Influence of the thermomechanical treatment on the microplastic behaviour of a wrought Al–Zn–Mg–Cu alloy

For plastic deformations smaller than the conventional limit of 0.2p.c. for the yield stress, the so-called microplastic behaviour of a rolled AlZnMgCu alloy is investigated experimentally. Tension and compression responses are compared along the rolling and the transverse directions. The alloy shows an asymmetric response in tension and compression (i.e., compressive stress minus tensile stress for a given absolute plastic strain is non-zero). Moreover, this asymmetry changes sign between the rolling and the transverse directions. The difference between tension to compression is observed to decrease as the conventional limit is approached. The influence of the heterogeneous microstructure of the alloy and the fabrication process on these asymmetries is discussed. Modelling of the material response based on a self-consistent scheme is used to estimate the internal stresses resulting from the thermomechanical treatment, and also to investigate the influence of the heterogeneous elastoplastic behaviour of two types of constitutive grains (recrystallised and unrecrystallised). For a plastic strain smaller than 0.02p.c., the microplastic behaviour is not well described with the adopted model since the underlying assumption of uniform stress and strain fields per phase is questionable in this initial level of plasticity. However, the model shows that the asymmetries observed at plastic strains ranging from 0.02p.c. to 0.2p.c. are consistent with the intragranular stresses developed during the stretching step.