The flow stress of aluminium and copper at high temperatures

Abstract

The ratio of the flow stress at a high temperature to that at O°C has been measured for polycrystalline Al and Cu, and for single crystals of Al. In Al the flow stress decreases ratio strongly with temperature above a certain critica1 temperature (about 250°C). In Cu a similar but more gradual decrease in the flow stress ratio is observed. The results are interpreted in terms of a model based on sessile vacancy jogs in screw dislocations, which has also been used by Hirsch (1960) and Mott (1960) to explain linear hardening. Below a certain critical temperature the screw dislocations advance with the aid of stress leaving a dipole or a row of vacancies behind; in this region the flow stress will be nearly temperature independent. Above the critical temperature the jogs advance with the aid of thermal activation, vacancies being emitted by and moving away from the jogs in the same thermally activated process, and the flow stress decreases strongly with temperature. The activation energy for this process is that of self-diffusion. An analysis of the experimental results in tern of this model shows that the mean vacancy jog height for A1 is 2–6 Burgers vectors, and for Cu only 1–2 Burgers vectors. Models involving the climb of edge dislocation are also discussed and found to be less satisfactory, although they cannot be ruled out completely.