Strengthening mechanisms of T1 precipitates and their influence on the plasticity of an Al–Cu–Li alloy

This paper presents a systematic study of the influence of the state of precipitation on the plasticity of an Al–Cu–Li alloy. By varying the heat treatment, a variety of T1 precipitate morphologies are obtained. Atomic resolution electron microscopy observations show that T1 precipitates remain shearable even during early stages of over-ageing when their thickness is of several nanometres. They appear to be sheared only by single dislocations at a given location, which prevents catastrophic strain localisation at a microscopic scale. In later stages of over-ageing, the study of macroscopic strain hardening rate and of slip line localisation strongly suggests a transition to precipitate by-passing. The influence of the strengthening mechanism on strain reversal experiments (Bauschinger effect) is discussed.