CALPHAD-enabled design of metallic glasses with target properties: Gold-based glass for jewellery

Abstract

The compositions of current metallic glasses are designed to ensure glass formation, and they can be far from optimal for intended applications. The composition changes needed to improve specific properties can lead to a loss of glass-forming ability. There is a need to develop new glass-forming systems with excellent intrinsic properties, without a costly and time-intensive trial-and-error approach. A systematic composition-screening method is proposed that is fast and efficient in reducing the vast number of alloy systems to a few that merit detailed study. We first use a-priori parameters to identify potential glass-forming systems and then CALPHAD to identify potential specific glass-forming compositions within each system. The method is applied to develop a gold-based bulk metallic glass for luxury goods that meets the requirements to be classified as 18 karat (≥75 wt% Au). This novel Au-Ag-Pd-Cu-Ge-Si alloy shows good glass-forming ability and, as-targeted, its tarnish resistance is much improved compared to earlier Au-based metallic glasses. The improved tarnish-resistance is due to the addition of germanium, and the underlying mechanisms are explored.

Keywords

• A-priori screening
• Alloys by design
• Bulk metallic glasses
• CALPHAD
• Precious-metal alloys
• Tarnish resistance