Effects of Gd, Y, and La Rare-Earth Elements on the Microstructural Stability and Elevated-Temperature Mechanical Properties of AZ81 Magnesium Alloy
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The effects of separate additions of 1 wt pct Gd, Y, and La rare-earth (RE) elements on the microstructural stability and strength of a cast AZ81 alloy were investigated in the as-cast and annealed conditions. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and optical microscopy (OM) analyses along with shear punch testing (SPT) were employed to characterize the microstructural stability and mechanical properties of the studied alloys. The main microstructural constituent in the AZ81 base alloy, Mg17Al12 intermetallic phase, showed a relatively low thermal stability, resulting in a significant strength decline at high temperatures. The incorporation of the RE elements into the base alloy not only refined the microstructure but also improved the thermal stability and high-temperature mechanical properties of the RE-containing alloys. The observed enhancement in both stability and shear strength was attributed to the reduction in the volume fraction of β-Mg17Al12 and formation of the thermally stable Al2Gd, Al2Y, and Al11La3 intermetallic particles, which suppress the grain growth throughout the annealing process. Among the employed RE elements, La was found to be the most effective one in the retention of the initial fine microstructure as well as ultimate shear strength after long-term annealing at 400 °C. This is believed to be caused by the higher number density and more uniform dispersion of the Al11La3 particles in the Mg matrix in comparison with the Al2Gd and Al2Y particles, which showed a more sparse distribution of some agglomerated particles.
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