Inhomogeneities In Crystals Research Articles

Viscosity of the Electron Gas in Metals

With a view toward explaining measurements of ultrasonic attenuation in metals, the coefficient of shear viscosity of a free electron gas interacting with thermal phonons and local crystal inhomogeneities is computed. The methods employed are essentially those developed for the problem of electrical conductivity. By means of a variational principle, a general formal solution is obtained. The effective mean free path is found to be somewhat smaller and of a somewhat more complicated (though not qualitatively different) temperature dependence than that associated with electrical conduction. The effect of a transverse magnetic field is determined for the case when a time of relaxation exists.

The formation of mechanical twins

It is suggested that the nucleation of mechanical twins is controlled by large local stresses which are set up in a crystal round inhomogeneities of various kinds. An examination is made of the stress fields round two types of inhomogeneity arising during plastic deformation, the bounded slip band, and the kink band. It is shown that in hexagonal metals, twin nuclei on planes containing the slip direction of the dislocations in the inhomogeneity are unlikely to be formed. The theory is compared with available data on these metals, and critical experiments are suggested to distinguish between this hypothesis and that which proposes a macroscopic critical resolved twinning stress.