Modeling the atomic structure of an amorphous Co 25 Nb 75 alloy produced by mechanical alloying using an additive hard sphere model and RMC simulations K.D. Machado * , J.C. de Lima, C.E.M. de Campos, T.A. Grandi, A.A.M. Gasperini Depto de Fısica, Universidade Federal de Santa Catarina, Trindade, Cx. P. 476, 8804 0900 Florianopolis, Santa Catarina, Brazil Received 5 November 2003; in final form 15 December 2003 Published online: 9 January 2004 Abstract The local atomic order of an amorphous Co 25 Nb 75 alloy produced by mechanical alloying was studied by X-ray diffraction and modeled considering an additive hard sphere model and also reverse Monte Carlo simulations of its X-ray structure factor. From both methods the partial G ij ðrÞ functions and S ij ðKÞ factors were determined. From these functions, coordination numbers and interatomic distances for the first neighbors were calculated and compared to each other. The alloy has a weak chemical short-range order and a preference for unlike pairs. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction Mechanical alloying (MA) [1] is an efficient method for synthesizing crystalline [2–4], amorphous [5–8], sta- ble and metastable solid solutions [9,10]. MA has also been used to produce materials with nanometer sized grains and alloys whose components have large differ- ences in their melting temperatures and are thus difficult to produce using techniques based on melting. The few thermodynamics restrictions on the alloy composition open up a wide range of possibilities for property combinations [11], even for immiscible elements [12]. The temperatures reached in MA are very low, and thus this low temperature process reduces reaction kinetics, allowing the production of poorly crystallized or amorphous materials. Co–Nb is one of many systems of interest due to their superior magnetic properties [13]. Its equilibrium phase diagram shows three stable phases Co 7 Nb 6 , a-Co 2 Nb and Co 3 Nb, and at least one high-temperature alloy, b-Co 2 Nb [14]. Besides the techniques used to produce Co–Nb alloys, which are described in [14], Co–Nb amorphous films have been formed by ion-beam mixing (IM) of multilayer films [13]. The amorphization range is determined to vary from 23 to 80 at.% Co. Zeng and Pan [13] have also shown that induced growth of such amorphous films can be performed by the ion beam assisted deposition (IBAD) technique. We used MA to produce an amorphous Co 25 Nb 75 alloy, which was studied using an additive hard sphere (AHS) model [15] (see Section 2.2) and reverse Monte Carlo simulations [16–18] of its XRD structure factor SðK Þ. From the AHS model and RMC simulations, the partial structure factors S ij ðK Þ and the partial pair distribution functions G ij ðrÞ were determined and coordination numbers and interatomic distances were found. In addition, the bond angle distribution functions H ijl ðcos hÞ were also obtained from the simulations. 2. Calculation details 2.1. Faber and Ziman structure factors According to Faber and Ziman [19], SðK Þ is ob- tained from the scattered intensity per atom I a ðK Þ through * Corresponding author. Fax: +55-48-3319068. E-mail address: kleber@fisica.ufsc.br (K.D. Machado). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.12.072 Chemical Physics Letters 384 (2004) 386–390 www.elsevier.com/locate/cplett