Order in the AlloyCu3Au

Electron diffraction by the transmission method has been used to investigate superstructure as produced in the alloy ${\mathrm{Cu}}_3$Au by various heat treatments. Thin films of this composition, which have never been heated and are not thoroughly mixed, become homogenized and develop unmistakable order when heated at 169°C for 16 hours, but films heated only at lower temperatures for 16 hours do not yield certain evidence of order. Sharper superstructure rings are produced by films which have been annealed at higher temperatures, and from the evidence of the diffraction patterns we estimate that perfect order is developed by heating at 270°C for 16 hours. Diffraction patterns from films which have been quenched from temperatures above 400°C exhibit, in addition to rings of the disordered face-centered cubic structure, a very diffuse band inside the (111) ring. A similar diffuse diffraction feature is obtained from films while at various elevated temperatures up to 560°C. When an initially ordered specimen is heated to progressively higher temperatures, the originally sharp (100) and (110) superstructure rings becomes gradually broader and less well-defined, merging finally into a single diffuse band. Increase in breadth of the rings becomes noticeable first, for a film in equilibrium, at a temperature a little above 325°C. At about 370°C these rings have become very poorly resolved; but the resolution continues to become poorer, although at a much less rapid rate, up to 560°C which is the limit of our observations. There seems to be no sudden change in the diffraction pattern corresponding to the alleged Curie point of order.