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Effect of Zn addition, strain rate and deformation temperature on the tensile properties of Sn–3.3 wt.% Ag solder alloy
Materials Characterization 58 (2007) 323 – 331
Effect of Zn addition, strain rate and deformation temperature on the tensile properties of Sn–3.3 wt.% Ag solder alloy A. Fawzy Physics Department, Faculty of Education, Ain Shams University, Cairo, Egypt Received 11 April 2006; received in revised form 16 May 2006; accepted 20 May 2006
Abstract Stress–strain characteristics of the binary Sn–3.3 wt.% Ag and the tertiary Sn–3.3 wt.% Ag–1 wt.% Zn solder alloys were investigated at various strain rates (SR, ε·) from 2.6 × 10− 4 to 1.0 × 10− 2 s− 1 and deformation temperatures from 300 to 373 K. Addition of 1 wt.% Zn to the binary alloy increased the yield stress σy and the ultimate tensile stress σUTS while a decrease of ductility (total elongation εT) was observed. Increasing the strain rate (ε·) increased both σy and σUTS according to the power law σ = C ε·m. A normal decrease of εT with strain rate was observed according to an empirical equation of the form εT = A exp (− λε·); A and λ are constants. Increasing the deformation temperature decreased both σy and σUTS in both alloys, and decreased the total elongation εT in the Zn-free binary alloy, whereas εT was increased in the Zn-containing alloy. The activation energy was determined as 41 and 20 kJ mol− 1 for these alloys, respectively. The results obtained were interpreted in terms of the variation of the internal microstructure in both alloys. The internal microstructural variations in the present study were evaluated by optical microscopy, electron microscopy and X-ray diffraction. The results show the importance of Zn addition in enhancing the mechanical strength of the Sn–3.3 wt.% Ag base alloy. © 2006 Elsevier Inc. All rights reserved. Keywords: Strain rate; Temperature; Strength; Ductility; Eutectic; Solder alloy
1. Introduction Because of their unique combination of material properties and low cost, Sn–Pb solder alloys are important materials for assembling electronic components on printed circuit boards [1]. However, there are environmental and health hazards associated with the toxicity of lead present in the solder, and the potential for small levels of Pb to damage the nervous system [2,3]. The search for potential replacement alloys has covered a wide area of extensive research in recent
E-mail address: [email protected]. 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.05.013
years [4–9]. New Pb-free solder alloys, which offer superior mechanical properties, have resulted from these efforts. A Sn–3.5 wt.% Ag alloy was found to have superior strength compared to the Sn–Pb solder alloys [7–10]. It appears, moreover, that the mechanical properties of this alloy can be significantly adjusted by controlling the solidification rate, and hence the microstructure [11]. It was found [11] that water quenching (solidification rate 24 K/s) yielded a nonequilibrium microstructure of β-tin dendrites surrounded by a fine eutectic mixture of Ag3Sn particles dispersed in the tin-rich matrix. Air cooling (at 10 K/s) also yielded coarser β-tin dendrites, surrounded by a coarser eutectic mixture of Ag3Sn
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Table 1 Chemical compositions of the alloys: Sn–3.3 wt.% Ag and Sn–3.3 wt. % Ag–1 wt.% Zn Alloy
Sn
Ag
Zn
Pb
Sn–3.3 wt.% Ag Sn–3.3 wt.% Ag–1 wt.% Zn
96.66 95.63
3.28 3.29
0.00 1.02
0.05 0.04
performed to understand the effect of Zn addition on the stress–strain characteristics of a Sn–3.3 wt.% Ag solder alloy at various strain rates ranging from 2.6 × 10− 4 to 1.0 × 10− 2 s− 1, over a temperature range from 300 to 373 K. 2. Experimental procedures
particles dispersed in the tin-rich matrix. Under equilibrium conditions, the phase diagram of Sn–Ag system at the eutectic composition predicts a microstructure consisting of a mixture of Ag3Sn (ε-phase) and tin [12,13]. It was found [9,14] that the presence of Zn in a Sn– Ag solder alloy results in an increase in its mechanical strength. Zn additions to a eutectic Pb–10 wt.% Sn alloy have also been shown to improve the high temperature creep strength [15]. On the other hand, the addition of 1.5 wt.% Zn to a Pb–1.5Sb alloy improves the ductility, but leads to a decrease of the flow stress [16]. It is well known that a potent means for combating these types of issues is through careful selection of alloying additions. Motivated by these apparently contradictory findings between a softening and a hardening effect of Zn addition, the present study was
The Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys were prepared from high purity Sn, Ag and Zn (purity 99.99%). The appropriate weights of the elements for the binary alloy were well mixed with CaCl2 flux to prevent oxidation in a graphite mold and kept at 100 K above the melting temperature of Ag. Casting into stainless steel molds was done in air with a cooling rate of 10 K/s. Zn was added to a portion of the binary alloy to form the second alloy, Sn–3.3 wt.% Ag–1 wt.% Zn, at 100 K above the melting temperature. Chemical analysis of the two solder alloys was found to be close to the desired constitution (Table 1). The ingots, as bars of the two alloys, were cold drawn into 0.6-mm diameter wire. Specimens with a gauge length of 50 mm were prepared for tensile testing. Uniaxial tests were performed using an Instron-type
Fig. 1. Microstructure of: (a) OM for the as-cast Sn–3.3 wt.% Ag, (b) OM for the as-cast Sn–3.3 wt.% Ag–1 wt.% Zn solder alloy, (c) TEM for Sn– 3.3 wt.% Ag solder alloy, (d) OM for Sn–3.3 wt.% Ag after heating at 373 K for 45 min and (e) OM for Sn–3.3 wt.% Ag–1 wt.% Zn solder alloy after heating at 373 K for 45 min.
A. Fawzy / Materials Characterization 58 (2007) 323–331
machine with experimental error less than ± 3%. Prior to the tensile tests, each specimen was held for about 30 min at the test temperature, in a furnace attached to the machine, in order to achieve thermal equilibrium. Tensile tests were performed at 300, 323, 348 and 373 K. During the test, the deformation temperature was controlled within ± 2 K. All specimens were strained to fracture with strain rates ranging from 2.6 × 10− 4 to 1.0 × 10− 2 s− 1. The microstructure of the as-cast solders was investigated using optical microscopy (OM), transmission electron microscopy (TEM) and X-ray diffraction. Prior to OM observations, the samples were polished with a diamond paste, then etched with a dilute nitric acid and alcohol solution. For the TEM observa-
325
tions of the as-cast alloys, thin foil samples of 3-mm diameter were mechanically thinned, and electropolished to perforation using a jet technique in a solution containing 80% acetic acid and 20% perchloric acid. 3. Results and discussion 3.1. Microstructure Fig. 1 shows optical and electron micrographs of the microstructure of the two solder alloys. The microstructure of the binary eutectic Sn–3.3 wt.% Ag solder alloy is shown in Fig. 1a. The light dendrite arms are the
Fig. 2. X-ray diffraction patterns for the as-cast Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn showing the existence of the intermetallic compound Ag3Sn in: (a) Sn–3.3 wt.% Ag and (b) Sn–3.3 wt.% Ag–1 wt.% Zn solder alloys.
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β-Sn phase while the dark interdendritic regions contain the eutectic mixture of the Ag3Sn particles dispersed within the Sn-rich matrix (close to pure Sn). This is in good agreement with the structure revealed by other studies [6–11]. The addition of 1 wt.% Zn was found to suppress the formation of β-Sn dendrites and yields a uniform dispersion of the Ag3Sn intermetallic compound within the Sn-rich mixture producing a networklike microstructure with the β-Sn as shown in Fig. 1b. This was interpreted in terms of the very low solubility of Zn in solid Sn while large amounts of Zn are readily soluble in solid Ag [9]. The existence of the Ag3Sn intermetallic compound was observed in the present work by electron microscopy and X-ray investigation as illustrated by the electron micrograph shown in Fig. 1c and the X-ray diffraction patterns given in Fig. 2. Evolution of the microstructure after 45 min at 373 K is shown in Fig. 1d and e for the alloys without and with the Zn addition, respectively. 3.2. Tensile response Fig. 3 shows typical representative stress–strain curves of the: (a) Sn–3.3 wt.% Ag and (b) Sn–3.3 wt.%
Fig. 4. Stress–strain curves of (a) Sn–3.3 wt.% Ag and (b) Sn–3.3 wt. % Ag–1 wt.% Zn solder alloys obtained at T = 323 K.
Fig. 3. Stress–strain curves of (a) Sn–3.3 wt.% Ag and (b) Sn–3.3 wt.% Ag–1 wt.% Zn solder alloys at SR (ε· = 1.9× 10−3 s−1).
Ag–1 wt.% Zn solder alloys at different strain rates ranging from 2.6 × 10− 4 to 1.0 × 10− 2 s− 1 at a constant deformation temperature, 323 K. Fig. 4 shows the representative stress–strain curves of the same alloys at deformation temperatures of 300, 323, 348 and 373 K at a constant strain rate of 1.9 × 10− 3 s− 1. From these figures, it is noted that the stress–strain parameters, the flow stress σUTS, the yield stress σy and the total elongation εT of both alloys are strongly affected by the variation of the strain rate and deformation temperature. For both alloys at the same deformation temperature, increasing the strain rate (Fig. 3) gives rise to higher values of both σy and σUTS and a decrease in εT. At the same strain rate, raising the deformation temperature (Fig. 4) results in a continuous softening; a decrease in σy and σUTS was observed. From this figure, an abnormal decrease of εT with increasing deformation temperature in the Zn-free alloy (Fig. 4a) was observed while a normal increase of εT with deformation temperature in the Zn-containing alloy (Fig. 4b) was observed.
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As for most metals and alloys, solder alloys experience simultaneous work hardening and dynamic recovery when they are deformed. The work hardening and dynamic recovery have opposite effects on the
327
mechanical properties of the solder alloys, where the former hardens the material while the latter leads to softening. Hence, the stress–strain curves obtained here present the combined effects of both factors. Moreover,
Fig. 5. (a) Variation of the UTS with strain rate tested at 323 K for Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys, (b) the relation between log σUTS and Log ε· at different testing temperatures and (c) variation of the strain rate sensitivity index m with the deformation temperature for both alloys.
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dislocation annihilation seems to occur more rapidly than dislocation generation during deformation. Therefore, at higher temperatures the lower strain rate provided a longer time for the dynamic recovery to occur, resulting in the decrease in the σy and σUTS as observed in Figs. 3 and 4. 3.3. Strain rate and temperature dependence of the ultimate tensile stress (UTS) Fig. 5a shows the relation between the strain rate (SR) and the ultimate tensile stress (UTS) for both the Zn-free and the Zn-containing alloys at different deformation temperatures. From this figure, increasing SR (ε·) increases the UTS in both alloys. This is because increasing strain rate is accompanied by an increase in the dislocation density. As these disloca-
tions move they become tangled. It is then more difficult for other dislocations to glide through the material, especially at the lower deformation temperatures. Note that the Zn-containing solder alloy samples exhibited higher UTS than those exhibited by the Znfree alloy at all deformation temperatures. The higher values of the UTS in the Zn-containing alloy may be attributed to the difference in the microstructures of these alloys (see Fig. 1a, b). The refinement and uniform distribution of the intermetallic Ag3Sn particles in the Zn-containing alloy seem to provide a greater measure of dispersion strengthening due to the finer particle size in the ternary alloy compared to the binary alloy. From Fig. 5b, it can be seen that the logarithm of the σUTS increases linearly with the logarithm of the SR (ε·). Consequently, the relationship between UTS (σUTS) and
Fig. 6. Yield stress (σy) as a function of (a) strain rate at different testing temperatures, (b) deformation temperature at different strain rates for both Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys.
A. Fawzy / Materials Characterization 58 (2007) 323–331
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the SR (ε·) for both alloys can be expressed by the equation [17]:
was found to be consistent with the variation of εT with temperature in both alloys, as was observed in Fig. 4.
rUTS ¼ C e:m ;
3.4. Strain rate and temperature dependence of the yield stress σy
ð1Þ
where C is a constant and m is the strain rate sensitivity index. Values of the index m can be obtained from the slopes of the log σUTS–log ε· relationships shown in Fig. 5b. Fig. 5c shows the variation of m with deformation temperature for both alloys. From this figure, it is clear that m decreases with temperature in the Zn-free alloy while it increases with temperature in the Zn-containing one. It is well known that m values describe the capacity of the alloy for necking resistance [18]. Hence, the resistance to necking was found to increase with temperature in the Zn-containing alloy while it decreases in the Zn-free one. This observation
The yield stress σy as a function of both strain rate and deformation temperature is shown in Fig. 6a and b. From this figure, it is clear that increasing the strain rate (Fig. 6a) and/or decreasing the deformation temperature (Fig. 6b) increases the yield stress σy. The similar effect of temperature and strain rate on the yield stress can be understood by considering the deformation as a stressassisted and thermally activated process. Hence, at high strain rates the yield stress increases since there is limited time for the motion of dislocations. Similarly, at low deformation temperatures the yield stress increases
Fig. 7. Variation of the total elongation εT with the strain rate ε· at different testing temperatures for both Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt. % Zn alloys.
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due to the lower thermally activated dislocation motion. This makes it more difficult for other dislocations to glide through the material. Increasing the deformation temperature will rearrange the dense dislocation networks formed by strain hardening into simple and less tangled networks. This reduces the lattice energy and, therefore, it is not surprising to observe the lower values of yield stress at higher deformation temperatures, since the dislocations have much more freedom to move and overcome obstacles through climb and cross slip. From Fig. 6a and b, it is clear that σy values for the Zn-free alloy are lower than those of the Zn-containing alloy. This is explicable in terms of the microstructure since the microstructure of the Zn-containing alloy is characterized by the existence of the fine Ag3Sn particles in the Sn matrix which act as pinning centers for the mobile dislocations. 3.5. Strain rate and temperature dependence of the total elongation εT The strain rate dependence of the total elongation εT is depicted in Fig. 7a and b for both alloys. It was found to obey an empirical relation of the form: eT ¼ A exp ðke: Þ
grains, as observed in Fig. 1e. This distribution enhances the grain boundary migration and the resulting increase in grain size seems to be effective in increasing the elongation with temperature in the Zn-containing alloy. 3.6. Kinetic analysis For the eutectic Sn–Ag alloy strained at temperatures greater than half of the melting temperature, the strain rate ε· and the flow stress σUTS may be related to the deformation temperature with the help of the kinetic rate equation [7,19] e: ¼ A ðrÞ1=m expðQ=RT Þ
ð3Þ
where A is a constant, R is the gas constant, m is the strain rate sensitivity index and Q is the activation energy. Fig. 8a and b shows the relation between log ε· and 1000 / T for both the Sn–3.3 wt.% Ag and the Sn– 3.3 wt.% Ag–1 wt.% Zn alloys. The strain rate values were obtained from Fig. 7a and b at a total elongation of ≈ 80 × 10− 3. This elongation was selected because it was
ð2Þ
where A and λ are constants depending on the tensile test conditions. From this figure, it is clear that the total elongation in both alloys decreases with increasing strain rate. However, in the Zn-free alloy, the values of εT are lower with increasing temperature at all strain rates (Fig. 7a), while in the Zn-containing alloy the values of εT increase with temperature at all strain rates (Fig. 7b). This difference in the relationship between tensile elongation as a function of strain rate and temperature seems to depend on the difference in the microstructure of the alloys. As previously mentioned, the addition of Zn suppresses the formation of β-Sn dendrites and yields a fine, uniform distribution of Ag3Sn particles distributed through the Sn matrix (see Fig. 1b); this enhances the resistance to dislocation motion. Raising the temperature of the binary alloy resulted in a coarsening and/or coalescence of the eutectic regions (see Fig. 1d) [11]. Hence, relatively large areas with a uniform distribution of eutectic mixture can be obtained at higher temperatures, leading to an increased pinning effect by the Ag3Sn particles and, consequently, a decrease of εT with temperature. Conversely, raising the temperature of the Zn-containing alloy coalesces the βSn dendrites at the grain boundaries as well as within the
Fig. 8. Reciprocal temperature dependence of the strain rate ε· for both Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys.
A. Fawzy / Materials Characterization 58 (2007) 323–331
in the middle of the elongation range. From the slopes of the straight lines obtained in Fig. 8, activation energies of 41 and 20 kJ mol− 1 are obtained for the Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys, respectively. These values were found to be close to those reported for the grain boundary sliding mechanism in Sn-based alloys and were found to be in agreement with those obtained in other researches [20,21]. 4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8]
The main conclusions to be drawn from this investigation may be summarized as follows: 1. The yield stress σy and ultimate tensile stress σUTS were found to increase with an increase in the strain rate ε· and/or a decreasing in deformation temperature in both the Zn-free and the Zn-containing alloys. 2. Significant improvement in the tensile strength of Sn–3.3 wt.% Ag alloy was realized by the 1 wt.% Zn addition. 3. Total elongation in the Zn-free alloy was found to decrease with deformation temperature whereas it increases in the Zn-containing alloy. 4. Values of 41 and 20 kJ mol− 1 were obtained for the activation energy of deformation in the Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys, respectively. 5. The tensile strength appears to be a sensitive parameter of morphology and processing parameters of both Sn-based alloys examined in this program. Acknowledgment The author would like to express his deep thanks to Prof. F. Abd El-Salam for his guidance and encouragement through the study of the present work.
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[9] [10] [11] [12] [13] [14] [15] [16]
[17] [18]
[19] [20] [21]
Vianco PT, Frear DR. J Met 1993;45:14. Napp DP. SAMPEJ 1996;32:59. Kariya Y, Otsuka M. J Electron Mater 1998;27(11):1229. W-Yang, Mesler RW, Felton LE. J Electron Mater 1995;24 (10):1465. Lee SM. Jpn J Appl Phys Part 1 1996;35(7):3999. McCormack M, Kammlott GW, Chen HS, Jin S. Appl Phys Lett 1994;65(10):1233. Lang Fengqun, Tanaka Hiroyuki, Munegata Osamu, Taguchi Toshihiko, Narita Toshio. Mater Charact 2005;24:223. McCormack M, Jin S, Kammolott GW, Chen HS. Appl Phys Lett 1993;63:15. McCormack M, Jin S. J Electron Mater 1994;23(7):635. Igoshev VI, Kleiman VI. J Electron Mater 2000;29(2):244. Osha F, Williams JJ, Chawla N. JOM 2003 (June):56. Massalski TB, editor. Binary alloys phase diagrams. Material Park (OH): American Society of Metals; 1990. p. 94. Oh C-S, Shim J-H, Lee B-J, Lee DN. J Alloys Compd 1996;238:155–66. Kariya Yoshiharo, Otsuka Mashiza. J Electron Mater 1998;27 (11):1229. El-Daly AA, Addel-Daiem AM. Phys Status Solidi A 2003;198 (1):56. El-Daly AA, Abdel-Daiem AM, Esaadani EA, Adel-Rahman AN, Mohamed SM. Mater Chem Phys 2003;83:96. El-Daly AA, Abdel-Daiem AM, Esaadani EA, Adel-Rahman AN, Mohamed SM. Mater Chem Phys 2003;27(11):1229. Dieter GE, editor. “Mechanical metallurgy”. SI Metric Edition: McGraw-Hill Book Company (UK) Limited; 1998. p. 297. Cham KC, Tong GA. Strain rate sensitivity of high-strain rate super plastic A 16061/20SiCw composite under uniaxial and equiaxial tension. Mater Lett 2001;51:389. Plumbridge WJ. J Mater Sci 1996;31:2501. El-Daly AA. Phys Status Solidi A 2003;200(2):333. El-Daly AA. Phys Status Solidi A 2004;201(9):2035.
Effect of Zn addition, strain rate and deformation temperature on the tensile properties of Sn–3.3 wt.% Ag solder alloy A. Fawzy Physics Department, Faculty of Education, Ain Shams University, Cairo, Egypt Received 11 April 2006; received in revised form 16 May 2006; accepted 20 May 2006
Abstract Stress–strain characteristics of the binary Sn–3.3 wt.% Ag and the tertiary Sn–3.3 wt.% Ag–1 wt.% Zn solder alloys were investigated at various strain rates (SR, ε·) from 2.6 × 10− 4 to 1.0 × 10− 2 s− 1 and deformation temperatures from 300 to 373 K. Addition of 1 wt.% Zn to the binary alloy increased the yield stress σy and the ultimate tensile stress σUTS while a decrease of ductility (total elongation εT) was observed. Increasing the strain rate (ε·) increased both σy and σUTS according to the power law σ = C ε·m. A normal decrease of εT with strain rate was observed according to an empirical equation of the form εT = A exp (− λε·); A and λ are constants. Increasing the deformation temperature decreased both σy and σUTS in both alloys, and decreased the total elongation εT in the Zn-free binary alloy, whereas εT was increased in the Zn-containing alloy. The activation energy was determined as 41 and 20 kJ mol− 1 for these alloys, respectively. The results obtained were interpreted in terms of the variation of the internal microstructure in both alloys. The internal microstructural variations in the present study were evaluated by optical microscopy, electron microscopy and X-ray diffraction. The results show the importance of Zn addition in enhancing the mechanical strength of the Sn–3.3 wt.% Ag base alloy. © 2006 Elsevier Inc. All rights reserved. Keywords: Strain rate; Temperature; Strength; Ductility; Eutectic; Solder alloy
1. Introduction Because of their unique combination of material properties and low cost, Sn–Pb solder alloys are important materials for assembling electronic components on printed circuit boards [1]. However, there are environmental and health hazards associated with the toxicity of lead present in the solder, and the potential for small levels of Pb to damage the nervous system [2,3]. The search for potential replacement alloys has covered a wide area of extensive research in recent
E-mail address: [email protected]. 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.05.013
years [4–9]. New Pb-free solder alloys, which offer superior mechanical properties, have resulted from these efforts. A Sn–3.5 wt.% Ag alloy was found to have superior strength compared to the Sn–Pb solder alloys [7–10]. It appears, moreover, that the mechanical properties of this alloy can be significantly adjusted by controlling the solidification rate, and hence the microstructure [11]. It was found [11] that water quenching (solidification rate 24 K/s) yielded a nonequilibrium microstructure of β-tin dendrites surrounded by a fine eutectic mixture of Ag3Sn particles dispersed in the tin-rich matrix. Air cooling (at 10 K/s) also yielded coarser β-tin dendrites, surrounded by a coarser eutectic mixture of Ag3Sn
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Table 1 Chemical compositions of the alloys: Sn–3.3 wt.% Ag and Sn–3.3 wt. % Ag–1 wt.% Zn Alloy
Sn
Ag
Zn
Pb
Sn–3.3 wt.% Ag Sn–3.3 wt.% Ag–1 wt.% Zn
96.66 95.63
3.28 3.29
0.00 1.02
0.05 0.04
performed to understand the effect of Zn addition on the stress–strain characteristics of a Sn–3.3 wt.% Ag solder alloy at various strain rates ranging from 2.6 × 10− 4 to 1.0 × 10− 2 s− 1, over a temperature range from 300 to 373 K. 2. Experimental procedures
particles dispersed in the tin-rich matrix. Under equilibrium conditions, the phase diagram of Sn–Ag system at the eutectic composition predicts a microstructure consisting of a mixture of Ag3Sn (ε-phase) and tin [12,13]. It was found [9,14] that the presence of Zn in a Sn– Ag solder alloy results in an increase in its mechanical strength. Zn additions to a eutectic Pb–10 wt.% Sn alloy have also been shown to improve the high temperature creep strength [15]. On the other hand, the addition of 1.5 wt.% Zn to a Pb–1.5Sb alloy improves the ductility, but leads to a decrease of the flow stress [16]. It is well known that a potent means for combating these types of issues is through careful selection of alloying additions. Motivated by these apparently contradictory findings between a softening and a hardening effect of Zn addition, the present study was
The Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys were prepared from high purity Sn, Ag and Zn (purity 99.99%). The appropriate weights of the elements for the binary alloy were well mixed with CaCl2 flux to prevent oxidation in a graphite mold and kept at 100 K above the melting temperature of Ag. Casting into stainless steel molds was done in air with a cooling rate of 10 K/s. Zn was added to a portion of the binary alloy to form the second alloy, Sn–3.3 wt.% Ag–1 wt.% Zn, at 100 K above the melting temperature. Chemical analysis of the two solder alloys was found to be close to the desired constitution (Table 1). The ingots, as bars of the two alloys, were cold drawn into 0.6-mm diameter wire. Specimens with a gauge length of 50 mm were prepared for tensile testing. Uniaxial tests were performed using an Instron-type
Fig. 1. Microstructure of: (a) OM for the as-cast Sn–3.3 wt.% Ag, (b) OM for the as-cast Sn–3.3 wt.% Ag–1 wt.% Zn solder alloy, (c) TEM for Sn– 3.3 wt.% Ag solder alloy, (d) OM for Sn–3.3 wt.% Ag after heating at 373 K for 45 min and (e) OM for Sn–3.3 wt.% Ag–1 wt.% Zn solder alloy after heating at 373 K for 45 min.
A. Fawzy / Materials Characterization 58 (2007) 323–331
machine with experimental error less than ± 3%. Prior to the tensile tests, each specimen was held for about 30 min at the test temperature, in a furnace attached to the machine, in order to achieve thermal equilibrium. Tensile tests were performed at 300, 323, 348 and 373 K. During the test, the deformation temperature was controlled within ± 2 K. All specimens were strained to fracture with strain rates ranging from 2.6 × 10− 4 to 1.0 × 10− 2 s− 1. The microstructure of the as-cast solders was investigated using optical microscopy (OM), transmission electron microscopy (TEM) and X-ray diffraction. Prior to OM observations, the samples were polished with a diamond paste, then etched with a dilute nitric acid and alcohol solution. For the TEM observa-
325
tions of the as-cast alloys, thin foil samples of 3-mm diameter were mechanically thinned, and electropolished to perforation using a jet technique in a solution containing 80% acetic acid and 20% perchloric acid. 3. Results and discussion 3.1. Microstructure Fig. 1 shows optical and electron micrographs of the microstructure of the two solder alloys. The microstructure of the binary eutectic Sn–3.3 wt.% Ag solder alloy is shown in Fig. 1a. The light dendrite arms are the
Fig. 2. X-ray diffraction patterns for the as-cast Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn showing the existence of the intermetallic compound Ag3Sn in: (a) Sn–3.3 wt.% Ag and (b) Sn–3.3 wt.% Ag–1 wt.% Zn solder alloys.
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β-Sn phase while the dark interdendritic regions contain the eutectic mixture of the Ag3Sn particles dispersed within the Sn-rich matrix (close to pure Sn). This is in good agreement with the structure revealed by other studies [6–11]. The addition of 1 wt.% Zn was found to suppress the formation of β-Sn dendrites and yields a uniform dispersion of the Ag3Sn intermetallic compound within the Sn-rich mixture producing a networklike microstructure with the β-Sn as shown in Fig. 1b. This was interpreted in terms of the very low solubility of Zn in solid Sn while large amounts of Zn are readily soluble in solid Ag [9]. The existence of the Ag3Sn intermetallic compound was observed in the present work by electron microscopy and X-ray investigation as illustrated by the electron micrograph shown in Fig. 1c and the X-ray diffraction patterns given in Fig. 2. Evolution of the microstructure after 45 min at 373 K is shown in Fig. 1d and e for the alloys without and with the Zn addition, respectively. 3.2. Tensile response Fig. 3 shows typical representative stress–strain curves of the: (a) Sn–3.3 wt.% Ag and (b) Sn–3.3 wt.%
Fig. 4. Stress–strain curves of (a) Sn–3.3 wt.% Ag and (b) Sn–3.3 wt. % Ag–1 wt.% Zn solder alloys obtained at T = 323 K.
Fig. 3. Stress–strain curves of (a) Sn–3.3 wt.% Ag and (b) Sn–3.3 wt.% Ag–1 wt.% Zn solder alloys at SR (ε· = 1.9× 10−3 s−1).
Ag–1 wt.% Zn solder alloys at different strain rates ranging from 2.6 × 10− 4 to 1.0 × 10− 2 s− 1 at a constant deformation temperature, 323 K. Fig. 4 shows the representative stress–strain curves of the same alloys at deformation temperatures of 300, 323, 348 and 373 K at a constant strain rate of 1.9 × 10− 3 s− 1. From these figures, it is noted that the stress–strain parameters, the flow stress σUTS, the yield stress σy and the total elongation εT of both alloys are strongly affected by the variation of the strain rate and deformation temperature. For both alloys at the same deformation temperature, increasing the strain rate (Fig. 3) gives rise to higher values of both σy and σUTS and a decrease in εT. At the same strain rate, raising the deformation temperature (Fig. 4) results in a continuous softening; a decrease in σy and σUTS was observed. From this figure, an abnormal decrease of εT with increasing deformation temperature in the Zn-free alloy (Fig. 4a) was observed while a normal increase of εT with deformation temperature in the Zn-containing alloy (Fig. 4b) was observed.
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As for most metals and alloys, solder alloys experience simultaneous work hardening and dynamic recovery when they are deformed. The work hardening and dynamic recovery have opposite effects on the
327
mechanical properties of the solder alloys, where the former hardens the material while the latter leads to softening. Hence, the stress–strain curves obtained here present the combined effects of both factors. Moreover,
Fig. 5. (a) Variation of the UTS with strain rate tested at 323 K for Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys, (b) the relation between log σUTS and Log ε· at different testing temperatures and (c) variation of the strain rate sensitivity index m with the deformation temperature for both alloys.
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dislocation annihilation seems to occur more rapidly than dislocation generation during deformation. Therefore, at higher temperatures the lower strain rate provided a longer time for the dynamic recovery to occur, resulting in the decrease in the σy and σUTS as observed in Figs. 3 and 4. 3.3. Strain rate and temperature dependence of the ultimate tensile stress (UTS) Fig. 5a shows the relation between the strain rate (SR) and the ultimate tensile stress (UTS) for both the Zn-free and the Zn-containing alloys at different deformation temperatures. From this figure, increasing SR (ε·) increases the UTS in both alloys. This is because increasing strain rate is accompanied by an increase in the dislocation density. As these disloca-
tions move they become tangled. It is then more difficult for other dislocations to glide through the material, especially at the lower deformation temperatures. Note that the Zn-containing solder alloy samples exhibited higher UTS than those exhibited by the Znfree alloy at all deformation temperatures. The higher values of the UTS in the Zn-containing alloy may be attributed to the difference in the microstructures of these alloys (see Fig. 1a, b). The refinement and uniform distribution of the intermetallic Ag3Sn particles in the Zn-containing alloy seem to provide a greater measure of dispersion strengthening due to the finer particle size in the ternary alloy compared to the binary alloy. From Fig. 5b, it can be seen that the logarithm of the σUTS increases linearly with the logarithm of the SR (ε·). Consequently, the relationship between UTS (σUTS) and
Fig. 6. Yield stress (σy) as a function of (a) strain rate at different testing temperatures, (b) deformation temperature at different strain rates for both Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys.
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the SR (ε·) for both alloys can be expressed by the equation [17]:
was found to be consistent with the variation of εT with temperature in both alloys, as was observed in Fig. 4.
rUTS ¼ C e:m ;
3.4. Strain rate and temperature dependence of the yield stress σy
ð1Þ
where C is a constant and m is the strain rate sensitivity index. Values of the index m can be obtained from the slopes of the log σUTS–log ε· relationships shown in Fig. 5b. Fig. 5c shows the variation of m with deformation temperature for both alloys. From this figure, it is clear that m decreases with temperature in the Zn-free alloy while it increases with temperature in the Zn-containing one. It is well known that m values describe the capacity of the alloy for necking resistance [18]. Hence, the resistance to necking was found to increase with temperature in the Zn-containing alloy while it decreases in the Zn-free one. This observation
The yield stress σy as a function of both strain rate and deformation temperature is shown in Fig. 6a and b. From this figure, it is clear that increasing the strain rate (Fig. 6a) and/or decreasing the deformation temperature (Fig. 6b) increases the yield stress σy. The similar effect of temperature and strain rate on the yield stress can be understood by considering the deformation as a stressassisted and thermally activated process. Hence, at high strain rates the yield stress increases since there is limited time for the motion of dislocations. Similarly, at low deformation temperatures the yield stress increases
Fig. 7. Variation of the total elongation εT with the strain rate ε· at different testing temperatures for both Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt. % Zn alloys.
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due to the lower thermally activated dislocation motion. This makes it more difficult for other dislocations to glide through the material. Increasing the deformation temperature will rearrange the dense dislocation networks formed by strain hardening into simple and less tangled networks. This reduces the lattice energy and, therefore, it is not surprising to observe the lower values of yield stress at higher deformation temperatures, since the dislocations have much more freedom to move and overcome obstacles through climb and cross slip. From Fig. 6a and b, it is clear that σy values for the Zn-free alloy are lower than those of the Zn-containing alloy. This is explicable in terms of the microstructure since the microstructure of the Zn-containing alloy is characterized by the existence of the fine Ag3Sn particles in the Sn matrix which act as pinning centers for the mobile dislocations. 3.5. Strain rate and temperature dependence of the total elongation εT The strain rate dependence of the total elongation εT is depicted in Fig. 7a and b for both alloys. It was found to obey an empirical relation of the form: eT ¼ A exp ðke: Þ
grains, as observed in Fig. 1e. This distribution enhances the grain boundary migration and the resulting increase in grain size seems to be effective in increasing the elongation with temperature in the Zn-containing alloy. 3.6. Kinetic analysis For the eutectic Sn–Ag alloy strained at temperatures greater than half of the melting temperature, the strain rate ε· and the flow stress σUTS may be related to the deformation temperature with the help of the kinetic rate equation [7,19] e: ¼ A ðrÞ1=m expðQ=RT Þ
ð3Þ
where A is a constant, R is the gas constant, m is the strain rate sensitivity index and Q is the activation energy. Fig. 8a and b shows the relation between log ε· and 1000 / T for both the Sn–3.3 wt.% Ag and the Sn– 3.3 wt.% Ag–1 wt.% Zn alloys. The strain rate values were obtained from Fig. 7a and b at a total elongation of ≈ 80 × 10− 3. This elongation was selected because it was
ð2Þ
where A and λ are constants depending on the tensile test conditions. From this figure, it is clear that the total elongation in both alloys decreases with increasing strain rate. However, in the Zn-free alloy, the values of εT are lower with increasing temperature at all strain rates (Fig. 7a), while in the Zn-containing alloy the values of εT increase with temperature at all strain rates (Fig. 7b). This difference in the relationship between tensile elongation as a function of strain rate and temperature seems to depend on the difference in the microstructure of the alloys. As previously mentioned, the addition of Zn suppresses the formation of β-Sn dendrites and yields a fine, uniform distribution of Ag3Sn particles distributed through the Sn matrix (see Fig. 1b); this enhances the resistance to dislocation motion. Raising the temperature of the binary alloy resulted in a coarsening and/or coalescence of the eutectic regions (see Fig. 1d) [11]. Hence, relatively large areas with a uniform distribution of eutectic mixture can be obtained at higher temperatures, leading to an increased pinning effect by the Ag3Sn particles and, consequently, a decrease of εT with temperature. Conversely, raising the temperature of the Zn-containing alloy coalesces the βSn dendrites at the grain boundaries as well as within the
Fig. 8. Reciprocal temperature dependence of the strain rate ε· for both Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys.
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in the middle of the elongation range. From the slopes of the straight lines obtained in Fig. 8, activation energies of 41 and 20 kJ mol− 1 are obtained for the Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys, respectively. These values were found to be close to those reported for the grain boundary sliding mechanism in Sn-based alloys and were found to be in agreement with those obtained in other researches [20,21]. 4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8]
The main conclusions to be drawn from this investigation may be summarized as follows: 1. The yield stress σy and ultimate tensile stress σUTS were found to increase with an increase in the strain rate ε· and/or a decreasing in deformation temperature in both the Zn-free and the Zn-containing alloys. 2. Significant improvement in the tensile strength of Sn–3.3 wt.% Ag alloy was realized by the 1 wt.% Zn addition. 3. Total elongation in the Zn-free alloy was found to decrease with deformation temperature whereas it increases in the Zn-containing alloy. 4. Values of 41 and 20 kJ mol− 1 were obtained for the activation energy of deformation in the Sn–3.3 wt.% Ag and Sn–3.3 wt.% Ag–1 wt.% Zn alloys, respectively. 5. The tensile strength appears to be a sensitive parameter of morphology and processing parameters of both Sn-based alloys examined in this program. Acknowledgment The author would like to express his deep thanks to Prof. F. Abd El-Salam for his guidance and encouragement through the study of the present work.
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