The nanoscale mechanisms of strengthening and ductility enhancement in an Al-Mg-Si-Mn alloy on processing by high-pressure torsion

In this work we show that high-pressure torsion (HPT) processing of an Al-1.0at%Mg-0.8at%Si-0.2at%Mn in T6 state causes an 81% increase in ultimate tensile strength (UTS) to reach 559 MPa, whilst, remarkably, retaining a very good ductility of 14%. After an initial decrease in ductility for up to 1 HPT turn, further deformation up to 5 turns remarkably causes increases in ductility as well as increases in hardness and UTS. Analysis by X-ray diffraction line broadening and transmission electron microscopy show that during HPT processing a high density of dislocations are produced, and the grain size decreases from 3 & mu;m for T6 sample to around 286 nm for 5r-HPT sample. Atom probe tomography reveals solute atoms strongly segregate on ultrafine-grained boundaries for a 5r-HPT sample. A model for the strengthening mechanisms is established to evaluate contribution of dis-locations, grain boundaries, solid solution atoms, precipitates and solute (co-)clusters. We show that in the HPT processed 6xxx-series Al-Mg-Si alloy the strengthening caused by solute segregation at dislocations and on grain boundaries is the dominant strengthening mechanism. This work illustrates a rare case of a beneficial concomitant increase of strength and ductility during HPT. The result is an AA6xxx material with strongly increased strength and very good ductility as compared to the widely employed commercially available AA6xxx-T6 alloys.