Sammendrag
By first-principles calculations, a systematic study has been undertaken to investigate the combined effect of vacancies and Mg impurities on the generalized planar fault energy (GPFE) of pure Al. It was predicted that introduction of a single vacancy at the stacking fault plane can decrease the GPFE and enhance twinning propensity of Al. Furthermore, vacancies were shown to exhibit a natural/activated Suzuki segregation feature towards the intrinsic/extrinsic stacking fault of Al, respectively. Along with an increasing vacancy content localized near the deformation plane a noticeable decrease in the intrinsic stacking fault energy γISFE was obtained, which potentially may induce more split-up of dislocations into partials. However, a further increase of twinnability (based on the predicted GPFE curves) with increasing vacancy concentration was not found. More interestingly, we discovered a coupled Suzuki segregation behavior of Mg solutes and vacancies towards the intrinsic stacking fault of Al. A systematic analysis of the twinnability parameter τa on basis of the modified GPFE curves under the influence of Mg solutes and vacancies clearly demonstrates the general difficulty of deformation twinning of Al-Mg alloys. Moreover, a decreased γISFE with Mg alloying as well as with the presence of vacancies may serve, at least partially, to explain the high work-hardening rate and the formation of band structures in Al-Mg alloys processed by severe plastic deformation (SPD).