The influence of MN additions on the deformation behaviour of an AlMgSi alloy

Abstract

The deformation behaviour at low plastic strains of three AlMgSi alloys in the fully aged condition has been studied. The MN-free alloy (BD3) plastically deforms to form narrow slip bands whose spacing decreases slightly with increasing strain. This alloy fails intergranularly at low strain because the high stress concentrations at the head of the slip bands nucleate voids at grain boundary precipitates which then propagate by coalescence within the PFZ. The two Mn-bearing alloys (BD3 and BDS) contain volume fractions of 0.004 and 0.011 respectively of 0.1 μm dia. incoherent particles of α-Al 12Mn 3Si. as well as the age-hardening phase. At small strains these alloys develop slip bands of slightly smaller spacing than those in BD3. out with increasing strain the slip band spacing decreases until at 9% strain are no longer resolvable in BD8. The Mn-phase thus causes lateral spreading of the slip bands, giving rise to significantly smaller stress concentrations at the head of the slip bands, and whose magnitude is further reduced by the smaller grain size of these alloys. A semi-quantitative assessment of these two contributions to the local grain boundary stress has been made. It is concluded that intergranular failure in BD6 and BD8 is suppressed at low strains because the associated stress concentrations are insufficient to produce decohesion at the grain boundary precipitates.