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

摘要

The amorphous 18 K white gold alloy of the composition Au_(49)Ag_(5.5)Pd_(2.3)Cu_(26.9)Si_(16.3) 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℃ 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 Au_(49)Ag_(5.5)Pd_(2.3)Cu_(26.9)Si_(16.3) 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 18 K amorphous white gold alloy.