Table of contents

A survey is made of recent experimental work on slip in single crystals of metal. Temperature has a great influence on the plastic properties, and the behaviour of different metals with the same crystal structure depends principally on the ratio of the temperature at which the experiment is conducted to the melting temperature of the metal. The experimental results are discussed in terms of the dislocation theory of slip.

A discussion is given of the rate of flow in metal single crystals. Flow is believed to be due to the presence of dislocations; the rate of production and rate of movement of dislocations are treated.

Various workers have assumed that slip in a metal is propagated by the motion of dislocations through the lattice. The geometrical and mechanical properties of some types of dislocation have been investigated. It is shown that plane systems of simple dislocations can break up a crystal into crystallites of the kind discussed by Bragg⁽⁸⁾·

Calculations are made of the size of a dislocation and of the critical shear stress for its motion.

This paper contains a discussion of some of the unsolved theoretical problems connected with the solid state of matter. One of these is the occurrence of small crystallites even in the most perfect crystals. Reasons are given for attributing such imperfections in crystals to the conditions of growth from the melt. It is pointed out that a study of the conditions prevailing at the boundaries between crystallites might throw light on slip and on the appearance of slip bands - properties of crystals which have not yet been adequately explained. The nature of melting and the converse process of solidification require further study. Local melting may occur in crystals owing to slip and be a factor in producing strain hardening. A method is given of evaluating the relation between stress and strain outside the usual elastic limits and at temperatures approaching the melting temperature.

This paper summarizes and discusses data on diffusion constants in ionic and metallic systems in terms of the equation D = D₀e^-E/RT. Theories of diffusion are outlined, and expressions for the diffusion constant together with the relationship of the diffusion constant to other physical quantities are considered. Similarities between diffusion and chemical reaction are brought out by considering the entropy of activation and periodicity curves for D₀.

When an alloy in which precipitation is to take place is cooled, the process only begins with an observable velocity when the temperature has fallen by a definite amount below the temperature at which the single-phase alloy is in equilibrium. This fact is explained by a theory of nuclear formation. Also attempts may be made to calculate the rate at which nuclei form.

The ageing of aluminium alloys containing 4 per cent of copper is discussed. At room temperature the copper atoms segregate on the (100) planes of the crystal, but no new phase is formed. At 200° c. a pseudomorphic form of CuAl₂ is precipitated.

Intermediate metastable structures resulting from the partial decomposition of solid solutions have been found in the Cu-Ni-Al, Fe-Ni-Al and Fe-Cu-Ni systems. In each case a single-phase structure, stable at high temperatures, breaks up into two phases on cooling. The first stage in the decomposition is the formation of islands, in which atoms are segregated prior to precipitation.

1. Islands of CuNi₂Al segregate from a copper-rich solid solution.

2. Islands of FeNiAl segregate from an iron-rich solid solutions. These islands are recognized by the formation of the superlattices characteristic of the precipitating phases.

3. During the decomposition of FeCu₄Ni₃ into two phases, a laminated structure is produced, in which copper-rich laminae alternate with laminae rich in iron and nickel. Each has a tetragonal structure with the same a axes as in the former solid solution; in the copper-rich laminae c > a; in the other laminae c < a.

The process of diffusion being assumed to result simply in an interchange of atoms, an estimate is made of the internal strains produced by precipitation. According to the dislocation theory these strains are responsible for the hardness of the material, and so the degree of hardening can be predicted.

An estimate is given of the way in which the strain energy associated with a particle of precipitate depends upon the shape of the particle. It is shown that the energy can only be reduced if the precipitate forms in flat plates, and then only in the case of particles which have broken away from the lattice of the matrix. This dependence of strain energy on shape causes the precipitate to consist of plates of a shape similar to that observed in Widmanstätten structures.

It is suggested that a cold-worked metal is in dynamical rather than statical equilibrium. An analogy with the state of affairs in superlattices is made.

A survey is made of changes in crystalline structure of polycrystalline metals and large single crystals subjected to deformation by cold-work and by static and cyclic stressing, observations being made by a combination of mechanical, metallographic and x-ray tests. It is shown that the grains may be dispersed into crystallites characterized by a lower limiting size depending on the metal and that it is possible to observe internal lattice strains which it is considered are related to the process of strain hardening. It is also shown that the response of the crystalline structure depends on the manner and particularly the speed of application of stress, and that it is possible in this way to inhibit the dispersed crystallite formation and suppress the primitive yield-point of a metal, thus varying the hardness independently of the external deformation.

Measurements of the intensities of x-ray reflections from cold-worked metals indicate that in this respect the lattice distortion resembles a frozen heat motion. Experiments by Hengstenberg and Mark, Boas, Brill, Fricke and his collaborators are quoted in addition to those by the writer. It is shown that with filed powders of Cu, Ni and Rh, the intensity effect decreases in this order.

Measurements of the widths of the reflections from Cu, Ni and Rh, particularly rhodium, are discussed in relation to various theories to account for the broadening of x-ray reflections and it is shown that the most satisfactory explanation appears to be one in which it is supposed that the mean lattice parameter varies from one crystalline to another. It is also shown that the broadening increases in the order Cu, Ni, Rh.

The question why an increase in the intensity effect is accompanied by a decrease in the broadening of the reflections and the relation of these phenomena to the cohesional properties of the metals is briefly considered.

The effect of a single crystal boundary on the critical shear strength of a metal is investigated.

The internal mechanism of glacier flow has been studied by examining the crystal textures of samples of snow and ice from different parts of Alpine glaciers, and by taking strain-measurements within the glaciers themselves. The results show that under the influence of glacier flow firn, a compacted snow covering the upper regions of glaciers, is deformed through relative motion of individual ice crystals, whereas the ice which forms the bulk of the glacier yields by crystal deformation and crystal growth on the one hand, and by slip over large thrust planes on the other hand.

The magnetic properties from which a numerical value can be obtained for the internal stress σ_i are collected together, and the 180° walls are considered in detail. Their energy is determined theoretically and experimentally. They play a decisive role in the understanding of the coercive force.

The various causes of internal friction in solids are discussed. Internal friction depends in general on thermal currents which occur owing both to the inhomogeneous strains set up by the vibrations in the solid, and to the variations in the elastic constants from point to point in polycrystalline materials. The influence of ferromagnetism also is considered.

The Fourier transform of a crystal is a representation which, if fully known, would be equivalent to, and give the same information as, the usual description of a crystal. At the same time the Fourier transform is very closely related to the diffraction properties and it offers the best survey of the diffraction data obtained, and of those missing, for a discussion of crystal shape and of crystal imperfection. The desirability of an exact quantitative x-ray study of the perfection of an individual metal single crystal before and during plastic deformation is urged.