THE DEVELOPMENT OFSPANGOLD

Ira M. Wo ff & Michael B. Cortie

Physical Metallurgy DivisionMINTEK

Randburg, South Africa

The underlying mechanisms of martensitic phase transformations in

gold alloys have attracted a great deal of study. Whereas their

technological exploitation in shape-memory devices has found only

limited application, the Spangold concept introduces a novel

application — that of using the transformation to decorate gold

ornaments. This article examines the attributes of suitable alloys and

the development of a prototype alloy for use in jewellery.

44 (' Gold Bull., 1994, 27 (2)

INTRODUCTION

The Spangold concept is described in a companionpublicaton [1] and is the subject of recent patentsapplied for by Mintek [2], In essence, Spangold refersto a family of gold alloys formulated to undergo aphase transformation. This change in the crystal struc-ture, known in simple terms as a martensitic transfor-mation, manifests itself as a change in the surface reliefof the alloys. The modification of the surface texturegives rise to a decorative glitter, or 'spangle', whïch isintrinsic to the alloys, and gives them their name. Theaesthetic properties of Spangold find application injewellery.

While in principle the Spangold concept can beextended to embrace all the systems that exhibitmartensite-like transformations, those transforma-tions associated with intermetallic compounds of goldare of particular interest. This article examines someof the specific requirements for exploiting the Span-gold effect for ornamental purposes and shows that,apart from the dictates of caratage and aesthetics, al-loys based on intermetallic compounds can also beusefully engineered in terras of other properties, suchas castability, density, and wear resistance. Althoughthese alloys are extremely versatile, their properties candiffer significantly from those of conventional jewel-lery alloys and, by their nature, require differentmanufacturing and finishing techniques.

BACKGROUND

The aesthetic properties of gold, revered for its deepyellow colour, have been the wellspring for much ofthe characterization work on gold alloys and com-pounds. The development of Spangold issues from anage-old fascination with the decorative nature of goldcompounds.

Intermetallic compounds based on AuAl2, werenoted for their striking purple colour [3] as early as1891. Intermetallic components are based aroundsimple, but thermodynamically stable, stoichiometriccombinations of the constituent elements. The diver-

sity and range of gold compounds have been describedin an extensive literature [4].

Martenstic phase transformations in discrete goldcompounds were identified by Chang and Read [5]in the early 1950s. Since then, their discovery hasevolved into the science of shape-memory alloys andrelated phenomena in a wide range of systems. Nu-merous potential shape-memory applications forgold-base alloys have been put forward, since they areperceived to have advantages in components wherecolour and resistance to corrosion and tarnishing areimportant. Whereas the original gold-cadmium com-pounds of Chang and Read were relatively brittle, andwere studied in single-crystal form, a number of rela-tively malleable alloys have been identified sub-sequently. For example, the Fulmer Research Institutepatented alloys containing between 40 and 60 percent gold by mass and proposed their use in jewellery,citing applications such as the gripping of jewels inmounts [6, 7]. Recently new gold-base shape-memoryalloys for use in ornamental items have been claimedby Japanese manufacturers [8, 9]. In these develop-ments, interest focussed on the mechanical and shape-memory properties.

The Spangold concept exploits the martensiticphase transformation, common to shape-memory al-loys, for decorative purposes. The phase transforma-tion distorts the original crystal structure, and inducesa surface relief pattern. Under ideal conditions. this issufficiently pronounced to obtain dispersion of te-flected light, giving the alloys their characteristic glit-ter. Not all martensitic transformations are suitable inthis regard. The phase changes can yield a range ofmartensitic variants, depending on the alloy composi-tion. In other systems. the mechanical, environ-mental, and optical properties of the alloys are notcom- patible with jewellery applications.

A detailed review of the crystallographic theory ofmartensitic transformations is beyond the scope ofthis article. Instead, the focus is on the qualitative at-tributes of the phase transformations exploited inSpangold, and their application in a prototype alloysystem.

Gold Bull., 1994, 27 (2) 45

GOLD ALLOYS FOR JEWELLERY

Gold alloys suitable for use in jewellery have certainminimum requirements, based on their intrinsic valueand appearance. While some of these attributes maybe obvious, .they bear stating because the properties ofthe envisaged intermetallic compounds can be verydissimilar to those of conventional alloys. Table 1 con-tains a list, by no means exhaustive, of what may beclassified as primary and secondary properties.

Table 1

Properties of gold jewellery alloys

Prirnary tSecondary

Colour WorkabilityCaratage Castability

Inertness to environment Strength/Wear resistance

Owing to the nature of bonding in intermetalliccompounds, their intrinsic properties are frequentlyinappropriate for structural applications. Moreover

— while colour is of almost unique importance ingold alloys, not all gold alloys and intermetalliccompounds exhibit any colour. Rather, the elec-tronic structure normally gives rise to metallic silveror grey tones;

— intermetallic compounds usually have limited mal-leability, a restricted composition range, and cansuffer from environmental degradation, such as`intermetallic pest';

— owing to their high thermodynamic stabilities, in-termetallics often have high melting ranges.

Furthermore the Spangold alloys must undergo a suit-able transformation, which is manifest at the surfaceas macroscopic strain resulting in the necessary opticalproperties.

PHASE TRANSFORMATIONSIN GOLD ALLOYS

Only a relatively small number of metallic alloys under-go a martensitic phase transformation. In the simplest

form, a martensitic phase transformation may be con-sidered to be a displacive solid-state transformation,without diffusion. More accurately, the mechanismis described as `invariant plane strain' and manifesteitself macroscopically as a shape deformation of thesurface. On a microstructural scale, the martensiteappears as packets of lenticular laths, plates, or twins.

The non-ferrous martensites can be loosely classi-fied into three groups, according to the descriptionprovided by Delaey et al. [10]. The alloy systems be-longing to the first group are all terminal solid solu-tions baséd on elements that in the pure form exhibitan allotropic phase transformation. Those belongingto the second group have in common a parent body-centred cubic (bcc) beta phase, which is intermetallic.Finally the third group is characterised by a cubic-to-tetragonal (or orthorhombic) phase transformation.This deviation from a cubic unit cell is often small,and the transformed phases are sometimes referred toas quasi-martensites [10].

The gold alloys that exhibit a martensitic transfor-mation generally, but not exclusively, derive from theclass of alloys known as beta-phase electron com-pounds (group two). At elevated temperaturen, thecompounds exist as a bcc beta phase, which transformsto a martensitic phase under the appropriate coolingconditions. In most cases, the beta phase exhibitsatomic ordering, in which case the martensite phaseis allo ordered.

Phase transformations that are phenomenologi-cally akin to martensitic transformations, but are oftentreated separately in the literature, include the devel-opment of structural order in certain alloy systems. Inparticular, the ordering reactions of the compoundAuCu have received a great deal of attention in thisregard [11-14]. It is now rigorously known that Au-Cu alloys of roughly equiatomic composition formtwo complex ordered tetragonal or orthorhombicphases on cooling from the disordered face-centredcubic parent phase. Both of these ordering reactionsgive rise to characteristic relief effects on a polishedsurface, and generate internally twinned plates of theAuCu phase [11]. The mechanism of transformationnecessarily requires relocation of neighbouring atoms.However, since the relocation does not contribute tothe transformation shape change, the operative crys-tallographic theory has been argued to be consistentwith a marsensitic shear transformation [14]. For the

46 ro Gold Bull., 1994, 27 (2)

Figure 1

The characteristic `basket-weave'structure of the martensite incopper-base alloys, shown for a Cu-Zn Al-brass

purposes of this discussion, we therefore refer to thetransformations in AuCu as being martensitic. Incharacter, the AuCu system is most closely related toa group three martensite. The similarity between theordering reaction in AuCu and a martensitic transfor-mation also finds expression in other martensitic or-dering reactions (Co-Pt, Mg-Cd, Fe3Al) [15].

Our studies have shown that the shape deforma-tion effect in AuCu alloys can be enhanced by ternaryalloying. Since the AuCu phases centre around a goldcomposition of about 75 wt.% (or 18 carat), theyforen an admirable basis for the development of theSpangold concept in goldewellery, and this systemreceives further attention here.

Finally, of peripheral interest is the phenomenonof transformation reactions that are confined to thesurface region. The removal of transformation strainsat the surface can cause a twinning reaction to a depthof some 7 to 10 nm beneath the surface. The structureof the surface martensite is reported to he very de-pendent on the surface orentation, and is unrelatedto that of the bulk martensite, if indeed the alloy un-dergoes a bulk transformation [10]. Spontaneousphase transformations that are limited to the surfacehave been recognised in other alloys (Lovey et al.[16]). These are of interest regarding the Spangoldconcept, since they hold out the potential of combin-ing the decorative effect with the properties of the base

alloy. Ths also suggests the possibility of coating sub-strates with alloy films that undergo martensitic trans-formations.

CHARACTERIZATION OFMARTENSITIC STRUCTURES IN GOLD

The martensitic transformation of the parent phaseusually occurs over a characteristic range of tempera-tures, commencing at the martensitic start (M S) temperature, and proceeding to completion at the finish(Mf) temperature. Similarly, heating of the martensitephase progressively restores the parent phase, al-though, as will be seen, the morphology of the surfaceexhibits varying degrees of reversibility. In theory, thealloys can be cycled through the transformation anindefinite number of times.

Martensite structures may be distinguished on thebasis of their internal microstructure. Broadly speak-ing, the martensites exhibit internally faulted or inter-nally twinned structures. Copper-base martensitesusually conform to a faulted structure and appear asplates in a 'basket-weave' pattern under the micro-scope. A typical example is shown in Figure 1.

In contrast, internally twinned martensites aredentifiable by their strongly aligned twin-band struc-

tures. The intermetallic Au-Cd and Au-Mn alloys fallinto this class. The transformations experiencevery little elastic constraint and entire grainscan undergo transformation to a single mar-tensite crystal instead of to a number ofsmall discrete plates [17]. When nucleationoccurs at several points within the grain, in-tersecting families of twins occur (Fig. 2a).

Characteristic martensitic structures,and the accompanying change in the reliefof a polished surface, are illustrated in Fig-ure 2 for alloys of Au-Mn and Au-Ti. lt willbe apparent that a diverse range of physicalstructures occurs and that not all of these aredetectable with the naked eye. Whereas some,like the copper-base alloys in Figure 1, ex-hibit a coarse structure with marked surfacerelief, the surface distortion wrought byother phase changes is on such a fine scale asto be visible only under a microscope.

Gold Bull., 1994, 27 (2) 47

PHASE TRANSFORMATIONSIN THE Au—Cu SYSTEM

The phase transformations in the Au -Cu system giverise to features that are ideally suited to the Spangold.concept, in that. they occur at modest temperaturesand the surface structures can be readily regenerated.The alloys also display some unique transformationcharacteristics. Consideration is given here to the mi-crostructural development of the pseudo-binary Au-Cu-Al system, selected to have a fineness of nominally18 carats, as a prototype of the Spangold concept.

As noted above, in alloys containing in the regionof 75 wt.% gold, cooling from the random fcc parentphase leads to ordering of the Ll o type below about380 °C, and to the formation of an orthorhombicAuCu(II) phase berween 380 and 410°C. In the caseof the Ll o reaction, the surface relief is caused by me-chanical twinning subsequent to ordering,but the AuCu(II) reaction tilts the surface asan ordered region grows from the cubicphase [15]. It has been noted [15] that the - 'growth of the ordered plates as long bandsparallel to a pseudo-habit plane closelymatches the transformation characteristicsof copper-zinc alloys. The plates possessan internally twinned structure, and havebeen postulated to grow by the gliding oftransformation dislocations at a race deter-mined by the rate of ordering at the disloca-tions [14]. r.

The mechanistic route giving rise to in-variant-plane strain in AuCu alloys is there-fore characterised by an ordering reaction thataccompanies the martensitic shear transfor-mation. The shape deformation consequentlyproceeds from the ordered intermetalliccompound rather than the disordered par-ent phase, although for practical purposesthe spangle is generated in a similar fashion,

Figures 2a and b

Twinned martensitic structures in alloys based onAu-Mn (a), andAu-Ti (b)

namely by cooling from above the ordering tempera-ture.

The effect of small additions of aluminium on thephase change in 18 carat Au-Cu alloys, as manifest onpolished surfaces, is shown in Figure 4. Followingidentical heat treatments, aluminium additions causea progressive change of the strongly aligned twinstructure, from one of incipient twinning and tiltingof the surface to a structure more closely resemblingthe broad bands found in the copper-base alloys. Rela-tive to the AuCu-base alloy, additions of aluminiumpromote complete transformation, probably as a re-sult of a shift in the phase boundaries, effectively en-hancing the kinetics. At the same time, the surfacerelief is accentuated, which results in a more effectivescattering of incident light. Thus, by appropriate al-loying, the martensitic structure can he modified toexhibit more pronounced optical properties.

48 ro Gold Bull., 1994, 27 (2)

Figures 3a, b and cVariation of the martensitic structure as a

function of aluminium content in 18 caratAu-Cu-Al alloys. Following identical heattreatments, the microstructural developmentshows(a) incipient twinning and tilting of thesurface,(b) modification of the twin structure, and(c) complete transformation with broad twinbands.

THE CONCEPT OF

ANTI-SPANGLE

lt will be understood that modification ofthe surface topography of the 18 carat Au-Cu-Al alloys can be brought about in twoways. If in the first instance, a polished sur-face of the parent phase is slowly cooledthrough the transformation temperature re-gime, the transformation distorts the sur-face, creating the desired spangle. If thespangle created by the transformation is nowpolished flat, the surface once again distortsif the alloy is heated back through the trans-formation temperature. In the latter case,the martensite is converted back to the par-ent phase, thus reversing the transformationand, in effect, reversing the original distor-tion. This gives rise to a microstructure thatis optically similar to the martensitic struc-ture, and which is designated `anti-spangle'for conveniente.

Under ideal conditions, the anti-spangleonce again reverts to a smooth surface oncooling but, in practice, there is some con-straint which prevents the plates from fullyrecovering the original relief. On cooling,new martensitic plates may nucleate as a re-sult of the remnant `anti-spangle' plates.Thus the surface topography may comprisea combination of the two effects. The fadedremnant anti-spangle plates or twins can bediscerned in the background of the trans-formed structure in Figure 4.

Gold Bull., 1994, 27 (2) 49

The properties of the intermetallic Spangoldalloys can be modified in a number of otherimportant respects. For the AuCu alloys, ithas been leen that the addition of alu-minium shifts the transformation reactionand also influences the transformation ki-netics. However, small alloying additionsalso change

— the mechanical properties (Fig.6),— the castability (in terms of fluidity and

melting point),— the colour (Fig.7).

Figure 4

Complex anti-spangle' structure derived Erom superimposof the martensitic structure on the reverse transformation

cycling

The hysteresis in the reverse transformation canbe attributed to `lattice invariant strain', or lattice de-fects such as twins, stacking faults and/or dislocationsintroduced by the transformation, and which zend toprevent complete reversibilty. This leads tothe development of an unusual surface mor- +^.phology. Criss-crossing lates can give rise fto a type of `basket-weave' structure (Fig. 4),which accounts for the apparent anomaly ofplates lying across one another. In effect, thisfurther enhances the spangle. `:..

The `memory' capability of the alloyscan be manifest in other ways. For example,the transformation can be induced by local-sed stresses, such as those that occur duringmachining.

Hardness of Ordered Cornpounds

ition It has Iong been recognized that there existsduring a maximum in the hardness of gold alloys

that have been suitably heat-treated, at acomposition of around 75 wt.% Au (see for

example [18]) This has been linked to the develop-ment of structural order of the type discuseed above,and constitutes an important mechanism for achiev-ing higher strengths in gold alloys [19, 20].

When an alloy that has been quenchedto retain the parent structure is machinedbefore heating, stress-induced martensitegives rise to a surface relief that is superim-posed on the transformed martensitic struc-ture on heating. The result is a distortedplate structure that lacks the desirable defi-nition of spangle, and results in the appear-ance of a wrinkled surface (Fig. 5).

Figure 5

Distorted surface caused by partial stress-induced transformation

50 Gold Bull., 1994, 27 (2)

Al, wt% :

Figure 6

Hardness of transformed 18 carat Au-Cu-Al alloys asa function of composition

That an ordering reaction precedes or coincideswith the transformation in Au-Cu-AI alloys seemscertain from observations that the react ion can be sup-pressed by prolonged annealing in the disordered par-ent phase, followed by quenching. Transformation canthen be induced by heating to within the temperaturerange 200 - 300 °C which suggests that thermal acti-vation of the ordering reaction precedes trans- forma-tion.

The attainment of higher hardnesses via the or-dering reactions has important advantages. These in-clude greater wear-resistance and the ability of thealloys to be finished with a high lustre.

I0yellow-blu® 0----0. polishd

Q — nangI d

}^ellpvv-k?Éu^ % Fr red-gre^r

red-grr`Er

4 1 2 3 4 5. 6 7 8 :9 V0

Castability of the Intermetallics

As a result of their electronic configuration, the inter-metallic compounds in general frequently exhibit un-usual physical properties. One such property is theirhigh degree of fluidity in the molten condition. Thisis a function of the free-electron to atom ratio, whichapproaches zero as the ratios of constituent elementsapproach stoichiometry.

In practical terms, the width of the 'mushy' zoneon the equilibrium phase diagram contracts until, atstoichiometry, an intermetallic compound may meltcongruently.

The low surface tension imparts good fluidity tothe metal, and enhances castability. Thin, intricateshapes are therefore readily cast.

A further advantage of the Au-Cu-AI system is therelatively low melting range of the alloys, determinedby differential thermal analysis to be between 715 and770 °C, depending on composition. This further fa-cilitates manufacture.

Colour

The Jeep yellow colour of gold is due to the absorp-tion properties of its electronic structure. In effect,gold and topper have much higher reflectivities forthe low-energy end of the visible spectrum, confer-ring the unique property of colour on them [21].

Q Y 1 3 4 5 6. 7 9 10.

Al, wt%

Figures 7a and bColour co-ordinates and reflectivity of 18 carat Au-Cu-Al alloys plotted in CIELab notation,

showing the effect of composition:(a) effect ofAl content on co-ordinates a and b, (b) effect ofAl content on co-ordinate L

ro Gold Bull., 1994, 27 (2) 51

Figure 8

Exemple from the Spangold collection:18 ct dagger, with `red' Spangold blade, finished with red ivory wood,

rubies, red tourmaline, and yellow gold

Alloying additions to copper and gold progressively`bleach' their colour by modifying the electronic bandstructure, shiftïng the reflected wavelengths to theultra-violet region of the spectrum. The colour is alsochanged by the formation of an intermetallic com-pound, which produces an entirely new band struc-ture. The intense 'purple gold' and `blue gold' coloursof AuAl2 and AuIn2 respectively, result from a rise inthe reflectivity towards the violet end of the spectrum[22]. This causes the red and the violet wavelengthsto be strongly reflected, resulting in a strildng reddish-purple colour in AuAl2.

The addition of copper to gold shifts the reflec-tivity to slightly lower energies, creating the colour of`red' gold. It is therefore interesting to note that thereplacement of copper with aluminium in the inter-metallic AuCu-AuAl pseudo-binary leads to a shift inthe reflected colour, namely from yellow to red to pur-ple to white (Fig. 7). The progression appears to emu-late the AuAl2 compound. (From a consideration ofthe Au-Al phase diagram, it will be leen that AuAl2 is

a constituent phase at the copper-poor end of thepseudo-binary 18 carat series, but does not appear toplay a role at the lower aluminium levels.)

As an additional feature of interest, it is worthnoting the apparent shift in the colour broughtabout by the spangling transformation. The Au-Cu-Al system therefore joins the ranks of the colouredintermetallics. This effect has also been recognizedin copper-base alloys, and forms the basis of severalpatents [23, 24].

APPLICATION OF SPANGOLD

The incorporation of the Spangold effect in selectedjewellery items is shown in Figures 8 and 9. TheSpangold is offset against conventional 18 caratgold, as well as a variety of other finishes such asred-ivory wood, rhodium-plating and preciousstones. The vivid and scintillating contrast of the

Spangold finish is eloquent tes-timony to the attractiveness ofthese gold alloys.

St1MMARY

The Spangold concept embodiesa unique combination of attributesinherent in certain alloys. Thesenclude their spectacular trans-formation properties, colour,and variety of finishes.

The characterization of sut-able transformation features ledto the successful development ofprototype Spangold alloys. In ad-dition to exploiting their novelfinish, the use of the prototypeSpangold alloys in jewelleryalso realizes the practical appli-cation of structural gold basecompounds.

Apart from their opticalqualities, the Spangold alloysbased on intermetallic com-

52 (' Gold Bull., 1994, 27 (2)

Figure 9

Exemple from the Spangold collection.-Tension bracelet, finished with yellowgold

pounds can exhibit a number of othersalient features pertinent to their ap-plication in jewellery. These includedensity, lustre, castabilty, and wear-re-sistance. These gold compounds possessa wide and novel range of properties andare likely to enjoy continuing interest.

ACKNOWLEDGMENTS

It is a pleasure for the authors to thankthe following people and institutionsfor their interest and assistance through-out a wide-ranging research and devel-opment programme:World Gold Council, for jewellery ap-plication support.The Chamber of Mines of South Africa,for the Joan of gold.Mr Kurt Donau, who designed andcrafted the exhibits.Thïs paper is published by permissionof Mintek.

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54 ro Gold Bull., 1994, 27 (2)