Abstract
The composition Au49Ag5.5Pd2.3Cu26.9Si16.3 (at.%) is of interest as the basis for the development of gold-based bulk metallic glasses for application in jewellery. In-situ heating in transmission electron microscopy (TEM) and differential scanning calorimetry (DSC, both conventional and fast) are used to obtain a comprehensive characterization of the decomposition on heating a melt-spun glass of this composition. Linking TEM with DSC over a range of heating rates 0.083‒2000 K s‒1, allows the sample temperature in the TEM heating stage to be calibrated. On heating up to melting, the glass decomposes in up to four stages: (1) complete transformation to single-phase nanocrystalline (Au,Cu)7Si; (2) grain growth of this phase; (3) precipitation of (Pd,Ag)Si, reducing the supersaturation of silicon in the (Au,Cu)7Si matrix; (4) with the precipitate phase remaining stable, decomposition of the matrix to a mixture of (Au,Ag)8Cu2, AuCu and Cu3Au phases. At all stages, grain diameters remain sub-micrometre; some of the stable nanocrystalline microstructures may themselves be of interest for applications. The characterization of the decomposition can assist in the optimization of the glass composition to improve tarnish-resistance, while retaining adequate glass-forming ability, formability in thermoplastic processing, and resistance to crystallization.
Highlights
The first reported metallic glass (MG) formed by rapid solidification had the composition Au75Si25 [1]
We identify the phases formed upon heating an Au49Ag5.5Pd2.3Cu26.9Si16.3 MG ribbon using in-situ transmission electron microscopy (TEM)
The critical was 20,000 K s–1 for the self-doped glass (SDG) and 6000 K s–1 for the chemically homogeneous glass (CHG) [23]
Summary
The first reported metallic glass (MG) formed by rapid solidification had the composition Au75Si25 (all compositions are given in nominal at.%) [1]. This alloy has low glass-forming ability (GFA) and the glass has poor thermal stability, crystallizing in 24 h at room temperature (RT) [1]. Interest in gold-based MGs has increased with the discovery of compositions showing higher GFA that permit bulk formation with minimum crosssection thickness of several mm. Its high gold content (76.3 wt.%), equivalent to more than 18 carat, permits use in jewellery [2], for which it is attractive (as expected in general for a MG) because of its high hardness (relative to crystalline gold alloys) conferring scratch resistance.
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