P.J. Goodhew

Helium bubble nucleation at grain boundaries

Abstract Helium bubble nucleation has been studied at grain boundaries in a ternary austenitic steel after helium implantation in the temperature range 450–600°C. Transmission electron microscopy shows that all interfaces except the coherent twin are preferred nucleation sites for bubbles, and that the density of bubbles is greater on interfaces containing resolvable grain boundary dislocations (GBDs), A model has been developed which accounts for the bubble density as a function of GBD spacing. This model implies that there is a positive binding energy between He and GBDs until the dislocation spacing becomes very small. The diffusion coefficient of helium along GBDa is much lower than that arising from interstitial diffusion in the lattice. It appears that helium migrates in grain boundaries and along GBDs by a slow vacancy-type mechanism.

Growth of inert gas bubbles after implantation

Abstract The cavity distributions produced by the implantation of 500 keV helium into nickel and subsequent heat treatment have been studied by cross-sectional TEM. The development of bubbles in the injected gas layer on annealing has been modelled and can only be accounted for in terms of both migration and coalescence and vacancy supply from the surface. Migration is inhibited for small bubbles by the very high gas density in the cavity. The implications of these observations on the interpretation of bubble growth experiments are discussed.

Direct evidence for the Brownian motion of helium bubbles

Abstract The Brownian motion of gas bubbles in a bee metal is demonstrated for the first time. Helium bubbles in vanadium have been shown to migrate in a random walk manner at 0.55 T m by in-situ hot stage electron microscopy. The growth of bubbles by migration and coalescence has been observed and the kinetics of migration have been shown to be consistent with either a surface diffusion or an interface controlled rate limiting process.

The growth of helium bubbles in niobium and Nb-1% Zr

Abstract Helium bubbles have been precipitated in niobium and Nb-1% Zr by room-temperature irradiation with α-particles followed by isothermal annealing at 1050°C. It has been established that the bubbles grow by a migration and coalescence mechanism. After long annealing times all the bubbles are strongly faceted but many are extremely elongated. From the growth rates of the bubble populations it is possible to establish that the rate-controlling mechanism is surface diffusion for small bubbles and facet nucleation for larger bubbles. A model for the growth of elongated bubbles by facet nucleation is developed and from it the energy of an edge on a {100} facet can be estimated as 1.2 × 10 −11 J/m for niobium. In the Nb-1% Zr alloy the edge energy is at least four times greater.

LOW-ENERGY (2-5 KEV) ARGON DAMAGE IN SILICON

The damage caused by low-energy argon bombardment of silicon has been investigated by Rutherford back-scattering and transmission electron microscopy. At the doses required for sputter cleaning, gas bubbles are formed even at argon energies as low as 2 keV. The bubbles, and the argon they contain, cannot be annealed out below 1100 degrees C, although a significant amount of gas is trapped more weakly in other sites and is released at lower temperatures. The stability of the bubbles is explained in terms of the knock-in of oxygen from the native oxide during bombardment. The formation of bubbles cannot be avoided unless the angle of incidence of the argon is almost glancing.

On the core structures of misfit dislocations in grain boundaries

Abstract The core widths of grain boundary misfit dislocations in [001] twist boundaries close to coincidence site lattice misorientations have been studied by transmission electron microscopy on thin-film bicrystals of gold. Matching of experimental dark-field weak-beam images with profiles computed using the dynamical theory of electron diffraction shows that the core width for the 36·9°, σ = 5 boundary is ∼ 5 nm and may be even greater for the 22·6°, σ = 13 and 28·1°, σ = 17 boundaries. The results suggest that the coincidence site lattice plus dislocation network model of high-angle grain-boundary structures is applicable only to small angular ranges of misorientation close to angles corresponding to coincidence site lattice boundaries with relatively low energies.

A description of the migration and growth of cavities

Abstract A model which describes the migration of cavities and the growth of cavity populations is presented. Previously the mechanisms of cavity migration and the processes of cavity growth have been considered individually but little attention has been given to how they might combine. This paper shows how these mechanisms can be combined and compares the predictions of the model with experiments on cavity migration in uranium dioxide, copper and steel and cavity growth in nickel.

Helium bubble growth in 316 stainless steel

Abstract A systematic TEM investigation has been made of helium bubble growth in type 316 stainless steel. Commercial stainless steel samples have been vacuum annealed following room temperature helium implantation to a concentration of 5 × 1026 He m−3. The bubble growth kinetics have been determined by measuring the mean bubble radius at annealing times in the range 1 to 200 h for temperatures of 873, 923 and 1023 K. At the lower two temperatures the bubble growth mechanism is believed to be migration and coalescence, with the migration limited by volume diffusion of the metal atoms. Four additional anneals for 2 h at temperatures in the range 923-973 K have been made to determine the activation energy for diffusion. This is found to be 4.9 eV (assuming the migration is limited by volume diffusion). At 1023 K and for annealing times of less than 8 h, bubble growth appears to occur mainly by the acquisition of thermal vacancies from the sample surface. At longer annealing times the bubble migration become...

Dislocation networks in (111) twist boundaries

Abstract A study has been made of (111) twist boundaries in bicrystals of gold with misorientations close to zero and close to the twin misorientation (60°). Closely spaced networks of dissociated dislocations enclosing intrinsic stacking faults in the low-angle boundaries were observed. In the near-twin boundaries networks of perfect interfacial (DSC) dislocations were found but none were observed in the hexagonal arrangement as expected. These observations are interpreted both in terms of conventional dislocation dissociation reactions involving steps at the interface and in terms of the displacement space and symmetry arguments presented by Pond (1977).

Cavity growth mechanism maps

Cavity growth and shrinkage are of great technological and scientific interest in many areas of physical metallurgy. Small cavities, of diameter 10 ..mu..m or often much less, occur during creep deformation, during the later stages of sintering and as a result of radiation damage. They may arise as a side effect of ion implantation, or this technique may be used to produce them for experimental purposes. Similarly, quenching has been used to introduce and then study the behaviour of voids. The term cavity covers a whole spectrum of defects, which may or may not contain gas. There will be a gas pressure which exactly balances the surface tension forces of the gas/solid interface and a cavity containing gas at this pressure is usually said to be an equilibrium bubble. If the cavity contains more gas than this it is an overpressurised bubble and if it contains less it is strictly an underpressurised bubble. The limiting case of an empty cavity is clearly a void but in practice this term is often applied to any underpressurised bubble. Most cavities are thought to contain at least a small amount of gas, since it is generally agreed that a few gas atoms aremorexa0» necessary for the nucleation of even a void. Other sources of gas within cavities are ion implantation (for example resulting from alpha particle bombardment) and the trapping of air (or other gas) during the closure of porosity in the early stages of sintering. There may also, at elevated temperatures, be a vapour pressure component in the total gas content of the cavities.«xa0less