Implication of peritectic composition in historical high-tin bronze metallurgy

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<ul><li><p>sniwoKo</p><p>Received 3 April 2009</p><p>1. Introduction</p><p>Archaeological evidence has it that, from within a few</p><p>CuSn phase diagram in Fig. 1 [2] shows that this composition</p><p>points and better flow properties inside the mold, has long</p><p>or elaborate surface decoration, especially in ancient China</p><p>examining microstructures of two Korean bronze bowls dated</p><p>M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 1 2 6 8 1 2 7 5</p><p>ava i l ab l e a t i enced i rec t . com</p><p>omcorresponds approximately to the upper limit to avoidformation of the phase in normal bronze casting, which istoo brittle to accommodate impact loading either in fabrica-tion or in use. Therefore the CuSn alloys with tin contentsignificantly above 10%, termed high-tin bronze, is fabricated</p><p>to the 12th to 14th century AD, he found the application of hotforging and quenching on alloys of about 20% tin. Furtherevidence was given on the use of a similar technology inIslamic Iran [5,6], Thailand [7,8], Central Asia [9,10], India [11],and Korea [12,13]. It is argued on archaeological evidence thatcenturies after the first discovery of tin (Sn) alloying in copper(Cu) during the 4th millennium BC, the tin contents near 10%had long been perceived as a composition for the bestmechanical property, i.e., strength without brittleness [1].(The tin content in this article is based onweight fraction.) The</p><p>where lead (Pb) was frequently added for further improvementof casting properties [3].</p><p>Surprisingly, high-tin alloys were often forged to producesome special bronze artifacts. Voce [4] may have been the firstto report examples of forged high-tin bronzes in 1951. Byprimarily by casting to circumvent the prthe brittle phase. High-tin bronze, with</p><p> Corresponding author. Tel.: +82 41 860 2562;E-mail addresses: (J.S</p><p>1 Tel.: +82 42 864 4511; fax: +82 42 863 54882 Tel.: +82 42 862 2415; fax: +82 42 863 5488</p><p>1044-5803/$ see front matter 2009 Elsevidoi:10.1016/j.matchar.2009.05.009been used in the casting of bronze objects with complex shapeHardnessFracture toughnesstemperatures followed by quenching to examine the variation of microstructure, hardnessand fracture characteristics. The results show that hardness increases with tin content andalmost reaches the upper limit at 22% tin. Evidence has been found that the small-scale dendrites spanning across the former grains that were transformed tomartensite serve asinterlocking micro-bridges and thereby substantially reinforce the boundary strength toenhance fracture toughness. This effect is extremely sensitive to the fraction and can bestbe obtained in alloys of near 22% tin. This specific composition, termed peritectic, seemsoptimal for sufficient strength without serious brittleness, and allows objects for a similarpurpose to be made with less material. The choice of near peritectic composition inhistorical high-tin bronze metallurgy constitutes an excellent example of humanadaptation to harsh environments where access to tin was limited and material cost hadto be minimized.</p><p> 2009 Elsevier Inc. All rights reserved.Received in revised form 14May 2009Accepted 16 May 2009</p><p>Keywords:Historical high-tin bronzemetallurgyPeritectic compositionForgingQuenchingArticle history: Bronze alloys of varying tin contents from 0% to 28% were cast and then heated at elevatedImplication of peritectic compobronze metallurgy</p><p>Jang Sik Parka,, Cheol Woo Parkb,1, Keun JuaDepartment of Metallurgical Engineering, Hongik University, ChochbDaejeon Science High School, Yuseong-gu, Daejeon, 305-338, South</p><p>A R T I C L E D A T A A B S T R A C T</p><p>www.e l sev i e r. coblem arising fromits lower melting</p><p>fax: +82 41 866 8493.. Park), petermyst@hanm..</p><p>er Inc. All rights reservedition in historical high-tin</p><p>e Leeb,2</p><p>n, Choongnam, 339-701, South Korearea</p><p>/ l oca te /matcha rthis high-tin technology was practiced in India, Thailand, andCentral Asia from as early as the 1st millennium BC. Goodway</p><p> (C.W. Park), (K.J. Lee).</p><p>.</p></li><li><p>A TM A T E R I A L S C H A R A C T E R I Zand Conklin [14] and Sun and Wang [15] discussed thetechnical aspects of this technology applied in makingmusical instruments in the Philippines and in China. It is ofsignificance that previous studies consistently reported high-tin bronze objects of near peritectic composition, 22% tin,shaped by forging and then finished by quenching from the+ field of the CuSn phase diagram. This technology offorging and quenching is in strong contrast to that oftraditional China, based on casting and the ternary CuSnPb alloys with substantial variation in alloy composition [3].</p><p>The consistency found in the forged high-tin bronzes thathave been made in Korea for more than 1000 years suggeststhe presence of restrictions enforcing the selection of thespecific tin contents and the associated thermo-mechanicaltreatments. Without doubt the increased tin content isbeneficial in casting, but it can be detrimental to mechanicalworking unless the temperature is properly controlled, not tomention the disadvantage of high material cost due to thegeneral tin shortage in pre-industrial Korea. Forging at the+ phase field followed by quenching is then understood asan effort to keep away from the brittle phase in fabricationand use. This does not explain, however, the narrow range ofcompositions and temperatures consistently selected in</p><p>Fig. 1 CuSn phase diagram (quoted from Meallography1269I O N 6 0 ( 2 0 0 9 ) 1 2 6 8 1 2 7 5preference. The equilibrium CuSn phase diagram contains awide range of other tin contents and temperatures that cansuppress the formation and, at the same time, provideseemingly better material property or better economy. Thisstudy probes the implication behind the selection of peritecticcomposition in the high-tin bronze technology where theunique thermo-mechanical treatments of forging andquenching are necessary elements in fabrication. The CuSnalloys with varying tin content were prepared and giventhermal treatments for the control of microstructure. Hard-ness measurements were made on specimens with varyingmicrostructure and their fracture characteristics were exam-ined on polished surfaces as well as on fractured surfaces. Theresults were then compared with those obtained fromexamining bronze artifacts made in the Koryo (9181392) andChoseon (13921910) dynasties of Korea [16].</p><p>2. Experiments</p><p>Alloys were made to the target compositions as specified inTable 1 using copper and tin ingots of commercial purities. Thetin contents were chosen to cover the whole alloys that are</p><p>and microstructure of ancient and historic metals [2]).</p></li><li><p>typically used for making bronze objects. For each of the 10alloys of 0% to 28%Sn, a total of 1.0 to 1.5 kg ingots of copper andtin were charged in a clay crucible to be placed in an airenvironment of the electric furnace set at 1150 C. The molten</p><p>supports with a space of approximately 6mm. The tin contentswere estimated by the energy dispersive spectrometer (EDS)equipped in the SEM to bewithin a few tenths of a percent of thetarget compositions. Hardness measurements were made onspecimens in as-polished conditions using the B-scale Rockwell</p><p>Table 1 Rockwell B-scale hardness numbers measured from the CuSn alloys with varying tin contents and thermaltreatments.</p><p># Sn % As-cast Quenched at 750 C Quenched at 700 C Quenched at 610 C</p><p>1 0 8.3 (8.5, 8.0) a</p><p>2 5 24.3 (22.5, 26)3 10 52.3 (50.5, 54) 44.5 (45.0, 44.0)4 15 69.5 (69.5, 70.5, 68.5) 64.5 (63.5, 65.0) 63.5 (63.5, 63.5)5 17 80.8 (81, 80.5) 78.0 (77.0, 79.0) 74.0 (73.5, 75.5) 70.0 (73.0, 67.0, 70.0)6 20 93.8 (93.5, 94) 97.5 (98.0, 97.0) 96.3 (96.5, 96.0) 90.8 (90.5, 91.0)7 22 99.5 (99.5, 99.5) 103.5 (103.5, 103.5) 105.5 (105.0, 106.0) 92.3 (92.0, 92.5)8 24 104.0 (103.5, 104.5) 107.5 (108.0, 107.0) 104.5 (106.0, 103.0)9 26 113.8 (113.5, 114) 112.0 (112.0, 112.0)10 28 114.0 (114.0, 114.0) 112.0 (112.0, 112.0)</p><p>a Data in () are the hardness numbers from individual measurements.</p><p>1270 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 1 2 6 8 1 2 7 5alloy was stirred for mixing before it was cast in quartz orgraphite tubes approximately 1 cm in diameter and 15 cm inlength. The resulting alloys in long cylindrical form were thencut into short circular buttons 0.5 to 1 cm in height to be used asspecimens for thermal treatmentsat 610 C, 700 Cand750 C forapproximately 1 h before being quenched in water. Thesetemperatures correspondapproximately to the lower andupperlimit for practical thermal treatments to induce themartensiticphase transformation. Some of the specimens thus preparedweremounted and polished following standardmetallographicprocedures and then etched, if needed,with a solution of 100mldistilled water, 30 ml hydrochloric acid and 10 g ferric chloride.Theirmicrostructureswere examined in the opticalmicroscopeand the scanning electronmicroscope (SEM). Some of the alloyswere made into rectangular blocks approximately 55 mmin cross section and 10 mm in length with a notch placed inthe middle of the length, to be used as specimens for theexamination of fracture surfaces in the SEM. Fracture occurredby applyingan impact on anotched specimenplacedacross twoFig. 2 Hardness (Rockwell B-scale) versus tin content in theCuSn alloys. The inset shows the variation of hardness overthe tin content from 0% to 28%.Fig. 3 Optical micrographs showing cracks formed near theindentation mark during the Rockwell B-scale hardnessmeasurements on the CuSn alloys. (a) 22% Sn as-cast (200),(b) 10% Sn quenched at 750 C (200), (c) 24% Sn quenched at750 C (200), (d) 26% Sn quenched at 700 C (25).</p></li><li><p>hardness tester with an indenter of 1/16 inch diameter steelsphere exerting a 100 kg load.</p><p>3. Results</p><p>Table 1 presents hardness numbers, both those from indivi-dual measurements and their averages, versus compositionsand thermal histories. Fig. 2 shows the variation of hardnesswith tin contents from 15% to 25%, and the inset covers the</p><p>whole range from 0% to 28%. Several important facts are foundin Table 1 and Fig. 2. The as-cast hardness varies almostlinearly with tin contents up to 26%. The hardness ofquenched specimens increases sharply with tin contentsbetween 17% Sn and 20% Sn. In hardness, the quenchedspecimens are comparable to or even better than the as-castones only in the narrow range, particularly from 20% to 24%Sn. The specimens quenched at 610 C have lower hardnessthan those quenched at 700 C, most significantly in 22% Sn.The specimens quenched at 750 C produces similar results in</p><p>1271M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 1 2 6 8 1 2 7 5Fig. 4 Optical micrographs showing the structures of the CuSn(100), (b) 17% Sn (100), (c) 20% Sn (100), (d) 22% Sn (100), (e)alloys quenched at 700 C and an EDS spectrum. (a) 15% Sn24% Sn (100), (f) EDS spectrum from (d).</p></li><li><p>hardness and microstructure to those quenched at 700 C,particularly in 20% and 22% Sn alloys.</p><p>The specimens were then examined in the optical micro-scope for any deformation or cracks in the vicinity of hardnessindentation marks. Arrows in Fig. 3ad, optical micrographs,locate cracks or deformation. Fig. 3a, taken from the etchedsurface of an as-cast 22% Sn specimen, shows crackspropagating through the + eutectoid between the regions.Fig. 3b, showing the as-polished surface of a 10% Sn specimenquenched at 750 C, contains cracks and slip bands runningparallel in the region. This specimen, with its low tin contentand the thermal treatment, is expected to consist mostly ofthe phase possessing substantial ductility as is implied inthe slip bands. The cracks in Fig. 3b, however, demonstratethat sites prone to fracture still remain, most probably at thegrain boundary areas, although no second phase is visible inthe micrograph. Similar cracks were also observed in 15% Snand 17% Sn specimens that were thermally treated. On thecontrary, no cracks are found in Fig. 3c, the etched surface of a24% Sn specimen quenched at 750 C. Instead, a grainboundary, deformed and made visible during the hardnessmeasurement, was recognized. Similarly no cracks butdeformed grain boundaries are found in Fig. 3d, the as-polished surface of a 26% Sn specimen quenched at 700 C.The contrast around the indentation mark indicates theoccurrence of plastic flow. Fig. 3c and d shows that themartensite and phases are able to accommodate a certain</p><p>amount of plastic deformation and have some resistance tocrack formation within grains although their boundaryregions are vulnerable to fracture.</p><p>Fig. 4ae illustrations are optical micrographs showing thevariation ofmicrostructureswith tin contents from 15% to 24%in specimens quenched at 700 C. The specimens consist ofthe and -martensite phases. The fraction ofmartensite, notsignificant in Fig. 4a of a 15% Sn specimen, increases rapidlywith tin contents, filling the whole specimen at 24% Sn. The15% Sn specimen in Fig. 4a still contains a noticeable amountof the second phase, martensite, in inter-dendritic regionseven after being heated for 1 h at 700 C. Boundaries betweenthe former grains are visible only in Fig. 4c of the 24% Snspecimen; although unobservable in the micrographs theyshould exist in the others as well. Fig. 4f, an EDS spectrumtaken fromFig. 4d, containsmajor peaks at copper and tin, andthe tin level inferred from the spectrumwas close to the targetcomposition of 22%.</p><p>Fig. 5ad, showing SEM micrographs in stereopairs, pro-vides a true visualization of irregular fracture surfaces whenviewed througha stereo-viewer. Fig. 5a, fromanas-cast 22%Snspecimen, shows arrays of broken columns protruding upabove the base surface. When the figure is carefully examinedthrough a stereo-viewer, signs of significant plastic flow can beobserved in the column regions, in contrast to the bottomsurface that is covered with cleaved planes, which is char-acteristic of brittle fracture. Comparison of this figure with</p><p>1272 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 1 2 6 8 1 2 7 5Fig. 5 SEM stereopair micrographs showing fracture surfaces oquenched at 750 C (200), (c) 20% Sn quenched at 700 C (150), (f the CuSn alloys. (a) 22% Sn as-cast (500), (b) 22% Snd) 24% Sn quenched at 700 C (35).</p></li><li><p>Fig. 3a, which shows the propagation of cracks in the as-cast22% Sn specimen, will make it clear that the columnscorrespond to the phase in the form of dendrites while thecleaved planes represent the eutectoid regions. Fig. 5bd, fromspecimens quenched from700 C, is clearly distinguished fromFig. 5a in microstructure. The 22% Sn specimen shown inFig. 5b contains two regions with different morphologicalcharacteristics. The top region shows inter-granular fractureoccurring along the former grain boundaries. The smallcolumns protruding up above the base surface are dendritesembedded in the martensite phase across the grain boundarybefore they were fractured. They have signs of substantialplastic deformation, implying that they serve to increase theadhesive force of the boundaries. The bottom region...</p></li></ul>


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