Abstract
The rate sensitivity of flow stress was investigated as a function of pre-strain for solution-treated and furnace-cooled aluminium alloy (2017), over a temperature range encompassing the regime of jerky flow (Portevin-Le Chatelier effect). In strain rate change (increase) tests, the stress increased at first and then decreased gradually to the stress level determined by the strain and the strain rate. This fact suggests that the stress relies on the strain rate in two manners; One is the “frictional stress”, which is assumed to be uniquely determined by the strain rate, and the other is the “interaction stress” between dislocations and solute atoms or precipitates, which is assumed to approach with time the limiting value determined by the strain rate when this is changed. The relation between stress and log (strain rate) was linear in each case but its slope was positive for the former and negative for the latter. For solution treated specimens the slope of frictional and total stress was about 3 MPa and −2 MPa, respectively, at room temperature. The absolute values of these were unaffected by the strain rate up to ∼10−2 s−1, but increased slightly with strain.
A temperature range in which the total rate sensitivity was negative was about from 200 K to 400 K for solution treated specimens and from 230 K to 500 K for furnace-cooled specmiens. These ranges were wider than that in which P-L effect was observed; only at room temperature for the former, and between 228 and 398 K for the latter at the strain rate equal to 10−4 s−1. Therefore, it is impossible to explain the P-L effect only by the condition that the rate sensitivity is negative.
