Basic and Applied Research: Section I Thermodynamic Optimization of the Ni-Zn System George Penev Vassilev, Tomas Gomez-Acebo, and Jean-Claude Tedenac (Submitted 21 September 1999; in revised form 15 February 2000) Optimization of thermodynamic and phase diagram data has been performed and consistent sets of coefficients for the calculation of the phase equilibria in the system Ni-Zn have been obtained using the program BINGSS. The  phase has been modeled as a stoichiometric compound (NiZn 8 ). The binary liquid and the solid Ni-based solutions have been treated as disordered substitutional phases. The intermediate ,  1 , and  compounds have been modeled as phases with substitutional defects and vacancies on two sublattices. The calculated phase diagram and thermodynamic quantities are in excellent agreement with the experimental data. but later studies have revealed a homogeneity range. [11,17] 1. Introduction Hansen and Anderko [1] accepted the homogeneity range 45.5 to 52 at.% Zn as well as vertical phase boundaries with the The experimental studies on the Ni-Zn system have been neighboring  and  phases. summarized by a number of authors, [1–4] and one of us has A summary of the experimental data about the invariants already performed thermodynamic evaluations of this sys- is presented in Table 1. We have converted the compositions, tem [5,6] modeling all phases as substitutional solutions. originally [9,10,12–15] given in weight percents, to mole fractions The purpose of the present work is to perform an optimiza- using the standard atomic masses A for Ni and Zn (A Ni = tion through coupling of phase diagram and thermochemical 58.69 g/mol, A Zn = 65.39 g/mol; Barin [22] ). data using the program BINGSS, [7,8] in order to calculate sets The crystal structure of the  1 phase is a face centered of coefficients describing the thermodynamic properties of tetragonal one, AuCu type, L1 0 . [12–15,23–26] A perfectly all nickel-zinc phases. ordered state for such a phase is possible at the stoichiometric composition only. Liang et al. [27] and Lau et al. [28] suggested that the predominant defects in this phase should be of substi- 2. Experimental Information tutional type and derived pertinent equations describing its thermodynamic properties.  Phase. This is a high-temperature phase, which is not 2.1 Phase Diagram Data stable below 948 K. [1,3,12,13,26,28,29] It is formed by a peritectic Liquidus and Solidus. The liquidus and the solidus lines reaction (Ni) + L ⇔  and can participate in a peritectoid have been relatively well studied. Thermal analysis (TA), reaction, producing  1 phase (Ni) +  ⇔  1 , a eutectic differential thermal analysis (DTA), metallographic methods, reaction L ⇔  + , and a eutectoid reaction  ⇔  1 +  and isothermal saturation of liquid Zn with Ni have been (Table 1). used. [9–16] The  phase is of the CsCl type, B2. [13,14,1,3,4] Chang The Solvus of the (Ni)-Fcc Solutions [ Phase, (Ni)]. and co-workers [30,31] have derived expressions to calculate its Tafel [9,10] reported the solubility of Zn in (Ni) from 1053 K thermodynamic properties by considering the substitutional to room temperature to be constant (42.3 at.% Zn). Tamaru [12] defects only. reported a temperature-independent solvus with 35.87 at.%  Phase. Numerous studies have been performed on Zn also. Nevertheless, Heike et al. [17] found the highest Zn the structure and the phase homogeneity region of the  concentration to be 40.4 at.% at 1313 K, diminishing at lower phase. [9–15,23,32–40] This phase has been initially ascribed to the temperatures. Similar results have been obtained through compound NiZn 3 (the congruent melting point corresponds to magnetic measurements by Schramm. [18] that composition) and a solubility toward the Zn-rich Our studies [16] have revealed that the solubility of Zn in side. [9,10,23] Considering these results, Bauer and Hansen [32] (Ni) is 29.6 at.% at 943 K and 32.4 at.% at 1073 K, in general have concluded that this solution extends also on the nickel agreement with the latter author. side. Tamaru, [12] Tamaru and Osawa, [25] and Heike et al. [13–15] A lower solubility has been claimed by Lihl [19,20] as well have determined experimentally the homogeneity range of as by Ali and Geiderikh, [21] but it has not been accepted this phase. by Massalski. [4] Schramm, [23] Lihl, [19,20] and Malaruka and Melikhov [34]  1 Phase. This phase exists in the equiatomic region. claimed that two different crystal structures ( and  1 ) have Initially, Tafel [9,10] has assumed that it is a line compound, been observed in the  solid solution. Morton [35,36] reported the existence of an inverse antiphase domain structure and suggested that a single -phase region should be considered. George Penev Vassilev, Faculty of Chemistry, University of Sofia, This view is accepted in the later phase diagram 1164 Sofia, Bulgaria; Tomas Gomez-Acebo, CEIT and University of assessments. [3,4,6] Navarra, 20018 San Sebastian, Spain; and Jean-Claude Tedenac, USTL, LPMC, CC003, Cedex 5, 34095 Montpellier, France. In our study of the Ni-Zn layer growth kinetics, [38] we Journal of Phase Equilibria Vol. 21 No. 3 2000 287