Concerning the mechanics of type-A portevin-Le chatelier plastic flow initiation in Al 6061

Abstract

The mechanics involved in the development of type-A Portevin-Le Chatelier plastic flow in Al 6061 alloy tensile specimens were investigated using strain differential measurements. Specimens with tapered as well as uniform diameter gage sections were used. In the former the Lüders bands tend to move into a negative strain gradient, while in the latter they move into a positive gradient. In both cases plastic instability leads to strain rate fluctuations that grow catastropically into the Lüders phenomena. Once formed the bands tend to move under a stress increment that is nearly independent of the strain and only weakly dependent on strain rate. The near constancy of the stress required to drive the bands can account for the increase in Lüders strain, Δ∈_L, with increasing number of band passages, and the fact that Δ∈_L increases as a band moves into a positive strain gradient, while it decreases when it moves into a negative gradient.