Thermodynamic descriptions of the binary Ni–Sn and ternary Cu–Ni–Sn systems over entire composition range: A revisit
Introduction
Cu–Ni–Sn alloy offers a unique combination of outstanding strength, excellent electrical conductivity and high corrosion resistance, which has become one of the most important substitutes for copper beryllium alloy in electronic instruments [[1], [2], [3]]. Moreover, Cu–Ni–Sn is also a key system for lead-free soldering [4,5], and has attracted increasing attentions in recent years [[5], [6], [7]]. Accurate thermodynamic information is necessary to improve the ability to predict the behavior of new materials [[8], [9], [10], [11]]. Thus, the phase stability and phase equilibria of the Cu–Ni–Sn system are of significant scientific and technological interests.
To date, three research groups [4,12,13] have performed thermodynamic studies of the Cu–Ni–Sn ternary system. The first thermodynamic description of the Cu–Ni–Sn system was reported by Miettinen [12] in 2003. Nevertheless, this description only covered the Cu–Ni rich side and the γ phase was stable in the liquid phase at high temperatures (above 2590 °C). The artificial stabilization of the γ phase at high temperatures was afterwards observed based on the thermodynamic description proposed by Yu et al. [4]. After that, Wang et al. [13] re-optimized this system by considering their own measured phase equilibrium data. In their assessment, the ternary intermetallic phase and ternary solubilities in binary compounds were not considered. However, the latest investigations [6] proved that extensive ternary solubilities exist in the boundary binary compounds. Thus, to re-assess the Cu–Ni–Sn ternary system over the whole composition range by considering the latest experimental data is necessary as well as of great importance.
To obtain an accurate thermodynamic description for the Cu–Ni–Sn ternary system, a reliable phase equilibrium information is necessary. Although the Cu–Ni–Sn system has been experimentally investigated by a large number groups [7,[14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]], some discrepancies still remain. The first discrepancy is related to the existence of intermediate ternary phase. Three intermediate ternary phases, τ1 (Ni5CuSn2), τ2 (Ni2CuSn) and τ3 (NiCu2Sn), have been reported in the Cu–Ni–Sn system. The τ1 and τ2 phases were first reported by Pak et al. [14,15], and then confirmed by a subsequent investigation [6]. Although the τ2 compound was experimentally observed by these two research groups [6,14], its detailed crystal structure is unknown. The Heusler phase τ3 was observed in several studies [[16], [17], [18]] but its existence was not confirmed in later research works [6,14,15]. The second main discrepancy is about the phase equilibria at low temperatures. The phase relationships at 220 °C on the Cu–Ni rich side obtained in a recent investigation [6] are quite different from the ones at 240 °C as reported in Ref. [19]. One of the major controversies is whether continuous solid solution can form between Cu3Sn (LT) and Ni3Sn (LT) phases or not. Consequently, all these discrepancies in Cu–Ni–Sn system should be clearly clarified before subjecting to thermodynamic modeling.
Inspired by the above considerations, the present work aims to 1) evaluate the existing thermodynamic descriptions for the sub-binary systems (Cu–Ni, Cu–Sn and Ni–Sn), 2) perform a critical review of the phase equilibria in the Cu–Ni–Sn ternary system and clarify the aforementioned discrepancies, i.e. the existence of intermediate ternary phases and the phase relationship at low temperatures, and 3) obtain a self-consistent thermodynamic description for the Cu–Ni–Sn ternary system based on the reviewed experimental data over the whole composition range.
Section snippets
Evaluation of sub-binary systems
To obtain a thermodynamic description of the Cu–Ni–Sn ternary system, accurate thermodynamic descriptions for sub-binary systems (i.e., Cu–Ni, Cu–Sn and Ni–Sn) are necessary. Mey [33] reassessed the Cu–Ni binary system by considering all the available experimental data. The calculated Cu–Ni phase diagram shows good agreement with most of the experimental phase equilibrium data. Therefore, the thermodynamic parameters of the Cu–Ni system evaluated by Mey [33] were adopted in this work. The
Thermodynamic models
According to the literature review in Section 2, the phase equilibria of the ternary Cu–Ni–Sn system are quite complicated. There include 14 stable phases, namely Liquid, Fcc, BCT(Sn), Bcc_A2, γ(D03), Cu10Sn3, Cu41Sn11, Cu3Sn, η (Cu6Sn5_HT, Ni3Sn2_HT), Ni3Sn2_LT, Cu6Sn5_LT, Ni3Sn_LT, Ni3Sn4 and τ1. The Liquid, Fcc, BCT, Bcc_A2 and γ phases are described with the substitutional solution model. Considering the negligible solubility of the third element in Cu6Sn5_LT, Cu41Sn11 and Ni3Sn2_LT, these
Ni–Sn binary system
The thermodynamic parameters in the Ni–Sn binary system were optimized using the PARROT module in the Thermo-Calc software [54] that minimizes the sum of the squares of the differences between the experimental data and calculated values. During the assessment, the thermodynamic parameters in the liquid phase are firstly adjusted to remove the artificial miscibility gap at high temperatures by considering the reported thermodynamic properties [21,55,56] and phase equilibrium data [48,[57], [58],
Conclusion
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The reported thermodynamic descriptions of sub-binary systems in the Cu–Ni–Sn ternary system were evaluated. The thermodynamic parameters of the Ni–Sn binary system were updated yielding significant improvements in the description of the Fcc phase boundary. The artificial miscibility gap in the liquid phase at high temperatures was successfully removed.
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The experimental phase equilibria in the Cu–Ni–Sn ternary system were critically reviewed, and the existent discrepancies for the phase
Data availability statement
The raw data required to reproduce these findings are available to download from the electronic material attached to this paper.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The financial supports are from Hebei Provincial Science and Technology Program of China (Grant No. E2019202234), National Joint Engineering Research Center for Abrasion Control and Molding of Metal Materials (Grant No. HKDNM2019019), and Key Laboratory of Lightweight and High Strength Structural Materials of Jiangxi Province (Grant No. 20171BCD4003). Y. Tang acknowledges the financial support from Yuanguang fellowship released by Hebei University of Technology.
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