Study on the microstructure, properties, and thermal compression deformation behavior of Mg-Gd-Nd-Zr magnesium alloys

Abstract

The Mg-Gd-Nd-Zr system alloy exhibits a remarkably pronounced precipitation strengthening effect, with great potential for the development of high-strength, heat-resistant Mg alloys. The present investigation of this system alloy has been chiefly concentrated on the high Gd content (> 10 wt.%). Nevertheless, the high Gd content results in the high density and cost of the alloys, which is not conducive to their application. This research therefore aims at the development of a low Gd content ( 10 wt.%) Mg-Gd-Nd-Zr alloy based on the Mg-7Gd-xNd-0.5Zr (x = 0, 0.5, 2, 3.5 wt.%) alloy. The alloys were prepared by the casting method and subjected to a solid solution treatment (525 °C / 6 h) and an aging treatment (225 °C / 10 h). Subsequently, the microstructure of the alloys was observed and analyzed by optical microscopy (OM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and the mechanical properties of the alloys were tested. Furthermore, the thermal deformation behavior of the alloys, as well as the dynamic precipitation and dynamic recrystallization (DRX) behavior during thermal deformation, were investigated by OM, SEM, TEM, and electron backscatter diffraction (EBSD) based on thermal compression tests. The results demonstrate that the addition of Nd to the as-cast alloy promotes the precipitation of the Mg5Gd phase and refines the grains. The alloy's strength is enhanced by the combined effects of precipitation strengthening and grain refinement strengthening. The strength of the cast alloys after solution and aging treatment (T6) was significantly improved, with the Mg-7Gd-2Nd-0.5Zr (GN2) alloy exhibiting the best mechanical properties (UTS 20°C = 328 MPa, YS 20°C = 208 Mpa, El 20°C = 6.5%; UTS 250°C = 294 Mpa, YS 250°C = 181 Mpa, El 250°C = 8.5%). The precipitation of the β′ phase, which has good thermal stability and matches well with the α-Mg matrix, is a crucial factor in the significant increase in strength of the T6 alloys. Furthermore, the optimal thermal deformation condition for the solid solution alloy GN2 is 450 ~ 500°C and 0.003 ~ 0.05 s-1. During thermal deformation, the alloy forms textures, and an increase in the deformation temperature and a decrease in the strain rate result in a reduction in texture strength. Significant DRX and dynamic precipitation behaviors are observed during the hot deformation of the alloy. At low deformation temperatures and high strain rates, DRX is dominated by the discontinuous dynamic recrystallization (DDRX) mechanism. As the temperature increases and the strain rate decreases, the DDRX mechanism gradually changes to continuous dynamic recrystallization (CDRX). At temperatures exceeding 450°C and strain rates exceeding 1 s-1, dynamic precipitation is observed to become significantly weakened. Precipitate particles larger than 200 nm in size induce DRX nucleation, and those located at the grain boundaries of DRXed grains can pin the grain boundaries, limiting the growth of DRX grains and refining DRXed grains. Precipitates with sizes smaller than 200 nm slow down dislocation motion, thus retarding DRX. Overall, the Mg-7Gd-2Nd-0.5Zr alloy has the potential for the development of a high-strength and heat-resistant Mg-Gd-Nd-Zr system alloy with a low Gd content. Furthermore, this study provides a theoretical foundation for subsequent improvements and optimizations of the alloys and serves as a reference for other similar studies.