Simultaneous increase of tensile strength and ductility of Al-Si solid solution alloys: The effect of solute Si on work hardening and dislocation behaviors

Si is one of the most common alloying elements in Al alloys. Usually, it is added to Al alloys to produce precipitates (e.g., Mg2Si) and/or improve the castability of casting Al alloys. In this work, we report a significant effect of Si as a solute element on work hardening and dislocation behaviors of Al-(0.5-1.5 wt%)Si alloys, which has not been well documented yet. It is shown that compared to the pure Al, the Al-Si solid solution alloys exhibit a simultaneous increase of tensile strength and ductility upon uniaxial tensile tests. This trend is enhanced with increasing Si concentrations. The ultimate tensile strength (UTS) and uniform elongation (UE) of the Al-1.5 wt% Si alloy are 121 MPa, and 27% which are 33% and 12% higher than those of the pure Al, respectively. The simultaneously increased UTS and UE are ascribed to the enhanced work hardening by the solute Si. It was found that the solute Si significantly changes the dislocation configuration from dislocation cells in the pure Al into a more uniform distribution of loose dislocation tangles in the Al-Si alloys upon plastic deformation. Such a change increases the dislocation density and thus enhances the work hardening of these alloys. The first-principles calculations reveal that the solute Si weakens the redistribution of electrons on the stacking-fault plane and remarkably reduces the stacking-fault energy (SFE) of the Al-Si alloys. The decreased SFE suppresses the dislocation cross-slip. As a result, the dislocations tangle in the grain interiors more uniformly, which impedes the dislocation motion and enhances the work hardening of the Al-Si alloys.