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Interconnection of thermal parameters, microstructure and mechanical properties in directionally solidified Sn–Sb lead-free solder alloys
Materials Characterization 106 (2015) 52–61
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Interconnection of thermal parameters, microstructure and mechanical properties in directionally solidified Sn–Sb lead-free solder alloys Marcelino Dias a, Thiago Costa a, Otávio Rocha b, José E. Spinelli c, Noé Cheung a,⁎, Amauri Garcia a a b c
Department of Manufacturing and Materials Engineering, University of Campinas — UNICAMP, 13083-860 Campinas, SP, Brazil Federal Institute of Education, Science and Technology of Pará — IFPA, 66093-020 Belém, PA, Brazil Department of Materials Engineering, Federal University of São Carlos — UFSCar, 13565-905 São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Received 18 February 2015 Received in revised form 7 May 2015 Accepted 12 May 2015 Available online 14 May 2015 Keywords: Lead free solder alloys Solidification Reverse dendritic-cellular transition Mechanical properties
a b s t r a c t Considerable effort is being made to develop lead-free solders for assembling in environmental-conscious electronics, due to the inherent toxicity of Pb. The search for substitute alloys of Pb–Sn solders has increased in order to comply with different soldering purposes. The solder must not only meet the expected levels of electrical performance but may also have appropriate mechanical strength, with the absence of cracks in the solder joints. The Sn–Sb alloy system has a range of compositions that can be potentially included in the class of high temperature solders. This study aims to establish interrelations of solidification thermal parameters, microstructure and mechanical properties of Sn–Sb alloys (2 wt.%Sb and 5.5 wt.%Sb) samples, which were directionally solidified under cooling rates similar to those of reflow procedures in industrial practice. A complete high-cooling rate cellular growth is shown to be associated with the Sn–2.0 wt.%Sb alloy and a reverse dendrite-to-cell transition is observed for the Sn–5.5 wt.%Sb alloy. Strength and ductility of the Sn–2.0 wt.%Sb alloy are shown not to be affected by the cellular spacing. On the other hand, a considerable variation in these properties is associated with the cellular region of the Sn–5.5 wt.%Sb alloy casting. © 2015 Elsevier Inc. All rights reserved.
1. Introduction The prediction of morphological transitions in as-solidified microstructures is of great interest for the assessment and design of the corresponding mechanical properties. The mechanisms that govern such transitions, are generally based on operational parameters such as temperature gradient GL, growth rate VL, cooling rate (Ṫ = GL VL) and alloy composition [1–3]. The variation of such parameters leads to a large spectrum of structures, with consequence effects on the material behaviour. These structures can be analysed as macrostructure (order of millimetres) and microstructure (order of micrometres). Elongated columnar grains, equiaxed grains or a mixture of both may characterize as-solidified components. The arrangement of these grains is known as macrostructure, and the morphological transition as CET (columnar-to-equiaxed transition). In terms of the microstructure that is formed within individual grains there are, in most cases, either a dendritic or a cellular network of continuously varying solute content, second phases, and possibly porosity and inclusions. However both morphologies can occur under a wide range of growth rates with a cellular/dendritic transition (CDT) occurring with increasing VL [4]. Additional increase in VL changes the dendritic front back to cellular, characterizing a reverse transition that leads to the so-called high velocity ⁎ Corresponding author. E-mail address: [email protected] (N. Cheung).
http://dx.doi.org/10.1016/j.matchar.2015.05.015 1044-5803/© 2015 Elsevier Inc. All rights reserved.
cells [5]. Experimental studies on this reverse transition are rare in the literature for metallic systems. In industrial practice, attempts are made not only to produce wholly columnar or wholly equiaxed structures but also only dendrite or cell microstructures. There is interest in knowing whether transition in structures occurs, which may provoke efforts in order to understand the associated solidification conditions. In the intervening years, significant advances in the insight of microstructural evolution during alloy solidification have been made by generations of researchers. Most of the works have been focused on the study of CET [1,6–11] whilst CDT [2,12–16] has remained poorly reported in literature. The interest on the near peritectic Sn–5.5 wt.%Sb Pb-free solder alloys is related to high temperature electronic applications, especially on step soldering technology. This soldering method occurs at various steps with successively lower soldering temperatures for latter steps in order to prevent remelting of the earlier solder joints. Thus, different solders with scaled melting points are used [17,18]. The melting points of Sn–Sb alloys are higher than that of several Pb free solder alloys and due to this fact, Sn–Sb alloys have been used in earlier solder joints as high-temperature solders during step soldering processes. In compliance with recent Pb-free requirements for electronic products, Sn–Sb alloys are potential candidates that could substitute the most common high-temperature high-lead solders, containing 85–97 wt.%Pb [19]. Due to the fact that Sb is an effective solid solution strengthener in Sn [20], tin–antimony solders are generally stronger than the Sn–Pb
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Fig. 1. Schematic vertical upward directional solidification casting assembly and mould details (right side).
failure susceptibility. The mechanical properties of a joint are ultimately controlled by microstructural features such as the matrix morphology (cell, dendrite) and second-phase particles. The amount, shape, and arrangement of these structural features are particularly important from the point of view of mechanical behaviour. In order to understand how the microstructure evolves, especially during service, it is fundamental firstly, to understand how the original structures are generated during solidification of molten solders. Thus, any appropriate analysis should commence at this point. Because of the reduced size of the electronic interconnections, physical measurements of the thermal profile during the solidification are a difficult task without interfering into the phenomenon. In this sense, experiments using devices based on directional solidification have been successfully used to encompass a wide range of solidification conditions, including those of reflow procedures in industrial practice. There are basically two approaches used in the investigation of the directional solidification: (i) experiments under steady state heat flow conditions performed in a Bridgman type apparatus [24–27]; (ii) under non-steady conditions performed in cooled chill moulds [28–33]. In this latter trend of directional solidification, particularly concerning solder alloys, some studies have been reported in the literature [14,34,35]. The classical Sn–Pb solder alloy system has been investigated and compared with theoretical dendritic and cellular growth models [14]. The authors have measured cellular and primary dendritic spacings in dilute Sn–Pb alloys directionally solidified under unsteady-state heat flow conditions, and correlated with solidification thermal parameters. CDT was
Fig. 2. Sn–Sb phase diagram with the analysed alloys Sn–2 wt.%Sb and Sn–5.5 wt.%Sb indicated by dashed lines.
solders [21]. Additionally, the near-peritectic composition has advantages of better room temperature creep resistance and ductility, and also higher microstructure stability compared with conventional Sn– Pb solders [18,19,22,23]. The reliability of lead-free solder interconnections is not limited only to electrical performance but also to the minimization of mechanical
Table 1 Chemical composition (wt. %) of metals used to prepare the alloys. Element
Pb
Fe
Sb
Sn
Cd
Ni
Cu
Bi
Zn
Si
Sn Sb
0.0469 0.215
0.0081 0.075
0.0005 Balance
Balance –
0.00001 –
0.0001 0.034
0.0047 0.034
0.0046 –
0.0001 –
0.009
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Fig. 3. Sequence used to obtain the samples for tensile test, microstructural characterization, segregation and phase analyses.
observed to occur for a range of values of parametric factors which include thermal gradient, growth rate and solute content, given by η = (GL / VL)(1 / Co), in such a way that for η N 1.0 a cellular structure is observed and for η b 0.7 a dendritic structure prevails. Moura and collaborators [34], using a similar aforementioned solidification device, also observed CDT in the Pb-free Sn–0.7 wt.%Cu eutectic solder: dendrites prevailed for growth rates higher than 0.50 mm s− 1 whilst cells were found for VL b 0.35 mm s− 1 and the microstructural transition (CDT) occurred between these range of growth rates. In the present investigation two monophasic Sn-rich Sn–Sb alloys (2.0 and 5.5 wt.%Sb) are directionally solidified (DS) under transient heat flow conditions, with a view to permitting the microstructural evolution under a wide range of cooling rates. The study aims to analyse microstructural features such as morphologies and scales of the Snrich matrix along the length of the DS castings. Careful determinations of thermal solidification parameters and characterization of the phases forming the microstructural pattern are performed. Experimental growth laws correlating cellular (λc) and primary dendritic spacings (λ1) with the cooling rate (Ṫ) and growth rate (VL), as well as the thermal parameters conducive to morphological transitions are envisaged. The study also aims to establish experimental interrelations of the local length scale of the microstructure along the DS casting length and the corresponding tensile mechanical properties (ultimate-σuand yield-σy-tensile strengths and elongation-to-fracture -δ).
cover made of an insulating material was used as a thermal barrier to reduce heat losses at the metal/air surface. The bottom part of the mould was closed with a 3 mm thick carbon steel sheet. The alloys were melted in situ, and the lateral electric heaters had their power controlled in order to permit the desired superheat to be achieved. To start
2. Experimental procedure In order to promote vertical upward solidification, an apparatus was designed in such a way that heat is directionally extracted by the bottom of the casting (Fig. 1). The imposed transient directional solidification (DS) results in a range of as-solidified microstructures at different cooling rates in a single casting experiment. The experiments were performed with monophasic Sn–Sb alloys having the following nominal compositions: Sn–2 wt.%Sb and Sn–5.5 wt.%Sb as indicated by the dashed lines in the phase diagram of Fig. 2. The chemical compositions of commercially pure metals that were used to prepare these alloys are shown in Table 1. The DS casting assembly (shown schematically in Fig. 1) has been detailed in previous studies [31,36]. The apparatus consists of a vertical cylindrical mould with heat being extracted only by a water-cooled bottom, promoting upward directional solidification. A stainless steel mould was used, having an internal diameter of 50 mm, a height of 110 mm and a wall thickness of 3 mm. The inner vertical surface was covered with a layer of insulating alumina to minimize radial heat losses, thus permitting unidirectional heat flow to be attained. A top
Fig. 4. Experimental cooling curves for eight thermocouples inside the casting positioned from the metal/mould interface. Tp is the initial melt temperature. TL is the liquidus temperature. TS is the solidus temperature. a) Sn–2 wt.% Sb alloy b) Sn–5.5 wt.% Sb alloy.
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Fig. 5. Typical examples of the observed columnar macrostructures (a) Sn–2 wt.%Sb; b) Sn–5.5 wt.%Sb alloy.
solidification, the electric heaters were disconnected, and at the same time, the controlled water flow was initiated. The initial melt temperatures were standardised at 10% above the alloy liquidus temperatures. As shown in Fig. 1, a set of fine J-type thermocouples, sheathed in 1.6 mm outside diameter (OD) stainless steel tubes, were inserted in the geometrical centre of the cylindrical mould cavity along its length at different positions from the heat-extracting surface at the bottom of the casting. All the thermocouples were connected by coaxial cables to a data logger interfaced with a computer, capable of automatically record temperature data at a frequency of 5 Hz. The cylindrical ingots were sectioned on a mid-longitudinal plane, mechanically polished using abrasive papers and etched in a sequence of solutions to reveal the macrostructure: (1) 100 ml H2O, 10 ml HNO3, 4 g (NH4)2MoO4 ~ 5 min; (2) 1:1 proportion of glacial acetic
acid and hydrogen peroxide (H2O2) ~ 3 min; (3) 10 ml HCl, 140 ml H2O, 10 g CuCl2 ~ 5 min; and (4) Kroll etching (20 ml HF, 10 ml HNO3, 70 ml H2O) ~5 min. The microstructural characterization of the directionally solidified alloy castings was performed by extracting samples at different transverse sections along the casting length, as shown in Fig. 3. The interval of the analysed sample positions, from the metal/mould interface, was established in about 3 mm in order to cover the positions where the cooling rate suffers a great variation. On the other hand, where the cooling rate almost stabilizes, from this position on, an interval of 10 mm was adopted. The samples were polished with 100, 200, 400, 600, 800 and 1200 grit SiC papers, and then finely polished with diamond paste (1 and 3 μm). The sample surfaces were then subjected
Fig. 6. Experimental position of the liquidus isotherm as a function of time. R2 is the coefficient of determination.
Fig. 7. Growth rate, VL, as a function of position (P) from the cooled bottom of the DS castings for monophasic Sn–Sb alloys during upward vertical directional solidification.
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to ultrasonic cleaning before etching with a solution of 2 ml hydrochloric acid (HCl), 3 ml nitric acid (HNO3), ethyl alcohol to achieve 100 ml. The etching was performed by carefully immersing the samples in the etchant for only a few seconds and about 30 s for the Sn– 5.5 wt.%Sb and Sn–2.2 wt.%Sb alloy samples, respectively, followed by cleaning the sample surface with running water and ethanol. Optical microscopy was performed using an Olympus Inverted Metallurgical Microscope (model 41GX). The cell spacing (λ c ) and the primary dendritic arm spacing (λ1) were measured from the optical images of the as-solidified samples (about 40 independent readings for each selected position, with the average taken to be the local spacing). The triangle method [37] was used in order to perform such measurements. A triangle is formed by joining the centres of the cross sections of three neighboring primary dendritic branches and the sides of the triangle correspond to λ1. Square central parts of the transversal samples, shown schematically in Fig. 3, were cut by a precision saw into pieces of about 3.0 mm of thickness and investigated by a Rigaku Rix 3100 X-Ray Fluorescence Spectrometer to estimate their average concentrations through an area of 100 mm2 probe. This permits the occurrence of macrosegregation along the casting length to be investigated. The same samples were used for XRD analyses. X-ray diffraction (XRD) patterns were obtained by a Panalytical X'pert PRO diffractometer in the 2θ range from 25° to 95° using Cu Kα radiation with a wavelength of 0.15406 nm. Transverse specimens for tensile tests were extracted from different positions along the DS casting length (see Fig. 3). These specimens were prepared according to specifications of the ASTM Standard E 8M/04 and tested in a MTS 810 machine at a strain rate of about 3 × 10−3 s−1. In order to ensure reproducibility of the tensile results, three specimens were tested for each selected position in order to determine ultimate/ yield tensile strengths and elongation. 3. Results and discussion Typical examples of experimental cooling curves for the eight thermocouples inserted into the casting, during solidification of the Sn–2 wt.%Sb alloy, are shown in Fig. 4. Fig. 5 shows typical examples of the resulting macrostructures of the DS castings of the Sn–2 wt.%Sb and Sn–5.5 wt.%Sb alloys. It can be seen that columnar structures characterize the entire casting length. The thermocouple readings, collected during solidification, were used to generate plots of position (P) from the metal/mould interface as a function of time (t) corresponding to the liquidus front of every alloy passing by each thermocouple. A numerical technique, based on the minimum square method, was used to fit mathematical power functions of the form P(t) = a tb (a; b are constants) on these experimental plots (see Fig. 6). The derivative of these functions with respect to time gave values for the growth rate (VL), as shown in Fig. 7. Moreover, the data acquisition system employed permitted accurate determination of the slope of the experimental cooling curves. Hence, the cooling rate (Ṫ) was determined along the casting lengths, by considering the thermal data recorded immediately after the passage of the liquidus front by each thermocouple (Fig. 8). In both Figs. 7 and 8 it can be seen that the use of a water cooled mould imposes higher values of growth rates and cooling rates, respectively, near the casting surface and a decreasing profile along the casting due to the increasing thermal resistance of the solidified shell with distance from the cooled surface. It can also be seen that the Sn–2 wt.%Sb alloy exhibits values of VL and Ṫ that are much higher than the corresponding values of the Sn–5.5 wt.%Sb alloy for positions closer to the casting surfaces. This seems to be associated with the higher wettability of the Sn– 2 wt.%Sb alloy, since the differences in both thermal parameters are significant only for regions that are close to the cooled surface of the casting. The directionally solidified alloy castings had local compositions examined along the length with a view to investigating the eventual
Fig. 8. Cooling rate, Ṫ, as a function of position (P) from the cooled bottom of the DS castings for monophasic Sn–Sb alloys during upward vertical directional solidification.
occurrence of long-range segregation. The alloy compositions along the casting length were determined by X-ray fluorescence and are shown in Fig. 9. It can be seen that the solute concentrations of Sb and Sn are essentially constants as a function of position from the cooled surface of the castings, and can be roughly represented by horizontal lines, i.e. the vertical directional solidification has not induced solutal convection conducive to macrosegregation profiles. Actually, Sn, the solvent is the rejected element at the solidification interface during the progress of solidification since the partition coefficient (k) is higher than 1.0. In fact, the corresponding k value of 1.18 cannot be considered a strong partition coefficient, since segregation does not occur for k = 1. Fig. 10 shows some typical transverse and longitudinal microstructures obtained along the length of the DS castings. Both alloys have the microstructures characterized by a Sn-rich matrix (Sn-β phase) surrounded by the Sn–Sb phase. The main difference is associated with the morphology of the Sn-rich matrix. The Sn–2 wt.%Sb alloy (Fig. 10a) exhibits a fully cellular microstructure along the entire casting. On the other hand, in the case of the Sn–5.5 wt.%Sb alloy casting (Fig. 10b), for regions closer to the cooled surface of the casting, the matrix has a cellular microstructure followed by a transition to a dendritic microstructure, characterizing an unexpected reverse dendritic/cellular transition. Cahn [4] reported that as the solidification velocity is increased for a given alloy composition and temperature gradient (GL), one observes a transition in structure: planar, cellular, dendritic,
Fig. 9. Composition profiles of Sn and Sb along the length of the DS castings (Sn–2 wt.%Sb and Sn–5.5 wt.%Sb alloys).
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cellular, and planar. According to Fig. 7, high values of growth rate can be realized near the metal/mould interface, which permits, if considering G constant at a first moment, to conclude that the cells located in this region are high velocity cells. Trivedi et al. [5] pointed out that the reverse transition from dendrite to cell is difficult to observe in situ for metallic systems and the that the scale of the microstructure is extremely fine and associated with high growth rates. However, the so-called high-velocity cells occurred in the present study for the Sn–5.5 wt.%Sb alloy DS casting even for moderate values of both growth rate (VL N 0.4 mm/s) and cooling rate (Ṫ N 1.2 K/s). The dendritic region occurred for Ṫ b 0.9 K/s or VL b 0.3 mm/s, and the transition region over which one structure changes into another was located in the intermediate range of the cited values.
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Fig. 11 shows the XRD patterns of the Sn–Sb alloys. The samples examined in the present study exhibit the presence of peaks associated with Sn(β) and SnSb phases as previewed by the phase diagram of Fig. 2. Although the solidification conditions were far from the equilibrium, the Sb2Sn3 phase has not been detected. Five different positions were examined along each Sn–Sb alloy casting, encompassing a representative range of cooling rates. There were no clear tendencies regarding the intensities of peaks for the different positions from the casting cooled surface. With a view to analysing the role of the solidification thermal parameters on the scale of the as-solidified microstructure, the cellular (λc) and primary dendritic arm (λ1) spacings have been measured along the casting length. Fig. 12 shows that the variations in λ1 and λc
Fig. 10. Typical microstructure of Sn–Sb alloys — P is the position from the cooled bottom of the casting; λc is the cellular spacing; λ1 is the primary dendritic arm spacing; VL is the growth rate; Ṫ is the cooling rate. a) Sn–2 wt.%Sb alloy b) Sn–5.5 wt.%Sb alloy.
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Fig. 10 (continued).
with the growth rate (VL) may be characterized by − 1.1 power laws, which apply for both cellular and primary dendritic growth. The same exponent was reported in the literature, e. g. Rocha et al. [14] and Cruz [38], concerning the dendritic growth of Al–Cu and Al–Sn directionally solidified under unsteady-state conditions and Rosa and coauthors [39] for the cellular growth of dilute Pb–Sb alloys. The scale of the microstructures of the so-called high-velocity cells determined for the Sn–5.5 wt.%Sb alloy is in the range 15–35 μm, which are finer than those of the Sn–2.0 wt.%Sb alloy, i.e., 28–60 μm, considering the same range of VL values from 0.4 to 0.7 mm/s. The increase in the alloy solute content is associated with a decrease in the cellular spacing. The majority of industrial solidification processes occur under unsteady state regime, where temperature gradient and growth rate cannot be controlled and vary freely with time. For such heat flow conditions a
more appropriate solidification thermal parameter should encompass both the growth rate and the thermal gradient, as is the case of cooling rate that synthesizes both parameters (Ṫ = GL·VL). Correlations between λ1, λc and Ṫ have been established, as shown in Fig. 13, which are characterized by −0.55 power laws. The same exponent was also reported in the literature to apply for a number of binary alloys [40]. The cellular growth is observed to prevail in the Sn–5.5 wt.%Sb alloy casting for cooling rates higher than 1.2 K/s. As the cooling rate increased after the transition range, the cellular spacing decreased. The well-known Hall–Petch equation shows that the yield strength or flow stress is proportional to the reciprocal of the square root of the grain diameter, which means that the influence of any parameters other than the grain boundary is not included in the analysis. Inside the grains the effects of morphology and scale of the phases forming the microstructure,
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Fig. 11. Typical XRD patterns of (a) Sn–2 wt.%Sb and (b) Sn–5.5 wt.%Sb alloys for different positions along the casting length.
e.g. cellular or dendritic networks of continuously varying solute content, second phases, eutectic mixtures, and eventually porosity and inclusions, cannot be neglected on the resulting mechanical properties. Many experimental studies in the literature show that microstructure–mechanical properties relationships can be represented by Hall–Petch formulae, which depend on cellular, dendritic or eutectic interphase spacings [32, 41–43]. Goulart and collaborators [32] analysing Al–Fe hypoeutectic alloys, reported that ultimate tensile strength, elongation and yield stress are significantly affected by the cellular spacing. In their investigation, it has been observed that finer cellular spacings lead to improvements in elongation, ultimate and yield tensile strengths whilst an opposite effect is observed for coarser cells. The results of tensile tests of the present study are summarized in Figs. 14–16, where the ultimate tensile strength (σu), yield tensile strength, σy = 0.2 (0.2% proof stress) and elongation (δ)
are related to the cellular spacing or the primary dendrite arm spacing. It can be realized that σu, σy and δ for the Sn–2 wt.%Sb alloy were not affected by variations in the cellular spacing. On the other hand, for the Sn– 5.5 wt.%Sb alloy with the decrease in λc, σu and σy increase and δ decreases. However, after the cellular to dendritic transition, the aforementioned mechanical properties were also not affected by the scale of the dendritic structure (which is coarser than the cellular one). The SnSb phase is located along the intercellular/interdendritic regions and is supposed to act as a reinforcement phase. The lower the spacing characterizing the scale of the β-Sn matrix, more homogeneously distributed would be the SnSb phase with a consequential more efficient role in blocking dislocations. This is so for the cellular region of the Sn–5.5 wt.%Sb alloy, as reflected by the Hall–Petch experimental equations of Figs. 14b and 16. However, after the morphological transition from cells to dendrites,
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Fig. 12. Cellular (λc) and primary dendritic (λ1) spacings as a function of growth rate for the directionally solidified Sn–Sb alloys. R2 is the coefficient of determination.
which occurred with the decrease in cooling rates (hence producing dendritic regions with coarser spacings as compared to those of the previous cellular region), the distribution of the reinforcing phase seems not to be so efficient, thus leading to essentially constant mechanical properties. For the Sn–2 wt.%Sb alloy, despite the occurrence of fine cells, it seems that the lower volumetric fraction of the SnSb phase was not enough to provide a significant reinforcement, and the mechanical properties resulted essentially constants with the decrease in cellular spacing, as shown in Figs. 14a and 15. According to Figs. 14 and 16, in the range associated with highvelocity cells of the Sn–5.5 wt.%Sb alloy, the experimentally determined average σu, σy and δ values varied from 35–47 MPa, 31–40 MPa and 25– 43%, respectively. Such mechanical property values are quite reasonable as compared with those reported for the same alloy in the NIST database for solders [44] which indicates 35.2 MPa, 25.7 MPa and 22%, respectively. This demonstrates that the cellular morphology is able to improve the mechanical properties of the Sn–5.5 wt.%Sb alloy. This morphology can be obtained for cooling rates higher than 1.2 K/s.
4. Conclusions The following conclusions can be drawn from the present experimental investigation:
Fig. 13. Cellular (λc) and primary dendritic (λ1) spacing evolutions versus Ṫ for the Sn–Sb alloys. R2 is the coefficient of determination.
Fig. 14. Ultimate (σU) and yield (σy) tensile strength and as a function of cellular (λc) and primary dendritic (λ1) spacings for (a) Sn–2.0 wt.%Sb and (b) Sn–5.5 wt.%Sb solder alloys. R2 is the coefficient of determination.
• The relatively high cooling rates imposed by the water-cooled directional solidification apparatus used in the experiments were enough to guarantee the onset of high-cooling rate cells in the Sn– 5.5 wt.%Sb alloy, which occurred for Ṫ N 1.2 K/s. With the subsequent
Fig. 15. Elongation-to-fracture (δ) as a function of cellular (λc) spacing for the Sn–2 wt.%Sb alloy.
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Fig. 16. Elongation (δ) as a function of cellular (λc) and primary dendritic (λ1) spacings for the Sn–5.5 wt.%Sb alloy. R2 is the coefficient of determination.
decrease in cooling rate during solidification, a transition cell/dendrite was shown to occur, i.e. the dendritic region occurred for Ṫ b 0.9 K/s. For the Sn–2 wt.%Sb alloy a cellular microstructure prevailed along the entire DS casting. • After the microstructural transition zone in the Sn–5.5 wt.%Sb alloy casting, correlations between the cellular spacing and tensile testing results — σu, σy and δ — permitted Hall–Petch type equations to be proposed relating λc to these mechanical properties. • The mechanical properties remained unchanged for both the dendritic region (which was shown to be coarser than the cellular one) of the Sn–5.5 wt.%Sb alloy casting and along the entire Sn–2 wt.%Sb alloy casting regardless of fine or coarse cellular structures.
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El-Taher, Microstructural evolution and tensile properties of Sn–5Sb solder alloy, containing small amount of Ag and Cu, J. Alloys Compd. 509 (2011) 4574–4582. [19] G. Zeng, S. McDonald, K. Nogita, Development of high-temperature solders: review, Microelectron. Reliab. 52 (2012) 1306–1322. [20] M.M. El-Bahay, M.E. El Mossalamy, M. Mahdy, A.A. Bahgat, Study of the mechanical and thermal properties of Sn–5 wt.%Sb solder alloy at to annealing temperatures, Phys. Status Solidi A 198 (2003) 76–90. [21] R.W. Messler, Joining of Materials and Structures: From Pragmatic Process to Enabling Technology, Butterworth-Heinemann, Oxford, 2004. [22] R.K. Mahidhara, S.M.L. Sastry, I. Turlik, K.L. Murty, Deformation and fracture behavior of Sn–5%Sb solder, Scr. Metall. Mater. 31 (1994) 1145–1150. [23] M.J. Esfandyarpour, R. Mahmudi, Microstructure and tensile behavior of Sn–5Sb lead-free solder alloy containing Bi and Cu, Mater. Sci. Eng. A Struct. 530 (2011) 402–410. [24] M. 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Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Interconnection of thermal parameters, microstructure and mechanical properties in directionally solidified Sn–Sb lead-free solder alloys Marcelino Dias a, Thiago Costa a, Otávio Rocha b, José E. Spinelli c, Noé Cheung a,⁎, Amauri Garcia a a b c
Department of Manufacturing and Materials Engineering, University of Campinas — UNICAMP, 13083-860 Campinas, SP, Brazil Federal Institute of Education, Science and Technology of Pará — IFPA, 66093-020 Belém, PA, Brazil Department of Materials Engineering, Federal University of São Carlos — UFSCar, 13565-905 São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Received 18 February 2015 Received in revised form 7 May 2015 Accepted 12 May 2015 Available online 14 May 2015 Keywords: Lead free solder alloys Solidification Reverse dendritic-cellular transition Mechanical properties
a b s t r a c t Considerable effort is being made to develop lead-free solders for assembling in environmental-conscious electronics, due to the inherent toxicity of Pb. The search for substitute alloys of Pb–Sn solders has increased in order to comply with different soldering purposes. The solder must not only meet the expected levels of electrical performance but may also have appropriate mechanical strength, with the absence of cracks in the solder joints. The Sn–Sb alloy system has a range of compositions that can be potentially included in the class of high temperature solders. This study aims to establish interrelations of solidification thermal parameters, microstructure and mechanical properties of Sn–Sb alloys (2 wt.%Sb and 5.5 wt.%Sb) samples, which were directionally solidified under cooling rates similar to those of reflow procedures in industrial practice. A complete high-cooling rate cellular growth is shown to be associated with the Sn–2.0 wt.%Sb alloy and a reverse dendrite-to-cell transition is observed for the Sn–5.5 wt.%Sb alloy. Strength and ductility of the Sn–2.0 wt.%Sb alloy are shown not to be affected by the cellular spacing. On the other hand, a considerable variation in these properties is associated with the cellular region of the Sn–5.5 wt.%Sb alloy casting. © 2015 Elsevier Inc. All rights reserved.
1. Introduction The prediction of morphological transitions in as-solidified microstructures is of great interest for the assessment and design of the corresponding mechanical properties. The mechanisms that govern such transitions, are generally based on operational parameters such as temperature gradient GL, growth rate VL, cooling rate (Ṫ = GL VL) and alloy composition [1–3]. The variation of such parameters leads to a large spectrum of structures, with consequence effects on the material behaviour. These structures can be analysed as macrostructure (order of millimetres) and microstructure (order of micrometres). Elongated columnar grains, equiaxed grains or a mixture of both may characterize as-solidified components. The arrangement of these grains is known as macrostructure, and the morphological transition as CET (columnar-to-equiaxed transition). In terms of the microstructure that is formed within individual grains there are, in most cases, either a dendritic or a cellular network of continuously varying solute content, second phases, and possibly porosity and inclusions. However both morphologies can occur under a wide range of growth rates with a cellular/dendritic transition (CDT) occurring with increasing VL [4]. Additional increase in VL changes the dendritic front back to cellular, characterizing a reverse transition that leads to the so-called high velocity ⁎ Corresponding author. E-mail address: [email protected] (N. Cheung).
http://dx.doi.org/10.1016/j.matchar.2015.05.015 1044-5803/© 2015 Elsevier Inc. All rights reserved.
cells [5]. Experimental studies on this reverse transition are rare in the literature for metallic systems. In industrial practice, attempts are made not only to produce wholly columnar or wholly equiaxed structures but also only dendrite or cell microstructures. There is interest in knowing whether transition in structures occurs, which may provoke efforts in order to understand the associated solidification conditions. In the intervening years, significant advances in the insight of microstructural evolution during alloy solidification have been made by generations of researchers. Most of the works have been focused on the study of CET [1,6–11] whilst CDT [2,12–16] has remained poorly reported in literature. The interest on the near peritectic Sn–5.5 wt.%Sb Pb-free solder alloys is related to high temperature electronic applications, especially on step soldering technology. This soldering method occurs at various steps with successively lower soldering temperatures for latter steps in order to prevent remelting of the earlier solder joints. Thus, different solders with scaled melting points are used [17,18]. The melting points of Sn–Sb alloys are higher than that of several Pb free solder alloys and due to this fact, Sn–Sb alloys have been used in earlier solder joints as high-temperature solders during step soldering processes. In compliance with recent Pb-free requirements for electronic products, Sn–Sb alloys are potential candidates that could substitute the most common high-temperature high-lead solders, containing 85–97 wt.%Pb [19]. Due to the fact that Sb is an effective solid solution strengthener in Sn [20], tin–antimony solders are generally stronger than the Sn–Pb
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Fig. 1. Schematic vertical upward directional solidification casting assembly and mould details (right side).
failure susceptibility. The mechanical properties of a joint are ultimately controlled by microstructural features such as the matrix morphology (cell, dendrite) and second-phase particles. The amount, shape, and arrangement of these structural features are particularly important from the point of view of mechanical behaviour. In order to understand how the microstructure evolves, especially during service, it is fundamental firstly, to understand how the original structures are generated during solidification of molten solders. Thus, any appropriate analysis should commence at this point. Because of the reduced size of the electronic interconnections, physical measurements of the thermal profile during the solidification are a difficult task without interfering into the phenomenon. In this sense, experiments using devices based on directional solidification have been successfully used to encompass a wide range of solidification conditions, including those of reflow procedures in industrial practice. There are basically two approaches used in the investigation of the directional solidification: (i) experiments under steady state heat flow conditions performed in a Bridgman type apparatus [24–27]; (ii) under non-steady conditions performed in cooled chill moulds [28–33]. In this latter trend of directional solidification, particularly concerning solder alloys, some studies have been reported in the literature [14,34,35]. The classical Sn–Pb solder alloy system has been investigated and compared with theoretical dendritic and cellular growth models [14]. The authors have measured cellular and primary dendritic spacings in dilute Sn–Pb alloys directionally solidified under unsteady-state heat flow conditions, and correlated with solidification thermal parameters. CDT was
Fig. 2. Sn–Sb phase diagram with the analysed alloys Sn–2 wt.%Sb and Sn–5.5 wt.%Sb indicated by dashed lines.
solders [21]. Additionally, the near-peritectic composition has advantages of better room temperature creep resistance and ductility, and also higher microstructure stability compared with conventional Sn– Pb solders [18,19,22,23]. The reliability of lead-free solder interconnections is not limited only to electrical performance but also to the minimization of mechanical
Table 1 Chemical composition (wt. %) of metals used to prepare the alloys. Element
Pb
Fe
Sb
Sn
Cd
Ni
Cu
Bi
Zn
Si
Sn Sb
0.0469 0.215
0.0081 0.075
0.0005 Balance
Balance –
0.00001 –
0.0001 0.034
0.0047 0.034
0.0046 –
0.0001 –
0.009
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Fig. 3. Sequence used to obtain the samples for tensile test, microstructural characterization, segregation and phase analyses.
observed to occur for a range of values of parametric factors which include thermal gradient, growth rate and solute content, given by η = (GL / VL)(1 / Co), in such a way that for η N 1.0 a cellular structure is observed and for η b 0.7 a dendritic structure prevails. Moura and collaborators [34], using a similar aforementioned solidification device, also observed CDT in the Pb-free Sn–0.7 wt.%Cu eutectic solder: dendrites prevailed for growth rates higher than 0.50 mm s− 1 whilst cells were found for VL b 0.35 mm s− 1 and the microstructural transition (CDT) occurred between these range of growth rates. In the present investigation two monophasic Sn-rich Sn–Sb alloys (2.0 and 5.5 wt.%Sb) are directionally solidified (DS) under transient heat flow conditions, with a view to permitting the microstructural evolution under a wide range of cooling rates. The study aims to analyse microstructural features such as morphologies and scales of the Snrich matrix along the length of the DS castings. Careful determinations of thermal solidification parameters and characterization of the phases forming the microstructural pattern are performed. Experimental growth laws correlating cellular (λc) and primary dendritic spacings (λ1) with the cooling rate (Ṫ) and growth rate (VL), as well as the thermal parameters conducive to morphological transitions are envisaged. The study also aims to establish experimental interrelations of the local length scale of the microstructure along the DS casting length and the corresponding tensile mechanical properties (ultimate-σuand yield-σy-tensile strengths and elongation-to-fracture -δ).
cover made of an insulating material was used as a thermal barrier to reduce heat losses at the metal/air surface. The bottom part of the mould was closed with a 3 mm thick carbon steel sheet. The alloys were melted in situ, and the lateral electric heaters had their power controlled in order to permit the desired superheat to be achieved. To start
2. Experimental procedure In order to promote vertical upward solidification, an apparatus was designed in such a way that heat is directionally extracted by the bottom of the casting (Fig. 1). The imposed transient directional solidification (DS) results in a range of as-solidified microstructures at different cooling rates in a single casting experiment. The experiments were performed with monophasic Sn–Sb alloys having the following nominal compositions: Sn–2 wt.%Sb and Sn–5.5 wt.%Sb as indicated by the dashed lines in the phase diagram of Fig. 2. The chemical compositions of commercially pure metals that were used to prepare these alloys are shown in Table 1. The DS casting assembly (shown schematically in Fig. 1) has been detailed in previous studies [31,36]. The apparatus consists of a vertical cylindrical mould with heat being extracted only by a water-cooled bottom, promoting upward directional solidification. A stainless steel mould was used, having an internal diameter of 50 mm, a height of 110 mm and a wall thickness of 3 mm. The inner vertical surface was covered with a layer of insulating alumina to minimize radial heat losses, thus permitting unidirectional heat flow to be attained. A top
Fig. 4. Experimental cooling curves for eight thermocouples inside the casting positioned from the metal/mould interface. Tp is the initial melt temperature. TL is the liquidus temperature. TS is the solidus temperature. a) Sn–2 wt.% Sb alloy b) Sn–5.5 wt.% Sb alloy.
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Fig. 5. Typical examples of the observed columnar macrostructures (a) Sn–2 wt.%Sb; b) Sn–5.5 wt.%Sb alloy.
solidification, the electric heaters were disconnected, and at the same time, the controlled water flow was initiated. The initial melt temperatures were standardised at 10% above the alloy liquidus temperatures. As shown in Fig. 1, a set of fine J-type thermocouples, sheathed in 1.6 mm outside diameter (OD) stainless steel tubes, were inserted in the geometrical centre of the cylindrical mould cavity along its length at different positions from the heat-extracting surface at the bottom of the casting. All the thermocouples were connected by coaxial cables to a data logger interfaced with a computer, capable of automatically record temperature data at a frequency of 5 Hz. The cylindrical ingots were sectioned on a mid-longitudinal plane, mechanically polished using abrasive papers and etched in a sequence of solutions to reveal the macrostructure: (1) 100 ml H2O, 10 ml HNO3, 4 g (NH4)2MoO4 ~ 5 min; (2) 1:1 proportion of glacial acetic
acid and hydrogen peroxide (H2O2) ~ 3 min; (3) 10 ml HCl, 140 ml H2O, 10 g CuCl2 ~ 5 min; and (4) Kroll etching (20 ml HF, 10 ml HNO3, 70 ml H2O) ~5 min. The microstructural characterization of the directionally solidified alloy castings was performed by extracting samples at different transverse sections along the casting length, as shown in Fig. 3. The interval of the analysed sample positions, from the metal/mould interface, was established in about 3 mm in order to cover the positions where the cooling rate suffers a great variation. On the other hand, where the cooling rate almost stabilizes, from this position on, an interval of 10 mm was adopted. The samples were polished with 100, 200, 400, 600, 800 and 1200 grit SiC papers, and then finely polished with diamond paste (1 and 3 μm). The sample surfaces were then subjected
Fig. 6. Experimental position of the liquidus isotherm as a function of time. R2 is the coefficient of determination.
Fig. 7. Growth rate, VL, as a function of position (P) from the cooled bottom of the DS castings for monophasic Sn–Sb alloys during upward vertical directional solidification.
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to ultrasonic cleaning before etching with a solution of 2 ml hydrochloric acid (HCl), 3 ml nitric acid (HNO3), ethyl alcohol to achieve 100 ml. The etching was performed by carefully immersing the samples in the etchant for only a few seconds and about 30 s for the Sn– 5.5 wt.%Sb and Sn–2.2 wt.%Sb alloy samples, respectively, followed by cleaning the sample surface with running water and ethanol. Optical microscopy was performed using an Olympus Inverted Metallurgical Microscope (model 41GX). The cell spacing (λ c ) and the primary dendritic arm spacing (λ1) were measured from the optical images of the as-solidified samples (about 40 independent readings for each selected position, with the average taken to be the local spacing). The triangle method [37] was used in order to perform such measurements. A triangle is formed by joining the centres of the cross sections of three neighboring primary dendritic branches and the sides of the triangle correspond to λ1. Square central parts of the transversal samples, shown schematically in Fig. 3, were cut by a precision saw into pieces of about 3.0 mm of thickness and investigated by a Rigaku Rix 3100 X-Ray Fluorescence Spectrometer to estimate their average concentrations through an area of 100 mm2 probe. This permits the occurrence of macrosegregation along the casting length to be investigated. The same samples were used for XRD analyses. X-ray diffraction (XRD) patterns were obtained by a Panalytical X'pert PRO diffractometer in the 2θ range from 25° to 95° using Cu Kα radiation with a wavelength of 0.15406 nm. Transverse specimens for tensile tests were extracted from different positions along the DS casting length (see Fig. 3). These specimens were prepared according to specifications of the ASTM Standard E 8M/04 and tested in a MTS 810 machine at a strain rate of about 3 × 10−3 s−1. In order to ensure reproducibility of the tensile results, three specimens were tested for each selected position in order to determine ultimate/ yield tensile strengths and elongation. 3. Results and discussion Typical examples of experimental cooling curves for the eight thermocouples inserted into the casting, during solidification of the Sn–2 wt.%Sb alloy, are shown in Fig. 4. Fig. 5 shows typical examples of the resulting macrostructures of the DS castings of the Sn–2 wt.%Sb and Sn–5.5 wt.%Sb alloys. It can be seen that columnar structures characterize the entire casting length. The thermocouple readings, collected during solidification, were used to generate plots of position (P) from the metal/mould interface as a function of time (t) corresponding to the liquidus front of every alloy passing by each thermocouple. A numerical technique, based on the minimum square method, was used to fit mathematical power functions of the form P(t) = a tb (a; b are constants) on these experimental plots (see Fig. 6). The derivative of these functions with respect to time gave values for the growth rate (VL), as shown in Fig. 7. Moreover, the data acquisition system employed permitted accurate determination of the slope of the experimental cooling curves. Hence, the cooling rate (Ṫ) was determined along the casting lengths, by considering the thermal data recorded immediately after the passage of the liquidus front by each thermocouple (Fig. 8). In both Figs. 7 and 8 it can be seen that the use of a water cooled mould imposes higher values of growth rates and cooling rates, respectively, near the casting surface and a decreasing profile along the casting due to the increasing thermal resistance of the solidified shell with distance from the cooled surface. It can also be seen that the Sn–2 wt.%Sb alloy exhibits values of VL and Ṫ that are much higher than the corresponding values of the Sn–5.5 wt.%Sb alloy for positions closer to the casting surfaces. This seems to be associated with the higher wettability of the Sn– 2 wt.%Sb alloy, since the differences in both thermal parameters are significant only for regions that are close to the cooled surface of the casting. The directionally solidified alloy castings had local compositions examined along the length with a view to investigating the eventual
Fig. 8. Cooling rate, Ṫ, as a function of position (P) from the cooled bottom of the DS castings for monophasic Sn–Sb alloys during upward vertical directional solidification.
occurrence of long-range segregation. The alloy compositions along the casting length were determined by X-ray fluorescence and are shown in Fig. 9. It can be seen that the solute concentrations of Sb and Sn are essentially constants as a function of position from the cooled surface of the castings, and can be roughly represented by horizontal lines, i.e. the vertical directional solidification has not induced solutal convection conducive to macrosegregation profiles. Actually, Sn, the solvent is the rejected element at the solidification interface during the progress of solidification since the partition coefficient (k) is higher than 1.0. In fact, the corresponding k value of 1.18 cannot be considered a strong partition coefficient, since segregation does not occur for k = 1. Fig. 10 shows some typical transverse and longitudinal microstructures obtained along the length of the DS castings. Both alloys have the microstructures characterized by a Sn-rich matrix (Sn-β phase) surrounded by the Sn–Sb phase. The main difference is associated with the morphology of the Sn-rich matrix. The Sn–2 wt.%Sb alloy (Fig. 10a) exhibits a fully cellular microstructure along the entire casting. On the other hand, in the case of the Sn–5.5 wt.%Sb alloy casting (Fig. 10b), for regions closer to the cooled surface of the casting, the matrix has a cellular microstructure followed by a transition to a dendritic microstructure, characterizing an unexpected reverse dendritic/cellular transition. Cahn [4] reported that as the solidification velocity is increased for a given alloy composition and temperature gradient (GL), one observes a transition in structure: planar, cellular, dendritic,
Fig. 9. Composition profiles of Sn and Sb along the length of the DS castings (Sn–2 wt.%Sb and Sn–5.5 wt.%Sb alloys).
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cellular, and planar. According to Fig. 7, high values of growth rate can be realized near the metal/mould interface, which permits, if considering G constant at a first moment, to conclude that the cells located in this region are high velocity cells. Trivedi et al. [5] pointed out that the reverse transition from dendrite to cell is difficult to observe in situ for metallic systems and the that the scale of the microstructure is extremely fine and associated with high growth rates. However, the so-called high-velocity cells occurred in the present study for the Sn–5.5 wt.%Sb alloy DS casting even for moderate values of both growth rate (VL N 0.4 mm/s) and cooling rate (Ṫ N 1.2 K/s). The dendritic region occurred for Ṫ b 0.9 K/s or VL b 0.3 mm/s, and the transition region over which one structure changes into another was located in the intermediate range of the cited values.
57
Fig. 11 shows the XRD patterns of the Sn–Sb alloys. The samples examined in the present study exhibit the presence of peaks associated with Sn(β) and SnSb phases as previewed by the phase diagram of Fig. 2. Although the solidification conditions were far from the equilibrium, the Sb2Sn3 phase has not been detected. Five different positions were examined along each Sn–Sb alloy casting, encompassing a representative range of cooling rates. There were no clear tendencies regarding the intensities of peaks for the different positions from the casting cooled surface. With a view to analysing the role of the solidification thermal parameters on the scale of the as-solidified microstructure, the cellular (λc) and primary dendritic arm (λ1) spacings have been measured along the casting length. Fig. 12 shows that the variations in λ1 and λc
Fig. 10. Typical microstructure of Sn–Sb alloys — P is the position from the cooled bottom of the casting; λc is the cellular spacing; λ1 is the primary dendritic arm spacing; VL is the growth rate; Ṫ is the cooling rate. a) Sn–2 wt.%Sb alloy b) Sn–5.5 wt.%Sb alloy.
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Fig. 10 (continued).
with the growth rate (VL) may be characterized by − 1.1 power laws, which apply for both cellular and primary dendritic growth. The same exponent was reported in the literature, e. g. Rocha et al. [14] and Cruz [38], concerning the dendritic growth of Al–Cu and Al–Sn directionally solidified under unsteady-state conditions and Rosa and coauthors [39] for the cellular growth of dilute Pb–Sb alloys. The scale of the microstructures of the so-called high-velocity cells determined for the Sn–5.5 wt.%Sb alloy is in the range 15–35 μm, which are finer than those of the Sn–2.0 wt.%Sb alloy, i.e., 28–60 μm, considering the same range of VL values from 0.4 to 0.7 mm/s. The increase in the alloy solute content is associated with a decrease in the cellular spacing. The majority of industrial solidification processes occur under unsteady state regime, where temperature gradient and growth rate cannot be controlled and vary freely with time. For such heat flow conditions a
more appropriate solidification thermal parameter should encompass both the growth rate and the thermal gradient, as is the case of cooling rate that synthesizes both parameters (Ṫ = GL·VL). Correlations between λ1, λc and Ṫ have been established, as shown in Fig. 13, which are characterized by −0.55 power laws. The same exponent was also reported in the literature to apply for a number of binary alloys [40]. The cellular growth is observed to prevail in the Sn–5.5 wt.%Sb alloy casting for cooling rates higher than 1.2 K/s. As the cooling rate increased after the transition range, the cellular spacing decreased. The well-known Hall–Petch equation shows that the yield strength or flow stress is proportional to the reciprocal of the square root of the grain diameter, which means that the influence of any parameters other than the grain boundary is not included in the analysis. Inside the grains the effects of morphology and scale of the phases forming the microstructure,
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Fig. 11. Typical XRD patterns of (a) Sn–2 wt.%Sb and (b) Sn–5.5 wt.%Sb alloys for different positions along the casting length.
e.g. cellular or dendritic networks of continuously varying solute content, second phases, eutectic mixtures, and eventually porosity and inclusions, cannot be neglected on the resulting mechanical properties. Many experimental studies in the literature show that microstructure–mechanical properties relationships can be represented by Hall–Petch formulae, which depend on cellular, dendritic or eutectic interphase spacings [32, 41–43]. Goulart and collaborators [32] analysing Al–Fe hypoeutectic alloys, reported that ultimate tensile strength, elongation and yield stress are significantly affected by the cellular spacing. In their investigation, it has been observed that finer cellular spacings lead to improvements in elongation, ultimate and yield tensile strengths whilst an opposite effect is observed for coarser cells. The results of tensile tests of the present study are summarized in Figs. 14–16, where the ultimate tensile strength (σu), yield tensile strength, σy = 0.2 (0.2% proof stress) and elongation (δ)
are related to the cellular spacing or the primary dendrite arm spacing. It can be realized that σu, σy and δ for the Sn–2 wt.%Sb alloy were not affected by variations in the cellular spacing. On the other hand, for the Sn– 5.5 wt.%Sb alloy with the decrease in λc, σu and σy increase and δ decreases. However, after the cellular to dendritic transition, the aforementioned mechanical properties were also not affected by the scale of the dendritic structure (which is coarser than the cellular one). The SnSb phase is located along the intercellular/interdendritic regions and is supposed to act as a reinforcement phase. The lower the spacing characterizing the scale of the β-Sn matrix, more homogeneously distributed would be the SnSb phase with a consequential more efficient role in blocking dislocations. This is so for the cellular region of the Sn–5.5 wt.%Sb alloy, as reflected by the Hall–Petch experimental equations of Figs. 14b and 16. However, after the morphological transition from cells to dendrites,
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Fig. 12. Cellular (λc) and primary dendritic (λ1) spacings as a function of growth rate for the directionally solidified Sn–Sb alloys. R2 is the coefficient of determination.
which occurred with the decrease in cooling rates (hence producing dendritic regions with coarser spacings as compared to those of the previous cellular region), the distribution of the reinforcing phase seems not to be so efficient, thus leading to essentially constant mechanical properties. For the Sn–2 wt.%Sb alloy, despite the occurrence of fine cells, it seems that the lower volumetric fraction of the SnSb phase was not enough to provide a significant reinforcement, and the mechanical properties resulted essentially constants with the decrease in cellular spacing, as shown in Figs. 14a and 15. According to Figs. 14 and 16, in the range associated with highvelocity cells of the Sn–5.5 wt.%Sb alloy, the experimentally determined average σu, σy and δ values varied from 35–47 MPa, 31–40 MPa and 25– 43%, respectively. Such mechanical property values are quite reasonable as compared with those reported for the same alloy in the NIST database for solders [44] which indicates 35.2 MPa, 25.7 MPa and 22%, respectively. This demonstrates that the cellular morphology is able to improve the mechanical properties of the Sn–5.5 wt.%Sb alloy. This morphology can be obtained for cooling rates higher than 1.2 K/s.
4. Conclusions The following conclusions can be drawn from the present experimental investigation:
Fig. 13. Cellular (λc) and primary dendritic (λ1) spacing evolutions versus Ṫ for the Sn–Sb alloys. R2 is the coefficient of determination.
Fig. 14. Ultimate (σU) and yield (σy) tensile strength and as a function of cellular (λc) and primary dendritic (λ1) spacings for (a) Sn–2.0 wt.%Sb and (b) Sn–5.5 wt.%Sb solder alloys. R2 is the coefficient of determination.
• The relatively high cooling rates imposed by the water-cooled directional solidification apparatus used in the experiments were enough to guarantee the onset of high-cooling rate cells in the Sn– 5.5 wt.%Sb alloy, which occurred for Ṫ N 1.2 K/s. With the subsequent
Fig. 15. Elongation-to-fracture (δ) as a function of cellular (λc) spacing for the Sn–2 wt.%Sb alloy.
M. Dias et al. / Materials Characterization 106 (2015) 52–61
Fig. 16. Elongation (δ) as a function of cellular (λc) and primary dendritic (λ1) spacings for the Sn–5.5 wt.%Sb alloy. R2 is the coefficient of determination.
decrease in cooling rate during solidification, a transition cell/dendrite was shown to occur, i.e. the dendritic region occurred for Ṫ b 0.9 K/s. For the Sn–2 wt.%Sb alloy a cellular microstructure prevailed along the entire DS casting. • After the microstructural transition zone in the Sn–5.5 wt.%Sb alloy casting, correlations between the cellular spacing and tensile testing results — σu, σy and δ — permitted Hall–Petch type equations to be proposed relating λc to these mechanical properties. • The mechanical properties remained unchanged for both the dendritic region (which was shown to be coarser than the cellular one) of the Sn–5.5 wt.%Sb alloy casting and along the entire Sn–2 wt.%Sb alloy casting regardless of fine or coarse cellular structures.
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