Ancient Metal Mirror Alloy Revisited: Quasicrystalline Nanoparticles Observed J.A. SEKHAR, 1,2,3 A.S. MANTRI, 2 S. YAMJALA, 4 SABYASACHI SAHA, 4 R. BALAMURALIKRISHNAN, 4 and P. RAMA RAO 5,6 1.—Institute of Thermodynamics and Design, Cincinnati, OH 45215, USA. 2.—University of Cincinnati, Cincinnati, OH 45221, USA. 3.—MHI Inc., Cincinnati, OH 45215, USA. 4.—Defence Metallurgical Research Laboratory, Hyderabad 500058, India. 5.—International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad 500005, India. 6.—e-mail: [email protected]This article presents, for the first time, evidence of nanocrystalline structure, through direct transmission electron microscopy (TEM) observations, in a Cu- 32 wt.% Sn alloy that has been made by an age-old, uniquely crafted casting process. This alloy has been used as a metal mirror for centuries. The TEM images also reveal five-sided projections of nano-particles. The convergent beam nano-diffraction patterns obtained from the nano-particles point to the nano- phase being quasicrystalline, a feature that has never before been reported for a copper alloy, although there have been reports of the presence of icosahedral ‘clusters’ within large unit cell intermetallic phases. This observation has been substantiated by x-ray diffraction, wherein the observed peaks could be indexed to an icosahedral quasi-crystalline phase. The mirror alloy casting has been valued for its high hardness and high reflectance properties, both of which result from its unique internal microstructure that include nano-grains as well as quasi-crystallinity. We further postulate that this microstructure is a conse- quence of the raw materials used and the manufacturing process, including the choice of mold material. While the alloy consists primarily of copper and tin, impurity elements such as zinc, iron, sulfur, aluminum and nickel are also present, in individual amounts not exceeding one wt.%. It is believed that these trace impurities could have influenced the microstructure and, consequently, the properties of the metal mirror alloy. INTRODUCTION With the advent of the Bronze Age around 3500 Before Common Era (BCE), mirrors made of copper and its alloys (mainly bronze) were being crafted in regions like Mesopotamia, China, Korea, India, Ja- pan and Egypt. 1–10 The mirror alloy reported in this article was cast in the town of Aranmula in Kerala, India. This is the only known location where such alloy mirrors are still produced today. Bronze mir- rors of high tin composition have been in existence for close to 4000 years and reports indicate that the earliest use was in China. Thus, the composition may have been transmitted to southern India from China, although again there is no clear evidence of this except for noting that Chinese fishing nets are known to have found their way to the Kerala coast, presumably during the same time period. While the people of Aranmula believe the composition to be locally developed and divine, it is perhaps a rea- sonable assumption that the origin of the composi- tion might have actually been in China and traveled to India via Africa; however, the clay in Kerala may have aided the important zinc addition that we re- port in this article; this allows the alloy to undercool and display icosahedral properties. Today, except for the Aranmula manufacturers, there are no other known producers of such alloys in the world. The binary phase diagram is known to contain regions of several phases with very large cubic cells, such as the d and f phases in the 21 at.% Sn range. 11–15 However, several significant variations (in liquidus slope and equilibrium phases) of the Cu- Sn binary phase diagram have been reported. 11,13,15 JOM, Vol. 67, No. 12, 2015 DOI: 10.1007/s11837-015-1524-3 Ó 2015 The Minerals, Metals & Materials Society 2976 (Published online July 14, 2015)
Transcript
Ancient Metal Mirror Alloy Revisited: QuasicrystallineNanoparticles Observed
J.A. SEKHAR,1,2,3 A.S. MANTRI,2 S. YAMJALA,4 SABYASACHI SAHA,4
R. BALAMURALIKRISHNAN,4 and P. RAMA RAO5,6
1.—Institute of Thermodynamics and Design, Cincinnati, OH 45215, USA. 2.—University ofCincinnati, Cincinnati, OH 45221, USA. 3.—MHI Inc., Cincinnati, OH 45215, USA. 4.—DefenceMetallurgical Research Laboratory, Hyderabad 500058, India. 5.—International AdvancedResearch Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad 500005, India.6.—e-mail: [email protected]
This article presents, for the first time, evidence of nanocrystalline structure,through direct transmission electron microscopy (TEM) observations, in a Cu-32 wt.% Sn alloy that has been made by an age-old, uniquely crafted castingprocess. This alloy has been used as a metal mirror for centuries. The TEMimages also reveal five-sided projections of nano-particles. The convergent beamnano-diffraction patterns obtained from the nano-particles point to the nano-phase being quasicrystalline, a feature that has never before been reported for acopper alloy, although there have been reports of the presence of icosahedral‘clusters’ within large unit cell intermetallic phases. This observation has beensubstantiated by x-ray diffraction, wherein the observed peaks could be indexedto an icosahedral quasi-crystalline phase. The mirror alloy casting has beenvalued for its high hardness and high reflectance properties, both of which resultfrom its unique internal microstructure that include nano-grains as well asquasi-crystallinity. We further postulate that this microstructure is a conse-quence of the raw materials used and the manufacturing process, including thechoice of mold material. While the alloy consists primarily of copper and tin,impurity elements such as zinc, iron, sulfur, aluminum and nickel are alsopresent, in individual amounts not exceeding one wt.%. It is believed that thesetrace impurities could have influenced the microstructure and, consequently,the properties of the metal mirror alloy.
INTRODUCTION
With the advent of the Bronze Age around 3500Before Common Era (BCE), mirrors made of copperand its alloys (mainly bronze) were being crafted inregions like Mesopotamia, China, Korea, India, Ja-pan and Egypt.1–10 The mirror alloy reported in thisarticle was cast in the town of Aranmula in Kerala,India. This is the only known location where suchalloy mirrors are still produced today. Bronze mir-rors of high tin composition have been in existencefor close to 4000 years and reports indicate that theearliest use was in China. Thus, the compositionmay have been transmitted to southern India fromChina, although again there is no clear evidence ofthis except for noting that Chinese fishing nets areknown to have found their way to the Kerala coast,
presumably during the same time period. While thepeople of Aranmula believe the composition to belocally developed and divine, it is perhaps a rea-sonable assumption that the origin of the composi-tion might have actually been in China and traveledto India via Africa; however, the clay in Kerala mayhave aided the important zinc addition that we re-port in this article; this allows the alloy to undercooland display icosahedral properties. Today, exceptfor the Aranmula manufacturers, there are no otherknown producers of such alloys in the world.
The binary phase diagram is known to containregions of several phases with very large cubic cells,such as the d and f phases in the 21 at.% Snrange.11–15 However, several significant variations(in liquidus slope and equilibrium phases) of the Cu-Sn binary phase diagram have been reported.11,13,15
JOM, Vol. 67, No. 12, 2015
DOI: 10.1007/s11837-015-1524-3� 2015 The Minerals, Metals & Materials Society
Some of these variations support no partitioning atthe 21 at.% composition whereas others show a wideseparation between solidus and liquidus. Conse-quently, it is not clear whether the binary alloypossibly also contains the same phases as the mirroralloy or whether small amounts of solute impuritiesinfluenced the differences in the reported phasediagrams and phases. This particular metal mirroralloy has also been extensively studied by Srini-vasan et al.,1,7,10 Pillai et al.,2 and Meeks,3 all hav-ing previously identified the major phase in themirror-casting alloy (or in solidified Cu-Sn binaryalloys) as the d phase with a large cubic lattice. Themirror alloy casting has also been reported tocontain trace elements like Zn, O and Ni in additionto the major constituents namely Cu and Sn. Mirroralloy compositions have been reported, in wt.%, ofCu 64.73%, Sn 32.47% and the rest 2.8% by Srini-vasan et al.1,7,10 and of Cu 70.4%, Sn 29.4%, Zn0.06%, Fe 0.034% and Ni 0.052% by Pillai et al.2
The present study was initially undertaken inorder to study the solidification sequence of themirror alloy. While doing so, remarkable new find-ings of nano-crystallinity and quasi-crystallinitywere found in the alloy. This is the first report ofthese findings. The discovery of nano-crystals andquasi-crystals for this composition is unexpected. Acited unpublished report in Ref. 7 suggests that thealloy could be superplastic, which is the only pre-vious indication of a very fine grain structure.
MATERIALS AND METHODS
Material
A metal mirror (Fig. 1. Aranmula Kannadi) wasused as the source of samples for all the investigationsreported here. The ancient and indigenous process ofcasting the metal mirror, which is being practisedeven to this day, has been handed down from genera-tion to generation, and is held as a closely guardedfamily secret. Therefore, it is not surprising that sev-eral descriptions, not always identical, have beenwritten about or demonstrated in videos, which areaccessible by an internet search with the key-phrasecontaining ‘‘Aranmula’’. The general process, as de-scribed by Pillai et al. in their article entitled ‘‘Ancientmetal-mirror making in South India: Analyzing amysterious alloy’’ published in JOM2 involves thefollowing stages: (1) the achievement of the right alloycomposition by melting copper and tin (both in theform of small pieces/chunks) together in the rightproportion ina ‘‘clay crucibleusinga furnacefiredwithcoconut-shell charcoal’’; (2) obtaining small pieces ofthe alloy by remelting the alloy in disc form andbreaking it down; and, finally, (3) remelting the alloyand casting it as the mirror with the required thick-ness. The ingenuity lies in the use of an integratedcrucible-mold assembly for the last stage without theuse of risering systems. For this purpose, the mold isformed with two burnt clay discs, each of which hasone polished surface (to be used as the interior sur-
face), separated using three spacers 120� apart andwith the height depending on the thickness of themirror to be cast. The mold is covered with clay on allsides except the ‘in-gate’, which acts as the connectorto the crucible and which is fashioned out of clay and isfilled with pieces of the alloy. The mouth of the crucibleis then covered with a piece of cloth and sealed withclay. This configuration allows the assembly to be he-ated in an open pit furnace with the crucible down andthe mold up, so that while the alloy in the cruciblemelts, the mold gets simultaneously preheated. Sub-sequently, the assembly is takenout of the furnace andgradually inverted to allow the molten alloy to fill themold cavity smoothly. It is important to note that therequired clay (for the crucible, the mold and for sealingthe crucible-mold assembly) is sourced from localpaddy fields. Upon cooling for nearly 2 days, thecasting is separated from the mold, cleaned and pol-ished as described in Pillai et al.2
Methods
The mirror disc was removed from the casing andcut into smaller pieces for further investigations. ABuehler Isomet precision saw was used for thispurpose. On account of the extreme brittle nature,and the size and shape of the mirror (a 3-mm-thickdisc with 50 mm diameter), the speed of cutting waskept low to avoid breakage.
The composition of the metal alloy was deter-mined from a powdered sample piece using con-ventional wet chemical analysis. Approximately10 mm 9 10 mm 9 3 mm samples were made foroptical and scanning electron microscopy andhardness measurements. Hardness measurementswere made using a Knoop hardness tester usingloads varying from 1000 gf to 50 gf.
Fig. 1. Metal mirror in its casing.
Ancient Metal Mirror Alloy Revisited: Quasicrystalline Nanoparticles Observed 2977
Due to the highly brittle nature of the sample, it waschallenging to prepare the transmission electron mi-croscopy (TEM) samples. A thin electron transparentlamella measuring about 15 lm 9 5 lm was preparedusing FEITM QuantaTM 3D, a dual beam equipment.Another TEM sample was prepared by ion beam thin-ning using a Gatan PIPSTM instrument. Both sampleswere observed in a FEITM Tecnai G2-20T 200 kV TEMequipped with an EDAX 4000 Genesis EDS system.Forcross-correlation and for low magnification studies, thesame samples were also investigated in a FEITM
Quanta 400 scanning electron microscope (SEM)equipped with an EDAX Genesis 4000 EDS system, aswell as a Carl Zeiss Supra 55 Field Emission Gun SEM,equipped with an Oxford EDS system.
X-ray diffraction studies were performed on apowder sample using a Philips X’Pert h � h powderdiffractometer operated at 40 kV and 55 mA, usingCu-Ka radiation. The powder was prepared bygrinding the samples using a pestle and mortar(with particle size between 0.1 lm and 40 lm)suitable for XRD use. The powder was then spreadout evenly on a glass slab using double stick tape.
The reflectance properties of the mirror alloy wereobtained with a Variable Angle SpectroscopicEllipsometer (VASE�). The beam diameter for theellipsometer was about 1 mm. Comparative mea-surements were also made on a copper-standardpolished to lower than 0.1 lm finish with colloidalsilica on a lapping cloth.
RESULTS
Composition
Chemical analysis performed on the sampleshows that it is essentially a Cu-Sn alloy containingabout 32 wt.% Sn (Table I). Additionally, the alloywas also found to contain traces of Zn, Ni and Al,with their individual content not exceeding0.1 wt.%. These results are broadly in agreementwith those reported by Pillai et al.2
Multi-scale Microstructure
Optical microscopy indicated that themicrostructure consisted of large rivulet dendritesencompassing rosette dendrites and small agglom-
eration of equiaxed dendrites where the two types ofdendritic regions come together. A typical opticalmicrograph is shown in Fig. 2. It is seen that therivulet dendrites have sizes approaching a mil-limeter or more, whereas the rosette-like dendritesspan 200–600 lm tip-to-tip. The rivulet and rosettedendritic features were uniformly noted across theface and cross-section.
Upon investigation in the SEM, the presence ofdark particles, a few lm in size, was observed nearthe center of several rosette-type dendrites (Fig. 3).These particles were found, by EDS, to predomi-nantly contain Zn and S (inset of Fig. 3), while only
Table I. Chemical analysis of the metal mirror alloy
Elements
Composition (wt.%)
This work Pillai et al.2a
Sn 32.1 ± 0.6 29.4Zn 0.082 ± 0.002 0.06Ni 0.068 ± 0.002 0.052Al 0.021 ± 0.002 Not detectedCu Balance 70.4
aPillai et al. also report P (0.02 wt.%) and Fe (0.034 wt.%) to bepresent in the alloy.
Fig. 2. Optical micrograph revealing the microstructure to be com-posed of features at two different length scales: larger cells/dendriteswith feature size >750 lm (boundaries indicated by short arrows),and smaller rosette–shaped dendrites (centers indicated by longarrows) contained within the larger features.
Fig. 3. Scanning electron micrograph (at higher magnification rela-tive to Fig. 2) showing the rosette-shaped dendrites in greater detail.The centers of these dendrites were often found to contain ‘dark’particles (arrowed) that were identified, through EDS (inset), to berich in Zn and S. In contrast, the adjacent matrix showed peakscorresponding to only copper and tin.
Sekhar, Mantri, Yamjala, Saha, Balamuralikrishnan, and Rao2978
a few lm away, the adjacent matrix showed evi-dence for the presence of only the main elements,namely Cu and Sn. It is interesting that the alloy,overall, contains less than 0.1 wt.% Zn, but theamount of zinc in the particles found in the rosettedendrites suggests that the particles are practicallypure ZnS. Their location suggests that these parti-cles might have played a significant role in thenucleation of the rosette dendrites. Occasionally,particles rich in either nickel, or, iron and alu-minum, were found in close proximity to the Zn- andS-rich particles.
TEM investigation of the sample showed a pre-ponderance of nano-particles (Fig. 4), typically withsize less than 10 nm, within the dendritic regions.Selected area electron diffraction (inset of Fig. 4)obtained from these particles displays a family ofrings indicating that the nano-particles are orientedsomewhat randomly with respect to the beamdirection. The current work is the first to report,unambiguously, the presence of nano-particles inthe metal mirror alloy.
A closer examination reveals that at least a few ofthe particles observed in the TEM micrographs ap-pear to be five-facetted, suggesting that these nano-particles could be quasi-crystalline. Some of theseparticles are indicated with arrows in Fig. 4. In ourefforts to obtain diffraction from a single nano-par-ticle, we realized that the particles were too fine andthat the smallest (and maximally converged) sizeelectron beam still illuminated a few rather than asingle nano-particle. Figure 5 shows a typical nano-diffraction pattern, obtained from such a condition.Analyzing the diffraction pattern, we observe thatthere are four spots spaced 72� apart lying on asingle circle; this implies that they may have beenobtained from a quasi-crystal. Further, we also ob-served that the diffraction spots along a givendirection (designated AB in Fig. 5) in reciprocalspace are not equally spaced, another indication ofquasi-crystallinity. Thus, the nano-diffraction studysupports the proposition that the metal-mirror alloycontains quasi-crystalline nano-particles.
X-ray Diffraction
X-ray diffraction from powder samples providedstrong and sharp peaks (Table II) that could not beclearly indexed to any known periodic crystal sys-tem. The peaks also did not convincingly index toknown diffraction lines of the phases in the Cu-Snbinary alloy system or to a known crystalline lattice.An icosahedral crystal (iQc)16 match was thensought with the q vector technique described inRefs. 17–24. The q vector ratios for diffraction linesobtained from the powder sample are shown inTable II. The peak ratios are calculated based onthe methods provided in Ref. 19 with the choice of aprimary qmax = 0.2135 nm (also shown in Table II).It is seen that the majority of peaks |q/qmax| matchthe expected |q/qmax| values of Table III for
indexing to an icosahedral phase. This phase hasbeen labeled as the d(I) phase.
Hardness
The Knoop hardness (KH) was recorded andfound to vary between 422 KH and 611 KH forloads varying from 1000 gf to 50 gf, respectively.The Knoop indentation was not symmetric. Asmaller Knoop zone always encompassed the re-gion with the more interdendritic material (i.e.sampled a harder material). The average Knoop
Fig. 4. TEM bright field image obtained from an intra-dendritic regionshowing the presence of nano-particles with size less than 10 nm.Inset the electron diffraction pattern that exhibits rings indicating thatthe nano-particles are randomly oriented with respect to the beamdirection. Some of the particles (arrowed) clearly reveal five-facettedfeatures suggesting that these might be quasi-crystalline.
Fig. 5. Nano-diffraction pattern obtained by converging the electronbeam such that it covers only a few nano-particles. The blue lines arespaced 36� apart; consecutive arrowheads make an angle of 72�with respect to each other.
Ancient Metal Mirror Alloy Revisited: Quasicrystalline Nanoparticles Observed 2979
hardness was approximately about 550 KH. Thisis consistent with the hardness of 520–540 VHNunder 50 gf reported by Pillai et al. for the mirroralloy.2 In comparison, commercial tin-bronzecrystalline alloys with marginally lower tin con-tent (by wt.%), display a much lower hardness,e.g. UNS C90300: 86–89 Cu/7.5–9 Sn/3–5 Zn: 70BHN (�90HK); UNS C90700: 89 Cu/11 Sn/<0.5Zn, <0.5 Fe: 80 BHN; UNS C90500: 86–89 Cu/9–11 Sn/1–3 Zn (alpha + delta alloy): 75 BHN. It isnoted that the hardness is between 70 and80BHN for these alloys, corresponding to approx-
imately 80–100 HK. The much higher hardness ofabout 550 KH of the mirror alloy is consistentwith the literature25–28 on iQc phases which havebeen shown to display higher hardness comparedto their crystalline counterparts. For instance,Bhaduri and Sekhar25 have reported a hardnessof between 3 GPa and 4 GPa for quasi-crystals inthe Al-Cu-Li system. These hardness values cor-respond to 306–408 VHN or 317–425 KH,approximately. Thus, the metal mirror alloy ex-hibits a hardness higher than the quasi-crystalsreported in the Al-Cu-Li system.
Table II. XRD data from a powdered mirror sample arranged in the order of intensity
Intensity Pos. (�2Th.) d-spacing (A) |q/qmax| I phase index
100 42.345 2.134743 1 1v1 or 3V2 or 1E1 or 1F187.5846 42.325 2.134743 1 1v1 or 3V2 or 1E1 or 1F172.2347 24.005 3.706029 0.576019 3v155.0791 32.645 2.742208 0.778476 2F249.2099 35.695 2.514574 0.848948 B*39.2776 23.985 3.709074 0.575546 3v127.76524 24.045 3.699955 0.576965 3v123.02483 47.195 1.925183 1.108852 2V1 or 2F221.4447 84.785 1.142999 1.867668 2F419.63883 80.795 1.189075 1.795297 3V417.3814 62.555 1.484352 1.438165 2ndE3 or 3V316.70429 49.255 1.849375 1.154305 2ndE2 or 2F2
A Philips X’Pert h � h Powder diffractometer was used, operated at 40 kV and 55 mA, using Cu-Ka radiation (k = 1.5406 A). Please referto Table III for the q/qmax ratio for comparison with the iQc phase index column shown in this table. The row marked B* is possibly fromthe Eta phase, which is likely to be present. The last column contains the q vector ratios from the model shown in Table III. Thedesignation represents the order, type of model and the row number for that model.
Table III. Wave vector transfer q vectors for various orders in the vertex, edge and face models for a iQcphase
Sekhar, Mantri, Yamjala, Saha, Balamuralikrishnan, and Rao2980
Reflectance
The reflectance properties of the mirror alloy wereobtained with a Variable Angle SpectroscopicEllipsometer (VASE�). The beam diameter for theellipsometer was about 1 mm. Comparative mea-surements were also made on a polished copper-s-tandard, polished to less than 0.1 lm polish withcolloidal silica on a lapping cloth. The reflectanceresults shown in Fig. 6 confirmed those reported inRefs. 29, 30 that there is a significant loss ofreflection at small wavelengths in the visible regionfor pure copper.31 In contrast, the metal mirror alloyshowed more uniform reflectance across the entirevisible spectrum. One conclusion that could bereached when comparing the pure copper and themirror alloy reflectance properties is that, at lowerwavelengths of light, the pure copper has compar-atively more free electron-based absorption whencompared to the mirror alloy. Such an observation issuggestive of a more spherical Brillouin zone and auniform photonic band gap32 noted in iQc alloysover translationally periodic ordered alloys.
DISCUSSION
Known quasi-crystalline phases have been solidi-fied both by slow and rapid cooling techniques.33,34
However, except for a few studies,33 the formation ofiQc phases are more commonly observed with the ra-pid cooling from the liquid state or crystallization froma glassy state. It is remarkable and unanticipated thata clay-mold slow casting of the mirror alloy displaysquasi-crystalline and nano-crystalline phases.
The Cu-32 wt.% Sn binary a displays a widesolidification range allowing for significant residual
liquid even after considerable solid formation. Themirror alloy appears to have solidified at very slowrates with periodic temperature plateaus.1,2 Nosignificant macrosegregation is noted of the kindcommonly seen with tin-containing alloys. Thecurvature in the dendrites may be a result of thefluid flow35 resulting from the slow casting process,but it is not clear whether this also led to grainbreakage and distribution or if it influenced under-cooling.
The precise reasons for nano-crystal and iQc for-mation are unclear but may in general involve liq-uid clustering at this composition, significantundercooling, easy nucleation conditions36,37 anddifficult growth38–40 of aperiodic phases. The Cu-32 wt% Sn composition has been noted to displayliquid state clustering.41–44 Bronze alloys have alsobeen reported to contain icosahedral clusters insidelarge cubic unit-cells.45,46 Alloys that contain zincpromote undercooling over comparable alloys thatdo not.47 Differential scanning calorimetric studiesthat were performed on this alloy at 10 K/min wereinconclusive regarding undercooling tendencies.Many zinc-containing copper-tin solder alloys areknown to undercool but only at high tin contents.48
Although all of these possibilities point to reasonsthat may explain both the nano-crystallinity andquasi-crystallinity, the issue merits further inves-tigation.
SUMMARY AND CONCLUSION
In the present work, we have investigated themicrostructure, hardness and reflectance propertiesof an ancient metal mirror alloy that has beenmanufactured using an indigenous process held as a
Fig. 6. Comparison of the reflectance of the Aranmula Mirror before and after polishing with respect to pure copper. The numbers 65, 70 and 75in the legends refer to three different incident angles.
Ancient Metal Mirror Alloy Revisited: Quasicrystalline Nanoparticles Observed 2981
closely guarded family secret by a group of artisansin the town of Aranmula in Kerala, South India. Themajor observations and conclusions are the follow-ing:
� The material is essentially a copper (30–32 wt.%)-tin binary alloy that contains traces ofzinc, nickel and iron, each in amounts notexceeding 0.1 wt.%. While present in only smallquantities, these trace elements might neverthe-less significantly influence the microstructure,and therefore the properties of the mirror alloy.
� The alloy revealed a cast dendritic microstruc-ture, one that is quite complex with largermillimeter-sized dendrites encompassing smal-ler rosette-shaped dendrites with ZnS particlesat their centers and a few equiaxed grains.
� The extensive presence of sub 10-nm-sized par-ticles throughout the dendritic regions is noted.This is the first report of nano-crystalline fea-tures in this ancient alloy. Several of the parti-cles also displayed five-facetted features,suggesting that the phase is quasi-crystalline.Convergent beam nano-diffraction patternsshowed some diffraction spots, which are rotatedby 72� from each other. The patterns also showeddiffraction spots along a given direction that areunequally spaced. These nano-diffraction resultssupport the proposition that the metal-mirror alloycontains quasi-crystalline nano particles.
� X-ray diffraction of powdered samples enabledunambiguous indexing of a majority of the peaksto a quasi-crystalline phase.
Based on the above, we conclude that our obser-vations yield sufficient proof that the ancient metal-mirror alloy contains nano-particles that are likelyto be quasi-crystalline. We suggest, based on theavailable literature, that the superior hardness andreflectance properties, so essential for a mirror, aredue to the effects of quasi-crystallinity and nano-crystallinity.
ACKNOWLEDGEMENTS
Three of the authors (SY, SS and RB) thank Dr.Amol A. Gokhale, Director, DMRL, for permittingthe publication of this article. JS and SM gratefullyacknowledge MHI Inc. funding provided by Dr. A. A.Vissa; MHI-03/2009. Appreciation for assistanceprovided by ARCI Director Dr. G. Sundararajan isexpressed by PRR.
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