X-Ray Diffraction Measurements of Silver during Slow Heating to the Melting Point

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Measurements of the x-ray lattice parameter, the Bragg diffraction peak intensity, and the diffraction peak width were made during the very slow quasiequilibrium heating of high purity silver (99.999+% pure) to the point where melting was observed. Back-reflection measurements were made using a 〈;422〉 reflection from a small homogeneously heated single crystal region under conditions allowing continuous visual observation of the x-rayed region. A specially developed technique allowed several thousand meassurements to be made in a temperature interval starting about 4°C below the onset of melting. Normal lattice expansion (to within ±Δa/a=3.8×10—6) was observed to within at least a few thousandths of a degree (°C) of the temperature at which melting was first clearly detected visually. The half-width of the Bragg peak remained constant over this range. A 50% decrease in intensity, however, was observed starting about 0.08°C below the point at which melting was first observed. This result was attributed to the gradual onset of two-phase alloy melting which was not immediately detected visually. This was consistent with chemical analysis which revealed the presence of sufficient impurities (mainly ∼1 ppm oxygen) to have produced a two-phase melting range between solidus and liquidus of the order of a tenth of a degree (°C). It was generally concluded that there was no evidence from the x-ray measurements of any anomalous lattice behavior (premelting phenomena) in silver at temperatures as close to the solidus absolute temperature as a few parts per million (or possibly less). This was consistent with the expected first-order nature of the transition.