Growth Of Vacancy Clusters Research Articles

On the production and annealing of bombardment induced surface damage on a nickel (110) surface

The production and annealing of damage on a nickel (110) surface has been studied with low energy ion scattering (LEIS) and the results are discussed and compared with previously reported LEED, LEIS and TEM results. It is concluded that the production of damage on crystal surfaces which remain crystalline under ion bombardment may be explained in terms of the nucleation and growth of vacancy clusters. It is found that the damage, as observed by ion scattering, saturates at a level which does not depend on such bombardment conditions as temperature or ion species. The experiments indicate that at saturation, the surface is in a state of dynamic equilibrium in which the rates of creation and loss of surface pits are equal. Expressions are derived to explain both the present and previously reported ion scattering results. The annealing measurements show that two different anneal processes can be distinguished.

Radiation-induced diffusion of point-defects during low-temperature electron irradiation

Abstract Electron irradiation damage in lead was studied in a high-voltage electron microscope at temperatures between 8 and 300 K. The formation and growth of interstitial clusters was observed during irradiation at temperatures between 8 and 165 K and of vacancy clusters between 50 and 165 K, i.e. at temperatures well below annealing stage III (at ∼170K in lead). The observation of vacancy cluster growth is interpreted as evidence for preferential absorption of interstitials at sample surfaces. By a combination of above-and sub-threshold irradiations and in situ annealing treatments it could be shown that the point-defect motion responsible for cluster formation below Stage III is of radiation-induced nature. The importance of radiation-induced diffusion for the low-temperature irradiation and the annealing behaviour of lead and gold is discussed. Various mechanisms which may be responsible for radiation-induced defect migration and its main characteristics are considered.

A positron annihilation study of the annealing of, and void formation in, neutron-irradiated molybdenum

Abstract Positron annihilation has been used in a study of the annealing of molybdenum irradiated at 60°C to the low dose of 1·5 × 1018 fast neutrons cm−2. On the basis of changes observed in positron lifetimes and intensities, it was possible to identify the defect mechanisms responsible for the various annealing stages. In particular it was possible to follow the growth of vacancy clusters with increasing temperature, and to establish the formation of voids above 580°C.

Transmission electron microscope studies of point defect clusters in fcc and bcc metals

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.

Vacancy cluster growth during quenching

The growth of vacancy clusters in a quenched metal crystal as a function of temperature is described using a continuum model. Existing vacancy precipitation embryos are assumed to grow by the diffusion of excess monovacancies in concentration gradients according to Fick's laws. The temperature derivatives of cluster radii have been calculated, and the radii plotted versus temperature for spherical clusters in a normal metal (e.g. Al). The suggested model predicts a rapid growth already during the quenching process, in accordance with experimental results. L'accroissement des amas de lacunes dans un cristal métallique trempé est donné comme fonction de la température par un modèle continuum. On suppose que les germes de précipitation de lacunes grandissent dans les gradients de concentration selon les lois de Fick. Les dérivées de température des rayons des amas sont calculées et les rayons sont portés vers la température pour l'accroissement sphérique dans un métal modèle (par exemple Al). Le modèle proposé prédit un accroissement rapidedé jà pendant la trempe selon l'évidence expérimentale.

Effect of vacancy clusters on yielding and strain hardening of copper

Abstract Clustering of excess vacancies in copper single crystal specimens was followed by resistivity, x-ray small angle scattering, and stress—strain curve observations. No change in yield stress or strain hardening rate was associated with excess vacancies when they existed primarily as single vacancies, divacancies or perhaps slightly larger aggregates. An increase in the critical resolved shear stress at the yield that was independent of the orientation of the tensile axis accompanied the growth of vacancy clusters in the size range 10 A to 30 A. Larger clusters also were associated with orientation dependent changes in the strain hardening characteristics.