Dynamic strain ageing and crack propagation in the 2091 AlLi alloy

Abstract

Dynamic strain aging (DSA) phenomena, which stem from the dynamic interaction of mobile dislocations and diffusing solute atoms, are observed in many dilute alloys at intermediate temperatures. In uniaxial deformation, DSA may lead to a negative strain rate sensitivity (SRS) of the flow stress and to the occurrence of jerky flow, alias the Portevin-Le Chaetelier (PLC) effect. As little is known to date about the influence of DSA and the PLC effect on fracture-related properties, the aim of the present study is to investigate ductile fracture in a commercial Al-Li alloy, the 2091 alloy. In the standard treatment conditions used all through this work, this alloy exhibits serrated yielding in uniaxial deformation, associated with a depletion of the SRS. The present study is, therefore, focused on properties related to one single heat treatment, for which fracture is purely intragranular. Experimental results are reported and discussed on the influence of strain rate and temperature on both uniaxial deformation and the tearing behavior of the 2091 alloy. Evidence is found for an anomalous behavior of the tearing resistance in the temperature domain where DSA occurs. Further, heterogeneous deformation is shown to take place in such conditions in the plastic zone ahead ofmore » the crack tip region.« less