AbstractA mechanistic model that describes the microscopic mechanisms underlying plastic instability in precipitation strengthened Al–Li based alloy systems is proposed in this work. The model is based on experimental observations from high resolution nanoindentation tests and transmission electron microscopy (TEM) based methods, including in situ TEM tensile straining. These experiments show that dynamic strain aging (DSA), which is widely accepted as the underlying mechanism for plastic instability, cannot sufficiently account for the occurrence of plastic instability in Al–Li based alloy systems. It is proposed that an altogether different mechanism controls plastic instability, namely a diffusion-controlled pseudo-locking mechanism that accompanies order hardening. This mechanism does not require the concurrent operation of DSA by Li, which may be a nonviable mechanism given the low binding energy of Li to dislocation cores, for plastic instability to occur.