Ekasit Nisaratanaporn

An Improvement in Tarnish and Corrosion Resistance of 94Ag-4Zn-Cu Alloys with Sn Addition

The tarnish and corrosion resistance of 94Ag-4Zn-Cu-Sn alloys and Ag-5.89Sn alloy compared with Ag-5.95Cu alloy were investigated. The tarnish results show that the DE* value of Ag-5.95Cu alloy is higher than those of 94Ag-4Zn-Cu-Sn alloys and Ag-5.89Sn alloy due to the sulfide formations such as Ag2S, Cu2S and CuS. The DE* value significantly decreases with increasing tin content. This is attributed to the matrix enrichment of tin which protects the sulfur reaction on surface. From corrosion test results, 94Ag-4Zn-Cu-Sn alloys and Ag-5.89Sn alloy provide the noble shift in the corrosion potentials and pitting potentials but the negative shift in the corrosion current density compared with Ag-5.95Cu alloy. Corrosion rate of 94Ag-4Zn-Cu-Sn alloys and Ag-5.89Sn alloy decrease markedly compared with Ag-5.95Cu alloy because it depends on the alloying elements and the microstructural changes. Due to high solubility of zinc and tin, the microstructures of tin-rich alloys consist of the higher portion of single phase and less eutectic structure than that of Ag-5.95Cu alloy.

Effects of Copper (Cu), Indium (In), Tin (Sn), Antimony (Sb) and Zinc (Zn) in Sterling Silver Alloys on Mechanical Properties Improvement

Abstract The jewelry industry has been searching for sterling silver with high hardness values as it is believed by the industry to have direct effect on scratch resistance as well as improved mechanical properties such as “spring” property. Even though the direct relationship between hardness and how springy the jewelry articles are could not be linked. However it is a conventional wisdom of goldsmith to observe the usual trend for alloys with higher hardness to be more elastic. Moreover it is a common and cheap test to apply to a jewelry article. Therefore it is common find jewelry manufacturers to discuss and compare hardness values. The current commercial alloys could reach 140–170 HV after careful heat treatments and/or mechanical treatments. In our research, we focused on a systematic alloy development study of ultra-hard silver alloy systems. These systems could reach high hardness values in as-cast state without subsequent post-processing treatments. The complex system comprises of silver content in the range of 80.0–92.5 wt.- % with supersaturated matrix. Twenty-two alloying element candidates and fifteen supersaturated ternary and quaternary compositions were investigated. Vickers hardness tests were performed on prototyped alloy ingots. X-ray diffraction analysis was used to measure the lattice distortion as well as to identify second or third phases in the alloys, if they existed. Metallography and electron microscopes were used to identify the microstructures. Seven developed alloy systems were presented in this paper as the investigation led to the discovery of a quaternary ductile alloy system with the hardness value of 142 HV as cast. The microstructure of the quaternary alloy was mostly single phase and these preliminary results suggested the alloy could be a good candidate for jewelry application.

Effects of Zn/Cu Ratio and Silicon on Microstructure, Mechanical Properties, Tarnish and Corrosion Resistance of As-Cast 940 Silver Alloys

Abstract This work studies the effects of Zn/Cu ratio and silicon on the improvement of mechanical properties, tarnish and corrosion resistance of 940 silver alloys. The silver alloys to be investigated are divided into 2 groups. The group 1 alloys are Ag-Cu-Zn alloys. The group 2 alloys are Ag-Cu-Zn-0.02Si alloys. The microstructures of all alloys to be tested look similar and consist of a Ag-rich matrix (α phase) and Cu-rich eutectic structure (α + β phase). The proportion of eutectic structure decreases by increasing Zn/Cu ratio. Increasing Zn/Cu ratio significantly decreases hardness, ultimate tensile strength, but markedly increases elongation. Addition of 0.02wt.- %Si to Ag-Cu-Zn alloys has no effect on hardness and ultimate tensile strength, but decreases elongation. The tarnish and corrosion resistance of Ag-Cu-Zn alloys in 1 % NaCl solution at 298 K significantly increases with increasing Zn/Cu ratio. Addition of silicon also improves tarnish resistance.

Multi-Level Porosity Silver Foams by Powder Processing Method

Abstract Silver foams with 70% porosity have been produced by the sintering and dissolution process (SDP) of fine silver powder. The powder was obtained from the reduction of rod-shaped silver sulfate, using glycerol mixed with sodium hydroxide as reducer and reaction accelerator, respectively. The foam shows three-level porosities, ranging from nanometer to millimeter level. The nanopores were created during the reduction process. The micropores were formed by the dissolution of remaining silver sulfate. The millipores were made from the removal of disaccharide powders. The pore architecture is unique and can be tailored by varying material and process parameters such as silver sulfate and disaccharide morphologies, reduction temperature and time as well as volume fraction of materials.

Microstructural and Mechanical Development of As-Cast and Heat-Treated 935AgCu Alloys

Microstructural and mechanical development of the various heat-treated 935 Ag-Cu alloys were explored. The heat-treatment processes were applied viz. holding before quenching for 2 and 15 min, homogenization at 750 °C for 60 min followed by water quenching, and aging at 350 °C for 15 to 60 min followed by water quench. It was found that the specimens with copper addition gave a high resiliency in all heat treatment conditions however the effect of homogenization with aging treatment fostered deep blemish or fire scale on its surface. The amplifications of resiliency of 935 heat-treated AgCuSn and AgCuBe alloy were increased but elongation values were slightly reduced. The 935 AgCuBeSn specimens produced adequate resiliency and elongation after aging at 350 °C for 15 to 60 min followed by water quenching. Partial dissolution and spheroidization of eutectic phase were occurred by homogenization treatment at 750 °C. Aging treatment promoted precipitation of 3-20 nm fcc (Cu,Sn)–rich precipitates and engendered an improvement of hardness, yield strength and the modulus of resilience or resiliency.

Experimental Verification for Solid Fraction Measurement in Semi-Solid Silver Metal Processing in Comparison with Theoretical Thermodynamics Model

The actual volume fraction of the solid in semi-solid metal processing is critical to achieving the most effective semi-solid metal processing. The semi-solid slurry with different solid fractions shows different ability to fill molds. To determine suitable processing conditions for semi-solid sterling silver (92.5 wt.%), thermal analysis (TA), experimental verification in actual investment casting machine as well as thermodynamics models were performed. Classical Gibbs free energy calculation and Scheil-Gulliver modeling were used to estimate the relationship between solid fraction and temperature. In order to obtain the relationship between temperature and solid fraction, comparisons were made amongst different techniques. The investigated 925 silver alloy was heated to 950 °C, and gas bubbling apparatus was used to introduce argon gas into the melt. Cooling curves for the metal were recorded with two thermocouples, one at the center of the melt volume and one beside the containing crucible wall. Different cooling curves and temperature profiles are presented from various tests to compare solid fractions. It was found that the latent heat of fusion strongly affected the solid fraction in addition to thermodynamic contribution. To adequately express the solid fraction relationship, the thermodynamics modeling needed modifications that took into account the cooling effects as well as temperature gradient in the crucible.

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

Microstructure and Compressive Properties of Open-Cell Silver Foams with Different Pore Architectures

Abstract Hollow silver particles with three different shapes, i. e., spherical, cylindrical, and octahedral, were used as raw material to produce open-cell silver foams. The shape difference of the particles resulted from the reduction of different shaped silver sulfates. Fabrication of silver foams was carried out using SDP process with disaccharide particles as space holder. The resultant silver foams show high porosity with distinctive pore microstructures. The examination of pore structure by SEM shows that there are three levels of porosities which are unique for each type of foam. The foams, using spherical and cylindrical silver particles, have a larger surface area and a higher density than the foam with the silver particle of octahedral shape. A control of pore architecture can be performed by tailoring material and process parameters. The difference in pore architecture resulted in a different compressive behaviour of the foams. As expected, the foam, using octahedral silver particles, has lower compressive strength than the others, mostly due to lower foam density.

Effect of Al and Ti Additions in Cast Nickel Base Alloy, Grade Hastelloy X by Arc Melting Process on Microstructures and Oxidation Behavior at 900°C and 1000°C

The nickel base alloy, grade Hastelloy X was modified by Aluminum and Titanium additions by means of vacuum arc melting process in order to improve microstructural characteristics and oxidation resistance. The arc melted Hastelloy X was added Aluminum and Titanium each for 2%, 4% and 6% by weight. Then all specimens were performed with heat treatment, which consists of solutioning treatment at 1125°C for 24 hours and precipitation aging at temperatures of 760°C, 800°C and 845°C for 24 hours. Both aluminum and titanium additions resulted in network intermetalic phase formation, namely, σ-phase, throughout the matrix. Furthermore, the addition of both elements provided the better oxidation resistance for the alloys.

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.