Influence of Spinodal Decomposition on Dislocation Mechanisms in Binary Al-Mg Alloys

Al-Mg alloys are attractive materials due to their high solid solution strengthening, good strain hardening, and high formability. In an effort to shift alloying limits to higher Mg contents, we observe that spinodal decomposition in highly concentrated Al-Mg binary alloys causes compositional and structural modulations that change plastic flow and strain hardening. The underlying deformation mechanisms are not well understood, limiting further development of these materials. Here, we investigate an Al-9.3Mg (at.%) model alloy which undergoes spinodal decomposition and compare its plastic response to that of an Al-6.6Mg (at.%) solid-solution reference alloy. Upon tensile loading, Al-6.6Mg shows extensive cross-slip via {111} planes, while dislocations in Al-9.3Mg develop wavy slip patterns. We observe a new double cross-slip mechanism, from a {111} glide plane onto a {011} cross-slip plane and back to a parallel {111} glide plane, an effect not reported before for such alloys. We explain this in terms of a spinodal structure with periodic Mg fluctuation along the <100> direction. These spinodal fluctuations create lattice distortions at the coherent compositionally diffuse interfaces between the Mg-rich (10.9 at.%) and -lean (7.3 at.%) regions due to the significant atomic size difference (21%) of Al and Mg atoms. The dislocation segments on the {011} cross-slip plane, connecting the dislocation portions on two parallel primary {111} slip planes, are jogs. These jogs create a drag force, hence reducing dislocation mobility. This effect promotes dislocation accumulation, contributing to the enhanced work hardening rate observed for Al-9.3Mg. In summary this study reports a new jog formation mechanism via {011} cross-slip planes and the corresponding strengthening effects in highly concentrated Al-Mg alloys.