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Hardening of Low-Alloyed Gold’, Proceedings of Gold 2003: New Industrial Applications for Gold, Vancouver, Canada 16 J Hardening of Introduction The increase of hardness, strength and wear resistance of low Low-Alloyed Gold alloyed gold alloys for jewellery applications could be of particular interest for the growing markets in India, the Middle East and Asia, where high caratages (21/22-24ct) traditionally are preferred over the stronger but less precious Jörg Fischer-Bühner counterparts with 18ct or lower caratages (1). Although soft Forschungsinstitut für Edelmetalle und Metallchemie gold alloys may be preferred due to tradition or culture, the (FEM) overall trend towards miniaturisation of cross sections and The Research Institute for Precious Metals and Metal diameters of jewellery pieces, in part related to a growing Chemistry market for hollow jewellery, demands higher strength than Schwäbisch Gmünd, Germany currently available with conventional alloys. Furthermore, E-mail: [email protected] low-alloyed gold alloys are of considerable interest for technical applications, where a combination of high strength, Abstract ductility (forming operations) and electrical conductivity is The development of strengthened high carat Au alloys required (2). for jewellery applications is an interesting but Pure gold is extremely soft in the as-cast and annealed challenging task, with significant potential for further states. Copper and silver, which are the main alloying development of new age-hardenable alloys. The paper elements in gold jewellery alloys, contribute some solid first presents an overview on the possibilities and solution strengthening, with copper being more effective limitations to harden 22ct Au by conventional means, than silver due to the larger atomic size difference with gold. i.e. by alloying with conventional base metals (solid In 18ct jewellery alloys (75% Au*), a pronounced solution hardening) and in combination with heat strengthening effect occurs during low temperature ageing, treatment processes (age-hardening). The paper then which is based on the formation of ordered AuCu-phases (3). refers to 22ct up to 24ct Au and reports on the This requires a minimum of 5% copper in 18ct alloys, but is enhancement of the age-hardening effect of the becoming more pronounced for alloys with larger copper primary base metals by additions of selected contents (4). As recently reviewed, the combination of order secondary elements. It is suggested that the and precipitation hardening is the predominant prevailing age-hardening mechanism in these types strengthening mechanism in 18ct and lower carat jewellery of alloys consists in precipitation of intermetallic alloys (5). phases formed by the primary and secondary For high carat gold alloys, however, the possibilities to additions. harden an alloy are limited due to the low amounts of alloying elements that can be added to the alloy, otherwise falling below the required caratage. This holds especially for 24ct jewellery alloys with a minimum of 99-99.5% Au depending on individual national legislation. Former studies (6,7,8) that are summarised in another source (1), have focused on microalloying additions of (light) metals with a promising potential for solid solution hardening due to a large difference in atomic size and weight to pure gold. Other studies focused on microalloying with rare-earth, refractory or light metals/elements. Additions of elements with a negligible solubility in gold at low temperatures introduce a potential for precipitation hardening by formation of gold-based ordered (so-called intermetallic) phases or eutectic structures. In fact, most of this work initially was based on corresponding alloy development for electronic applications. Whereas the industrial exploitation of these types of strengthened alloys in electronics is successful (2), the suitability for jewellery applications seems to be limited. Possible reasons are: detrimental effects of the (micro-) alloying additions on melting, casting and working * Compositions are referred to as wt% throughout the entire paper. Gold Bulletin 2005 • 38/3 120 properties, as well as reduction in strength after remelting of with a Ni:Si-ratio of 2:1 (14); the formation of Ni2Si-phases material due to loss of alloying elements. Moreover, some of was confirmed later by transmission electron microscopy the alloying additions suggested are rare and expensive. (TEM) investigations. An important side-effect for the More recently, the development of 24ct gold alloys with electronic applications of copper alloys is, that the Cu-matrix increased hardness and good suitability for jewellery is essentially free of alloying elements after the age- applications has been reported. The hardening effect, which hardening treatment, which finally yields optimum electrical is obtained by alloying with Co+ Sb and suitable heat conductivity together with high strength. treatment (9), is based on precipitation hardening and is From the results and experience available outlined above, most pronounced in combination with preceding cold- it is obvious that the development of strengthened high working. carat Au alloys for jewellery applications, without detrimental Comparably little work on hardening 22ct Au alloys has effects on colour, melting and working properties, is still an been carried out (or published) so far (1). The development interesting but challenging task, with a significant potential of alloys which can be hardened by heat treatment to for further development of new age-hardenable alloys. hardness levels comparable to 18ct alloys has been reported The main aim of the present study was to overview the recently, but corresponding details on alloy compositions possibilities and limitations to harden 22ct Au by were available only after finalisation of the present work conventional means, i.e. by alloying with conventional base (10,11,12) and will be referred to later in this publication. metals (solid solution hardening) and in combination with The practical approach in development of age-hardenable heat treatment processes (age-hardening or precipitation alloys consists in alloying with a single element or base metal hardening). With a view to the practical aspects, ‘difficult’ “Me”, often far beyond the corresponding limits of solubility additions (e.g. highly reactive metals) were not considered, in the main metal matrix (here: Au), and identification of so the alloys discussed in this study should not require optimum subsequent age-hardening heat treatment sophisticated alloying equipment or techniques and should parameters to precipitate finely distributed “Me”-rich be suitable for a ‘normal’ jewellery production environment. particles. From the development of advanced age- We have in a second step tried to enhance the hardening hardenable high-conductivity high-strength copper alloys for effect of the base metals by addition of selected further connector applications (e.g. 13) it can be concluded, elements. This part of the work covered 22-24ct alloys, with however, that the age-hardening process can be significantly the overall aim of studying if the hardening approach altered or enhanced by further additions of suitable elements described above for copper base alloys can be successfully “A”. These elements possess a large tendency to form finely transferred to low alloyed gold alloys. dispersed MexAy-phases in the Cu matrix with the primary The work has been funded by the World Gold Council base metal “Me”. Phosphides and silicides of Co, Ni and Fe under G.R.O.W. Project RP 03-02: ‘Hardening of high carat have been identified to give a high potential for age- gold alloys by conventional means’ from 2002-2003. This hardening in Cu. In almost all cases, the most pronounced publication is an update of a paper presented at the age-hardening effect is observed if the atomic ratio x:y of the conference ‘New industrial applications for Gold’ in 2003 additions “Me” and “A” correspond to the atomic (15) and focuses on the basic metallurgical aspects. The stoichiometry of the expected reaction phase MexAy. Figure 1 results of extended trials on property testing of selected illustrates this for Ni- and Si-additions in copper: the 22ct alloys for jewellery applications have been presented maximum age-hardenability is obtained for compositions elsewhere (16, 17). 4 25 50 75 100 125 3 2 150 % Silicon 125 1 100 75 50 25 246810 % Nickel Figure 1 Age-hardening contribution in Copper by joint Ni- and Si- additions (14)* Gold Bulletin 2005 • 38/3 121 Solid solution hardening of 22ct Au metallographic cross sections of the as-cast button samples. A large share of jewellery is produced by investment casting In addition, the solution treated state was investigated, i.e. without any subsequent heat treatment or mechanical after annealing at 700-900°C for usually 1h. In order to working. The drawback for 22ct gold jewellery is a low as-cast evaluate the hardness in a ‘realistic’ as-cast state (i.e. after hardness, which typically is around 60 HV1 depending on the investment casting), test samples and some jewellery ring silver:copper ratio. Therefore the objective of a first series of samples were cast for selected alloys, using a standard experiments has been to increase the hardness in the as-cast investment casting process (Indutherm vacuum equipment, state without impairment of colour, investment casting graphite crucibles and stoppers, Argon protective gas, properties etc. The hardness in an as-cast state is determined gypsum bonded investment). Further experiments for a primarily by solid solution hardening, although some
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