Characterization of structure and energy of Ti x C y cluster at early formation stage in iron matrix by molecular dynamics Yanan Lv, Peter D. Hodgson, Lingxue Kong, Weimin Gao ⇑ Institute for Frontier Materials, Deakin University, Waurn Ponds, VIC 3216, Australia article info Article history: Received 1 April 2014 In final form 29 May 2014 Available online 6 June 2014 abstract The structure, energy and bonding property of Ti x C y clusters formed in iron matrix were studied through molecular dynamics (MD) simulation method. The initial clusters with 1D-linear, 2D-ring, and 3D-tetra- hedral structures were determined and their stability was calculated. The effect of temperature on the stability of the clusters was also discussed. In addition, the dissociation path of TiC clusters in iron matrix and the corresponding energy variation were analyzed. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction The initial stage of precipitate formation in alloys during quench/ageing process is key in the establishment of the final mor- phology and spatial distributions of mature precipitates, which lar- gely determines the mechanical properties of alloys. In the stage, aggregation of a small number of atoms occurs [1] and the aggre- gates are named precipitate clusters [2]. The precipitate clusters keep susceptibly growing during subsequent processing, serving as the precursor to further precipitation [3,4]. In this context the titanium carbide particles have been identified with an atom probe field ion microscope [5] and their role for strengthening the low carbon steel has been investigated [6,7]. Understanding the morphology and property of the initial con- stitutes of titanium carbides in iron matrix is the basis for investi- gating the formation and property of the precipitates. Early studies on the titanium carbide clusters are mainly focused on the crystal growth, equilibrium structures, and the dependence of structural and electronic properties on cluster size [8–10] and typically pres- ent some specific compositions, such as TiC x (x= 2–5) [11], TiC 3 and TiC 4 [12], TiC n (n 6 8) [13], Ti 4 C 4 and Ti 14 C 13 [14], Ti 8 C 12 [15], Ti 6 C 6 and Ti 12 C 12 [16]. All these studies were carried out for the independent titanium carbide clusters, i.e., there are only Ti and C atoms in the systems, without considering the influence of the iron matrix. However, it can be anticipated that the morphology and property of titanium carbide clusters in iron matrix are differ- ent with the independent ones, when considering the interaction of the clusters with iron atoms. The influence of iron atoms is more significant for small clusters due to the large contribution of the interatomic interaction of the iron atoms with the constituent atoms of the clusters to the total potential. However, by far no attention has been put on the structure and property of titanium carbide clusters in iron matrix. It has been recognized that the growth of titanium carbide clus- ters are greatly based on the combination of the small clusters formed at the earliest stage of precipitation. However, the atomis- tic behavior of the clustering phenomenon and the physical prop- erties of the small clusters have been poorly understood as it is very challenging to identify them experimentally. The primary focus of this Letter is on the structures and energies of the small titanium carbide clusters in iron matrix. The main goal is to deter- mine the favorable structure of the titanium carbide clusters through examining the various kinds of cluster isomers and com- pare the stability of the isomers based on the formation energy and the dissociation path and energy. 2. Computational methods 2.1. MD methodology Simulations were performed with an open molecular dynamics source LAMMPS [17]. The potential of 2NN.MEAM [18] was used to describe the interatomic interaction between the atoms in the Ti–C–Fe system. As a modification of the embedded-atom method (MEAM) [19], this potential considers the second nearest-neighbor interactions [18]: E u ðRÞ¼ F ð q o ðRÞÞ þ Z 1 2 /ðRÞþ Z 2 S 2 /ðaRÞ ð1Þ where Z 1 and Z 2 are the numbers of first and second nearest- neighbor atoms, respectively; F is the embedding function; q o is the background electron density for a given reference structure; R http://dx.doi.org/10.1016/j.cplett.2014.05.091 0009-2614/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Address: Institute for Frontier Materials, Deakin University, 75 Pigdons Road, Waurn Ponds 3216, Australia. Fax: +61 3 52271103. E-mail address: weimin.gao@deakin.edu.au (W. Gao). Chemical Physics Letters 608 (2014) 40–44 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett