Jewelry Applications Research Articles

Effects of Beryllium and Tin on Spring Property of Aged Silver Alloys 935 without Solutionization Treatment

Microstructural development and mechanical properties of as cast and heat treated 935 Ag-Cu and Ag-Cu-Be-Sn alloys without using solutionization were investigated. The conventional lost-wax process with vacuum assisted and induction heating has been performed for producing the as cast specimens. After pouring the melt into plaster mould, the casting has been prolonged for 15 minutes prior to quenching into water. Subsequently all specimens including the reference specimens, commercial specimens and Ag-Cu-Be-Sn additions specimens were aged at 250 and 350°C for a given period between 0 to 120 min according to normal aging procedure of the 935 AgCu alloy. In case of Be and Sn additions, the A1 alloy with aging at 350°C performed the combination of good mechanical properties and the surface finish with superior anti-tarnish and fire scale resistance and thus these are suitable for jewelry applications. Microstructures were characterized by using both scanning and transmission microscopes. The results show that the precipitates in the 935 AgCuBeSn alloy were detected and thus resulted in a better improvement in yield strength and modulus of resilience than those of the 935 AgCu alloy

Characterization of the brittle mechanism in a Au-Ge microalloy

In order to improve the hardness of pure gold, alloying with small amounts of germanium (Ge) is of great interest. However, the resulting embrittlement of the new alloy is undesirable for use in jewelry applications. Thus, it is necessary to elucidate the cause of this behavior in this microalloy in order to avoid it in future cases. The Au-Ge alloy was characterized using a combination of inductively coupled plasma mass spectrometry (ICP-MS) to investigate the nominal chemistry, Vickers microhardness to determine the alloy’s hardness in comparison to pure Au, X-ray diffraction (XRD) to find the phases/structures, along with light optical microscopy (LOM) and scanning electron microscopy (SEM) to explore the alloy’s surface features including grain size, and phase distributions. A heating microscope and differential thermal analyzer (DTA) were also used in order to determine the phase transformation behavior. Trace amount of Ge was found to render Au grains refined to the size of 10 to 160 μm. The corresponding microhardness was remarkably improved from 31 to 56 HV. The Au-Ge microalloy was found to exhibit Ge enrichment along the grain boundaries, resulting in an interdendritic, eutectic that forms throughout the microstructure. This phase distribution is likely the cause of the brittle mechanical behavior of the Au-Ge alloy.

Bulk metallic glasses based on precious metals: Thermal treatments and mechanical properties

The thermomechanical behavior of precious based metallic glasses has been investigated. Their compositions are free of nickel for biocompatibility. The gold-based BMG has a gold content resulting in 18 Karats alloy, a supercooled liquid region of 43 °C and a casting diameter up to 5 mm in rod. The compositions of platinum and palladium based BMGs are interesting as they can be formed into bulk glassy rods with diameter up to 15 and 30 mm respectively. The platinum-based BMG has a platinum content resulting in 850 Pt grade with a supercooling region reaching 58 °C. The palladium-based BMG is principally composed of 40 wt.% palladium and 32 wt.% platinum, with a large supercooling region reaching 73 °C.The thermoplastic deformation of these BMG has been examined using thermomechanical analyser (TMA) and the results show that the alloys can be easily processed in the supercooled liquid region. During thermal processing, crystallization must be controlled since it improves hardness and elastic modulus, but embrittles the alloys and stops the deformation. The high hardness of Au-, Pt- and Pd- base BMGs (respectively 340, 420, 460 HV) twice the value of conventional precious metals, coupled with good properties for superplastic forming in the supercooled liquid region made them promising materials for watch making and jewelry applications.

Thermodynamic Solidification Path Assessment and Microstructural Comparison of Deep Eutectic Au-Cu-Si System

Particularly in karat gold alloys in jewelry applications, many alloy formulations include a small amount of silicon for filigree castings and brightening effect [1]. On the other hand, some alloy manufacturers stayed away from silicon or set a maximum amount of silicon to avoid embrittlement [1-3]. However recent development of gold-based bulk metallic glass involved a large addition of silicon (16.3 at% Si) in the alloy formulation [4-6]. No silicon segregation had been observed in large ingots that were cooled at 100 K/s and above. In the current study of Au55Cu25Si20 alloy, extra silicon has been added to investigate the segregation effect using three different techniques for cross references: equilibrium solidification path, Scheil-Gulliver solidification with ThermoCalc® and microstructural study with varying cooling rates. The amounts of solidified primary and eutectic silicon were measured in three differently cooled ingots. Image analysis using ImageJ® was used to obtain size distributions, area fractions, and volume fractions of silicon in both primary silicon and eutectic silicon.

On the abnormal room temperature tarnishing of an 18 karat gold bulk metallic glass alloy

The amorphous 18K white gold alloy of the composition Au49Ag5.5Pd2.3Cu26.9Si16.3 at% possesses a premium white-gold color and a Vickers hardness value 1.5 times that of conventional white gold alloys. These properties, together with the low melting temperature of ca. 370°C and its good processability are highly desired for jewelry applications. Nevertheless, the premium white color of this alloy is unstable and turns into a yellowish brown color after some time. The intense tarnishing of the Au49Ag5.5Pd2.3Cu26.9Si16.3 at% bulk metallic glass alloy under various different environmental conditions has been investigated by SEM, STEM, TEM, and Auger spectroscopy (AES) and linked to a chemical interaction between the copper and the silicon of the alloy. A mechanism behind the tarnishing is discussed in detail and suggestions are made for future alloy development towards a tarnish resistant 18K amorphous white gold alloy.

The electrochemical corrosion behaviour of quaternary gold alloys when exposed to 3.5 % NaCl solution

Lower carat gold alloys, specifically 9 carat gold alloys, containing less than 40 % gold, and alloying additions of silver, copper and zinc, are commonly used in many jewellery applications, to offset high costs and poor mechanical properties associated with pure gold. While gold is considered to be chemically inert, the presence of active alloying additions raises concerns about certain forms of corrosion, particularly selective dissolution of these alloys. The purpose of this study was to systematically study the corrosion behaviour of a series of quaternary gold–silver–copper–zinc alloys using dc potentiodynamic scanning in saline (3.5 % NaCl) environment. Full anodic/cathodic scans were conducted to determine the overall corrosion characteristics of the alloy, followed by selective anodic scans and subsequent morphological and compositional analysis of the alloy surface and corroding media to determine the extent of selective dissolution. Varying degrees of selective dissolution and associated corrosion rates were observed after anodic polarisation in 3.5 % NaCl, depending on the alloy composition. The corrosion behaviour of the alloys was determined by the extent of anodic reactions which induce (1) formation of oxide scales on the alloy surface and or (2) dissolution of Zn and Cu species. In general, the improved corrosion characteristics of alloy #3 was attributed to the composition of Zn/Cu in the alloy and thus favourable microstructure promoting the formation of protective oxide/chloride scales and reducing the extent of Cu and Zn dissolution.

Multi-technique analysis of high quality HPHT diamond crystal

Fabrication of high quality diamond either for gemmological or for technological applications, is still a challenge. The control of impurity incorporation for doping or to modify the crystal colour should be still improved. The present contribution reports availability of nearly defect free HPHT (high pressure high temperature) diamond in terms of dislocations and point defects. Cathodoluminescence (CL) transitions related to point defects or dislocations (A-band) are not observed and only some individual dislocations are revealed by X-ray topography (topo-X). CL spectra are dominated by the excitonic-related transistions and the boron incorporation, estimated from the phonon-replica relative intensities, is around 1016cm−3 that corroborate the values estimated by FTIR. Such amount of B gives a pale blue colour to the diamond sintetized monocrystal. The latter crystals are grown at the Instituto de Monocristales S.L. for jewellery applications, but their use for semiconducting applications is also envisaged after the highlights of the present study.

Investigation of mechanical, corrosion and optical properties of an 18 carat Au–Cu–Si–Ag–Pd bulk metallic glass

The following article shows that amorphous gold based alloys are a promising material for jewelry applications. Specimens in form of amorphous plates were investigated with respect to their processibility and properties that are relevant for jewelry applications. Studies included thermophysical analysis, corrosion and mechanical behavior and spectrophotometric experiments. Compared to fine gold and traditional gold alloys, the most outstanding results were a hardness of up to 360 HV and a superior abrasion resistance of the 18 carat bulk metallic glass. Electrochemical corrosion tests revealed a dissolution rate similar to fine gold.

The aluminium-coppergold ternary system

Despite Au, Al and Cu being individually very well-known elements, their ternary phase diagram has not been studied in as much detail as those of many other Au-containing ternaries. Here we review what is known, and consider the prospects for technological exploitation of some of the ternary compositions. The components of greatest interest in Al-Au-Cu may be the β-phases, at least two of which have shape memory properties. Of these, ‘Spangold’, which has the nominal stoichiometry Au7Cu5Al4, has received some attention for jewellery applications, while the edge compound Cu3Al is a well-known shape memory composition with corresponding specialised industrial uses. The properties of other β-phase compositions in the system have been scarcely investigated. The system also contains an extensive γ-phase, Al4AuxCu9−x, where x ranges from 0 to ∼6.5, and the purple gold phase AuAl2.

Electrodeposited nanocrystalline bronze alloys as replacement for Ni

AbstractNanocrystalline white‐bronze, CuSn, electroplating was investigated as alternative to Ni plating as undercoat for noble metals in jewellery applications. A strongly acidic plating bath was developed with an organic additive to suppress hydrogen evolution and obtain bright coatings. Polarization curves indicate a strong inhibition by the organic additive at the tin equilibrium potential. Mass transfer controlled deposition yields nanocrystalline bronze coating with 0–65 wt% Sn. One or two low temperature equilibrium phases, high temperature equilibrium phases or a non‐equilibrium phase were identified by XRD analyses. Bright white bronze, 50 wt% Cu/50 wt% Sn, bronze coatings show improved tarnishing resistance and similar corrosion and wear resistance as undercoat for gold on actual jewellery parts compared to nickel. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Precious bulk metallic glasses for jewelry applications

Families of bulk metallic glass (BMG) forming alloys based on precious metal platinum and gold are introduced. The gold-based BMGs have a gold content resulting in 18 karat alloy. They show liquidus temperature as low as 370 °C, good processibility with a large supercooled liquid region reaching 60 °C and low critical cooling rate reflecting in maximum casting thickness of up to 5 mm. The platinum-based BMGs have a platinum content resulting in 850Plat grade. They have liquidus temperature as low as 522 °C, excellent processibility with a large supercooled liquid region reaching 98 °C and low critical cooling rate reflecting in maximum casting thickness of up to 20 mm. The alloys exhibit very desired mechanical properties for jewelry applications. Both alloys show very high strength, 1400 MPa for the Pt 57.5Cu 14.7Ni 5.3P 22.5 and about 1200 MPa for the Au 49Ag 5.5Pd 2.3Cu 26.9Si 16.3. Pt 57.5Cu 14.7Ni 5.3P 22.5 is the first monolithic BMG that shows, in addition to a high yield strength, a pronounced global plasticity of about 20%. Such a combination of high ductility, high strength, and large elastic limit, has not yet been observed in any metallic system. Vickers hardness of these precious BMGs are at least twice the value of conventional gold and platinum alloys. Both alloys exhibit properties ideal for superplastic forming in the supercooled liquid region such as low Tg, large supercooled liquid region, fragile liquid behavior, and the ability to process in air. The resulting net-shape superformed parts have negligible porosity and minimal internal stresses.

Hardening of Low-Alloyed Gold

The development of strengthened high carat Au alloys for jewellery applications is an interesting but challenging task, with significant potential for further development of new age-hardenable alloys. The paper first presents an overview on the possibilities and limitations to harden 22ct Au by conventional means, i.e. by alloying with conventional base metals (solid solution hardening) and in combination with heat treatment processes (age-hardening). The paper then refers to 22ct up to 24ct Au and reports on the enhancement of the age-hardening effect of the primary base metals by additions of selected secondary elements. It is suggested that the prevailing age-hardening mechanism in these types of alloys consists in precipitation of intermetallic phases formed by the primary and secondary additions.

Highly processable bulk metallic glass-forming alloys in the Pt–Co–Ni–Cu–P system

Highly processable bulk metallic glass alloys in the Pt–Co–Ni–Cu–P system were discovered. The alloys show low liquidus temperature below 900 K, excellent processability with low critical cooling rate reflecting in maximum casting thicknesses in quartz tubes of up to 20 mm, and a large supercooled liquid region. The Pt57.5Cu14.7Ni5.3P22.5 composition has a liquidus temperature of 795 K, a glass transition temperature of 508 K with a supercooled liquid region of 98 K. For medical and jewelry applications a Ni-free alloy, Pt60Cu16Co2P22 was discovered with a liquidus temperature of 881 K, a glass transition temperature of 506 K, and a supercooled liquid region of 63 K. Glass formation was observed in a wider composition range. Vickers hardness of these alloys is in the 400 Hv range. The alloys can be processed in the supercooled liquid region in air without any measurable oxidation. In this region, a large processing window is available in which the material does not embrittle. Embrittlement in these alloys is correlated with crystallization. It can be avoided as long as substantial crystallization does not take place during isothermal processing in the supercooled liquid region. Also, liquid processing can be performed in air when flux with B2O3.

Colour carbon coatings

In this paper, the unique properties of thin diamond-like layers are presented. Diamond-like carbon (DLC) films have been developed as colour coatings for a range of substrates. They can be used in a wide variety of applications, including jewellery and other applications where colours need to be combined in a tough, hard-wearing, aesthetically appealing and biocompatible application. DLC films were synthesised by the radio frequency plasma activated chemical vapour deposition (RFPACVD) technique, running at 13.56 MHz. The possibility of the repeatable manufacturing of such coatings makes them a new possible material in jewellery. The coatings have been examined using a range of characterisation tools, including X-ray elemental microanalysis, Raman spectroscopy, atomic force microscopy, colourimetric analysis, ellipsometry and the measurement of nanohardness and adhesion. The examinations show that the technology can be applied in many fields. An example of the jewellery application is afforded by the UK Millennium Medal, created by the well-known English artist Wendy Ramshaw OBE, who worked closely with our team.

Explosive joining of precious metals

Though it might appear merely as a curiosity at first sight, explosives have already been employed for the integral welding of dissimilar metals to yield functional composites for use mainly in shipbuilding, off-shore and chemical installations. We have now explored the use of this technology for the joining of thin precious metal sheets with other metals. A variety of two-coloured semi-fabricates with potential ornamental applications in jewelry were produced. The bond strength and the formability of the various materials combinations were examined.

Stable strengthening of 990-gold

The addition of 1wt% titanium to gold was found to substantially harden the precious metal and enhance its wear resistance, which makes it attractive for jewellery applications. The enhanced hardening is due to the formation of fine AU4Ti precipitates under the appropriate thermo-mechanical treatments. It has been shown that another hardening mechanism can be induced in wire having a diameter of less than 100μm, which is stable to heating in air up to 200°C. These observations are consistent with the formation of a network of titanium oxide inclusions up to 100μm below the free surface, which has been reported elsewhere. New results have indicated that thin foil of Au-1Ti, which has been subjected to similar thermo-mechanical processing, exhibits similar properties to those of fine wire of this material. Potential uses for the fine wire and foil are described.

A low melting point solder for 22 carat yellow gold

The gold-germanium-silicon alloy system has been demonstrated to be capable of furnishing solders melting below 450°C for gold jewellery with a fineness above 21 carats. The application of a thin gold plating to the alloys in foil form confers the requisite flow and spreading characteristics, when melted in a nitrogen atmosphere. Good colour matching can be achieved by a short post-soldering heat treatment, which modifies the alloy microstructure. The mechanical properties of these materials, as determined by shear and peel tests, are suitable for jewellery applications, although silicon and germanium differ considerably from gold with regard to their electrochemical potential, the continuous gold matrix ensures that the alloys maintain a high resistance to the acidic cleaning liquids used in domestic environments.

Intermetallice Compounds: Principles and Practice

Partial table of contents: STRUCTURAL APPLICATIONS Ni3Al as a Single Phase (C. Liu & D. Pope) Silicides (K. Kumar) IMC's as Precipitates and Dispersoids (A. Ardell) ELECTRO-MAGNETIC APPLICATIONS Superconductor Applications (J. Stekly & E. Gregory) Magneto-Optical Applications (W. McGahan) Thermoelectric and Electrical Applications (M. Vedernikov) CHEMICAL AND METALLURGICAL APPLICATIONS Coating Materials (J. Nicholls) Electrochemical Applications (A. Vijh) MISCELLANEOUS APPLICATIONS Jewelry Applications (W. Rapson) Dental Amalgams (R. Waterstrat & T. Okabe) Bearing and Tribological Applications (W. Glaeser).

Diffusion soldering

Diffusion soldering is a hybrid of diffusion bonding and soldering. The principle of the process is to run a minute volume of solder into a joint between components that are pressed together and to solidify the solder by conversion to high melting point phases through isothermal reaction with the substrates. Thereafter the joint will not remelt unless heated to the temperature at which the high melting point phases melt. Tin can be used to make diffusion soldered joints between components of pure gold.The objctive of the present study was to evaluate tin as a diffusion solder for joining items of 18 carat golds, representative of alloys widely used in the fabrication of jewellery. The permissible range of the process parameters have been identified and a process specification has been developed. Joints can be made by heating to 450°C, which is substantially lower than the temperature needed for brazing of jewellery using conventional carat filler alloys. The resulting joints are colour matched and have mechanical properties that are likely to be adequate for jewellery applications. Diffusion soldering with tin has also been applied successfully to join 22 carat jewellery and the new jewellery alloy of 990 gold.

Colour properties of detuned alternating multilayer systems

In this paper colour properties of non-absorbing detuned multilayer systems are studied. Attention is devoted to two-layer, three-layer and four-layer alternating systems placed on glass substrates. The dependences of the colour coordinates on the changes of all the thicknesses of the films forming the systems are investigated. The colour coordinate purity is discussed for the detuned systems mentioned. Comparison with the non-absorbing tuned alternating systems is carried out from the point of view of this purity. It has been found that the detuned systems can have a higher colour purity than the tuned ones even when the total number of films forming the tuned systems is greater than the number of films forming the detuned systems. This fact implies that detuned multilayer systems may be useful for some glass jewellery applications.