Table of contents

Two approaches to the calculation of point defect properties have often been adopted, with rather different aims: (i) the pair potential approach and (ii) the electron theory, characterized by the charge displaced by the defect. (i) is only quantitatively applicable to vacancies and interstitials when a 'rigid ion' or 'pseudoatom' model is valid. Alternative approaches based on Wannier functions and, for simple metals, on pseudopotentials, are discussed. A tight binding model for formation energies in transition metals is referred to and finally a theory in which relaxation effects can be approximately incorporated in a calculation of the charge displaced by a defect is outlined.

Reviews the investigation of monovacancies and divacancies in or near thermal equilibrium at high temperatures. The use of positron annihilation for determining vacancy formation energies is covered in considerable detail and put into perspective relative to other methods (eg thermal expansion, self diffusion, nuclear magnetic resonance). The experimental techniques for studying the trapping of positrons at vacancies and the trapping model used for analysing the experimental results are reviewed. A detailed discussion in terms of ion core size valency, and vacancy relaxation is given of the trapping criteria for different metals. It is shown that the experimental results on the existence or nonexistence of positron trapping by vacancies are well accounted for by the theoretical ideas. The formation energies of monovacancies in metals obtained by the positron annihilation techniques are presented and compared with those obtained by other methods.

Interatomic potentials and calculations for point defect energies and configurations in metals are reviewed. Initially semiempirical central two body potentials (Lennard- Jones, Morse, Born-Mayer) were used, but were not found to be very satisfactory. Empirical potentials based primarily on elastic constant data have been more successful for the calculation of vacancy energies but their agreement with interstitial properties is still uncertain. The use of non central empirical potentials and pseudopotentials is still under investigation.

A review of the present state of understanding of the defect reactions occurring during the annealing of irradiated metals is given. These reactions are presently understood best for recovery stage 1. In this stage-aside from close pair recovery-freely migrating interstitials react with vacancies, other interstitials and impurity traps. Stage II annealing involves primarily the rearrangement of the interstitial clusters nucleated in stage I. Growth of larger clusters is obtained at the expense of smaller ones. In addition, in samples where extensive trapping of interstitials at impurities has occurred in stage I, detrapping of the interstitials from these impurities is observed in stage II. For a number of metals there is now clear evidence that single vacancies become mobile in stage III and annihilate at the interstitial clusters remaining from lower temperatures.

The main features of the response function technique in defect lattice dynamics are outlined. Applications to such problems as the calculations of recoilless fractions in the Mossbauer effect are illustrated by considering a simple hypothetical defect consisting of a substitutional atom which differs from a host atom in mass and in being subject to an extra potential which is a function of only one component of its displacement. It is also shown how the Vineyard theory of diffusion can be turned into a practical computational technique by response function methods and the same defect, with the extra potential chosen to give two positions of equilibrium, is used to illustrate the procedure. A method is proposed for simplifying computer simulation studies of diffusion by an application of the response function idea.

Point defects can give rise to anelastic and internal friction phenomena, if several specific conditions are fulfilled. Conversely, the observation of these phenomena gives very useful information concerning the defect concentration, mobility and symmetry that are simultaneously detected. In metallurgical research, hetero interstitial investigations, performed by means of these techniques, reveal the impurity affinities and the precipitation kinetics. In radiation damage research, internal friction gives evidence of a free migrating defect in stage I_E of bcc and hcp metals, and in stage III of nickel and copper. Close pairs of surprising stability are often mentioned.

The progress made at Cornell University over the past few years in applying the field ion microscopy (FIM) technique to the study of point defects in irradiated or quenched metals is reviewed. The techniques involve essentially the atom by atom dissection of specimens by pulse field evaporation and the corresponding recording on film of the extensive number of FIM images produced at each stage of the process. The application of these techniques to the following problems are discussed: (i) the mobility of self interstitial atoms (SIA) in substage I_E of irradiated tungsten and platinum, (ii) the geometric configuration of the SIA, (iii) the point defect structure of depleted zones in tungsten and (iv) the properties of monovacancies and divacancies in quenched platinum.

Reviews the contribution that transmission electron microscopy has made to the understanding of the mobility and clustering of point defects in fcc and bcc metals. The first part of the paper considers the nucleation and growth of vacancy clusters in quenched fcc metals. In the second part the complex subject of irradiation damage is discussed with emphasis being placed on showing the inter-relationship existing between basic damage processes governing defect production and the visible defect structure. The third section discusses the observation of cluster growth and shrinkage in terms of the information they provide about defect mobility. In the final part of the paper a summary is presented of the main conclusions that can be drawn from electron microscope studies regarding the behaviour of vacancy and interstitial defects in fcc and bcc metals.

The theory of the diffuse scattering by point defects and by defect clusters is reviewed. For a small concentration of statistically distributed point defects the symmetry of the long ranging part of the displacement field and the strength of the defects can be obtained from scattering measurements close to the reciprocal lattice points (Huang scattering). In addition to the (symmetrical) Huang scattering an asymmetry of the scattering is observed. For defects with sufficiently large displacements, the sign of this asymmetric scattering determines the sign of the displacement field around the defect and its magnitude yields information about the magnitude of the displacements close to the defect. For defect clusters the diffuse scattering is strongly modified. The Huang scattering very near to the Bragg reflection increases in direct proportion to the number of point defects in the cluster. For larger distances q from the Bragg reflection the intensity decreases as 1/q⁴.

The following topics were dealt with: defect formation energy; positron annihilation techniques; metal recovery; vibrational properties of defects; internal friction phenomena; quantitative field ion microscopy; transmission electron microscopy; diffuse X-ray scattering. Individual papers within the subject scope of this journal will be abstracted in this or a subsequent issue.