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. For materials in general, the close correlation of in-situ TEM and DSC results should find wide use in characterizing complex transformation sequences.
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)
Using in-situ heating in transmission electron microscopy, supported by differential scanning calorimetry, we have made the first comprehensive characterization of the decomposition of the Au49Ag5.5Pd2.3Cu26.9Si16.3 metallic glass developed by Schroers et al By heating over the range from room temperature (RT) to melting at a wide range of rates = 0.083–2000 K s–1, it is possible to correlate the TEM and DSC studies in detail and thereby to obtain a reliable calibration of the sample temperature on the in-situ heating stage
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. The composition Au49Ag5.5Pd2.3Cu26.9Si16.3 developed by Schroers et al has been widely studied: on heating, the alloy has a wide supercooled liquid region suitable for thermoplastic processing [2] and good formability [3].
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