Microstructures of ancient and modern cast silver–copper alloys

Journal Article · Tue Apr 01 00:00:00 EDT 2014 · Materials Characterization

DOI:https://doi.org/10.1016/J.MATCHAR.2014.01.028· OSTI ID:22340353

Northover, S.M., E-mail: [e-mail] ^[1]; Northover, J.P., E-mail: [e-mail] ^[2]

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1. Materials Engineering, The Open University, Walton Hall, Milton Keynes MK7 6AA (United Kingdom)
2. Department of Materials, University of Oxford, Parks Rd, Oxford OX1 3PH,UK (United Kingdom)

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. - Highlights: • Similar twinned structures and oxidised surfaces seen in ancient and modern cast silver • General precipitation of fine Cu-rich rods apparently formed by discontinuous precipitation is characteristic of as-cast silver. • The fine rods are cube-cube related to the matrix in contrast with the eutectic. • The silver-rich phase remains supersaturated with copper. • Possibly age-related grain boundary features seen in ancient cast silver.

• OSTI ID: 22340353
• Journal Information: Materials Characterization, Vol. 90; Other Information: Copyright (c) 2014 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA); ISSN 1044-5803
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ANNEALING
CASTINGS
COPPER
COPPER ALLOYS
DENDRITES
ELECTRON DIFFRACTION
EUTECTICS
FCC LATTICES
GRAIN BOUNDARIES
MICROANALYSIS
OPTICAL MICROSCOPY
PRECIPITATION
RESONANCE IONIZATION MASS SPECTROSCOPY
SCANNING ELECTRON MICROSCOPY
SILVER
TRANSMISSION ELECTRON MICROSCOPY
X RADIATION
X-RAY DIFFRACTION