Elsevier

Thermochimica Acta

Volume 673, March 2019, Pages 185-191
Thermochimica Acta

Structural and physical studies of the Ag-rich alloys from Ag-Li system

https://doi.org/10.1016/j.tca.2019.01.016Get rights and content

Highlights

  • High temperature X-Ray Diffraction analysis of Ag-Li alloys.

  • Dilatometric measurements of Ag-rich alloys from Ag-Li system.

  • Electric conductivity and reversibility of Ag-rich alloys from Ag-Li system.

  • The structural and physical properties of Ag-rich alloys from Ag-Li system.

Abstract

The structural and physical properties of the Ag-rich alloys from Ag-Li system (Ag90Li10, Ag80Li20, Ag70Li30, Ag60Li40, Ag57Li43, Ag55Li45, Ag52Li48, and Ag50Li50) were performed with the use of the three different methods. All tested alloys were prepared from high purity metals (Ag and Li) by melting in a glove-box filled with high purity argon, with a very low concentration of impurities. The high-temperature X-ray diffraction investigations were conducted to confirm the structure of the prepared alloys. The thermal expansion measurements of the Ag-rich alloys were carried out by an optical, horizontal dilatometer at the temperature range from 298 to 665 K. Moreover, the electrical conductivity of investigated Ag-rich alloys was measured at room temperature. The results of X-ray diffraction phase analysis, thermal expansion and electrical conductivity measurements, obtained in these studies, may undermine the reliability of the present phase diagram of Ag-Li, which makes it necessary to verify the existing phase equilibria. Further thermodynamic and physicochemical studies of Ag-Li system are crucial to get complete data for the optimization of the thermodynamic properties and calculation of the reliable phase diagram of the Ag-Li system.

Introduction

Development of ecological technologies, energy sources and soldering materials plays an important role in present scientific research. Currently, Ag-Li alloys with a high concentration of Li are not applied, due to a high reactivity of Li with the oxygen, nitrogen, and moisture contained in the air. On the other hand, there is a large group of silver brazing alloys which contains lithium [1]. Moreover, a small addition of silver, lithium or both is used for modification of the mechanical properties at elevated temperatures, corrosion resistance and castability of magnesium alloys for the automotive, aviation, electronics and power industries, as well as for the production of medical equipment [[2], [3], [4], [5], [6], [7], [8], [9]]. One can also notice that the Ag-Li alloys are promising for energy storage as a negative electrode material in the new generation of Li-ion batteries [10,11]. Thus, the full knowledge of thermodynamics, physical and chemical properties, and also the phase diagram of the Ag-Li system is crucial.

First measurements of the Ag-Li system were made by Pastorello [12,13], in 1931. Based on the thermal and X-ray diffraction studies [12,13] the first phase diagram with two intermetallic phases (AgLi and AgLi3) was presented. Then, in 1953-4, Freeth and Reynor [14] published a different phase diagram, based on the microstructure, X-ray diffraction and thermal analysis results. They determined the maximum solubility of Ag in bcc-(Li) and Li in fcc-(Ag), which equalled 8 at. % in the eutectic reaction at c.a. 419 K and 39.2 at. % at 590 K, respectively. What is more, they also proposed to divide the existing in the Ag-Li system area of the phase stability of the γ-brass type into three separate phases γ1, γ2 and γ3. The range of homogeneity (at room temperature) for the γ3 phase was established between 63 and 73 at. % of Li, and for γ1 phase between 87 and 93 at. % of Li. Then, based on the available literature data, Pelton [15] proposed new version of the phase diagram of Ag-Li system, which contains seven phases: fcc-(Ag), bcc-(Li), Liquid, β(bcc_B2), γ1(AgLi2), γ2(AgLi3) and γ3(AgLi6). For calculations, Pelton [15] used not only the values presented in the Freeth and Reynor’s work [14] but also the values of the enthalpy of formation of solid fcc-(Ag) phases and liquid, gained at the temperature 623 and 1250 K by Predel et al. [16]. This phase diagram was generally accepted until the end of the last century [17]. Moreover, the authors [16] proposed the position of the liquidus and solidus curves between the liquid and fcc-(Ag) phase using the regular solution model. The other important thermodynamic properties such as the activity of Li in the liquid and solid solutions and enthalpy of mixing of Ag-Li liquid alloys were presented in [[18], [19], [20]].

The structure of the solid fcc-(Ag) was studied by Perlitz [21] and the lattice parameters for this phase were determined by Firth et al. [22] and Kelington et al. [23]. The short-range order parameters in the solid Ag-Li solutions were presented by Ruppersberg [24], and Migge and Andersen [25]. First attempts to define the structure of the γ3 phase (Ag30.2Li69.8) were done by Arnberg and Westman [26,27] with the use of X-ray diffraction analysis. They found that the γ3 phase had 26 atoms in the bcc lattice (I4¯3m). Noritake et al. [28] determined the γ-brass structure of Li64Ag36 alloy, by means of synchrotron powder diffraction and the Rietveld refinement. This structure coincides relatively with the phase stability of the γ3 phase. Pavlyuk et al. [29] critically analyzed the available data for Ag-Li system and proposed a new development of the phase diagram for this system, which was similar to this presented by Okamoto [17]. In the work [29] Ag-Li phase diagram contained four binary compounds β-AgLi and: γ1 (AgLi12), γ2 (Ag3Li10) and γ3 (Ag4Li9), which were closely related to each other and with the structure of the γ-brass. The β-AgLi phase crystallizes in the regular structure of the CsCl type (Pm 3¯ m, a = 3.169 Å) [30,31], which transforms into the tetragonal structure of the UPb type (I41/amd) at ambient conditions [29]. This phase transformation from regular to the tetragonal structure was found by [29] with the use of X-ray diffraction and by means of SHELXS and SHELXL programs [32], and the Rietveld refinement. The lattice parameters of the tetragonal structure of AgLi phase were: a = 3.9605(1), c = 8.2825(2) Å, and factors: R = 4.81, Rf = 4.87 [29]. The first suggestion about the instability of AgLi phase was proposed by Pauly [33], who has studied the X-ray diffraction patterns of pseudo-binary sections for AgLi-InLi, AgLi-AuLi and AgLi-LiTl systems. The author [33] noticed that the X-ray diffraction pattern of the ternary alloys close to the composition of the binary end-phase AgLi never reproduced the characteristic structure of the CsCl type. However, Pauly [33] suggested the existence of another undefined structure. This information inspired scientists to study the structure of an unknown phase β-AgLi. Wang et al. [34] found that the minimum value of the enthalpy of mixing of liquid Ag-Li alloys exists for XLi = 0.50, which presents a weak tendency to formation of short-range order in this solution with the maximum order in liquid phase surroundings. For the description of liquid solutions, the authors [34] used the modified quasi-chemical MQMPA model proposed by Pelton and Chartrand [35]. They also compared the calculated enthalpy of formation of solid fcc at 623 K with the Predel et al. [16] data, and the calculated activity of lithium with this measured by Becker et al. [18] at 830 K.

The aim of our work was the study of the physical properties of Ag-rich alloys of Ag-Li system and verification of existing phase areas occurring in this binary system up to XLi = 0.50 concentration range.

Section snippets

Materials and methods

The samples of Ag-Li alloys for the high-temperature X-ray diffraction measurements were prepared similarly to the one described in our previous works [36]. The specifications of pure, metallic elements used in all experiments are listed in Table 1. The alloys were prepared in a glove box filled with a high purity argon atmosphere (Table 1), with a trace concentration of impurities (O2 < 0.1 ppm, N2 < 0.1 ppm, H2O < 0.1 ppm). Weighed amounts of pure metals were melted in heat-resistant steel

Results and discussion

The X-ray diffraction patterns for the prepared samples are shown in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10. As can be seen in Fig. 2, Fig. 3, Fig. 4, Fig. 5 the X-ray diffraction patterns for alloys, containing from 10 to 40 at. % of Li presented only the solid solution of silver up to 493 K. These results confirmed that, at the measured range of temperature, the solid solution of silver (Ag) occurs up to c.a. 40 at. % of Li. These results are in good agreement

Conclusions

In this work, structural and physical properties of the selected Ag-rich alloys from the Ag-Li system were investigated with the use of the X-ray diffraction, dilatometric and electric methods. The X-ray diffraction patterns for alloys which contain from 10 to 40 at. % of Li presented only the solid solution of silver up to 693 K. In the case of the X-ray diffraction pattern of the alloys, which have from 45 to 50 at. % of Li, the AgLi intermetallic phase (tetragonal and cubic) were detected.

Acknowledgements

The work was partially supported by the Polish Ministry of Science and Higher Education in a period of the years 2013 – 2015 in the framework of Project No. IP2012 035572. The purchase of the optical dilatometer Misura® 3 FLEX-ODLT, which was used in the investigations, was financed by the European Regional Development Fund within the frames of Project POIG. 02.01.00-12-175/09.

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