Recovery mechanisms in nanostructured aluminium

Commercial purity aluminium (99.5%) has been cold rolled to a true strain of 5.5 (99.6% reduction in thickness). The material is very strong but low temperature recovery may be a limiting factor. This has been investigated by isothermal annealing treatments in the temperature range 5–100C. Hardness tests, microstructural investigations by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were carried out to identify and characterise possible recovery mechanisms.

Annihilation of zigzagged dislocations, positioned between deformationinduced lamellar boundaries of medium-to-high angles, and annihilation of dislocations in boundaries were found to be important recovery mechanisms, whereas other mechanisms, such as triple junction motion, subgrain coalescence, and boundary migration, were less important or negligible.

The recovery kinetics was analysed based on hardness data, showing that the apparent activation energy for recovery at low temperatures was 60–86 kJ mol1, consistent with thermally activated glide of jogged interior dislocations and the climb of dislocations in boundaries. These mechanisms are restricted by the presence of small intermetallic particles, which pin dislocations and boundaries and thereby raise the stability of the heavily deformed material.