The shearable–non-shearable transition in Al–Mg–Si–Cu precipitation hardening alloys: implications on the distribution of slip, work hardening and fracture

A systematic study has been conducted to evaluate the nature of the dislocation–precipitate interaction and its relationship to the mechanical properties for a commercial Al–Mg–Si–Cu alloy. A variety of experimental techniques employed including transmission electron microscopy, slip line observations and macroscopic work hardening behaviour. The results from this work indicate that a clear transition in macroscopic behaviour of the alloy can be observed when the precipitates become large enough so that they are not sheared by dislocations. Direct observations using a transmission electron microscope (TEM) indicate that the precursor to the Q phase becomes impenetrable to dislocations when its equivalent diameter is above 2.5–3.0 nm. The transition from shearable to non-shearable precipitates manifests itself in a number of ways including: (i) a change in the local distribution of slip from a banded to a more homogeneous structure and (ii) a characteristic change in macroscopic work hardening behaviour. In addition, observations on intergranular fracture suggest that the distribution of slip and the intrinsic fracture properties of the grain boundary are critical in controlling this process. Finally, an integrated view of the relationship between the basic dislocation–precipitate interaction and the global response of the alloy is rationalized.