As a symbol of prestige, a beautiful piece of jewelry, or an investment against economic uncertainty, gold has been a highly sought-after valuable metal in consumer market.
Gold is resistant to corrosion and not prone to irritate skin. Most importantly, its high degree of malleability gives rise to a diverse range of designs for personal ornaments, gifts to wedding couples or new born children.
The extreme malleability, ductility, and softness of pure gold make it practically difficult for jewelry applications. Also, to maintain the durability of the surface quality of the jewels made of pure gold is always a difficulty a user needs to face. The addition of alloying elements (other metals) to gold is used to increase the toughness and hardness of the metal. While almost any metal can be alloyed (melted) with gold, only a selected group of metals will not dramatically change the color or make the metal brittle. Also, according to the standard for jewelry, the percentage of alloying elements added will affect the karat of gold and hence the corresponding value. Thus, alloying process alone as the solution of enhancing the hardness of gold seems meeting the bottleneck.
Surface mechanical attrition treatment (SMAT) is one of the promising developed processes to form a nanocrystalline surface layer and refine grains in the subsurface layers, by actuating a number of spherical projectiles to impact the sample surface. This technique has been successfully applied in achieving surface nanocrystallization in a variety of materials including pure iron, pure titanium, pure copper, pure cobalt, aluminum alloy and stainless steel, and currently pure gold. Many experimental results show that the mechanical properties and performance of the materials could be significantly enhanced by means of the SMAT-induced surface nanocrystalline. For gold with the purity of 99.99%, the surface hardness has been successfully increased up 243% (Figure 1). The SMAT is an effective approach to upgrade the global properties of engineering materials without changing of the chemical constitution. As the SMAT is simple, flexible and low-cost, this technique showed its strong potential in the jewelry industries
To further enhance the hardness of Au-based precious metals, microalloy hardening obtained by the addition of other elements such as Titanium (Ti) or Chromium (Cr) combined with SMAT is employed. The gold alloy’s surface hardness has been increased by a factor of five from 28 HV to 150 HV and the total hardening thickness can exceed several hundred micrometers. The relationship between the improved mechanical behaviors and graded nanostructures to the composition of the microalloyed gold has been established (Figure 1). The outcome of this work paves a novel, environmentally friendly and effective way to generate microalloyed precious metals such as gold with high strength and high ductility potentially applicable to jewelry industries.
Figure 1 The effect of SMAT on the surface hardness of 24K, 22K and 18K Gold.
Publications
1. Li, X, Yang, H, Meng, F, Lu, J & Lu, Y 2019, 'Grain-size insensitive work-hardening behavior of Ag microwires', Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing, vol. 759, pp. 655-660.
2. Yang, H, Cao, K, Zhao, X, Liu, W, Lu, J & Lu, Y 2019, 'Brittle-to-ductile transition of Au2Al and AuAl2 intermetallic compounds in wire bonding', Journal of Materials Science: Materials in Electronics, vol. 30, no. 1, pp. 862-866.
3. Yang, HK, Cao, K, Han, Y, Wen, M, Guo, JM, Tan, ZL, Lu, J& Lu, Y 2019, 'The combined effects of grain and sample sizes on the mechanical properties and fracture modes of gold microwires', Journal of Materials Science and Technology, vol. 35, no. 1, pp. 76-83.
4. Sun, L, He, X & Lu, J 2018, 'Nanotwinned and hierarchical nanotwinned metals: A review of experimental, computational and theoretical efforts', npj Computational Materials, vol. 4, 6.