ESSC & DUPLEX 2019

We propose a new thermodynamic model description of carbonitride Cr3Ni2Si(N,C) “eta-phase” with M6C-type structure. Diamond-structured face-centered cubic Cr-Ni-Si Carbonitride eta-phase precipitates have been reported in some Si-containing 20-25 austenitic stainless steel grades [1] and irradiated type 316 [2]. This phase forms typically after long-term aging of at least several hundred hours. Once nucleated, the phase shows faster growth than nitride precipitates Z-phase [3]. Eta-phase will thus significantly affect microstructure and consequently influence creep strength of the material. Due to structural and morphologic similarities, eta-phase is often hard to distinguish experimentally from M23C6 and G-phase. A better knowledge of the theoretic stability of eta-phase, by evaluating its Gibbs energy computationally, is a valuable contribution for the prediction of phase relations and related microstructural development of stainless steels. The applicability of conventional M6X-model with 4 distinct crystallographic sites for -phase is discussed. It is shown that this model leads to an artificial phase stability and wrong composition at high nominal Si-contents. An improved model with 5 different crystallographic sites can solve this problem and reproduces the experimental tendencies of eta-phase stability. The site occupancies in the new eta-phase description were based on first-principles compound energies. With available compound energy models for intermetallic G-phase, and Z-phase nitride [4], relative precipitate stabilities as function of varied steel composition and heat treatments of stainless steels, as predicted by thermokinetic simulation, are discussed. The role of Si-segregation and altered diffusion behavior by irradiation for the precipitate evolution [5] is illuminated. [1] T. Sourmail, H.K.D.H. Bhadeshia, Metal. Mater. Trans. A, 36A (2005) 23. [2] J.M. Titchmarsh, T.M. Williams, “Precipitates in Neutron-Irradiated Austenitic Stainless Steel”. in: G.W. Lorimer, M.H. Jacobs, P. Doig, Eds., Quantitative Microanalysis with High Spatial Resolution, pages 223-228, London, 1981. Institute of Metals. [3] V. Vodarek, Minor phase evolution in AISI 316LN+Nb steels during creep at 650°C, Proceedings of Metal 2010, 18.-20.5. 2010, Roznov pod Radhostem, Chech Republic. [4] E. Povoden-Karadeniz, MatCalc Engineering Database ME-Fe, version 1.000. Wien, 2017. [5] J.-H. Shim, E. Povoden-Karadeniz, E. Kozeschnik, B.D. Wirth. Modeling precipitation thermodynamics and kinetics in type 316 austenitic stainless steels with varying composition as an initial step toward predicting phase stability during irradiation, Journal of Nuclear Materials, 462 (2015) 250.