Direct observations of the annealing of prismatic dislocation loops and of climb of dislocations in quenched aluminium

Abstract

A high temperature stage fitted to a Siemens Elmiskop 1 electron microscope has been used to study directly the annealing of prismatic dislocation loops in thin foils prepared from quenched and aged bulk specimens of aluminium. In the temperature range 170-200°c the loops are observed to shrink and eventually to disappear. Observations on dislocations other than loops also present in the thin foils show that they become smoother while the loops are disappearing. The results are interpreted in terras of climb of dislocations by vacancy emission, and in the case of loops in thin foils, where vacancy supersaturation effects are thought to be negligible, the climb rate is governed by the line tension and curvature of the loops. The radius r of a loop as a function of time t is given by the theoretical expression r = r ₀[1 - (t/τ)]1/2 where τ is the lifetime of a loop of initial radius r ₀ . The temperature dependence is primarily τ = τ₀ exp (E/k T) where E is the activation energy of self diffusion. Comparison with experiment gives estimates of the activation energy in the range 1· 2–1 · 3ev. Annealing experiments with bulk material have been carried out in parallel with the above experiments. They show that the loops anneal out in the same temperature range. The details are different however owing to vacancy supersaturation effects. Larger loops appear to grow at the expense of smaller ones. The theory gives a qualitative explanation of the annealing of electrical resistivity in quenched aluminium observed by other workers, although the shape of the isothermal annealing curve is difficult to predict quantitatively, owing to its dependence on the complex annealing behaviour of loops in bulk material. The experiments also suggest that the temperature rise of thin foils examined in the electron microscope due to heating by the electron beam is very small.