242 Chiang Mai J. Sci. 2012; 39(2)

Chiang Mai J. Sci. 2012; 39(2) : 242-253http://it.science.cmu.ac.th/ejournal/Contributed Paper

The Role of Boron on Grain Refinement in SterlingSilver AlloySiriwan Sakultanchareonchai [a], Natthaphol Chomsaeng [b], Meechai Thepnarat [b],Hiroki Kurata [c], Seiji Isoda [d], Torranin Chairuangsri*[b] and Ekasit Nisaratanaporn [a][a] Department of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok

10330, Thailand.[b] Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200,

Thailand.[c] Advance Research Center for Beam Science, Institute for Chemical Research, Kyoto University, Uji,

Kyoto 611-0011, Japan.[d] Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida-Ushinomiya, Kyoto 606-8501,

Japan.*Author for correspondence; e-mail: tchairuangsri@gmail.com

Received: 27 October 2011Accepted: 16 March 2012

ABSTRACTThe role of boron on grain refinement in the Sterling silver alloys grade 950

containing 3.9%Cu, 0.08%Si, 0.11%Sn and 1%Zn without and with boron addition wasstudied. Microstructure of the silver alloys was investigated by light microscopy,electron probe microanalysis, scanning and transmission electron microscopy. To studymicrostructural phenomena at grain boundaries, thin foils for transmission electronmicroscopy were prepared by focused ion beam method, cross-sectional along the grainboundaries. The results revealed that addition of boron at less than 10 ppm level into thealloy reduced the grain size by a factor of about three. Nano-scale copper-rich particleswere found at grain boundary vicinity in the silver alloy with boron addition.Enhancement of segregation ability of copper and a possibility of co-segregation ofboron and carbon with copper to grain boundaries leading to precipitation of fine particleswere therefore proposed. This supports the constitutionally-undercooled-zone mechanismof grain refinement.

Keywords: Grain Refinement, Sterling Silver, Boron, TEM.

1.INTRODUCTIONAn understanding on the role of boron

in some alloys has been well-established.Boron in steels has long been recognizedmainly with respect to hardenability [1]. Inferritic steels, boron is added to increase

the hardenability by segregating toprior austenite grain boundaries andthereby suppressing the nucleation offerrite [2,3]. Boron segregation andprecipitation of M23(B,C)6 at austenite grain

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boundaries upon cooling from theaustenitizing temperature have an influenceon hardenability of boron steels [4]. Boroncan substitute for carbon or nitrogen inMX compounds in some alloy and stainlesssteels [4]. In austenitic stainless steels,boron can be present either as a traceimpurity or as an alloying addition in lowconcentration range (<100 ppm by weight).The presence of Mo in austenitic stainlesssteel has a pronounced effect on thesubsequent precipitation, giving Mo-richborides of the M3B2 and M5B3 types andtetragonal M2B, whereas in Mo-free steels,the Cr-rich M2B generally forms with anorthorhombic structure [5]. Williams andTalks [6] suggested that B may stabilizeM23C6 and postpone the formation of sigmaphase in 316 type steels. In HSLA steels,Sojka et al. [7] noted a synergistic effect ofCu and B on considerable increment inthe yield strength of HSLA steels. In thenickel-base alloys, segregation of B to grainboundaries has also been reported [8,9]and Ni3B intermetallic increased wearresistance in nickel-based alloys [10].In directionally solidified high chromiumwhite irons, boron was observed tosegregate mainly to the austenite/carbideinterface, it was also detected within theeutectic carbide and rarely as a boroncompound [11]. In Fe-Ni based alloys,appropriate boron addition, which affectedthe formation of h phase and borideprecipitation, can be an effective way oflowering hydrogen embrittlement sensitivityof the alloys [12]. Grain refinement ofaluminium and aluminium alloys bytitanium-boride particles has been addressed[13]. The combined effect of increasedundercooling of the eutectic reaction andthe effective number density of eutecticnuclei in the presence of strontium andboron can be the refinement mechanism in

near-eutectic Al-Si alloy [14]. In electrodealloys, boron strongly promotes theformation of the amorphous phase inthe rapid-quenched alloy [15]. For titaniumalloys, it has been reported recently thatboron addition at ~0.1-0.2wt% significantlyrefined the microstructure in the as-castcondition facilitating the subsequentprocessing and an improvement onmechanical properties from the presenceof hard titanium boride particles [16-19].Boron addition also affected microstructureand mechanical properties of Ti-basedshape memory alloys [20].

However, the role of boron in silveralloys is not well-understood. Few literaturescan be found regarding an addition ofboron as a micro-alloying element inSterling silver alloy grade 950, whichcontains 95wt%Ag. For silver alloys, grainrefinement can lead to improvement oftensile strength and reduction of hottearing crack [21]. Some patents [22,23] alsoindicate effects of boron on grainrefinement in Sterling silver alloys, butthe mechanism of boron on grain refinementhas not been described. Because Sterlingsilver alloy grade 950 has been widelyproduced by the jewelry industry, the roleof boron in grain refinement is thereforeinvestigated in this study. Microstructuralinvestigation was performed mainly byelectron microscopy to study microstructuralphenomena at grain boundaries. Focusedion beam method was utilized to preparecross-sectional thin foils for transmissionelectron microscopy along the grainboundaries. Finally, the mechanism of Bon grain refinement in these silver alloys wasdiscussed.

2. MATERIALS AND METHODS2.1 Materials Preparation and ChemicalAnalysis

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The alloys were melted in a vacuuminduction furnace at 1100°C and pouredinto plaster molds at 580°C. After completesolidification for ten minutes, the silvertrees were washed without acid pickling.Table 1 shows the chemical composition oftwo Sterling silver alloys grade 950 preparedTable 1. Chemical composition of the Sterling silver alloys determined by ICP-OES.

Sterling SilverAlloys

Cu Zn Si Sn B(%wt) (%wt) (%wt) (%wt) (ppm)

950A950B

3.85 0.96 0.077 0.112 0 3.84 0.95 0.073 0.111 8.46

in this work, without and with boronaddition. Inductively coupled plasma - opticalemission spectroscopy (ICP-OES), PerkinElmer Model ICP-Plasma-1000, was usedfor chemical analysis. Specimens weredissolved in an aqueous solution of 7 vol%nitric acid and 3 vol% hydrochloric acid.

2.2 Light Microscopy (LM)Specimens were etched by an aqueous

solution of 3 wt% chromic acid and 5 vol%sulfuric acid. Etched specimens werecharacterized by Axio Lab.A1 Mat polarizedlight microscope. Grain size measurementwas determined following the ASTME112-96 (Reapproved 2004). A planimetricmethod was used by inscribing a circle ofknown area on a micrograph. The sum ofall the grains included completely within thefield plus one half the number of grainsintersected by the circumference of the areagives the number of equivalent whole grains.This equivalent number should be in between50-100 grains and is then multiplied by theJeffries’ multiplier to obtain the number ofgrains per square millimeter. At least 3 fieldswere counted to obtain an average value.

2.3 Scanning Electron Microscopy (SEM)Specimens were taken from the main

sprue adhered to the riser of the cast treesand cut as slices with 4-5 mm in thickness.They were ground by silicon carbidepapers down to 1200 grits, then polisheddown to μm diamond pastes. Polishedspecimens were investigated by a JEOL-JSM 5910LV scanning electron microscopein backscattered electron image (BEI)mode. The accelerating voltage was 15-20

kV.2.4 Electron Probe Microanalysis (EPMA)

Polished specimens were also usedfor EPMA-1610 Shimadzu to determineelemental mapping. The accelerating voltage,beam current and beam diameter were 15 kV,20 nA and 3 μm, respectively.

2.5 Focused Ion Beam (FIB)Preparation of thin foils for transmission

electron microscopy by twin-jet electro-polishing technique was tried, but did notgive satisfactory results in these cast silveralloys. Firstly, solute segregation in the alloysled to local pitting during electropolishing.Secondly, it was very difficult to obtain thinareas at grain boundaries because of theirrelatively large grain sizes. Therefore, FIBmethod was chosen. A Quanta 200 3D FIBmachine was utilized and operated at 30 kVwith Ga+ ion beam. Firstly, 20 μm-thickspecimens were mechanically polished andetched by an aqueous solution of 3 wt%chromic acid and 5 vol% sulfuric acid toreveal grain boundaries. They were mountedon copper grids, which were cut half. FIBmilling was then performed cross-sectionallyalong grain boundaries as shown schematicallyin Figure 1. The procedure A started fromusing high current (20 to 10 nA) to reducethe thickness down to about 3 μm, following

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by medium current (1 to 0.1 nA) down toabout 100 nm obtaining a wedge shape, andthen by low current (0.05 to 0.01 nA) down

to about 30 nm as shown in Figures 2(a-b).Alternatively, the procedure B started fromhigh current followed by medium current

Figure 1. Schematic illustration of specimen mounted on Cu grid, which is cut half, and FIBmilled to prepare thin foils for transmission electron microscopy.

Figure 2. Two procedures for preparing thin foils for transmission electron microscopy byFIB. (a) Procedure A used low current Ga+ beam in the last stage to obtain thin wedge-shapeareas. (b) Secondary ion image shows a thin area prepared by the procedure A. (c) ProcedureB used medium current, tilted Ga+ beam in the last stage to obtain thin diamond-shape areas.(d) Secondary ion image shows a thin area prepared by the procedure B. (e) A plot betweenthe beam current and the corresponding thickness of specimen in the procedures A and B.

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to obtain a wedge shape but, instead ofusing low current, the medium current canbe maintained and the Ga+ beam wasglanced by ±1 degree forming a diamondshape as shown in Figures 2(c-d). It shouldbe noted that grain boundaries were clearlyrevealed in secondary ion image, but notin secondary electron image. A plotbetween the Ga+ beam current and thespecimen thickness is given in Figure 2(e).

2.6 Transmission Electron Microscopy(TEM)

After FIB preparation, thin foils wereinvestigated by a JEM-2010 and a JEM-2010transmission-scanning transmission electronmicroscopes (TEM/STEM), operated at 200kV, for bright-field transmission electronmicroscopy (BF-TEM) image, selected areaelectron diffraction pattern (SADP) and energydispersive x-ray spectrometry in transmissionelectron microscopy (TEM-EDS).

Figure 3. Polarized light micrographs show macrostructure of the alloy 950A with the averagegrain size 1.85 mm (a) and the alloy 950B with the average grain size 0.64 mm (b).

3. RESULTS3.1 Macrostructure and Microstructureof the Alloys

Light micrographs of macrostructureshowing grain size of both alloys are givenin Figure 3. Average grain size of the alloys950A and 950B is 1.85 mm and 0.64 mm,respectively. Addition of boron reducedthe grain size of the Sterling silver alloy by afactor of approximately three. Figure 4illustrates SEM-BEIs showing microstructureof the alloys 950A and 950B. Betweendendritic arms, secondary phases wererevealed with darker contrast. Althoughaddition of boron affected the grain size,the distribution of secondary phases wasnot significantly affected. From elementalmapping by EPMA in Figure 5, the secondaryphases contain mainly Cu and Si. On the otherhand, Zn distribution is more uniform in themicrostructure and B is uniformly distributedin dendritic areas.

a

b

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Figure 4. SEM-BEIs show microstructure of the alloys 950A (a) and 950B (b), respectively.

Figure 5. BEI image (a) and elemental maps (b-e) in EPMA show the microstructure of thealloy 950B and distribution of Cu, Si, Zn and B, respectively.

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3.2 Transmission Electron MicroscopyFigures 6(a-b) are BF-TEM micrographs

comparing the structure of grain boundariesin the alloys 950A and 950B, respectively.Grain boundaries in the alloy 950A is lessundulating and no particle is observed along.On the other hand, grain boundaries in thealloy 950B is obviously undulating (actually thiscan also be seen in FIB image in Figure 2).Fine particles are evidenced as indicated byarrows in Figure 6(b). Figure 6(c) is anenlargement of the area within the border inFigure 6(b), whereas fine particle at the grainboundaries is revealed by Moire’ fringescontrast. Size of these particles is about 10-20nm. It is likely that grain boundaries in the alloy950B were also pinned by these particlesresulting in an undulating grain boundarystructure. Figures 6(d-e) are correspondingselected area diffraction patterns (SADPs) fromthe two silver grains marked as ‘Ag(1)’ and

‘Ag(2)’ in Figures 6(b-c), indicating theirstructure as fcc-Ag, and their zone axes areclose to [ ]Ag(1) and [ Ag(2), respectively.Figures 6(f-g) are corresponding SADPsfrom the particle and the two surroundingmatrix grains in Figure 6(c) and their indexing.Most strong reflections are from the silvergrains constructing the grain boundary andfrom double diffraction by {hkl}Ag(1) to{311}Ag(2), which arises from overlapping ofthe two silver grains at the grain boundary.One strong reflection from the particle isobserved as marked by ‘A’ in Figure 6(g) andhas a measured d-spacing of 2.08 close to{111}fcc-Cu (2.088 , JCPDS file No. 04-0836)or {137}copper boron (2.08 , JCPDS file No.26-1111). This reflection occurs in thedirection parallel to the {111}Ag(1) reflection(2.354 , JCPDS file No. 87-0720), resultingin parallel Moire’ fringes with a spacing ofabout 1.76 nm in Figure 6(c).

Figure 6. BF-TEM micrographs show a grain boundary in the alloy 950A (a) and 950B (b),respectively. Fine particles can be observed as marked by arrows in (b). A particle within theborder in (b) was enlarged in (c) and parallel Moire’ fringes are clearly seen. SADPs in (d-f) arecorresponding to the Ag(1), the Ag(2) and the particle in (c), respectively. Indexing of theSADP in (f) is given in (g). The reflection marked ‘A’ is a strong reflection from the particle.

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Figure 7 are TEM-EDS spectra takenfrom the matrix and the particle, shown inFigure 6. Characteristic peak of Cu L atabout 0.93 keV was clearly detected. Theresults suggest that these particles are Cu-richand also contains high carbon content.Overlapping of the carbon K peak to the

Figure 7. TEM-EDS spectra taken from the Ag matrix and the Cu-rich particle inFigure 6(c).

boron K peak leads to a difficultyin identifying boron in the particles byTEM-EDS. It is possible that, duringsolidification in the molds, these alloyingelements, which had been segregated to edgesof dendrites, precipitated as small particlesobstructing migration of dendritic edges.

By the use of FIB, the secondary phasesoccurred between dendritic arms havealso been revealed. Figure 8(a) is a lowmagnification BF-TEM image from an areacontaining secondary phases connected to agrain boundary in the alloy 950B. Enlargedimage of the area inside the border is given inFigure 8(b). Surprisingly, fine polycrystallineswith the size less than 100 nm were observed,not as the typical structure as expected in thelast stage of solidification e.g. eutectic orperitectic. This can be the effect of the focusedion beam preparation on inducing a changein microstructure of these secondary phases.The SADP as inset in Figure 8(b) can beindexed as a combined pattern of fcc-Cu

and fcc-Si. This is in agreement to the resultsfrom elemental mapping from EPMA inFigure 5, in which high content of Si andCu in the secondary phases was indicated.

4. DISCUSSIONSterling silver alloys have been developed

to improve their properties regardingtarnishing or sulphidation resistance, strengthand productivity without embrittlementduring solidification and hot working,so-called “hot tearing or hot shortness”[24,25]. Normally, Cu and Zn are two majorelements used in Sterling silver. Cu acts as astrengthening element. Zn increases castabilityby decreasing the melting point and improving

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Figure 8. BF-TEM micrograph at low magnification of an area of secondary phasesconnected to a grain boundary in the alloy 950B (a). Higher magnification of the area inthe border is shown in (b) and the corresponding SADP as inset. The ring pattern inSADP indicated a mixture of fine polycrystallines of fcc Cu and Si. Two large light-greyareas in (a) are holes in the specimen created by ion beam.

the fluidity of the molten alloys, levels upwhiteness, and also acts as a deoxidant [25].Recently, the alloying development is focusedon anti-tarnishing properties by alloyingwith Zn, Sn and Si. Tarnishing resistancecan be promoted by Si and Sn. Si is notonly known to perform deoxidization andbrightening functions in the Sterling silveralloys grade 950 [26], but also has a major

drawback on promoting large grain sizewhich leads to brittle behavior, especially hottearing [24,26]. Si content is consequentlylimited in the range of 0.01-2 wt% in castsilver alloys [23]. Grain refinement is anapproach to improve strength and to reducethe detrimental effects of the Si-containingSterling silver alloys. Many alloying elementsas grain refiner are therefore added to reduce

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susceptibility of the silver alloys to theformation of hot tearing crack, but theamount of usage is still ambiguous. Excessiveaddition will cause many intermetalliccompounds and addition in a micro-alloyinglevel is typically applied. Boron has beenknown [14] to inhibit grain growth in silveralloys, even at low concentration less than20 ppm and at temperatures used forsoldering in the jewelry trade. However, themechanism has not been addressed.

How the solute elements affect grainrefinement during solidification can bedescribed by two mechanisms [27]. Firstly,the segregating elements cause restriction tograin growth in the melt, thereby thetransformation process is slowed down andthere is more time for nucleation events tooccur. Secondly, segregation of the soluteelements leads to a so-called “constitutionallyundercooled zone” in front of the growingsolid-liquid interface and nucleation canoccur on nucleants, thereby interrupting thegrowth of previous grains. The role of boronon grain refinement in aluminium andtitanium alloys has been explained by theboride-particle theory [13, 16-19], but theconstitutionally undercooled zone has beenaddressed for the case of the near-eutecticAl-Si alloy [14]. In this work, the constitu-tionally-undercooled-zone mechanism canalso be the case to explain effects of boronon grain refinement of the Sterling silveralloys. Cu-rich particles were observed at andnearby grain boundaries only in the alloy950B with boron addition. Even though thepresence of boron at grain boundaries hasnot been evidenced, it can be proposed thatboron enhances segregating ability of Cuand C and may also co-segregate to grainboundaries. This can lead to a constitutionallyundercooled zone, within which Cu-richparticles can precipitate. These precipitatescan act as nucleants for nucleation or the

solid-liquid interface in the melt can bepinned by these particles, as observed inthe case of the solid-solid interface duringgrain growth in the solid state. The Cu-richprecipitate has also been found in Ag-Cualloy with beryllium addition, which improvedthe modulus of resilience and springproperty of the alloy [28]. In HSLA steels,Sojka et al.[7] noted a synergistic effect of Cuand B on considerable increment in the yieldstrength of HSLA steels, which effectivelylowers the austenite-to-ferrite transformationand precipitation strengthening due to e-Cuparticles. Even though the lattice spacing ofthe precipitate in this study can be indexedalso as e-Cu, borides or borocarbides canalso be the case as observed in other boron-containing steels. Therefore, synergistic effectsof Cu and some other alloying elements onthe microstructure and properties of Steringsilver alloys should be further investigated.

5. CONCLUSIONSAddition of boron at less than 10 ppm

level into the Sterling silver alloys grade 950can cause grain refinement and the grain sizewas reduced by a factor of about three. Nano-scale Cu-rich particles were found at grainboundary vicinity in the boron-containing alloy.Therefore, it can be proposed that boronenhances segregation ability of Cu andperhaps co-segregates with Cu and carbonto grain boundaries leading to precipitationof Cu-rich precipitates. This supports themechanism of grain refinement by aformation of nucleants within constitutionallyundercooled zone in front of the growingsolid-liquid interface leading to nucleationand interruption of growth of previous grains.

ACKNOWLEDGEMENTThe authors wish to acknowledge the

financial and facility support by the OldmoonCo., Ltd., the Thailand Research Fund

252 Chiang Mai J. Sci. 2012; 39(2)

(TRF), and the Innovative Metals ResearchUnit, Department of MetallurgicalEngineering, Chulalongkorn University,Thailand.

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