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Implication of peritectic composition in historical high-tin bronze metallurgy Jang Sik Park a, , Cheol Woo Park b,1 , Keun June Lee b,2 a Department of Metallurgical Engineering, Hongik University, Chochiwon, Choongnam, 339-701, South Korea b Daejeon Science High School, Yuseong-gu, Daejeon, 305-338, South Korea ARTICLE DATA ABSTRACT Article history: Received 3 April 2009 Received in revised form 14 May 2009 Accepted 16 May 2009 Bronze alloys of varying tin contents from 0% to 28% were cast and then heated at elevated temperatures followed by quenching to examine the variation of microstructure, hardness and fracture characteristics. The results show that hardness increases with tin content and almost reaches the upper limit at 22% tin. Evidence has been found that the small-scale α dendrites spanning across the former β grains that were transformed to martensite serve as interlocking micro-bridges and thereby substantially reinforce the boundary strength to enhance fracture toughness. This effect is extremely sensitive to the α fraction and can best be obtained in alloys of near 22% tin. This specific composition, termed peritectic, seems optimal for sufficient strength without serious brittleness, and allows objects for a similar purpose to be made with less material. The choice of near peritectic composition in historical high-tin bronze metallurgy constitutes an excellent example of human adaptation to harsh environments where access to tin was limited and material cost had to be minimized. © 2009 Elsevier Inc. All rights reserved. Keywords: Historical high-tin bronze metallurgy Peritectic composition Forging Quenching Hardness Fracture toughness 1. Introduction Archaeological evidence has it that, from within a few centuries 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 best mechanical property, i.e., strength without brittleness [1]. (The tin content in this article is based on weight fraction.) The CuSn phase diagram in Fig. 1 [2] shows that this composition corresponds approximately to the upper limit to avoid formation of the δ phase in normal bronze casting, which is too brittle to accommodate impact loading either in fabrica- tion or in use. Therefore the CuSn alloys with tin content significantly above 10%, termed high-tin bronze, is fabricated primarily by casting to circumvent the problem arising from the brittle δ phase. High-tin bronze, with its lower melting points and better flow properties inside the mold, has long been used in the casting of bronze objects with complex shape or elaborate surface decoration, especially in ancient China where lead (Pb) was frequently added for further improvement of casting properties [3]. Surprisingly, high-tin alloys were often forged to produce some special bronze artifacts. Voce [4] may have been the first to report examples of forged high-tin bronzes in 1951. By examining microstructures of two Korean bronze bowls dated to the 12th to 14th century AD, he found the application of hot forging and quenching on alloys of about 20% tin. Further evidence was given on the use of a similar technology in Islamic Iran [5,6], Thailand [7,8], Central Asia [9,10], India [11], and Korea [12,13]. It is argued on archaeological evidence that this high-tin technology was practiced in India, Thailand, and Central Asia from as early as the 1st millennium BC. Goodway MATERIALS CHARACTERIZATION 60 (2009) 1268 1275 Corresponding author. Tel.: +82 41 860 2562; fax: +82 41 866 8493. E-mail addresses: [email protected] (J.S. Park), [email protected] (C.W. Park), [email protected] (K.J. Lee). 1 Tel.: +82 42 864 4511; fax: +82 42 863 5488. 2 Tel.: +82 42 862 2415; fax: +82 42 863 5488. 1044-5803/$ see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2009.05.009 available at www.sciencedirect.com www.elsevier.com/locate/matchar

Implication of peritectic composition in historical high-tin bronze metallurgy

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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

ava i l ab l e a t www.sc i enced i rec t . com

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Implication of peritectic composition in historical high-tinbronze metallurgy

Jang Sik Parka,⁎, Cheol Woo Parkb,1, Keun June Leeb,2

aDepartment of Metallurgical Engineering, Hongik University, Chochiwon, Choongnam, 339-701, South KoreabDaejeon Science High School, Yuseong-gu, Daejeon, 305-338, South Korea

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +82 41 860 2562;E-mail addresses: [email protected] (J.S

1 Tel.: +82 42 864 4511; fax: +82 42 863 54882 Tel.: +82 42 862 2415; fax: +82 42 863 5488

1044-5803/$ – see front matter © 2009 Elsevidoi:10.1016/j.matchar.2009.05.009

A B S T R A C T

Article history:Received 3 April 2009Received in revised form 14May 2009Accepted 16 May 2009

Bronze alloys of varying tin contents from 0% to 28% were cast and then heated at elevatedtemperatures 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 αdendrites spanning across the former β 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 α 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.

© 2009 Elsevier Inc. All rights reserved.

Keywords:Historical high-tin bronzemetallurgyPeritectic compositionForgingQuenchingHardnessFracture toughness

1. Introduction

Archaeological evidence has it that, from within a fewcenturies 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.) TheCu–Sn phase diagram in Fig. 1 [2] shows that this compositioncorresponds approximately to the upper limit to avoidformation of the δ phase in normal bronze casting, which istoo brittle to accommodate impact loading either in fabrica-tion or in use. Therefore the Cu–Sn alloys with tin contentsignificantly above 10%, termed high-tin bronze, is fabricatedprimarily by casting to circumvent the problem arising fromthe brittle δ phase. High-tin bronze, with its lower melting

fax: +82 41 866 8493.. Park), petermyst@hanm..

er Inc. All rights reserved

points and better flow properties inside the mold, has longbeen used in the casting of bronze objects with complex shapeor elaborate surface decoration, especially in ancient Chinawhere lead (Pb) was frequently added for further improvementof casting properties [3].

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. Byexamining microstructures of two Korean bronze bowls datedto 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 thatthis high-tin technology was practiced in India, Thailand, andCentral Asia from as early as the 1st millennium BC. Goodway

ail.net (C.W. Park), [email protected] (K.J. Lee).

.

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Fig. 1 – Cu–Sn phase diagram (quoted from “Meallography and microstructure of ancient and historic metals [2]).

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and 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 theα+β field of the Cu–Sn phase diagram. This technology offorging and quenching is in strong contrast to that oftraditional China, based on casting and the ternary Cu–Sn–Pb alloys with substantial variation in alloy composition [3].

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 theα+β phase field followed by quenching is then understood asan effort to keep away from the brittle δ phase in fabricationand use. This does not explain, however, the narrow range ofcompositions and temperatures consistently selected in

preference. The equilibrium Cu–Sn phase diagram contains awide range of other tin contents and temperatures that cansuppress the δ 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 Cu–Snalloys 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 (918–1392) andChoseon (1392–1910) dynasties of Korea [16].

2. Experiments

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

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Table 1 – Rockwell B-scale hardness numbers measured from the Cu–Sn alloys with varying tin contents and thermaltreatments.

# Sn % As-cast Quenched at 750 °C Quenched at 700 °C Quenched at 610 °C

1 0 8.3 (8.5, 8.0) a

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)

a Data in () are the hardness numbers from individual measurements.

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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 moltenalloy 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 5×5 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 two

Fig. 2 – Hardness (Rockwell B-scale) versus tin content in theCu–Sn alloys. The inset shows the variation of hardness overthe tin content from 0% to 28%.

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

Fig. 3 – Optical micrographs showing cracks formed near theindentation mark during the Rockwell B-scale hardnessmeasurements on the Cu–Sn 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).

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hardness tester with an indenter of 1/16 inch diameter steelsphere exerting a 100 kg load.

3. Results

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

Fig. 4 – Optical micrographs showing the structures of the Cu–Sn(×100), (b) 17% Sn (×100), (c) 20% Sn (×100), (d) 22% Sn (×100), (e)

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

alloys quenched at 700 °C and an EDS spectrum. (a) 15% Sn24% Sn (×100), (f) EDS spectrum from (d).

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hardness and microstructure to those quenched at 700 °C,particularly in 20% and 22% Sn alloys.

The specimens were then examined in the optical micro-scope for any deformation or cracks in the vicinity of hardnessindentation marks. Arrows in Fig. 3a–d, optical micrographs,locate cracks or deformation. Fig. 3a, taken from the etchedsurface of an as-cast 22% Sn specimen, shows crackspropagating through the α+δ eutectoid between the α regions.Fig. 3b, showing the as-polished surface of a 10% Sn specimenquenched at 750 °C, contains cracks and slip bands runningparallel in the α region. This specimen, with its low tin contentand the thermal treatment, is expected to consist mostly ofthe α 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 γ phases are able to accommodate a certain

Fig. 5 – SEM stereopair micrographs showing fracture surfaces oquenched at 750 °C (×200), (c) 20% Sn quenched at 700 °C (150), (

amount of plastic deformation and have some resistance tocrack formation within grains although their boundaryregions are vulnerable to fracture.

Fig. 4a–e illustrations are optical micrographs showing thevariation ofmicrostructureswith tin contents from 15% to 24%in specimens quenched at 700 °C. The specimens consist ofthe α and β-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 β 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%.

Fig. 5a–d, 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

f the Cu–Sn alloys. (a) 22% Sn as-cast (×500), (b) 22% Snd) 24% Sn quenched at 700 °C (×35).

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Fig. 7 – Optical micrograph showing structure of the Choseonspoon in Fig. 6a (×500).

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Fig. 3a, which shows the propagation of cracks in the as-cast22% Sn specimen, will make it clear that the columnscorrespond to the α phase in the form of dendrites while thecleaved planes represent the eutectoid regions. Fig. 5b–d, 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 β grain boundaries. The smallcolumns protruding up above the base surface are α 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 of Fig. 5bshows fracture through the interior of the former β grains;it, too, has α dendrites that are plastically deformed andembedded in the base surface. The fracture surfaces in Fig. 5cand d reveal the drastic change in fracture mode due to thedifference in microstructures caused by the deviation in tincontents from 22%. Fig. 5c, from a 20% Sn specimen, consistsmostly of α dendrites withmany side branches, some of whichremain intact while others are broken. By referring to Fig. 4c,it becomes clear that the intact dendrites are those due tothe fracture occurring along the interface between the α andthe martensite phase. In this 20% Sn specimen, these inter-phase boundaries apparently act as a preferential site forthe creation and propagation of cracks, causing a substantialreduction in fracture toughness. This peculiar fracturemode isalso implied in Fig. 3b, which shows the formation of cracks

Fig. 6 – Bronze artifacts excavated from a historical site atCheongju, Korea. (a) Spoon made in the Choseon period (AD1392–1910); (b) and (c) vessels made in the Koryo period (AD918–1392).

even in a specimen with a lower Sn content, i.e., 10%. Fig. 5d,the fracture surface of a 24% Sn specimen, contains twodistinct regions, one with an irregular surface and the otherwith an even surface. The former results from trans-granularfracture while the latter corresponds to interfaces exposedupon fracture; the interfaces in this higher Sn alloy correspondto the boundary regions of the former β grains that underwentthe martensitic phase transformation. Evidently, the grainboundaries are vulnerable to crack formation, and causefracture toughness to decrease, which is also expected fromFig. 3c. Experimentswith the specimens exclusively consistingof the γ phase as in Fig. 3d produced similar results, indicatingthat they are also prone to grain boundary fracture.

Fig. 6a–c shows three bronze artifacts excavated from ahistorical site at the city of Cheongju [17], most typical of thoseused continuously for nearly a thousand years in Korea. Fig. 6apresents a spoon made in the Choseon period; Fig. 6b and cdisplays bowls with and without a stand, respectively, bothmanufactured in the Koryo period. The microstructures of theartifacts were almost identical and similar to that illustratedin Fig. 7, an optical micrograph taken from the spoon. Themicrostructure consists of the twinned α grains spread in themartensite background, and the dark areas near the top andbottomresulted fromcorrosion. Three things are evident in themicrostructure when compared with Fig. 4a–e; first, the spoonwas made of bronze alloys of near peritectic composition;second, it was shaped by hot forging; third, it was finished byquenching, probably at around 700 °C. Notice that the walls ofall the artifacts are extremely thin, on the order of 0.2 mm,except at the handle of the spoon. These thin walls must beunderstood as an effort to minimize material cost.

4. Discussion

High-tin bronze fabricated by forging is a subject of curiosityfor two reasons; technical difficulties arise from the formationof the δ phase that causes brittle fracture in fabricationand use, and access to tin during pre-industrial periods waslimited. Forging at the proper temperatures followed byquenching is a necessary requirement to avoid the δ phase.

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This does not explain, however, the specific tin contents andquenching temperatures chosen consistently, for instance, inKorea for more than a millennium. The experimental resultsdescribed above identify several critical factors leading to theestablishment of this unique historical high-tin technology.

The data presented in Table 1 show that hardness, regard-less of the nature of thermal treatments, increases with tincontents up to 22%, where it begins to level off rapidly andreaches the maximum at 26%. The α phase, due to its softmaterial property, can support only a limitedportionof the loadexerted, and high hardness is duemainly to the second phasessuch as the δ and β-martensite phases. In the case of as-castsamples where the δ phase operates as the second phase,hardness increases almost linearly over a wide range of tincontent. In quenched specimens, the fraction of the β-martensite varies rapidly from nothing to 100% within a shortrange between approximately 15% and 24% Sn. Table 1 showsthat only the narrow range of tin contents from 20% to 24% cansuppress the δ formation without sacrificing hardness.

The ability of quenched specimens to resist crack forma-tion, discussed by referring to Fig. 3b and c, deviates signifi-cantly from the prediction based on the α fraction. Formationof cracks was found more pronounced in specimens of lowertin contents. This somewhat unexpected result is explained bythe analysis of fracture surfaces. In lower tin specimensconsisting primarily of the α phase with a little martensite,fracture occurs predominantly along the inter-phase bound-aries between the α and martensite phases. This is due to thelimited fraction of the strong martensite phase. The boundaryregions then become more susceptible to fracture due toextensive deformation resulting from the reduced load-bearing capacity. Lowering the tin content below 20% istherefore not an effective choice to improve toughness. Infact, the increase in tin contents improves the resistance tocrack formation. This effect, however, is counteracted by theoccurrence of fracture along the former β grain boundaries,and with the increase in tin contents, the fracture toughnessbecomes more dependent on boundary strength. Evidence isobserved, particularly in 22% Sn specimens, that the small-scale α dendrites, spanning across the boundaries, act asmicro-bridges interlocking the adjacent grains. This α phaseapparently reinforces the boundary regions, but the effect isextremely sensitive to the α fraction and can best be obtainedat tin contents close to 22%.

The superiority of the specific tin content near 22% Sn is bestillustrated by the bronze objects in Fig. 6, which represent thetechnological tradition practiced in Korea for more than1000 years ever since it was introduced, most probably, in theUnified Silla period (AD 668–935) [13]. It is impressive thatobjects with their wall thickness of 0.2 mm or less weresuccessfully forged out of high-tin alloys for practical purposes.Evenaminor violationof theoptimal conditionswouldnot havebeen allowed in achieving this kind of extraordinary technolo-gical sophistication. Similar objects are still beingmade inKoreastrictly following the traditional way in both alloy compositionsand quenching temperatures. The only difference of signifi-cance is found in their wall thickness, which is 1 mm or above,i.e., more than five times thicker than those in Fig. 6. Privatecommunication with the traditional bronze artisans currentlyworking at the Yongin Folk Village in Korea reveals that the

technical skill for such thin walls has been lost because there isno reason, either practical or economic, to keep it. This is a clearreflection of economic factors serving as a restriction in theestablishment of the historical high-tin bronze metallurgy. Thebest way to cope with the general problem of tin shortage facedbymost of thepre-industrialworld, includingKorea,wouldbe tosave costly materials in exchange for the intensive laborrequired in pushing the related technical sophistication to thelimit set by nature.

5. Conclusion

The presentwork has found several critical factors causing thenarrow range around 22% Sn and 700 °C in the Cu–Sn phasediagram to be consistently selected as the best operatingconditions in historical high-tin metallurgy where forging andquenching constitute two major elements in fabrication.

1. Alloys of near peritectic composition are among the fewCu–Sn alloys that do not sacrifice strength by suppressingthe brittle δ formation through quenching. The Rockwell B-scale hardness of quenched specimens increases rapidlywith tin contents up to 22%, and then levels off to reach themaximum soon.

2. Examination of the fracture surfaces shows that boundaryregions of the former β grains can be reinforced by the αdendrites interlocking the adjacent grains. This effect isextremely sensitive to the α fraction and is obtained mosteffectively at around 22% Sn. A little deviation from thisspecific composition deteriorates the mechanical proper-ties represented by strength and toughness; deviation tolower tin content reduces strength and has an adverseeffect on toughness due to the fragile inter-phase bound-aries formed between the α and martensite phases;deviation to higher tin content increases hardness but isdetrimental to fracture toughness due to the weak β grainboundaries.

3. Quenching temperatures also affect hardness, particularlyin alloys of near peritectic composition. Higher hardness isobtained in quenching at 700 °C and 750 °C than at 610 °C.

The balance between strength and toughness is the keyelements for superior mechanical properties of high-tinbronzes. The best balance can be obtained at tin contentsnear 22% and quenching temperatures at around 700 °C. Thiscondition is necessary if the cost of material, rather than thatof labor, has to be minimized for better economy, asexemplified in the extremely thin walls of Korean bronzeartifacts made continuously over a millennium. The unusualconsistency found in this unique historical high-tin technol-ogy may be regarded as an excellent example of humanadaptation to strict technological and economic requirements.

Acknowledgements

The authors would like to thank the Jungang Research Centerof the Cultural Heritage for providing the bronze objectsexamined in this work. This work was supported by the Korea

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Research Foundation Grant funded by the KoreanGovernment(MOEHRD, Basic Research Promotion Fund) (KRF-2008-D00573-I00015).

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