Helical dislocations in quenched aluminium-4% copper alloys

Abstract

Experiments by transmission electron microscopy on thin foils of polycrystalline A1-4 % Cu alloys, quenched from various temperatures have revealed helical dislocation lines which are thought to be produced by condensation of vacancies on dislocations of predominantly screw character. The axes of the helices are parallel to the Burgers vector, i.e. parallel to [110] directions. In material quenched from 500°c and 540°c the helical dislocations vary in radius and pitch from about 250 Å to more than 1500 Å. Materiel quenched from 440°c contains irregular helices with pitch several hundred angstroms and much smeller radius. Preliminary estimates suggest that the vacancy concentration required to produce the observed climb is ∼3 × 10⁻⁵(500° and 540°c quenches) and ∼ 2 × 10⁻⁶(440°c quench). The helices appear to interact with each other and with other dislocations to produce intertwined helices and rows of loops. Observations of connected helices of opposite hand and of the lengths of helical dislocations suggest that loops from sources of slip active during quenching may be very elongated with long screw components. Dislocation loops are also observed. Some appear to be formed by interaction of helices, others from collapsed vacancy discs. The latter occur in some areas in bands parallel to the traces of {111} planes and are thought to be associated with slip bands. Elsewhere very few collapsed vacancy discs are observed. The observations suggest that nucleation of loops in the quenched alloy may be a difficult process, and that some vacancies may remain quenched in. The observations are also discussed in relation to precipitation effects.