Microstructural evolution and high temperature mechanical properties of cast Al-15Mg2Si-xGd in situ composites

The effects of 0.5, 1, 2 and 4 wt% Gd additions on the microstructural evolution and high-temperature mechanical properties of a cast Al-15Mg2Si in situ composite were investigated by shear punch testing (SPT) in the temperature range of 298-563 K. The microstructural studies indicated that Gd addition changes the irregular morphology of the primary Mg2Si particles to polyhedral shape, and reduces their volume fraction and average size. It also forms an MgGd intermetallic compound, the morphology of which changes with the Gd content of the composites. SPT data showed that the ultimate shear strength (USS) of the base composite increases after Gd addition. The highest strength belongs to the composite containing 0.5 wt% Gd with the lowest size and volume fraction of the Mg2Si, and the finest lamellar morphology of the MgGd intermetallic particles. Increasing Gd content to 4 wt%, however, decreases the strength due to the coarsening of the Mg2Si particles caused by the overmodification effect of Gd, as well as changing and coarsening of the morphology of MgGd intermetallic phase.