Precipitation phenomena in ultrafine grained Al–Mg–Si alloy with heterogeneous microstructure

Abstract In this study we discuss the effect of heterogeneous microstructure obtained by hydrostatic extrusion on precipitation phenomena in commercially available Al–Mg–Si alloy. After deformation double fibrous texture is present with fiber axes parallel to extrusion direction. Grains with dominance of low angle grain boundaries with orientation and high angle grain boundaries with orientation were found. HRTEM investigation revealed that the precipitation sequence is different than in conventional coarse grained materials and depends on local character of grain boundaries. The reason for this occurrence is the series of thermal shocks during deformation, which allows firstly Mg–Si co-clusters to be formed and secondly incorporates Cu atoms into them hindering the growth of copper free β″. Microhardness measurements for different aging time show that up to 4 h the precipitation hardening effect is stronger than weakening by dislocation annihilation or grain growth. After 14 h, precipitation condition in UFG samples remain almost unchanged but mechanical properties decrease.