370 Chiang Mai J. Sci. 2011; 38(3)

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1 370 Chiang Mai J. Sci. 2011; 38(3) Chiang Mai J. Sci. 2011; 38(3) : Contributed Paper Combined Effects of Boron, Sodium and Strontium on Grain Refinement of Sterling Silver Grade 950 Siriwan Sakultanchareonchai*[a], 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. *Author for correspondence; sakultan@gmail.com Received: 30 September 2010 Accepted: 3 December 2010 ABSTRACT Combined effects of B, Na and Sr on grain refinement of sterling silver grade 950 were investigated. The silver alloys with different composition of B, Na and Sr were prepared in three steps including pre-alloy preparation, master alloy preparation and finally sterling silver casting. Characterization was performed by inductively coupled plasma optical emission spectroscopy for determining chemical composition, and light microscopy, scanning electron microscopy and electron probe microanalysis for investigating microstructure. The tensile properties of the alloys were determined following the ASTM E 8M-96. The results revealed that micro-scale addition of B, Na and Sr plays a significant role in grain refinement. Effective grain refinement with bright surface appearance, no hot tearing crack and almost no micro-shrinkage can be achieved. The mechanism of grain refinement by combined effects of B, Na and Sr is yet to be understood, but it cannot be related to the distribution of micro-scale second phase. The highest ultimate tensile strength of the alloy achieved was about 178 MPa, nearly twice of that of the typical sterling silver alloy (about 90 MPa). This confirms advantages of the grain-refined alloy on jewelry application. Keywords: grain refinement, sterling silver, micro-alloying. 1. INTRODUCTION Silver has been widely used in the jewelry industry in Thailand because of its long-life decoration, luxury and low-cost comparing with other precious metals. There are several requirement on properties of silver jewelry following global demand, for example tarnish resistance, strength and spring properties. The development on alloy addition is still required for the last ten years by worldwide market. Silicon and zinc are normally added to sterling silver to increase tarnish resistance and casting ability. Zinc can reduce the melting point of alloys, level up whiteness, act as deoxidant and improve the fluidity of melting metal [1]. Silicon is known to perform useful deoxidization and brightening functions [1,2], but it also has a major drawback on promoting large as-cast

2 Chiang Mai J. Sci. 2011; 38(3) 371 grain size leading to brittle behavior especially hot tearing crack [2]. To overcome this problem, silicon content is controlled to be in the range of wt% in master alloys [3] and other alloying elements for grain refinement purpose can be added to reduce susceptibility to the formation of hot tearing crack, but the amount of usage is still ambiguous. Excessive addition will cause many intermetallic compounds instantly. Moreover, dramatic contraction of liquid metal during solidification can be spurred by inappropriate addition. Consequently, grain-refined alloys are always added in micro-alloying level. Somepatents [3,4] indicate effects of B on grain refinement in sterling silver. Unpublished works by one of the authors revealed that Na tended to increase solubility of boron in master alloys [5] and Ca can act as a grainrefiner [6], but the main problem of calcium usage is a contamination of Mg and Al in the pre-alloys so that Sr can be an alternative choice. In this work, combined effects of B, Na and Sr on grain refinement of Sterling silver grade 950 were investigated. 2. MATERIALS AND METHODS 2.1 Materials PREPARATION Raw materials of high purity grade were used. The chemical analysis of raw materials is given in Table 1. As a complicated casting process, materials preparation were separated into 3 steps including pre-alloy preparation, master alloy preparation and sterling silver casting. Table 1. Details of raw materials used in alloy preparation. Raw Materials Manufacture Grade Form Essay (%) Cu Pata Chemical & Machinery Co., LTD AR Rod >99.95 Zn Padang Industry Public Company Limited AR Ingot > Sn Pata Chemical & Machinery Co., LTD AR Ingot > Na Lucky Ocean Industrial LTD AR Ingot >99.98 Sr Lucky Ocean Industrial LTD AR Ingot >99.0 NaBH 4 AJEX FINECHEM (LABCHEM) AR Powder > Pre-Alloy Preparation Four types of pre-alloys were prepared from raw materials with the amount as given in Table 2. All are cast in an induction furnace with a vacuum-assisted system at the melting temperature as shown in the table.

3 372 Chiang Mai J. Sci. 2011; 38(3) Table 2. Chemical composition and melting temperature of pre-alloys. wt% by ICP-OES Pre-alloys Melting Temperature ( C) Sr Na B Si Sn Cu Cu-Sr 1, Bal. Cu-B-Na 1, Bal. Cu-Si 1, Bal. Sn-Na Bal Master Alloy Preparation Four types of master alloys were prepared from pre-alloys with the amount as given in Table 3 by casting in a vacuumassisted induction furnace at 1,200 C. Molten master alloys were dropped as granules for homogeneous mixing in the later step of sterling silver casting. 2.4 Sterling Silver Casting Five sterling silver alloys were cast with various materials as shown in Table 4 : one is the typical sterling silver alloy containing 5 wt% Cu (the alloy R) as the reference alloy for comparison, other four (the alloys A-D) are the alloys prepared from the master alloys for obtaining different composition of B, Na and Sr. All were melted in a vacuum induction furnace at 1,100 C with an induced vibration system. The molten metals were poured into a jewelry-plaster mold which heated at 580 C. After molten silver solidified completely, the silver tree was taken out from the mold and washed with high pressure water and then collected to further study. 3. MATERIAL CHARACTERIZATION 3.1 Chemical Composition Analysis Alloys were dissolved in an aqueous solution of 7%v nitric acid and 3%v hydrochloric acid. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Perkin Elmer Model ICP-Plasma -1000, was applied to determine chemical composition of the pre-alloys as shown in Table 2 and of the master alloys in Table Surface Appearance and Hot Tearing Crack Counting The silver trees of all five sterling silver alloys after casting were cleaned and counted by bear eyes for the number of hot tearing cracks as shown in Table Scanning Electron Microscopy (SEM) The specimens were taken from the portion of main sprue and cut as slices with 4-5 mm in thickness. The specimens were ground with silicon carbide papers, 180, 400, 600, 800 and 1,200 grits, respectively. They were then polished with 6, 3, 1 and ¼ m diamond pastes, respectively. No etching was performed. A JEOL, JSM 5910 LV-SEM was utilized and operated at 20 kv in backscattered electron image (BEI) mode to investigate eutectic and second phase distribution.

4 Chiang Mai J. Sci. 2011; 38(3) 373 Table 3. Weight percentage of pre-alloys used in preparation of master alloys and elemental analysis of master alloys. Master Alloys Pre-Alloys (wt%) Cu-Sr Cu-B-Na Cu-Si Zn Sn-Na Cu (%) Elemental Analysis by ICP-OES Na (ppm) B (ppm) Sr (ppm) Si (wt%) a Bal b Bal c Bal d Bal Table 4. Casting materials, grain size and number of hot tearing cracks in 950 sterling silver alloys. 950 sterling silver alloys Casting materials Grain size (mm) Average grain size(mm) Number of hot tearing cracks R 5 wt% Cu + 95 wt%ag > A 5 wt% master alloy a + 95 wt%ag B 5 wt% master alloy b + 95 wt%ag > C 5 wt% master alloy c + 95 wt%ag D 5 wt% master alloy d + 95 wt%ag Electron Probe Microanalysis (EPMA) The unetched specimens were also used for EPMA. A JEOL, JXA-8100 EPMA was utilised for determining element segregation and distribution. 3.5 Light Microscope (LM) The specimens were etched by an aqueous solution of 3 wt% chromic acid and 5 vol% H 2 SO 4. An Axio Lab.A1 MAT optical microscope was used in polarized light mode to reveal the microstructure. 3.6 Tensile Testing The ASTM E 8M-96 was followed for the tensile test. A universal testing machine, Instron Corporation, Series IX, was utilized. The test was performed at 150 kn load and 0.5 mm/min strain rate. Ultimate strength and yield strength were determined by the graphical method. 4. RESULTS AND DISCUSSION 4.1 Surface Appearance and Hot Tearing Crack Figure 1 compared the surface appearance of the silver alloys. Grey appearance indicating thick oxide layer on the surface can be seen in Figures 1(a), (b) and (e), which are the cases of the alloys A, B and R, respectively. The results indicated that a proper combination of Si, Zn and Sn with adding B, Na and Sr gave bright surface without hot tearing defects. On the other hand, dark surface with thick oxide film in the case of alloy R was activated by Cu additive. Thick oxide layer leads to a difficulty to remove the scale and then makes a disadvantage on increasing operating time and reducing productivity. Moreover, an increase in copper content gave less corrosion resistance and anti-tarnish property. Therefore, these alloys cannot be compatible to use as a long-life jewelry article with bright surface.

5 374 Chiang Mai J. Sci. 2011; 38(3) The alloy without Sr and enough B i.e. the alloys A and B had a large number of hot tearing crack. Actually, hot tearing phenomenon was intensified by higher Si addition (i.e. more than wt%si in 950 sterling silver), which related to the effect of Si to increase hot tearing sensitivity and grain growth in gold [2]. By adding Sr as in the alloys C and D, the number of hot tearing crack can be reduced, especially the alloy D in which no hot tearing crack was observed as in the typical sterling silver alloy. Figure 1. Surface appearance of 950 sterling silver alloys after casting without acid pickling : (a) alloy A, (b) alloy B, (c) alloy C, (d) alloy D and (e) alloy R. 4.2 Grain Size Figure 2 shows polarized-light micrographs revealing grain size of the alloys. The results from grain size measurement are given in Table 4. Although no hot tearing crack was found on the typical sterling silver alloy R, but its surface appearance is poor and its grain size is relatively large, at least 2.25 mm. Grain refinement is most successful in the case of the alloy D, in which the grain size at mm is about an order less than that of the alloy R. To reduce hot tearing defects, the appropriate amount of micro-alloying is required for extra fine grain such as in the case of alloy D, while non-enough micro-alloying, i.e. in the alloys A and B, still gave finer grain size, at least 1 mm, and released hot tearing defects. Although hot tearing crack in the alloys A and B was found, the bright surface from Si addition is still required for complicated manufacturing. Therefore, Si alloy has still been the important ingredient in sterling silver although it causes hot tearing and grain growth on cast metals. By observing characteristics of grain boundaries, it can be seen that highly undulating grain boundaries were found in the alloys C and D. This may imply pinning of grain boundaries in these alloys by nano-scale particle.

6 Chiang Mai J. Sci. 2011; 38(3) 375 Figure 2. Grain structure of 950 sterling silver alloys: (a) alloy A, (b) alloy B, (c) alloy C, (d) alloy D, (e) alloy R (polarised light images) and (f) alloy R (Macrostructure). 4.3 Phase Distribution and Element Mapping SEM-BEI micrographs showing distribution of second phase in the alloys are compared in Figure 3. Normally, a high density of second phase can be observed in grainrefined structure, especially in aluminium alloys [7]. Contrary, grain refinement effect in this work cannot be related in such a way to the distribution of second phase, obviously from Figure 3. Figure 4 displays the microstructure at grain boundaries at higher magnification in polarized light mode. Coarse distribution of second phase appears in the grain-refined alloy D instantly, while somewhat finer distribution of second phase was found in the alloy R with large grain size. Although high amount of Si increases the distribution of Si-rich phase which enlarge the size of second phase [5], there is no relation between grain refinement and phase distribution. Figure 5 is element mapping from EPMA of the alloy D showing enrichment of Cu and Si in the second phase. Boron, sodium and strontium are uniformly dispersed in the silver matrix. It is quite well-known that grain refinement in aluminum alloys is related to heterogeneous nucleation of micro-scale second phases e.g. TiB 2 or TiAl 3 [7, 8]. However, that mechanism cannot be applied for this study. The mechanism of grain refinement in the alloy C and D is not well-understood, but it may relate to solidsolution phenomenon or pinning by nano scale precipitates which are not revealed by EPMA.

7 376 Chiang Mai J. Sci. 2011; 38(3) Figure 3. Microstructure of 950 sterling silver alloys: (a) alloy A, (b) alloy B, (c) alloy C, (d) alloy D and (e) alloy R. (SEM-BEI). Figure 4. Comparison of second phase distribution in (a) alloy R and (b) alloy D (polarised light images).

8 Chiang Mai J. Sci. 2011; 38(3) Microshrinkage Observation In lost wax casting, the severity of liquidto-solid contraction can be disclosed by hot tearing crack and microshrinkage on specimen. Grain refinement can reduce or get rid of the hot tearing sensitivity and microshrinkage. Normally, small crack or microshrinkage will happen at the interdendritic area during solidification. In Figure 4, the large and long dendrite arms in the alloy R have higher sensitivity of microshrinkage than in those of the alloy D. Figure 6 confirmed that microshrinkage at the surface as a defect from casting of the alloy R happened while no such defect was observed on the surface of the alloy D. The serious case is the thin and complicate castings which require delicately casting surface and grain refinement to control this phenomena. The appropriate amount of grain refiners in the alloy D can get rid of almost all microshrinkage. Figure 5. Element mappings of the alloy D from EPMA.

9 378 Chiang Mai J. Sci. 2011; 38(3) Figure 6. Microshrinkage in (a) alloy R and (b) alloy D (SEM-SEI). 4.5 Tensile Properties Figure 7 shows ultimate tensile strength (UTS) and yield strength (YS) of the alloys. The average UTS of the alloy R (grain size at least 2.25 mm) is 90 MPa (MN/m 2 ), while that of the grain-refined alloy D ( mm grain size) is 178 MPa, increasing nearly 100%. Contrary, yield strength is comparable in all cases. This can be the effect of casting defects in the specimens. Grain refinement with appropriate chemical composition not only increases mechanical properties but also eliminates hot tearing and microshrinkage. From Table 4, hot tearing crack had been still remained in the castings with alloy A, B and C. Apparently, hot tearing not only relates to grain refinement, but also involves the other mechanisms such as thermal contraction at solidification, liquid film and oxide film formation,vacancy at grain boundary or solid/liquid interface and etc. [9]. Figure 7. Tensile strength and yield strength of 950 sterling silver alloys.

10 Chiang Mai J. Sci. 2011; 38(3) CONCLUSIONS 1. Micro-scale addition of alloying elements - B, Na and Sr to the 950 sterling silver alloy plays a significant role in grain refinement. The combined alloy D made from the master alloy with Na 105 ppm, B 12 ppm and Sr 151 ppm induced most effective grain refinement with bright surface appearance, no hot tearing crack, and almost no micro-shrinkage in the alloy. 2. The highest ultimate tensile strength (178 MPa) can be achieved in the alloy D, increasing nearly 100% as compared to that of the typical sterling silver alloy R (90 MPa). This is due to grain boundary strengthening mechanism. 3. The distribution of micro-scale second phase cannot be related to grain refinement phenomenon of the 950 sterling silver alloys. The mechanism of grain refinement using combined effects of B, Na and Sr may relate to solute segregation or pinning of nano-scale particles. ACKNOWLEDGEMENTS The authors express their gratitude to OLDMOON CO., LTD and the Thailand Research Fund (TRF) for materials and financial support, respectively. REFERENCES [1] Hywel J.A., WIPO. Patent 2006: WO2006/ [2] Mccloskey C.J., Welch R.P. and Aithal S., The effect of Silicon Deoxidation and Grain Refinement on the Production Performance of a 14 Karat Yellow Gold alloy, Gold Bulletin. 2001: 4-7. [3] Gamon P.J., UK Patent 2005: GB A. [4] Gamon P.J., WIPO. Patent 2007: WO2007/ [5] Nisaratanaporn E., The production of In-Si alloy for silver industry, Thailand Research Fund (TRF), Thailand, [6] Nisaratanaporn E., The production of anti-tarnish alloy with fine grain for silver industry, Thailand Research Fund (TRF), Thailand, [7] Kashyap K.T. and Chandrashekar T., Effects and Mechanisms of Grains Refinement in Aluminum Alloys, Bull. Mater. Sci., 2001; 24(4): [8] Mohanty P.S. and Gruzleski J.E., Mechanism of Grain Refinement in Aluminum, Acta Metal. Mater., 1995; 43(5): [9] Eskin D.G. and Katgerman L., A Quest for a New Hot Tearing Criterion, Metall. Mater. Trans., 2007; 38A: