Precipitation of niobium carbonitrides in ferrite: chemical composition measurements and thermodynamic modelling

Abstract

High-resolution transmission electron microscopy and electron-energy loss spectroscopy have been used to characterize the structure and chemical composition of niobium carbonitrides in the ferrite of a Fe-Nb-C-N model alloy at different precipitation stages. Experiments seem to indicate the coexistence of two types of precipitates: pure niobium nitrides and mixed substoichiometric niobium carbonitrides. In order to understand the chemical composition of these precipitates, a thermodynamic formalism has been developed to evaluate the nucleation and growth rates (classical nucleation theory) and the chemical composition of nuclei and existing precipitates. A model based on the numerical solution of thermodynamic and kinetic equations is used to compute the evolution of the precipitate size distribution at a given temperature. The predicted compositions are in very good agreement with experimental results.

Theoretical study of interface structure and energetics in semicoherentFe(001)/MX(001)systems (Fors1, Wahnström2 2010Phys. Rev. B

“…The inclusion of the misfit gives a significant contribution to the interface energy and must therefore be accurately accounted for in order to obtain a proper interface description. These findings will have important bearings on our ability to understand the early stage of precipitation and its pathways, 5 to thermodynamically model precipitate stability in Fe-M-X systems, 77 and to describe long-term microstructural evolution in steels. …”

Solute Behavior in the Initial Nucleation of V- and Nb-Containing Carbide

“…Binary MX-type carbides have been well characterized at nanoscale by high-resolution transmission electron microscopy (HRTEM), high angle annular dark field (HAADF), electron energy loss spectroscopy (EELS), and small-angle neutron scattering (SANS) and directly giving out the platelike crystal lattice morphology and quantitative composition distribution. [11][12][13][14][15][16] With regard to multicomponent carbides, the competitive affinity of alloy elements such as V, Nb, and Ti to C is strongly influenced by cohesive energy and electron structures at the coherent interfaces between ferrous matrix and MX-type carbides. The compositional and structural evolution of transient phases is also affected by the diffusion coefficient of the carbonforming elements from the aspect of kinetic behaviors.…”

Nucleation Rate and Number of Precipitates in V and Nb-Microalloyed Steels

“…6). The calculated critical radii are smaller than those measured in V-microalloyed steels, whose average size at the nose of the curve is approximately 6 nm [21]. Thus it is shown that the experimental t 0.05 time (P s curve) in Figs.…”

“…Thus it is shown that the experimental t 0.05 time (P s curve) in Figs. 1 and 2 does not exactly correspond to the start of nucleation, but that the first nuclei of precipitates form at much shorter times, in such a way that between the moment when the first precipitates form and the moment of t 0.05 corresponding to the nose of the P s curve, when the precipitates were analyzed, considerable growth of the precipitates has occurred [21].…”