Investigation of Selected Thermo-physical Properties of Ternary Sn-Zn-Al Alloys Using DTA

Transcription

1 Neželezné kovy a slitiny Non-ferrous Metals and Alloys ISSN Investigation of Selected Thermo-physical Properties of Ternary Sn-Zn-Al Alloys Using DTA Studium vybraných termo-fyzikálních vlastností ternárních slitin Sn-Zn-Al metodou DTA Ing. Bedřich Smetana, Ph.D., Ing. Simona Zlá, Prof. Ing. Jaromír Drápala, CSc., Vysoká škola báňská Technická univerzita Ostrava, Fakulta metalurgie a materiálového inženýrství, RNDr. Aleš Kroupa, CSc., Ústav fyziky materiálů AVČR Brno, RNDr. Kristina Peřinová, Ing. Rostislav Burkovič, CSc., Vysoká škola báňská Technická univerzita Ostrava, Fakulta metalurgie a materiálového inženýrství The paper deals with the investigation of temperatures and latent heats of phase transformations of two binary alloys Sn-Zn and five ternary Sn-Zn-Al alloys using the DTA method. The investigated alloys belong to the area of the so called eutectic valley of the Sn-Zn-Al system. DTA of investigated samples was performed at the rate of heating/cooling of 4 C/min. In this way there were obtained temperatures of phase transformations of the ternary system Sn-Zn-Al, temperature of ternary eutectics T E C, temperature of liquidus, and other transformations. Latent heats of melting and solidification were calculated. Latent heat of melting of alloys varies within the interval J.g -1, solidification within the range J g -1. Theoretical isoplethic phase diagrams were created with use of the software MTData and Pandat. Experimental data were compared with the calculated temperatures. It follows from bibliographic searches that description of behaviour of ternary system Sn-Zn-Al is still not quite known. The Sn-Zn-Al alloys could become a suitable alternative substitute of lead based solders for temperatures up to 350 C. Příspěvek se zabývá studiem teplot a latentních tepel fázových přeměn dvou binárních slitin Sn-Zn a pěti ternárních slitin Sn-Zn-Al s využitím metody DTA (diferenční termická analýza). Analyzované slitiny se vyznačují rozdílným obsahem jednotlivých prvků. Obsah hliníku ve vzorcích je v rozmezí 1-3 hm.%. Studované slitiny spadají do oblasti tzv. eutektického sedla systému Sn-Zn-Al. DTA studovaných vzorků byla provedena při rychlosti ohřevu/ochlazování 4 C/min v rozmezí teplot 20 C až do úplného roztavení vzorků. Pro experimentální měření byl využit systém pro simultánní termickou analýzu Setaram SETSYS 18 TM TG/DTA/TMA. Byly zjištěny teploty fázových přeměn ternárního systému Sn-Zn-Al, teplota ternárního eutektika T E1, C, teploty likvidu a dalších přeměn. Binární systém Sn-Zn představuje referenční systém. Na základě entalpické kalibrace byla vypočítána také latentní tepla tání a tuhnutí. Latentní teplo tání studovaných slitin se pohybuje v rozmezí hodnot J g -1, tuhnutí v rozmezí J g -1. Byly sestrojeny teoretické izopletické fázové diagramy s využitím výpočtového software MTData a Pandat. Experimentálně získané teploty byly porovnány s termodynamicky vypočítanými teplotami. Z literární rešerše plyne, že stále není zcela znám popis chování ternárního systému Sn-Zn-Al, zejména v oblasti eutektického sedla. Systém Sn-Zn-Al představuje možné řešení v oblasti aplikace bezolovnatých pájek především v elektrotechnickém a automobilovém průmyslu. Slitiny na bázi Sn-Zn-Al by mohly být vhodnou alternativní náhradou olovnatých pájek pro teploty do 350 C. 1. Introduction Al-Sn-Zn system is one of the possible candidates for lead-free solders for high-temperature applications in electrical engineering and in automotive industry. It is so particularly due to its substantially lower toxicity in comparison with the systems containing lead, with increased anti-corrosion effects of Al and also other properties of these systems, such as surface tension and wettability. Namely for these reasons the systems Al-Sn-Zn appear to be a suitable substitute for lead free solders. However, lack of basic experimental data about these ternary systems still persists, or these data differ [1-5]. This is caused mainly by considerable complexity of this system. In this work the investigation was focused especially on the area of the system Al-Sn-Zn, which deserves still more attention [1-5]. The investigation was focused on the area of the so called eutectic valley of the ternary Al-Sn-Zn system in the area of temperatures and latent heats of phase transformations. Two binary and five ternary Sn-Zn-Al alloys were investigated. Experimental measurements were made with use of DTA method and Setaram Setsys 18 TM system. Results of experimental analyses were compared with thermo-dynamic modelling with use of the software MTData and Pandat. Results of experimental measurements and thermo-dynamic modelling fill in and specify more precisely the existing database containing data of the Al-Sn-Zn system, and they can contribute to the increased precision of thermodynamic calculations, as well as to finding the suitable candidates for lead-free solders for high-temperature applications. 2. Binary Sn-Zn, Al-Sn, Al-Zn systems and ternary Sn-Zn-Al system 32

2 ISSN For characterisation of more complex systems it is necessary to know the behaviour of simpler systems. In case of the system Al-Sn-Zn knowledge of the binary Zn-Sn, Al-Sn and Al-Zn systems [6] is necessary. The binary Zn-Sn system is a simple system of eutectic type [6]. Eutectic reaction runs at the temperature of C. The binary Al-Sn system is also a simple eutectic system [6]. Eutectic reaction runs at C. Equilibrium phase diagram Al-Zn [6] comprises eutectic reaction running at the temperature of 381 C and eutectoid reaction running at the temperature of 277 C. 4. Results and discussion Neželezné kovy a slitiny Non-ferrous Metals and Alloys Thermal analyses resulted in the DTA-curves. The DTA-curves of the binary Sn-Zn alloys are shown in Figure 1. Tab. 1 Ternary eutectic reaction scheme Tab. 1 Reakční schéma ternární eutektické reakce Reaction E1 L (Al) + (Zn) + Sn T Comp. [at.%] Pha. Al Zn Sn A. 195 L [2] 196 L [7] 198 L [4] Ternary Al-Sn-Zn system was investigated and the obtained results were published e.g. in these works [1-5]. The Al-Sn-Zn phase diagram was calculated with use of experimental data and theoretical thermodynamic description [2]. Eutectic reaction of the Al-Sn-Zn system is an important reaction. Table 1 gives the frequently published temperatures of the ternary eutectic point. Temperature of the ternary eutectic is slightly lower than in the binary Sn-Zn system. In the ternary system one more horizontal line is distinct, which corresponds to the temperature of 278 C. It is a peritectic reaction of the second type [1, 3]. 3. Experiment, DTA method For preparation of alloys with the precisely defined composition the following metals were used of the following purity: Sn 4N5, Zn 4N, Al 3N5. The chosen binary and ternary alloys were prepared by melting in electric resistance furnace in a graphite crucible. Chemical composition of the prepared alloys is given in the tables 2 and 3. For DTA analyses cubes with approximate dimensions 2x2x2mm were made from the prepared alloys. Mass of the analysed samples was in the range from 100 to 150 mg. Differential thermal analysis of the investigated alloys was made in highly pure inert dynamic atmosphere of Ar (purity > %, gas flow rate was 2 l h -1 ). Analyses were made in corundum crucibles. The instrument was calibrated prior to measurement to the temperatures and latent heats of the standard metals with purity 5N (In, Sn, Bi, Pb, Zn). Two cycles of heating and cooling at the rate of 4 C min -1 were realised. Temperature range of the performed analyses was C. The first cycle was made from the temperature of approx. 20 C up to the temperature of 450 C, it was then followed by cooling to approx. 20 C and repeated heating from 20 C to approx. 450 C and cooling to the room temperature. Temperatures and latent heats from the second cycle of heating and cooling were evaluated. Negligible differences between the results of the first and the second run were observed. Fig. 1 DTA-curves of binary Sn-Zn alloys Obr. 1 DTA křivky binárních slitin Sn-Zn Resulting temperatures of phase transformations are given in Table 2, and comparison of experimental works with the respected binary Sn-Zn system [6] is given in Figure 2. Figure 3 shows curves obtained for the ternary Al Sn Zn alloys. Characteristic temperatures of phase transformations are given in Table 3. Table 5 contains the values of latent heats of melting and solidification (corresponding to the areas of peaks on the DTAcurves). Figure 4 gives a comparison of experimentally obtained temperatures with theoretically calculated isopleth of the Sn-Zn-Al phase diagram for the following contents of Al: 1, 1.5, 2 and 3 wt.% Al. The isopleths were calculated using the thermodynamic database developed within the scope of the COST MP0602 Action [8] and the data published in [2]. 4.1 Binary alloys Sn-Zn In the binary Sn-Zn alloys two thermal effects-peaks were observed, see Figure 1, which demonstrate the running phase transformations. Tab. 2 Temperatures of phase transformations of binary alloys Tab. 2 Teploty fázových přeměn binárních slitin sample B1 B2 binary alloys - temperatures DTA - heat DTA - cooling composition T E T L T L T E [wt.%] [at.%] 83.3Sn16.7Zn 73.3Sn26.7Zn 91.1Sn8.9Zn 84.9Sn15.1Zn

3 Neželezné kovy a slitiny Non-ferrous Metals and Alloys ISSN One peak is characteristic by its steep inclination both at heating and at cooling. Shape of such peak corresponds to an invariant reaction. In the binary Sn Zn system this peak corresponds to an eutectic reaction. The obtained eutectic temperature T E of the alloy B1 (table 2) is by 0.4 C higher, and in the alloy B2 it is by 0.5 C lower than in the generally used binary Sn Zn diagram [6] Sn Zn diagram, see Table 2, Figure 2. The second peak corresponds to the subsequent melting of the solid phase (heating), which is accomplished by complete melting of the binary alloy. Temperature of liquidus T L is in the alloy B C and in B2 206 C. observed at C (Figure 4A) corresponding to the transformation L+(Al)``+(Sn) L+(Al)``. At cooling the second peak represents the start of alloy solidification, which is terminated only by eutectic reaction. Temperatures of liquidus T L were obtained - see Table 2, Figure 2. More distinct shift of temperatures T L (heating) toward higher values is caused by a delay of heat transfer in the sample (limited by heat conductivity) and therefore by later melting of the sample (by detection of thermal effect at higher temperature). At cooling the samples were undercooled. The resulting temperatures are lower than the values of the temperatures obtained at heating and also than the values shown in the diagram in Figure Ternary alloys Sn-Zn-Al Up to four thermal effects (alloy T1, cooling) were observed in the ternary alloys, corresponding to the phase transformations, see Figure 3, and to the temperatures, see Table 3. Temperatures of the phase transformations are at cooling more distinctly shifted toward the lower values, see Figure 3 and Table 3. Temperature values of all phase transformations of the ternary alloys obtained at cooling therefore cannot be taken as authoritative. DTA curves obtained at cooling can be in spite of that very helpful at analysis of the curves obtained at heating. At cooling we can very often discern the peaks demonstrating the running phase transformations in the samples, particularly small thermal effects and transformation effects, which run in close proximity. Thanks to this it is possible to analyse even minimum changes of the peak shapes (at heating) and attribute to them the running phase transformation. Temperature of ternary eutectics T E1 is C (average value from temperatures T E1 in all the samples). Temperature approaches the best the value given in [4], see Table 1. At the same time this temperature is approx. by 1 C lower than the temperature of the binary eutectics. Decrease of eutectics temperature is caused by the presence of Al. Moreover in the alloys T3 a T4 a thermal effect was observed directly following the thermal effect of ternary eutectic reaction at the temperature corresponding to the temperature T P1, see Table 3. This effect corresponds to the transformation: L+(Al)``+(Zn) L+(Al)`` in the alloys T3 and T4, see Figure 4B, D. In the alloy T5 the second peak was Fig. 2 Comparison of experimental temperatures of phase transformations of two binary alloys with the generally used binary Sn-Zn system [6] Obr. 2 Srovnání experimentálních teplot fázových přeměn dvou binárních slitin s fázovým diagramem Sn-Zn [6] In the alloy T1 no change following directly after eutectic reaction was observed. The next phase transformation was observed at the temperature of 258 C. In this area the transformation L+(Al)``+(Zn) L+(Al)`` occurs. Second thermal effect was observed in the alloy T2, which was ended at the temperature T P1, C. The peak corresponds to the transformation Zn+(Al)``+L L+(Al)``. At cooling one more heat effect (in the alloy T1) at C was observed. It is not sure, to which transformation it should be attributed. In the alloy T1 the temperature T L was C; T2 (360 C) and T4 (379 C). The temperature of liquidus of the alloys T3 and T5 is only few degrees above the temperature of 260 C. Temperatures of liquidus correspond to the calculated liquidus or are very slightly higher by few degrees (see Figure 4). Resulting liquidus temperatures could be to a certain extent influenced by superheating [9] and by homogeneity of the analysed samples (chemical and structural). Presence of Al 2 O 3 micro-particles (identified in as-cast samples) could have had an appropriate influence. 34

4 ISSN Neželezné kovy a slitiny Non-ferrous Metals and Alloys Fig. 3 DTA-curves of ternary alloys Obr. 3 DTA křivky ternárních slitin Fig. 4 Comparison of experimental temperatures of phase transformations of the ternary alloys with thermodynamically calculated diagrams according to MTData and Pandat Software; Notes to legend: (Sn), (Zn), (Al), solid solutions of Sn, Zn and Al; (Al)`, (Al)``, solid solution of Al with high and low Al-content; L, melt Obr. 4 Srovnání experimentálně získaných teplot fázových přeměn ternárních slitin s termodynamicky vypočítanými diagramy dle software MTData a Pandat; pozn. k legendě: (Sn), (Zn), (Al), tuhé roztoky Sn, Zn a Al; (Al)`, (Al)``, tuhý roztok Al s vysokým a nízkým obsahem Al; L, tavenina 35

5 Neželezné kovy a slitiny Non-ferrous Metals and Alloys ISSN Tab. 3 Temperatures of phase transformations of ternary alloys Tab. 3 Teploty fázových přeměn ternárních slitin sample T1 T2 T3 T4 T5 composition [wt. %] [at.%] 82.2Sn16.0Zn1.8Al 69.0Sn24.4Zn6.6Al 83.3Sn13.5Zn3.2Al 68.3Sn20.1Zn11.6Al 88.8Sn10.2Zn1.0Al 79.5Sn16.6Zn3.9Al 90.8Sn6.9Zn2.3Al 80.2Sn11.2Zn8.6Al 94.5Sn4.2Zn1.3Al 87.4Sn6.9Zn5.7Al ternary alloys - temperatures DTA - heating DTA - cooling T E1 T P2 T P1 T L T L T P1 T P3 T P2 T E Latent heats of phase transformations Thermal effects of phase transformations (sum of thermal effects corresponding to the areas of peaks) are given in the table 4. Overall heat of melting in the binary alloy B1 is 83 J g -1 ; heat of solidification is 85 J g -1. In the binary alloy B2 the thermal effect corresponds almost completely to the eutectic melting/solidification. Latent heat of melting is 75 J g -1, that of solidification is 76 J g -1. The latent heat of melting/solidification in the ternary alloys is given also by the sum of all thermal effects (see Figures 1 and 3) detected at heating/cooling. The value of latent heat of melting/solidification in the binary alloys decreases with the increasing content of Sn. In the ternary alloys, the Al content varies and therefore it is impossible to describe the dependence of magnitude of latent heat on the Al content (it is necessary to consider the influence of both Sn and Al content). However, it is possible to state that thermal effect of melting/solidification decreases with the increasing content of Sn (decreasing content of Zn). The values of latent heats of solidification are higher than the values of latent heats of melting. This is particularly due to larger amount of accumulated heat in the samples, which is caused by undercooling of the investigated alloys. In the binary alloys the difference between heats of melting and solidification is small, on the other hand in the ternary alloys the difference between the heats of melting and solidification achieves even 14 J g -1 (in the alloy T4). This is caused by higher value of undercooling of the alloys containing Al. Tab. 4 Latent heats of melting and solidification of Sn-Zn-(Al) alloys Tab. 4 Latentní tepla tání a tuhnutí slitin Sn-Zn-(Al) Binary and ternary alloys - latent heats sample 5. Conclusions DTA - heating H M DTA - cooling H S [J g -1 ] [J g -1 ] B B T T T T T On the basis of the realised experiments and results obtained with use of DTA measurements, realised using the Setaram SETSYS 18 TM thermal analysis system, it is possible to draw the following conclusions: - temperature of eutectic reaction T E of the investigated binary alloys Sn-Zn was found to be C and C respectively, and it is in excellent agreement with the widely accepted temperature of eutectic reaction (198.5 C) [6], - temperatures of liquidus T L of binary alloys, obtained from the heating curves, were slightly higher than in the published Sn-Zn binary system [6]. This may be caused by delay of heat transfer in the sample, because of limited thermal conductivity of the sample, - the temperature of ternary eutectic reaction in the Sn-Zn-Al system was found to be C, approx. by 1 C lower than temperature of the binary eutectics of the system Sn-Zn (198.5 C). The decrease of 36

6 ISSN Neželezné kovy a slitiny Non-ferrous Metals and Alloys temperature is caused by addition of Al- other phase transformation reactions were observed in samples above the eutectic reaction: L+(Al)``+(Zn) L+(Al)`` L+(Al)``+(Sn) L+(Al)`` in the alloys T1 - T4 in the alloy T5 - in the ternary alloy T1 (at cooling) we have also observed one thermal effect (234 C) which it was impossible to attribute certain phase transformation to. - liquidus temperatures correspond to the calculated liquidus or are very slightly higher (by few degrees) - the following latent heats of melting were calculated: 83 J g -1 in the binary alloy B1 75 J g -1 in the binary alloy B J g -1 in the ternary alloys T1-T5 - an undercooling of the investigated alloys occurred during cooling, ternary alloys showed higher degree of undercooling in comparison with the binary alloys Sn-Zn, its value depends also on the Al content - the cooling curves allowed us to differentiate some overlapping thermal effects Generally, very good agreement was achieved between our own experimental results, the data published by other authors and the data calculated on the basis of thermo-dynamic models [2]. In the ternary alloy T1 we have also observed thermal effect which it was impossible to attribute certain phase transformation to. In the future, alloys with the higher chemical and structural homogeneity will be prepared. Alloys preparation will be realised in vacuum. Knowledge of alloy properties together with reliable thermo-physical data is necessary for possible application of ternary system Sn-Zn-Al in practice. The authors are grateful to A. Watson, University Leeds, UK for performing thermodynamic calculations. This work was carried out within the scope of the project COST MP0602 Advanced Solder Materials for High-Temperature Application, No. OC and OC and project of the Ministry of Education, Youth and Sports of the Czech Republic Processes of preparation and properties of highly pure and structurally defined materials, No. MSM Literature [1] Sebaoun, A., Vincent, D., Tréheaux, D. Al Zn Sn phase diagram-isothermal diffusion in ternary systems. Materials Science and Technology. April 1987, Vol. 3, pp [2] Fries, S.G., Lukas, H.L., Kuang, S., Effenberg, G. Calculation of the Al-Zn-Sn ternary system. The Institute of Metals, London, 1991, pp [3] Prowans, S., Bohatyrewicz, M. Układ aluminium cyna cynk. Arch. Hutn. 1968, Vol. 13, No. 2, pp [4] Nayak, A.K. Trans. Indian Inst. Met. 1975, Vol. 28, No. 2, pp [5] Protopopescu, H.M. at. Al. Aluminium-tin-zinc. Ternary alloys, VCH, Vol. 8, 1993, pp [6] Massalski, T.B., Okamoto, H. Binary alloy phase diagrams. American Society for Metals. Ohio, 1996, Second edition plus updates on CD-ROM. [7] Losana, L., Carozzi, E. The ternary alloys of aluminium-zinc and tin (in Italian). Gazetta chimica italiana. 1923, No. 53, pp [8] Dinsdale A.T., Watson A., Kroupa A., Vrestal J., Zemanova A., Broz P., COST MP 0602 thermodynamic database for hightemperature lead free solders, ver. 1.0, 2009 [9] Kwang-Lung, L., Li-Shiang, W., Tzy-Pin, L., The microstructures of the Sn-Zn-Al solder alloys. Journal of Electronic Materials. 1998, Vol. 27, No. 3, pp Recenze: RNDr. Kornel Csach, CSc. Ing. Jozef Miškuf, CSc. Místo konání : Hannover, Německo 37