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

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Abstract
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.