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Microstructures of ancient and modern cast silver–copper alloys

Northover, S. M. and Northover, J. P. (2014). Microstructures of ancient and modern cast silver–copper alloys. Materials Characterization, 90 pp. 173–184.

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The microstructures of modern cast Sterling silver and of cast silver objects about 2500 years old have been compared using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray microanalysis (EDX) and electron backscatter diffraction (EBSD). Microstructures of both ancient and modern alloys were typified by silver-rich dendrites with a few pools of eutectic and occasional cuprite particles with an oxidised rim on the outer surface. EBSD showed the dendrites to have a complex internal structure, often involving extensive twinning. There was copious intragranular precipitation within the dendrites, in the form of very fine copper-rich rods which TEM, X-ray diffraction (XRD), SEM and STEM suggest to be of a metastable face-centred-cubic (FCC) phase with a cube–cube orientation relationship to the silver-rich matrix but a higher silver content than the copper-rich β in the eutectic. Samples from ancient objects displayed a wider range of microstructures including a fine scale interpenetration of the adjoining grains not seen in the modern material. Although this study found no unambiguous evidence that this resulted from microstructural change produced over archaeological time, the copper supersaturation remaining after intragranular precipitation suggests that such changes, previously proposed for wrought and annealed material, may indeed occur in ancient silver castings.

Item Type: Journal Item
Copyright Holders: 2014 Elsevier Inc.
ISSN: 1044-5803
Keywords: sterling silver; castings; precipitation; twins; EBSD; TEM
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Engineering and Innovation
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Item ID: 39594
Depositing User: Shirley Northover
Date Deposited: 26 Feb 2014 16:17
Last Modified: 09 Dec 2018 02:41
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