Thermodynamic descriptions of the binary Ni–Sn and ternary Cu–Ni–Sn systems over entire composition range: A revisit

Abstract

In this study, the phase equilibria of the Cu–Ni–Sn ternary system reported in the literature were critically reviewed and the discrepancies between the results from different sources were clarified. The thermodynamic parameters of the sub-binary Ni–Sn system were updated using the CALculation of PHAse Diagram (CALPHAD) approach yielding significant improvements in the description of Fcc phase boundary. Besides, the artificial miscibility gap in Ni–Sn liquid phase obtained in the previous assessment was successfully removed. Subsequently, a set of self-consistent thermodynamic parameters for the Cu–Ni–Sn ternary system over the whole composition range was systematically optimized by considering the reviewed experimental information together with the updated thermodynamic description for the Ni–Sn binary system. Comprehensive comparison of the calculated and reported thermodynamic properties as well as isothermal and vertical sections shows that the experimental information is satisfactorily reproduced by the present modeling, further validating the thermodynamic descriptions developed in this work.

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.