Self-diffusion in Solid Metals

Abstract

THE “sagacity” with which atoms, or groups of atoms, oscillating about fixed points in the crystal lattice, refuse to exchange position with neighbouring atoms, is often regarded as one of the chief characteristics of the crystalline state. On the other hand, numerous cases are recorded in which crystalline bodies, for example, solid metals, penetrate into each other, in which, therefore, a replacement of the atoms of one metal by those of the other takes place. The classical experiments of Roberts-Austen on the diffusion of gold in lead bars are widely known. At a temperature as low as 100° he found the diffusion coefficient of gold in lead to be 2 × 10⁵ cm.² day¹, being thus only about 100,000 times smaller than that of sodium chloride in water. Several cases of interpenetration of solid metals have been recorded since, including the interesting case of the diffusion of thorium in heated tungsten wires, reported recently by Langmuir. But it must be noticed that from the rate at which one metal like gold diffuses in another like lead, no conclusion can be drawn about the velocity with which the atoms change their position either in a bar of pure lead or of pure gold; no conclusion can be drawn on the rate of self-diffusion in these elements.