Showing papers in "Advances in Physics in 1952"

The mean free path of electrons in metals

Abstract: (2001). The mean free path of electrons in metals. Advances in Physics: Vol. 50, No. 6, pp. 499-537.

Mathematical theory of stationary dislocations

Abstract: (1952). Mathematical theory of stationary dislocations. Advances in Physics: Vol. 1, No. 3, pp. 269-394.

On the generation of vacancies by moving dislocations

Abstract: New experiments of Molenaar and Aarts, Blewitt and others seem to confirm the view of the author, previously based only on the experiments of Gyulai and Hartly and Stepanow on sodium chloride, that vacant lattice sites, and possibly interstitial atoms, are generated during plastic flow in ductile crystals, particularly in metals. It is pointed out that the average temperatures near a moving dislocation are probably not sufficiently high to evaporate vacant lattice sites or interstitial atoms as a result of thermal effects alone. Instead, one apparently must conclude that the imperfections are generated either by purely geometrical means during the looping of dislocations about appropriate obstacles, as the result of dynamical instability in the motion of a dislocation, possibly near a jog, or in the very high thermal pulses or ‘spikes’ which are generated either in the zone where two dislocations of opposite sign annihilate one another or near impediments where dislocations are strongly curved. I...

Crystal growth and dislocations

Abstract: The theory of the growth of perfect crystals deriving from Gibbs and Volmer, shows that new crystal layers cannot be nucleated at an appreciable rate at low supersaturation. The essential truth of this theory is confirmed by the growth of crystals in polyhedra, which would otherwise be incomprehensible. But on this theory, by itself, the growth rate of such crystals would be virtually zero. This is not usually the case (though the necessary conditions have apparently been realized in Hawards experiments). The discrepancy is removed by considering that a real crystal contains dislocations and may therefore consist of only one layer of atoms perpetually overlapping itself. Such a crystal can grow to an unlimited size without any nucleation of new layers. The rate of growth of such a crystal can be calculated; it is proportional to the square of the supersaturation for low values and to the first power of the supersaturation for higher values in satisfactory quantitative agreement with the experimental results of Volmer and Schultze. The theory indicates that an almost flat crystal surface will show spiral growth steps, ending on dislocations. These are observed in many different cristals, confirming that this mode of growth is very common. Finally, some discussion is given to the way in which dislocations are produced in the growth process.

Surface effects in plastic deformation of metals

Abstract: Summary It is now accepted that the appearance of slip bands on the surface of a plastically deformed metal is evidence that the deformation is not homogeneous but is concentrated on relatively few atomic planes. Recent microscopical experiments have suggested that this conclusion is only valid in the later stages of deformation and that the first fractional per cent of strain is much more nearly homogeneous. Theories to account for both these stages of deformation are examined in the light of microscopical evidence. The validity of conclusions about internal processes based on experiments on the surface is discussed; it is shown that the surface finish affects not only the appearance of internal processes but also the processes themselves. In cases where the deformation is not homogeneous the balance of evidence is that it is also not continuous in time: instead, slip on an active slip plane tends to a limit which is reached either gradually or suddenly depending on the nature of the metal and the condit...

Properties of helium three at low temperatures

Abstract: (1952). Properties of helium three at low temperatures. Advances in Physics: Vol. 1, No. 2, pp. 209-268.