Diffraction of X-rays by crystals at elevated temperatures

Abstract

During an investigation of age-hardening of an alloy of aluminium with 4% copper, a large number of Laue photographs was taken and attention was directed to certain anomalous streaks on these photographs which altered as the process of hardening took place (Preston 1938). In addition to these streaks, which were associated with the chemical change in the crystal, there were others which did not seem to change with the hardness and which were also present on photographs of aluminium. A further investigation showed that when a single crystal of the alloy or of pure aluminium was photographed at a temperature of about 550° C, the intensity of these streaks was greatly increased. The purpose of the present paper is to give a description of this effect which has now also been observed in crystals of rocksalt, periclase, diamond and zinc. The presence of streaks running through the normal reflexions is a commonplace on Laue photographs; they were observed 25 years ago by Friedrich (1913) in photographs of KCl and NaCl. Ten years later Faxén (1923), in a theoretical investigation of the effect of temperature on the intensity of X-ray reflexion, referred to Friedrich’s observations and suggested that an attempt should be made to correlate these streaks with the temperature of the crystal. So far as I am aware this suggestion has never been carried out. The experiments described below show that Faxén was correct in his surmise that the streaks are connected with the temperature of the crystal.