Development and Validation of a ReaxFF Reactive Force Field for Fe/ Al/Ni Alloys: Molecular Dynamics Study of Elastic Constants, Diffusion, and Segregation Yun Kyung Shin,* ,‡ Hyunwook Kwak, § Chenyu Zou, ‡,† Alex V. Vasenkov, § and Adri C. T. van Duin* ,‡,† † National Energy Technology Laboratory-Regional University Alliance (NETL-RUA), Pittsburgh, Pennsylvania 15236, United States ‡ Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States § CFD Research Corporation, Huntsville, Alabama 35805, United States * S Supporting Information ABSTRACT: We have developed a ReaxFF force field for Fe/ Al/Ni binary alloys based on quantum mechanical (QM) calculations. In addition to the various bulk phases of the binary alloys, the (100), (110) and (111) surface energies and adatom binding energies were included in the training set for the force field parametrization of the Fe/Al/Ni binary alloys. To validate these optimized force fields, we studied (i) elastic constants of the binary alloys at finite temperatures, (ii) diffusivity of alloy components in Al/Ni alloy, and (iii) segregation on the binary alloy surfaces. First, we calculated linear elastic constants of FeAl, FeNi 3 , and Ni 3 Al in the temperature range 300 to 1100 K. The temperature depend- ences of the elastic constants of these three alloys, showing a decrease in C 11 , C 12 , and C 44 as temperature increases, were in good agreement with the experimental results. We also performed ReaxFF molecular dynamics (MD) simulations for Al or Ni diffusion in the system modeled as Al/Ni mixed layers with the linear composition gradients. At 1000 K, Al diffusivity at the pure Al end was 2 orders of magnitude larger than that in the Al trace layers, probably explaining the nature of different diffusion behavior between molten metals and alloys. However, the diffusivity of Ni at the pure Ni end was only slightly larger than that in the Ni trace layers at the system temperature much lower than the melting temperature of Ni. Third, we investigated the surface segregation in L1 2 -Fe 3 Al, Fe 3 Ni, and Ni 3 Al clusters at high temperature (2500 K). From the analysis of composition distribution of the alloy components from the bulk to the surface layer, it was found that the degree of segregation depended on the chemical composition of the alloy. Al surface segregation occurred most strongly in Fe 3 Al, whereas it occurred most weakly in Ni 3 Al. These results may support the segregation mechanism that surface segregation results from the interplay between the energetic stability of the ordered bulk phase and the surface reconstruction. In addition, the surface segregation induced the depletion layers of segregating metal species (Al in Fe 3 Al and Ni 3 Al, and Ni in Fe 3 Ni) next to the segregation layers. These simulation results qualitatively agreed with early experimental observations of segregation in Fe/Al/Ni binary alloys. I. INTRODUCTION The importance of the mechanical properties and surface oxidation of alloys has been emphasized in many studies since these properties play a crucial role in controlling, among others, catalytic reactions, corrosion resistance, and adsorption. 1-4 In particular, the surface oxide film formed on the late transition metal alloys such as Fe/Al/Ni binary alloys is known to offer enhanced corrosion resistance under high temperature and oxidizing environment. 5-8 In addition, the segregation and reconstruction at the surface and grain boundary induce changes in chemical and physical properties, consequently influencing adsorption and material embrittlement. Although the composition and structure on the alloy surfaces have been studied extensively, precise mechanisms for the formation of the protective oxide layer and the effect of the oxide layer on the alloys in oxidizing environment have not been postulated. Therefore, an improved understanding of the high temperature mechanical properties as well as corrosion resistance of Fe/Al/ Ni alloys is important for the applications of these alloys. Ab initio or DFT calculations have been employed to study the energetics and metal/oxide structures. However, these methods are in general only efficient handling very small systems (usually 1-100 atoms). Atomistic molecular dynamics (MD) methods using empirical force field have been widely Received: August 27, 2012 Revised: November 13, 2012 Published: November 20, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 12163 dx.doi.org/10.1021/jp308507x | J. Phys. Chem. A 2012, 116, 12163-12174