PHASE RELATIONS NEAR TERNARY EUTECTIC POINT IN THE AgIn-Sb SYSTEM

The results of the phase relations near ternary eutectic point in the Ag-In-Sb system are investigated in this paper. Phase equilibrium calculation was done using Thermocalc software and experimental DTA results for the chosen alloys in the isopleths with molar ration of In:Sb = 7:3; 9:1 and 1:1. The structural characteristics of these alloys have been investigated using light optic microscopy and scanning electron microscopy, while hardness measurements have also been done. Solidification path for three ternary alloys located on three different investigated isopleths was calculated using Pandat software.


Introduction
A need for a substitution of conventional lead-bearing solders stimulated widespread search for new lead-free materials, which can be used in automotive, avionics and electronic industry.As a response to NCSM Lead-Free Soldering Project in the United States [1] as well as JEIDA and JWES projects carried out in Japan [2], European Union created COST 531 Action, which main aim was to choose possible candidates for lead-free solders among several groups of alloys.It was soon established that the group of silver-based alloys with the additions of low-melting metals like Sn, Bi or In can be a good substitute for idustrial application.However, to adjust solidification temperature as well as the structure and mechanical properties of the joint, a wide range of other metals can be introduced into the alloy as a third component.Investigations of antimony addition to Ag-In system has been recently carried out by Buchtova et al. [3] and @ivkovi} et al. [4,5] Isothermal section of this system at 200 o C was experimentally determined, as well as the isophlet for the ratio of In : Sb = 1.In order to find out how the antimony addition will influence the temperature of the eutectic point in the binary Ag-In system and how phase relations after this addition may look like, the particular part of the ternary system with low Sb content was investigated in this work.Next, using previously optimized parameters [6] these results were compared with the phase relations derived from computer calculations.

Experimental procedure
The samples, of chosen alloys along the investigated sections with molar ratio of In:Sb equal to 7:3, 9:1 (the compositions and masses are presented in Table 1) and 1:1 (sample with 10, 20 and 40 wt%Ag), were prepared from Ag, In and Sb of 99.99% purity.The samples were prepared in an induction furnace under inert gas atmosphere, where an extra amount of antimony (about 2-3%) was added to compensate the weight loss due to the volatilization of antimony.
Differential thermal analysis (DTA) was performed using Derivatograph (MOM, Budapest).The experiments were carried out with the heating rate 10 o /min, using sintered Al 2 O 3 as the reference specimen.
Standard apparatus for Vickers hardness measurements was used in this kind of experiments.

Phase equilibrium determination
Differential thermal analysis (DTA) was performed on samples along two isophlets determined by In:Sb ratio 9:1 and 7:3, respectively, and the obtained results are gathered in Table 2.The evolution of phase equilibria with antimony addition, starting from binary Ag-In system (Fig. 1a), is shown in subsequent Figures 1a through 1d.In Figure 1d the isophlet for the ratio In:Sb =1:1, determined previously [3,4], is also shown for comparison.The results demonstrate the change of the position of the eutectic point with the increase of antimony content.
The Ag-In-Sb system consists of the following phases [7] Using the thermodynamic description of the ternary system, which was worked out in our earlier paper [6], the respective isopleths were calculated using ThermoCalc software and were compared with the experimental DTA results, as shown in Figures 1a -1d.
It was found that the calculated ternary eutectic point occurs at temperature of 414,9K at the composition equal to X Ag = 0,014 and X Sb =0,017 mole fractions.So, isothermal sections just above and below ternary eutectic temperature were calculated using Pandat [8] software and are shown in Fig. 2a and 2b.
The existence of calculated ternary eutectic point has been checked experimentally, using DTA and metallography, as shown in Fig. 3.The eutectic structure was proven and the eutectic temperature of 419.5K (or 146.5 o C) was obtained, which is in accordance with the value predicted by calculation.Also, this is in agreement with literature data presented in [9], in which the ternary eutectic in the Ag-In-Sb system is given at the temperature of 417K (or 144 o C).

Structural characteristics of the alloys
In order to find out how the solidification process of the alloy may depend on antimony addition, some of the alloys in the investigated sections were investigated using 163  light optic microscopy (Fig. 4), scanning electron microscopy (Fig. 5) and X-ray diffraction analysis (Table 3).
Reasonable agreement between the experimental data (LOM, SEM and XRD) and the results of calculations in the field near ternary eutectic point in the In-rich part of the investigated system was found.

Calculation of the solidification path
Solidification path for three ternary alloys located on three different isophlets, and investigated by metallographic and SEM methods was calculated using Pandat software [8].Scheil model of the alloy solidification was used during these calculations.The obtained results for three different solidification paths are shown in Fig. 6.Depending on In/Sb ratio different three-phase equilibrium is obtained at the end of the solidification process, which is in accordance to obtained experimental data.

Hardness measurements
Hardness tests of alloys chosen from three different investigated isopleths were  performed with standard apparatus used for Vickers hardness measurements.The results, gathered in Table 4, are shown in Fig. 7 for the section with In:Sb molar ratio equal to 1:1.Most probably, the increasing amount of HCP_A3 phase influence the increase of the hardness of the alloys.Also, the presence of the intermetallic phases may either decrease (AgIn 2 ) or increase (Ag 2 In) the hardness of the alloy.

Conclusions
This paper deals with the investigations of the phase relations near ternary eutectic point in the Ag-In-Sb system, which can be a potential lead-free solder.In the frame of experimental determination, DTA, LOM, SEM and hardness measurement were performed.Based on these results and the results of calculations, phase equilibrium for the isopleths with molar ration of In:Sb = 7:3; Table 3.The results of X-ray diffraction analysis for the samples with 10wt%Ag (section In:Sb = 1:1), 50wt%Ag (section In:Sb = 7:3) and 40wt%aAg (section In:Sb = 9:1) : FCC_A1 -solid solution based on silver, HCP_A3 -which continuously exists from the Ag-In side to the Ag-Sb side in the ternary Ag-In-Sb system, ZINCBLENDE_B3 -phase based on In and Sb as the main constituents in almost equiatomic composition, TETRAGONAL_A6 -phase based on indium, RHOMBOEDRAL_A7 -phase based on antimony, AGSB_ORTHO -phase based on silver and antimony, intermediate stoichiometric compounds Ag 2 In and AgIn 2 , and the liquid phase.

Fig. 2 .Fig. 3 .
Fig.2.a) Isothermal section of the ternary system at T =420 K; b) Isothermal section of the ternary system at T =414 K

Fig. 7 .
Fig.7.Dependence of hardness on composition for the alloys in section with molar ratio In:Sb=1:1

Table 1 .
The composition and masses of chosen alloys in the investigated sections with molar ratio of In:Sb equal to 7:3 (a) and 9:1 (b)

Table 2 .
DTA results obtained for investigated sections in the Ag-In-Sb system

Table 4 .
The results of hardness measurements