Phase Transformations in the Al-Mg Alloys Driven by High-Pressure Torsion

As-cast aluminum-magnesium alloys with 3, 5, and 10 wt% Mg are subjected to high-pressure torsion (HPT) at room temperature: 5 GPa, five turns with rotation speed of 1 rpm. HPT leads to the strong grain refinement for both (Al) solid solution and intermetallic beta-phase. Transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) are used to characterize the phase transitions under heating of the HPT-treated alloys. The decomposition of the solid solution in the ultrafine-grained Al-10 wt% Mg alloy obtained by HPT does not follow the equilibrium diagram. The results of DSC, XRD, and TEM show that the Mg-rich intermetallic phases appear during heating in the following sequence: GP zones -> beta -> epsilon -> beta -> gamma -> beta. The (Al)/(Al) grain boundaries (GBs) in the fine-grained Al-10 wt% Mg alloy after HPT and annealing up to 400 degrees C (i.e., in the solid solution area, far from solvus and solidus lines) contain the Mg-rich areas. This can be the result of possible GB phase transitions with the formation of Mg-rich GB phases. The GBs in the binary Al-Mg can contain the thin layers of a GB phase far away from the solvus line.