Effects of Gd, Y, and La Rare-Earth Elements on the Microstructural Stability and Elevated-Temperature Mechanical Properties of AZ81 Magnesium Alloy

Abstract

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.