Study on the Recrystallized Microstructure Evolutions of Mg-Gd-Based Alloys Using Pre-aging and Element Substitution

To improve the strength of Mg-Gd-based alloys, two important points must be considered. One is the grain refinement degree during hot processing, and the other is the thermal stability of fine grains during final annealing or cooling. In this work, six samples with three different compositions, Mg-13Gd (G13), Mg-9Gd-4Y (GW94), and Mg-10Gd-3Nd (GN103), and two different heat treatments, as-solutioned (AS) and as-aged (AA) states, were used to study the recrystallized microstructure evolutions. The results of hot compression at 753 K showed that the AA-type samples had higher recrystallization fractions compared to the AS-type samples. However, the results of annealing at 673 K showed that sharp grain growth occurred in the AA-G13 and AA-GW94 samples and that the yield strengths were much lower compared to the AS-type samples. The exciting finding was the excellent thermal stability in the AA-GN103 sample, reflected by the conspicuous co-segregation of Gd and Nd solutes at the grain boundaries. The AA-GN103 sample had no obvious grain growth even after annealing over 300 min, and it exhibited the highest yield strength. Therefore, the mixed additions of Gd and Nd are strongly recommended for developing Mg-RE alloys with high strength and thermal stability.