Aging Precipitation Behavior and Strengthening Mechanism of Mg-9Gd-2Nd-0.5Zr Alloy

The ageing precipitation behavior and the strengthening mechanism of the Mg-9Gd-2Nd-0.5Zr alloy at different temperatures and times was investigated by optical microscopy (OM), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), hardness testing and tensile properties testing. The results show that the as-cast microstructure of the alloys consists of primary α-Mg solid solution, skeleton-like eutectic phase of Mg₅ (Gd, Nd) and GdH2 compound derived phase, which collectively contribute to the inferior mechanical properties of the as-cast alloy. The grain size of the alloy increases after solid solution treatment and the yield strength of the alloy decreases substantially. When aged at 200 ℃ to 250 ℃, the grain size of the alloy remains virtually unchanged while a significant amount of β′ phases were precipitated. At the same time, the driving force of the second phase increasing with the increase of temperature and the peak aging time was shortened. The alloy aged at 250 ℃ for hours exhibits the optimal mechanical properties, with tensile strength, yield strength, and elongation reaching 321.3 MPa, 180.1 MPa, and 4.5%, respectively. These values represent increases of 101.8%, 75.8% and 200% compared to the as-cast alloy. When the aging temperature increases to 275 ℃, the peak aging time did’t shortened, the peak hardness decreased significantly and the precipitated phase rapidly changed from the β′ to the β₁ that has a weaker strengthening effect.