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_(57.5)Cu_(14.7)Ni_(5.3)P_(22.5) and about 1200 MPa for the Au_(49)Ag_(5.5)Pd_(2.3)Cu_(26.9)Si_(16.3). Pt_(57.5)Cu_(14.7)Ni_(5.3)P_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.