Work softening in aluminium and some dilute aluminium alloys

Abstract

In general, the results show that work softening is a feature of aluminium and some aluminium alloys. It is thought likely that it is to be found in other face centred cubic metals and further work here would be interesting. Work softening is closely associated with the formation of a cell structure and it is apparent that the stage at which work softening occurs can be influenced by the impurity content and by the rate of deformation. Both these conditions influence the cell formation and the Stage II/Stage III transition of a single crystal. Seeger, (8) has proposed that the cell structure is a result of cross slip of screw dislocations from pile-ups behind Lomer-Cottrell barriers. Hirsch, (9) proposed that the cell structure formed as a result of screw dislocations cross slipping from their original slip planes to form relatively strain free cell walls. Both these mechanism involve easy cross slip of large numbers of screw dislocations and this is just the effect proposed by Seeger to occur at stresses at which linear hardening changes to parabolic hardening, i.e. Stage III. However, observations made during work on single crystals have shown that the cell structure is well developed at the end of linear hardening and consequently, if these mechanisms for cell formation are correct, some other stress relaxation phenomena must be associated with the Stage II/Stage III transition. In the present work, the cell structure developed in the very early stages of deformation. It can be seen from the curves of hardness against rolling reduction, Fig. 2, for the four metals that these are all of the same pattern and that the stage at which work softening starts, i.e. the region of fluctuating hardness, is immediately preceded by a high rate of work hardening. This is similar to the high rate of work hardening which immediately precedes parabolic hardening in the stress/strain curve of a single crystal. It is therefore thought that the mechanism responsible for parabolic hardening and work softening could be the same. It is thought likely that work softening will occur whatever the mode of deformation provided that a cell structure is present and is maintained. It appears that the cell structure is likely to be maintained provided that the stress system does not become complex, as for example, in the necking of a tensile specimen, where the cell structure has been shown to disintegrate.