Kinetics of heterogeneous dislocation precipitation of NbC in alpha-iron C. Hin a,c, * , Y. Bre ´chet b , P. Maugis c,d , F. Soisson a a Service de Recherches de Me ´tallurgie Physique, DMN-SRMP, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France b SIMAP, Domaine Universitaire, 1130 rue de la Piscine, B.P. 75, 38 402 Saint Martin d’He `res Cedex, France c Arcelor Research, Voie Romaine-BP 30320, 57283 Maizie `res-les-Metz, France d ENSIACET-INPT, 118 Route de Narbonne, 31077 Toulouse cedex 4, France Received 13 April 2008; received in revised form 16 July 2008; accepted 19 July 2008 Available online 5 September 2008 Abstract We propose Monte Carlo simulations of the precipitation kinetics of NbC on dislocations in a-iron based on an atomistic description of the main mechanisms controlling the kinetic pathway. The algorithm takes into account realistic diffusion properties, with a rapid diffusion of C atoms by interstitial jumps and a slower diffusion of Fe and Nb atoms by vacancy jumps. A simple model of dislocation includes interactions with solute atoms, through local segregation energy and long range elastic fields. The relative importance of local segregation energies and long range elastic stresses on the precipitation characteristics, such as early segregation of carbon atoms on dis- locations, transient precipitation of metastable carbides, and homogeneous and heterogeneous NbC precipitation, is discussed. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Steels; Heterogeneous nucleation of phase; MC-simulations; Precipitation; Dislocation 1. Introduction The precipitation of carbides plays an important role in many steels since the mechanical properties depend both on the precipitate microstructure and on the interstitial carbon concentration. Experimentally, these carbides are often observed in the austenitic phase at high temperatures and in the ferritic phase at low temperatures. Low carbon steels, such as HSLA steels, containing chemical elements like Nb, V or Ti exhibit carbide precipitates that increase the alloy yield stress. Carbide precipitation can also be used to lower the carbon content in solution thus increasing formability as is the case in IF steels. One can also take advantage of that precipitation to control the recrystalliza- tion process. Modelling the precipitation kinetics as a func- tion of temperature and composition is a key feature in alloy design for modern steels. It has been experimentally proven that NbC precipita- tion occurs preferentially on dislocations rather than in an homogeneous manner [1–11]. In the present paper, we consider precipitation of niobium carbides on dislocations in a-iron. Precipitation depends on the local solute concen- tration. The local concentration near dislocations can differ substantially from the nominal one, due to segregation induced by both short range chemical interactions and long range elastic stresses. The long range elastic stresses may also influence the solute transport kinetics toward disloca- tions, generating a segregation of solute atoms which will form an atmosphere, known as a Cottrell atmosphere, around the dislocation [12]. A phenomenological contin- uum description for the formation of this segregation within the framework of diffusion theory has been estab- lished by Cottrell and Bilby [12]. The existence of Cottrell atmospheres has been accepted for some time, due to the theory’s ability to describe macroscopic effects such as post 1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.07.044 * Corresponding author. Address: Service de Recherches de Me ´tallurgie Physique, DMN-SRMP, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France. E-mail address: celine_hin@yahoo.fr (C. Hin). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 56 (2008) 5535–5543