨10.1080/14786430701805590⟩. ⟨hal-00513850⟩