Search

Additive Manufacturing (AM) allows to manufacture new designs and novel geometries interesting for jewelry and watchmaking items. However, pure gold and gold alloys are challenging materials to manufacture by Laser Powder Bed Fusion (LPBF). Due to the low absorptivity at 1 064 nm Infrared (IR) wavelength combined to high thermal conductivity, it is difficult to manufacture pure gold and gold-based alloys by this process. Recent evolutions in laser technology allowed to build a machine using a 515 nm "green" laser. By changing the wavelength (1 064 nm to 515 nm), absorptivity can be improved from 7 to 37%. This paper will focus on 18 karat gold single tracks analysis produced by both wavelengths on a steel substrate. Different melting states will be detailed and indexed in order to select, correctly, machine parameters for producing gold items. It will be shown that 515 nm laser is more adapted to the material than 1 064 nm laser for manufacturing gold alloys. [ABSTRACT FROM AUTHOR]

Gold and silver alloys are very difficult to process by laser melting because of their high reflectivity and thermal conductivity. The processing of 18-karat gold alloys was optimized in several steps by suitable alloying additions and a variation of the process parameters. The first step of the optimization was to improve the density and the surface quality of selective laser-melted (SLM) parts made of standard 3N yellow gold. The residual porosity was about 3-4%. In a second step, the gold alloy powder was surface treated to reduce the reflectivity and thereby increase the interaction with the laser beam. This resulted in a significant reduction of porosity to below 1%. Finally, the thermal conductivity of the alloys was reduced by microalloying. The type and amount of alloying elements were determined by means of thermodynamic simulations. Additions of iron and germanium were found to be suitable additions and resulted in a reduction of porosity to 0.3%. At the same time, the surface roughness was reduced significantly. [ABSTRACT FROM AUTHOR]

The synthesis of nanometric alloys, or nanoalloys, is gaining interest in nanoscience due to their new and attractive physical properties. Furthermore, they improve the efficiency and performance of the diverse nanotechnological applications in comparison with the nanostructures composed of only one material. This study presents a novel synthesis method that facilitates the production of Au-Ag random bimetallic nanoalloys with uniform shape at room temperature, using the reducing and stabilizing properties of low-toxicity molecules, such as sucrose, ascorbic acid, and rongalite. The images taken with a transmission electron microscope (TEM) showed spherical bimetallic nanoparticles between 7 and 12 nm. The presence of Au and Ag in the nanostructures was confirmed by EDS. The optical absorption analysis showed a band centered at 480 nm, associated with surface plasmon resonance in Au-Ag nanoalloys. A Raman band was observed around 98 cm after the nanoparticles were synthesized. Additionally, the behavior of the radial breathing modes (RBM) of mixed bimetallic nanoalloys of AuAg (con n = 0-20) clusters was analyzed. The analysis was performed with the density functional theory (DFT) at Becke level, three-parameter, Lee-Yang-Parr (B3LYP) in combination with basis set Los Alamos Laboratory 2 double ζ (LANL2DZ). The predicted vibrational modes associated with RBM are located around 90-100 cm approximately. This shows a good tendency and correlation with the Raman band detected experimentally at low wave numbers. [ABSTRACT FROM AUTHOR]

The microstructural evolution and shear strength of [Ti.sub.50][Ni.sub.50] SMA and 316L SS infrared brazed with a gold-based filler metal Au-22Ni-8Pd (BAu-6) at 1050°C for 180 and 300 s were investigated. The results of dynamic wetting angle measurement indicate that Au-22Ni-8Pd filler metal exhibits rather good wettability on [Ti.sub.50][Ni.sub.50] and 316L SS substrates if the specimens are brazed at 1050°C for longer than 100 s. Experimental results of SEM BEIs and EDS/WDS show that the joint brazed for 300 s is composed of a layer of [Ni.sub.3]Ti+ TiNiAu lamellae, a layer of TiNiAu and Ti[(Au, Ni).sub.4] intermetallics, and a layer of Ti[Ni.sub.2]Au one at [Ti.sub.50][Ni.sub.50] substrate side, and is a layer of the mixture of (Ni, Fe)+(Au) solid solutions and a layer of (Ni, Fe) solid solution with no interfacial reaction compound formed at 316L SS substrate side. All the formed lamellae/layers increase in thicknesses as the brazing time is prolonged, which prevents the brittle [Ni.sub.3]Ti lamellae from contacting the layer of Ti[Ni.sub.2]Au. Accordingly, the joint shear strength improves as the brazing time increases. The shear strength of a joint brazed for 300 s can reach 204 MPa, where the fracture crack begins around the interface of the Ti[Ni.sub.2]Au and layer TiNiAu/Ti[(Ni,Au).sub.4] layers. Cleavage-dominated fracture is observed in SEI fractographs. Keywords Infrared brazing * [Ti.sub.50][Ni.sub.50] * Stainless steel * Gold-based filler * Interfacial reaction * Microstructure, Introduction Equiatomic [Ti.sub.50][Ni.sub.50] shape memory alloy (SMA) has an excellent shape memory effect and pseudoelasticity [1, 2], and it also has good biocompatibility [3, 4]. The nominal composition of AISI [...]

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. Keywords Au-Ge microalloy * Brittle mechanism * Vickers hardness * Internal, Introduction Gold has been utilized in cultural, decorative, and financial applications since ancient times. Most of the gold used in today's jewelry industry is refined from Au deposits. According to [...]

Combinatorial approaches comprised of combinatorial magnetron co-sputtering deposition and fast screening methods are introduced to study color as a function of composition in Au-based alloys. The microstructures of the thin films and bulk alloys are identified by X-ray diffraction, and their colors of the alloys are characterized by optical reflectivity. The results reveal that when comparing microstructures and reflectivity, thin films are similar to bulk alloys. In Au-Ag-Cu solid solutions, the color of the ternary alloy follows the rule of mixture. For colors resulting from AuAl intermetallic, the color of an alloy scales with the percentage of the intermetallic phase and the deviation from its ideal binary composition. In the Au-Al-Cu library, we found a ∼90 % AuAl area fraction compositional window where copper addition can be tuned to improve mechanical properties while keeping purple color, even though Al and CuAl phases exist. Moreover, when comparing the color in Au-Cu-Si-Ag amorphous and crystalline state solid solution for the same composition, the colors are essentially identical. [ABSTRACT FROM AUTHOR]

Precious metals are widely used for a variety of applications, among which are luxury goods, biomedical devices, and electronic components. Requiring precise components, those sectors are usually looking into lost-wax casting, machining, and, more recently, metal injection moulding. Material or (thermal) energy consumption and demand for the aforementioned processes are mainly suitable for stable crystalline structures. The applicability of a field-assisted sintering technique (FAST)-Electro-Sinter-Forging (ESF)-to a high-karatage gold powder is explored in the present research work. FAST is characterized by extremely rapid processing times, also enabling the production of metastable phases. Different powders with different features (as atomized, ball-milled, and admixed with nanodiamonds) were taken into account and fast-sintered; and the resulting samples were analyzed in terms of final density, microstructure, and hardness. [ABSTRACT FROM AUTHOR]

Gold welding alloys, which are used in the production of both hollow and solid gold chains, affect the optical and mechanical properties of various gold products because the corrosion resistance of the individual links depends on these properties. It is important that welding alloys with high corrosion resistance do not degrade during or after the production process. The mechanical properties of gold welding alloy wires are strongly influenced by the alloy microstructure, which has a key role in both the machinability and the quality of the wires. In the presented work, various physical and mechanical properties of gold originating from different industrial deformation processes are evaluated. Specifically, various plastic deformation grades caused by different annealing and rolling steps are analyzed. The change of the temperature, time, and velocity parameters in the annealing and lamination processes leads to the formation of different levels of residual stresses in the material, which can generate a variation in the corrosion properties of the gold wires. The change in microstructure, due to the different annealing and rolling steps, is analyzed by optical microscope (OM) and SEM observations. The residual stresses are evaluated using XRD analysis, and the variations of the mechanical properties by micro-hardness tests. The corrosion resistance is evaluated by potentiodynamic polarization tests, in which an electrolyte solution is used to simulate human sweat. Small grain size and high homogeneity of the microstructure are preferred for the final products. In this study, the samples that have been produced are shown to have lower levels of residual stresses and feature higher corrosion resistance and more favorable mechanical properties. [ABSTRACT FROM AUTHOR]

Diffusion couples and equilibrated alloys were used to construct the isothermal phase diagram of Au-Al-Cu at 500 °C. Electron microprobe analyses were performed to determine the phase compositions and phase relationships. Two ternary phases and 10 three-phase equilibriums were determined in this study. Four additional three-phase equilibriums were estimated to meet the criteria for phase relationships. The δ (AuAl) phase exhibited a wide range of solubility, and the lattice parameters were examined by X-ray. The solubility ranges of the binary intermetallic phases were also determined. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the present study, nanocomposites containing Ag-Au alloy nanoparticle ensembles with various compositions on polytetrafluoroethylene were prepared by physical vapor deposition. After a certain time of immersion of the samples in water, oxidation and dissolution of the Ag nanoparticles (AgNPs) occurred, and changes in the morphology, optical properties and composition of the nanocomposites were examined using transmission electron microscopy, ultraviolet-visible spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. The composition-dependence and the time-dependence of the silver ion release were studied, and the concentration of the silver ions in water was detected using inductively coupled plasma mass spectrometry. The results indicate that with increasing gold fraction in the Au-Ag alloy nanoparticles, a strong improvement of the oxidation resistance of the AgNPs occurs. The dissolution of Ag is rapid at the first contact of the sample with water until a saturation state is reached. XPS synchrotron measurements with different excitation energies show that the depletion of Ag from the nanoparticles does not lead to the formation of a Au-rich shell and that the atomic mobility is high enough in the small nanoparticles of about 5 nm average size to equilibrate any concentration gradient. [ABSTRACT FROM AUTHOR]

A facile seed-growth method was developed to synthesize AgAu alloy and core/shell nanocrystals (NCs) using different-sized Ag NCs (6.1, 7.4, and 9.6 nm) as seeds and octadecylamine as the reducing agent, surface ligand, and solvent. Pre-synthesized Ag NCs acted as catalysts for the reduction of Au precursors at 130 °C. Transmission electron microscopy, energy-dispersive spectroscopy, and optical absorption spectroscopy were used to characterize as-synthesized NCs. Spherical AgAu alloy NCs were obtained when pre-synthesized 6.1 and 7.4 nm Ag NCs were used as seeds. While, if 9.6 nm Ag NCs were used as seeds, cubic Ag/Au core/shell NCs were finally obtained. The shapes and structures of AgAu NCs are related to the Ag seed sizes and the growth mechanism of alloy and core/shell NCs is discussed in detail. Different reaction temperatures were tested to optimize the synthesis of AgAu alloy NCs, and it was found that the optimum reaction temperature for the growth of AgAu alloy NCs is 130 °C. [ABSTRACT FROM AUTHOR]

The age-hardening characteristics of a dental low carat gold alloy with a dual hardener system of indium (In) and Cu (33.9 Au-26.2 Ag-20.28 Cu-9.8 Pd-7.8 In-2 Zn (at%)) were examined by observing the age hardenability and related phase transformation, microstructural changes and elemental distribution during the aging process at 400°C. The dual hardener system by the use of both In and Cu provided more powerful hardening effect compared to a single-hardener system of In or Cu, without the formation of a AuCu type ordered phase. The alloy showed apparent initial hardening, which was attributed to the pre-precipitation or zone formation by the help of quenched in excess vacancies. During the constant increase in hardness, the single parent phase separated into three phases, Au-Ag-based phase, Au-Cu-based phase containing Pd and In, and InPd-based phase, through a metastable state. Indium which was added as one of the hardeners induced initial grain boundary precipitation, followed by an expansion of the lamellar structure, which was responsible for softening. The alternative lamellar structure was composed of a Cu-rich layer (Au-Cu-based phase containing Pd and In) and an Ag-rich layer (Au-Ag-based phase) replaced partly by the InPd-based phase. Separation of the Ag-rich layer from the Cu-rich layer is based on the miscibility limit of Ag and Cu due to their eutectic property. [ABSTRACT FROM AUTHOR]

The age-hardening and overaging mechanisms related to the metastable phase formation by the decomposition of Ag and Cu in a dental casting gold alloy composed of 56Au-25Ag-11.8Cu-5Pd-1.7Zn-0.4Pt-0.1Ir (wt.%) were elucidated by characterizing the age-hardening behaviour, phase transformations, changes in microstructure and changes in element distribution. The fast and apparent increase in hardness at the initial stage of the aging process at 400°C was caused by the nucleation and growth of the metastable Ag-Au-rich phase and the Cu-Au-rich phase by the miscibility limit of Ag and Cu. The transformation of the metastable Ag-Au-rich phase into the stable Ag-Aurich phase progressed concurrently with the ordering of the Cu-Au-rich phase into the AuCu I phase through the metastable state, which resulted in the subsequent increase in hardness. The further increase in hardness was restrained before complete decomposition of the parent α0 phase due to the initiation of the lamellar-forming grain boundary reaction. The progress of the lamellar-forming grain boundary reaction was not directly connected with the phase transformation of the metastable phases into the final product phases. The heterogeneous expansion of the lamellar structure from the grain boundary caused greatersoftening than the subsequent further coarsening of the lamellar structure. The lamellar structure was composed of the Ag-Au-rich layer which was Cu-, Pd- and Zn-depleted and the AuCu I layer containing Pd and Zn. [ABSTRACT FROM AUTHOR]

The article presents research associated to gold. One research focuses on the solvent-free oxidation of primary carbon-hydrogen bonds in toluene using gold-palladium alloy nanoparticles. Another research delves into the photocatalytic activity of gold nanoparticles that are supported on titania. Research on the effect of gold on platinum oxidation in homogeneous gold-platinum electrocatalysts is also mentioned.

Infrared-brazing Fe3Al with Au-44Cu as filler metal has been investigated. The brazed joint consists mainly of a β-phase, Au8-xCu4+xAl4, caused by the dissolution of Al from Fe3Al substrate into the braze alloy. The depletion of Al from Fe3Al substrate results in the formation of a layer of β-phase particles dispersed in the Fe-rich phase. The highest shear strength for AuCu filler is 327 MPa for specimens infrared brazed at 880°C for 180 s. The brazed joint is mainly fractured along the central β-phase in which the fractograph exhibits quasi-cleavage with dimples. Increasing the brazing time or temperature will deteriorate the bonding strength of the joint, and the fracture mode is prone to cleavage of brittle fracture. Au- 44Cu filler demonstrates a great potential for bonding Fe3Al intermetallic compound. [ABSTRACT FROM AUTHOR]

The as-cast AuSn20 eutectic alloys prepared by four different solidification pathways have been investigated in terms of the microstructure and the high-temperature compressive behaviors. The primary phases appeared in the four alloys are very sensitive to the cooling rate, which decrease in the size and the volume fraction as the cooling rate increases. The morphologies of the primary ζ'-Au5Sn phase are in dendritic at low cooling rate and change to rosette-like at high cooling rate. When the cooling rate is about 3.5× 104 K/min, the primary ζ'-Au5Sn can be suppressed but small δ-AuSn particles appear instead as the primary phase. The compressive behaviors at 220°C exhibit a low yielding stress and a long stress platform for the alloy prepared by injection casting with a copper crucible, which indicates an advantageous processing route for the production of the AuSn20 strips or foils. [ABSTRACT FROM AUTHOR]

In this paper we report the results of a deep investigation of gold title in jewelry alloys by means of potentiometric titration. After reviewing the suitable reactions for such purpose we focused on the most profitable one, which involves the use of hexadecylpyridinium chloride (CPC). This cation gives rise to selective and quantitative precipitation of AuCI4. We completely revised a method previously proposed by W.S. Selig in the 80's, by improving the sampling preparation and by optimizing the titration steps. Then, the new proposed method was tested on a large number of different alloys, commonly used for goldsmithery purposes. The obtained gold titles were compared with the data achieved by fire assay, giving rise to a very good agreement. At the light of these results, the present method can be rightfully considered a cheaper and "greener" alternative to the traditional one. [ABSTRACT FROM AUTHOR]

The attractive color of blue and especially purple gold alloys has always intrigued jewelry designers and producers. The reality of using these colors for jewelry has been very limited, however, mainly due to the inherent brittleness and poor corrosion resistance of this special class of gold alloys. The paper reports on some improvements in crack resistance that are obtained by particular additions to 14k blue- and 18k purple-gold colored alloys. Opportunities to incorporate such colors in jewelry design with these modified, colored-gold alloys using investment casting (as well as the still existing limitations) are discussed. Special attention is given to bi-metal casting technologies that can be used to manufacture multi-colored jewelry. A protective, transparent and wear-resistant coating finally needs to be applied to minimize corrosion and improve long-term stability of the colorful pieces. [ABSTRACT FROM AUTHOR]

This study examined the age-hardening by miscibility limit in a multi-purpose dental gold alloy containing platinum. The hardness increased rapidly in the initial stage of the aging process, reached the maximum value, then decreased continuously with aging time. The significant hardness increase resulted from the heterogeneous precipitation of the Pt-rich β phase from the grain boundary of the Au-rich α1 matrix due to the miscibility limit of Au-Pt system. With increasing aging time, the fine Pt-rich β precipitates covered almost the whole matrix, and by further aging, the precipitates grew coarse. The microstructural coarsening reduced the interface between the Au-rich α1 matrix and the Pt-rich β precipitates, which released the lattice strains between the two phases, resulting in a softening effect. In the later stage of aging process, the Au-containing Pt3In particle-like structure was transformed into the Au-depleted particle-like structure containing relatively large amounts of Cu resulting from the overlapping miscibility limit of both Au-Pt and Ag-Cu systems, which was responsible for the slow decreasing rate in hardness in the later stage of aging. [ABSTRACT FROM AUTHOR]

CO is a useful probe in the characterization of surface properties of both metal and metal oxide via adsorption. Adsorption of CO was used to monitor the possible active site of an Au/TiO2 catalyst for the CO oxidation reaction. CO adsorption on the reduced catalyst results in the band at 2104 cm-1 indicative of Au0. During the reaction (in the presence of both CO and O2 present) the band is shifted to higher wave numbers indicating non-competitive adsorption on the surface of Au species. This study also reveals the relationship between the presence of CO (in the absence of oxygen) and the build-up of surface species such as bicarbonates, formates and carbonate species which decreases the activity of the catalyst. The presence of both the reduced and the cationic species of Au seem to be requirement for the activity of the catalyst. [ABSTRACT FROM AUTHOR]

Blue and purple gold alloys form in the alloying systems of gold with gallium/indium and aluminium respectively and are known to be very brittle and to possess low corrosion resistance. Taking into account these drawbacks this paper describes the results of a European funded research project. The properties of the blue and purple gold alloys and coatings such as corrosion resistance, metal release rates, hardness and colour and the influence of alloying additions on these properties are presented and discussed. Surface engineering techniques and investment casting were used for manufacturing of jewelry items with selectively coated coloured surface. Coatings of AuGa2 and AuIn2 blue gold alloys were applied on 18kt gold and Sterling silver jewelry by electroplating, laser/torch cladding or dipping into liquid gallium. The suitability of blue gold coatings for jewelry purposes will be discussed in the light of reliability and feasibility. The work consists of two parts. Part I describes properties and surface processing techniques while Part II deals with investment casting and related alloy design of coloured gold alloys. [ABSTRACT FROM AUTHOR]

The article offers highlights from the recent research published writings. The scientists from the Kunming Institute of Precious Metals in Kunming, China suggested a proposal on detection of gold in precious metal alloys using a potentiometric titration method. The scientists from the University of Melbourne School of Physics in Victoria presented an investigatory project on roughness determination for surface structure and internal inhomogeneity of the thin, nano-structured and opaque materials. The scientists from the Universidad Autonoma Metropolitana-Unidad in Ixtapalapa, Mexico featured a research study on determination of gold size particles in gold metal alloys through carbon monoxide solution, hydrogen chemisorption and transmission electron microscopy (TEM) spectroscopy.

A simple method to prepare the rambutan-like hybrid nanospheres of Au-P123 is presented. The rambutan-like hybrid nanospheres of Au-P123 were characterized by transmission electron microscopy (TEM), Electron diffraction patterns and UV-vis absorption spectroscopy. [ABSTRACT FROM AUTHOR]

Stress corrosion cracking (SCC) is a well known form of environmental attack in low carat gold jewelry. It is desirable to have a quick, easy and cost effective way to detect SCC in alloys and prevent them from being used and later failing in their application. A facile chemical method to investigate SCC of 9 carat gold alloys is demonstrated. It Involves a simple application of tensile stress to a wire sample in a corrosive environment such as 1-10 % FeCl3 which induces failure in less than 5 minutes. In this study three quaternary (Au. Ag, Cu and Zn) 9 carat gold alloy compositions were investigated for their resistance to SCC and the relationship between time to failure and processing conditions is studied. It is envisaged that the use of such a rapid and facile screening procedure at the production stage may readily identify alloy treatments that produce jewelry that will be susceptible to SCC in its lifetime. [ABSTRACT FROM AUTHOR]

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 jewelry 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. [ABSTRACT FROM AUTHOR]