high cooling rate, regular and plate like cells in sn-ni

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High Cooling Rate, Regular and Plate Like Cells inSn–Ni Solder Alloys

Marcella G. C. Xavier, Bismarck L. Silva, Amauri Garcia, and Jos�e E. Spinelli*

Broad ranges of cooling rates ( _T) 0.8–30.5 and 0.4–5.0 K s�1 are attainedduring directional solidification of eutectic Sn–0.2wt% Ni and hypereutecticSn–0.5wt% Ni alloys, respectively. A reverse high cooling rate cell-to-dendritetransition occurs for the eutectic composition and a transition from highcooling rate cells to plate like cells for the hypereutectic alloy. High coolingrate β-Sn cells are associated with cooling rates >5.5 and >2.7 K s�1 foreutectic and hypereutectic compositions, respectively. A processing diagram,relating the ‘ _T–Ni content’ space with the microstructural morphology, isproposed. A combination of plate like cells and plate NiSn4 eutectic phaseresults in higher ductility.

1. Introduction

An important reason to investigate Sn–Ni alloys relies oninformation that Ni is a very common substrate in electronicpackaging. The adoption of Ni as a substrate in industrialapplications related to Sn-based soldering is considered anadvantage due to the significant slower growth kinetics betweenNi and Sn as compared to that between Cu and Sn, for instance.Despite that, Sn–Ni alloys are also considered capable materialsto replace the traditional Sn–Pb solder as joint materials inelectronic products.[1,2] The formation/stability of intermetalliccompounds (IMCs) in Sn–Ni alloys (either growing primarily orforming the eutectic structure) was properly discussed in theliterature,[1–4] but the solidification morphologies of the β-Snphase are still unclear. The effects of cooling rate on themicromorphology of the Sn-rich matrix and on the evolution ofthe microstructure of Sn–Ni alloys have not been completelyinvestigated so far.

Prof. J. E. Spinelli, M. G. C. XavierDepartment of Materials EngineeringFederal University of S~ao Carlos-UFSCar13565-905 - S~ao Carlos-SP, BrazilE-mail: [email protected]

Prof. B. L. SilvaDepartment of Materials EngineeringFederal University of Rio Grande do Norte-UFRN59078-970 - Natal-RN, Brazil

Prof. A. GarciaDepartment of Manufacturing and Materials EngineeringUniversity of Campinas-UNICAMP13083-860 - Campinas-SP, Brazil

The ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adem.201701179.

DOI: 10.1002/adem.201701179

Adv. Eng. Mater. 2018, 1701179 © 21701179 (1 of 7)

During solidification of a monophasicalloy, two possible microstructural transi-tions involving plane front to cells mayoccur. If the growth rate, v, is increased themorphology of the solidification interfacecan vary from planar to cellular, todendritic, to cellular again and finallyachieving once more a planar front.The first transition at low ‘v’ is related tothe limit of constitutional supercoolingwhereas the second one (high ‘v’) is relatedto the limit of absolute stability.[5,6] Cellularsolidification refers to a particular mode ofgrowth in which the solid/liquid interfaceassumes the morphology of cells, which isconsidered the morphology that precedes

the dendritic structure in the presence of a positive temperaturegradient in the liquid. The parameter ‘α’ is derived from theconstitutional supercooling criterion[7] combining three varia-bles in its definition, which are temperature gradient (G), growthrate (v) and alloy composition (C0), as defined by Equation (1):

α ¼ GDkvmC0ð1� kÞ ; ð1Þ

where ‘k’ is the partition coefficient, ‘m’ is the liquidus slope, and‘D’ is the solute diffusivity in the liquid.

It is well known that the temperature gradient (G), the growthrate (v), and the cooling rate ( _T¼Gv) are the thermal parametersgoverning the morphology and scale of as-solidified micro-structures.[8–11]

From the reported experimental observations,[12,13] it can beinferred that a certain range of α allows a cellular morphology tobe obtained. For a regular array of cells, a critical α (α<< 1) mustbe reached. “Pox” structures or irregular cells are expected toprevail for α varying between this critical value and unity (1.0).However, there are still no systematic experimental or theoreticalstudies of the growth of regular-irregular cells on various alloyssystems. According to Xu et al.[14] the growth of cells may followthe path: poxes, plate-like cells and regular cells.

Despite the studies existing in the literature on stable cellconfigurations,[15] studies emphasizing the microstructuraltransitions close to the absolute stability limit of metallic alloysare very scarce. Knowledge on this field persists focused on Znalloys, Al alloys, and steels. For example, an investigation inwhich the growth velocities varied up to 4.8mms�1 with aZn–1.52wt% Cu alloy was performed by Ma et al.[16] Theyobserved a transition from regular cells to plate-like cells whenthe growth velocity exceeded 1.0mms�1. Another study[17]

revealed that the morphology of the Zn-rich phase of Zn–0.3wt

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% Mg and Zn–0.5wt% Mg alloys varied from high cooling rateplate-like cells (for cooling rates>9.5 and>24K s�1, respectively)to a granular-dendritic-like transition region for lower coolingrates.

Fu et al.[18] investigated the effect of cooling rate in as-castsamples of AISI 304 stainless steel. They reported the occurrenceof a transition from dendritic austenite to cellular austenite athigh cooling rates. In the case of austenitic steel welds, Inoueand Koseki[19] reported the growth of cellular austenite (γ) as asecondary phase formed at the boundaries of the precedingprimary ferrite (δ) during solidification. According to thisresearch, the formation of ‘vermicular’ or ‘lacy’ ferrite may beaffected by the cellular growth of austenite since the dominanceof one ferrite over the other may depend on the heat flowdirection and on the preferential growth directions of bothphases (γþ δ) during welding. The structural characteristics ofthese ferrites as well as of austenite can have significantinfluence on stainless-steels weld properties.

According to Kurz and Fisher[6] there are two possible types ofmorphological instabilities during growth of binary eutectics:single-phase or two-phase. A single-phase interface, for instance,can prevail due to long-range solute layer accumulation aheadthe solidification front. Under such conditions, one phasebecomes heavily constitutionally undercooled. Consequently,dendrites/cells formed by one phase and interstitial two-phaseeutectic may characterize the final structure.

To gain insight into the morphology of the Sn-rich phaseformed in Sn–Ni alloys, transient directional solidificationexperiments were carried out with Sn–0.2wt% Ni and Sn–0.5wt% Ni alloys. The study aims, firstly, typify the Sn-rich cells anddendrites by performing a complete search along the length ofthe Sn–Ni alloys castings. Secondly, to establish the variations ofcellular/dendritic spacings with cooling rate and, finally, outlinecorrelations between the local length scale of the microstructure,the formed IMCs and tensile mechanical properties.

2. Experimental Section

2.1. Directionally Solidified Castings

The eutectic Sn–0.2wt%Ni and the hypereutectic Sn–0.5wt%Nialloys have been assessed. So, two directionally solidified (DS)Sn–Ni alloys castings were produced by using a water-cooledsetup.[20] The referred solidification system gives rise tounsteady-state solidification conditions. The surface of thecopper bottom-part mold has been finished with a 1200 grit SiCabrasive paper. The following procedures were performed foreach alloy: firstly, the alloy was melted in situ by radial electricalwiring positioned around a cylindrical stainless-steel container.Secondly, when the melt temperature was about 10% aboveeither the eutectic (0.2Ni) or the liquidus (0.5Ni) temperatures,the electric heaters were disconnected and at the same time thewater flow at the bottom of the container was started, whichallows the onset of solidification. Finally, the evolution oftemperatures along the length of the casting was measured byfine type J thermocouples (0.2mm diameter wire), which wereplaced in the geometrical center of the cylindrical mold cavityalong its length. The temperature-time records were stored with

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a view to permitting the cooling rates to be determined.[17]

Cylindrical DS castings having the dimensions 140mm high� 55mm diameter were produced. Several specimens weresectioned from the bottom to the top of each casting with a viewto permitting the scale of the microstructures and phasesmorphologies to be assessed and correlated with the evolution ofthe cooling rate along the length of the castings.

2.2. Analytical Tools

Metallographic analysis was performed to reveal the microstruc-tural and morphological details of both examined Sn–Ni alloyscastings, using grinding and polishing steps combined with theetchant 92% (vol) CH3OH, 5% (vol) HNO3, and 3% (vol) HCl.Micrographs were obtained using a light microscope with acoupled optical image processing system Olympus, GX51(Olympus Co., Japan) and using a Field Emission Gun (FEG)� Scanning Electron Microscope SEM-EDS FEI (Inspect S50L).The triangle method was employed to determine both theprimary dendritic arm spacing (λ1) and the cellular spacing (λc)on transverse sections of the DS castings.[21] Both Sn–Ni alloyscastings were entirely characterized across their lengths sothat the microstructural morphologies and transitions weredetermined.

Transverse specimens extracted from different positionsalong the length of the DS castings were prepared accordingto specifications of the ASTM Standard E 8M/04 and tested in anInstron 5500Rmachine at a strain rate of about 1� 10�3 s�1. Theultimate tensile strength and elongation-to-fracture have beendetermined and related to average microstructural spacings. Thesub-size flat specimens for tensile tests were: length¼ 46mm,thickness¼ 2 and 8mm width of grip section with 25mm gagelength.

3. Results and Discussion

Two general microstructure features can be observed in thedirectionally solidified Sn–Ni samples, as shown in Figure 1.One is composed by the β-Sn matrix and other formed by theinterstitial eutectic β-Snþ (Cu,Ni)Sn. Primary Ni3Sn4 interme-tallic particles (IMCs) were found in the first examined positions(range of higher cooling rates) of the DS Sn–0.5wt% Ni alloycasting. This agrees with the study performed by Belyakov andGourlay[1] with Sn–Ni alloys, in which for hypereutecticcompositions and cooling rates between 1 and 10K s�1, a verylarge fraction of primary Ni3Sn4 IMCs grew to the detriment ofNiSn4 particles.

After examining various sections along the length of the DScastings, many morphologies have been identified, as shown inFigure 1. Parallel SEM and optical microstructures summarizethe morphological aspects for particular ranges of eutecticcooling rates. High cooling-rates cells of regular morphologycharacterize both examined alloys for samples from regionsclose to the water-cooled surfaces of the castings, that is, coolingrates>5.5 and>2.7K s�1 for the Sn–0.2wt%Ni and Sn–0.5wt%Ni alloys, respectively. Thus, the increase in the alloy Ni contentis shown to induce the growth of this type of cells for lowercooling rates.

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Figure 1. Typical microstructures along the length of the directionally solidified Sn–0.2 wt% Ni and Sn–0.5 wt% Ni solder alloys castings, highlightingthe different morphologies typifying the β-Sn phase and their prevalence in the microstructure as a function of cooling rate.


A bit farther away from the bottom surface of the Sn–0.5 wt% Ni alloy casting, which signifies lower cooling rates, aparticular Sn-rich plate-like cellular microstructure is formed,despite the non-equilibrium solidification conditions. Theseplates seem to grow side by side, as shown in Figure 1. Thedeep-etched SEM images permit the micromorphologicalaspects of the matrix to be emphasized, as previouslydiscussed. According to Curreri et al.,[22] the decrease in

Adv. Eng. Mater. 2018, 1701179 1701179 (

growth rate from a certain distance from the cooled surface ofthe DS casting may cause the cell trunk to lag behind (andbecome engulfed by) the dendrite growth rate, thus increasingthe spacing. This seems to happen from a certain positionalong the length of the Sn–0.2 wt% Ni alloy casting. Here,high cooling rate cells having regular morphology weretransformed into dendrites with the decrease in cooling rate,characterizing a reverse cell/dendrite transition.





For Pb-based alloys[23] the growth of plate-like cells has beenrelated to increase in the constitutional undercooling. In thiscase, the plane front was reported to become uneven. For highersolidification velocities, these cells may develop a regular cellularmorphology. Similar observations were reported by Xu et al.,[24]

with plate-like cellular arrangements being the favorablemorphology when the growth conditions approach those of astable planar solidification front. In an investigation on frontstability during solidification of a dilute SCN-acetone alloy,[25] abreak-down of the planar front was reported to occur with theprogressive increase in the solidification velocity, which leaded tothe formation of two-dimensional plate-cells followed by thegrowth of three-dimensional regular cells. The cited findingscorroborate with the morphologies observed in the present studyfor the hypereutectic Sn–0.5wt% Ni alloy, with the growth of

Figure 2. a) Cooling rate related to the eutectic interface during transient dcastings; b) Cellular/dendritic spacing as a function of cooling rate; and c) evoNi content.

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high cooling rate regular cells preceding the plate-like cellularmorphology.”

_T was determined along the casting length, by consideringthe thermal data recorded immediately after the passage of theeutectic front by each thermocouple. The values of _Twere plottedin Figure 2a against the position P in both Sn–0.2wt% Ni andSn–0.5wt% Ni alloys DS castings so that the influence of the Nicontent could be noticed. Figure 2b shows the experimentalscatters for cellular (either regular or plate-like) and primarydendritic spacings, λc and λ1, respectively, as a function of thecooling rate. The plots in Figure 2a allow resolving theexperimental cooling rates related to various positions alongthe casting lengths of each examined alloy.

The experimental growth relations as a function of cooling ratederived for both alloys can be represented by power functions

irectional solidification of the Sn–0.2 wt% Ni and Sn–0.5 wt% Ni alloyslution of microstructural morphologies with the cooling rate and the alloy




Figure 3. Composition of the intermetallics phases (gray color) forming the eutecticmixture in both a) the Sn–0.2wt%Ni and b) the Sn–0.5wt%Ni alloys.


having a�0.55 exponent. If a specific cooling rate is examined inthe plots of Figure 2b, the microstructural spacing of both alloysremains very close. Finally, Figure 2c shows a ‘ _T–Ni content’map

Figure 4. a) Representative tensile stress–strain plots, and interrelations betwλ1 spacing for the Sn–0.2 wt% Ni and Sn–0.5 wt% Ni alloys: b) σu versus (

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showing longitudinal microstructures and describing thevariation of the β-Sn morphology with cooling rate for both theeutectic and hypereutectic Sn–Ni alloys.

een the tensile mechanical properties and the cell� λc/primary dendritic–λc,1)

�1/2; and c) δ versus (λc,1)�1/2.





From the viewpoint of the formation of intermetallics,Figure 3 shows that very distinct eutectic phases have beenformed in each alloy casting. (Cu,Ni)6Sn5 fibers characterize theeutectic structure of the Sn–0.2 wt%Ni alloy while NiSn4 “plates”prevail in the Sn–0.5wt% Ni alloy, representing the typicalmicrostructures found along the length of the DS castings. Itseems that the dissolution of Cu (from the bottommold) into theSn–0.2wt% Ni alloy induced the growth of the “rod-like” (Cu,Ni)6Sn5 eutectic phase, as confirmed trough the present SEM-EDS results and through comparisons with similar resultsreported elsewhere.[1] The morphologies and the compositionsof the phases make possible a clear identification to beperformed.

Figure 4a depicts typical stress–strain curves related tospecimens extracted from the DS Sn–0.2 wt% Ni and Sn–0.5 wt% Ni alloys castings: close to the cooled bottom (position,P¼ 6mm) and close to the top (position, P¼ 90mm) of thecastings. In Figure 4b and c, the tensile strength (σu) andelongation to fracture (δ) are plotted as a function of themicrostructural spacings. These plots were conceived in such away that Hall–Petch type relationships could be established forboth Sn–Ni alloys. σu increases with decreasing λ1,c for bothSn–Ni alloy castings. A better distribution of the reinforcingeutectic phases in the microstructure due to the finer λ1,cseems to be the reason for improving the strength, as can beseen in Figure 4b. The load transfer from Sn-rich matrix to the

Figure 5. SEM fracture morphologies of the a), b) Sn–0.2 wt% Ni and c), ddifferent positions (i.e., a), c) 6mm and b), d) 90mm) from the cooled su

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rigid highly interconnected eutectic phases seems to cause thecontribution to the strength of Sn–Ni alloys. Oppositetendencies were noted for the variation in ductility ascompared to those established for the strength plots. Eventhough δ values of both evaluated alloys remained close forλc

�1/2>0.13, a reverse trend is shown to be associated withhigher λc, that is, for cellular regions in the Sn–0.5 wt% Ni alloycasting in which the cells have grown at slower coolingconditions. The hypereutectic alloy microstructures in theseregions consist of Sn-rich plate-like cells surrounded by theβ-SnþNiSn4 eutectic structure (see Figures 1; 3b). Bundles ofplate-like cells seem to grow side by side and in a coupled waywith the eutectic mixture during the directional growth,resembling a eutectic-like lamellar growth (see Figure 1).Owing to the increase in contact area between the phases, anenhancement effect on the sliding of the interfaces betweenadjacent phases is expected. Such complex interaction mayoccur especially during necking, with provides a directbeneficial effect to the ductility.[26] This seems to explain thedifference in ductility shown in Figure 4c.

Two different positions of each alloy casting were examinedregarding their SEM fracture surfaces, as can be seen inFigure 5. All specimens showed a prevalence of dimples,which indicates that a ductile mode of fracture occurred.The presence of primary Ni3Sn4 particles has been mainlynoted in the Sn–0.5 wt% Ni alloy samples related to the

) Sn–0.5 wt% Ni alloys corresponding to tensile specimens extracted atrface of the DS castings.





positions close to the bottom of the casting, as can be seen inthe inlet image of Figure 5c. It appears that some deep cavitieswere induced to occur throughout these particles, which aremostly typical of the microstructure of the Sn–0.5 wt% Ni alloy.

Elongated dimples are consistent with the plate-like cells onthe hypereutectic alloy microstructure, as can be seen inFigure 5d. Furthermore, rounded dimples in Figure 5a, forfractures referring to the position 6mm, may be associated withthe eutectic Sn–Ni alloy microstructure in such regions, that is,very fine high cooling rate cells.

4. Conclusions

High cooling-rate cells of regular morphology were shown tocharacterize the microstructure of the Sn-rich matrix of theSn–0.2wt% Ni and Sn–0.5wt% Ni alloys, that is, cooling rates>5.5 and>2.7 K s�1, respectively, indicating that the higher alloyNi content has induced the growth of this type of cells for lowercooling rates. The microstructure of the Sn–0.2wt% Ni alloycasting was shown to experience a transition from high coolingrate cells to dendrites with the decrease in cooling rate, whereasfor the Sn–0.5wt% Ni alloy casting a transition from highcooling rate cells to plate-like cells occurred. (Cu,Ni)6Sn5 fibersformed the eutectic structure of the Sn–0.2wt% Ni alloy whileNiSn4 plates prevailed in the eutectic mixture of the Sn–0.5wt%Ni alloy. Experimental growth laws relating the microstructuralspacings, λ1,c, to the cooling rate have been proposed, as well asHall–Petch type experimental equations relating tensile proper-ties to such spacings. The ultimate tensile strength was shown toincrease with the decrease in λ1,c and a combination of plate-likecells and plate NiSn4 eutectic phase was shown to result inhigher ductility.

AcknowledgementThe authors thank the financial support provided by FAPESP (S~ao PauloResearch Foundation, Brazil: grants 2015/11863-5; 2016/18186-1; 2016/10596-6; 2017/15158-0), CNPq, and CAPES.

Conflict of InterestThe authors declare no conflict of interest.

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Keywordsmicrostructure, Sn–Ni, tensile properties

Received: December 28, 2017Revised: February 14, 2018

Published online:

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high cooling rate, regular and plate like cells in sn-ni

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