Effect of Sr on Microstructure and Strengthening Mechanism of Al-4.6Mg Alloy.

The as-cast Al-4.6Mg alloy was subjected to deformation and sensitization-desensitization heat treatment, and then the microstructure and the enhancement mechanism of Sr were investigated by optical microscopy, scanning electron microscopy-energy-dispersive spectroscopy, electron backscatter diffraction, and transmission electron microscopy. The precipitation phases of Al-4.6Mg alloy were mainly β-AlMg, AlMn, and Al(Mn Cr), and the nanoscale precipitation phases were AlMn and AlMn. The formation of β-AlMg was hindered by the addition of 0.1 wt.% Sr. In addition, the precipitate phase AlSr and the nano-sized precipitate phase τ-AlMgSr were uniformly distributed in the spherical matrix. The addition of Sr promoted the redissolution of Mg atoms in Al-4.6Mg alloy, increasing the solubility of Mg in the α-Al matrix from 4.7 wt.% to 5.1 wt.%. The microstructure analysis showed that Sr addition inhibited the recovery and recrystallization of the alloy because the Sr element elevated the recrystallization temperature. As a result, the grain deformation was intensified, the grain size was decreased from 6.96 μm to 5.39 μm, the low-angle grain boundaries were increased from 78.7 at % to 84.6 at %, and the high-angle grain boundaries were increased from 21.3 at % to 15.4 at %. Furthermore, the mechanical properties of the alloy were significantly improved, and the plasticity degraded after the addition of the Sr element. The yield strength of the alloy was enhanced mainly through fine grain strengthening, dispersion strengthening, solid solution strengthening, and working hardening. The strengthening mechanisms were analyzed in detail.