Discontinuous yielding in an Al-Li-Zr alloy

A common problem encountered with Al-Li alloys is their comparatively low ductility and their tendency to unstable plastic behavior. In this work, the instability mechanism of an Al-Li-Zr alloy was investigated, using results of tensile tests with constant applied strain rates (ranging between 1.25 x 10 exp -5/sec and 1.25 x 10 exp -2/sec) to study the transition between the uniform and discontinuous plastic flows. It is shown that the strain rate dependence of the critical strains for the occurrence of unstable flow is consistent with the prediction of the Kubin and Estrin (1990) model of the Portevin-Le Chatelier (PLC) effect. It is concluded that it is the dynamic interaction of mobile dislocations with diffusing Li solute atoms which is responsible for the occurrence of unstable flows, while the observed loss of ductility can be ascribed to the combined effects of PLC and the shearing of coherent precipitates. 12 refs