Low-Temperature Grain Boundary Diffusion Data Measured from Historical Artifacts
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Low-Temperature Grain Boundary Diffusion Data Measured from Historical Artifacts R.J. Kremer, M.A. Dayananda and A.H. King School of Materials Engineering Purdue University, West Lafayette, IN 47907-2044 ABSTRACT Diffusion processes in typical metals are slow at room temperature but there are many applications for which very long-term use is envisaged and stability needs to be assured over a timescale of 10,000 years, where even slow processes can be important. It is common to perform accelerated tests at higher temperatures and extrapolate the necessary information from the measurements so obtained. We have tested the validity of this type of extrapolation for room-temperature, grain boundary diffusion in the copper-silver system, by measuring lowtemperature diffusion profiles in antique samples of Sheffield plate.
INTRODUCTION Reliable low-temperature diffusion data for metals and alloys are generally very difficult to obtain because of the long timescales involved. Accelerated tests at high temperatures are conventionally used, and the results extrapolated to the temperatures of interest, but the extrapolations are questionable because differing mechanisms (such as grain boundary diffusion) can become important at lower temperatures. Ancient artifacts provide an opportunity to verify extrapolated data, and diffusion has been studied, for example, in Etruscan tin-bronze mirror fragments [1]. The engine of the Wright flyer airplane has also been examined for fine zones of precipitation hardening resulting from 90 years of aging [2]. In this work, we focus specifically upon grain boundary diffusion in the copper-silver system, since this is expected to be the dominant mode of atomic transport at room temperature. Sheffield plate is a specific form of silver-plated copper, which provides a suitable opportunity to test the reliability of Cu-Ag grain boundary diffusion extrapolations. The Sheffield plating process was discovered in the 1740’s, and double sided plating began in the 1760’s. Sheffield plate items remained popular until electroplating was invented in the 1840’s. Sheffield plate was manufactured by bonding polished plates of sterling silver to slabs of copper, by heating to a temperature slightly above the eutectic to allow interfacial melting and the formation of a metallurgical bond. Initial melting between the copper and silver can occur at about 780oC, and subsequent solidification results in the formation of a thin layer of eutectic between the copper and silver plating. When finished, the plate was made into decorative objects using traditional silverworking methods [3]. A Sheffield plate object has a reasonably well-defined temperature history. One can assume Harrison type A (bulk-dominated) diffusion kinetics [4] for a short time during the high temperature conditions of the initial bonding of the two metals, followed by type C (grain boundary dominated) kinetics [4] at room temperature as a household item until the present day. Therefore, the grain boundaries within the Sheffield plate ob
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