Figure 1 shows the microstructure of as cast Mg – 0.43nd – XY – 0.08zn – 0.11zr alloy. It can be seen from the figure that the as cast microstructure of the alloys with different compositions is composed of α – Mg matrix and eutectic phase at the grain boundary. In addition, black Zr like nucleoplasms can be seen inside the α – Mg grains. With the increase of Y content from 0 to 0.12 at.%, the grain size of the alloy decreases from 39 μ m (Fig. 1a) to 30 μ m (Fig. 1D).
In the alloy containing y, some subgrains can be observed in the α – Mg grains, and the grain boundaries are composed of some gray acicular phases (Fig. 1b-d), and the number increases with the increase of Y content. According to the XRD results, the gray needle like phase is mg12nd (Fig. 2a), which is tetragonal (a = 1.031 nm, C = 0.593 nm, space group I4 / MMM). In addition, a new white phase was found at the grain boundaries of Mg – 0.43nd – 0.06y – 0.08zn – 0.11zr and Mg – 0.43nd – 0.12y – 0.08zn – 0.11zr alloys (scanning image in the upper right corner of Fig. 1c and D).
According to EDs results and XRD analysis results (Fig. 2), these phases are mg24y5. With the increase of Y content from 0.06% to 0.12%, the phase transformation becomes more and more fine and disperses at the grain boundary. Therefore, the following conclusions can be drawn: with the increase of Y content, mg24y5 phase is formed in the alloy structure, the grain is refined, and the number of acicular mg12nd phase in α – Mg grain increases.
Figure 3 shows the mechanical properties of as cast alloy. It can be seen from the figure that the yield strength of the alloy increases from 98.9 MPa to 109.7 MPA with the increase of Y content. The increase of yield strength is mainly due to the refinement and strengthening of α – Mg grains and the strengthening of acicular mg12nd phase. However, the brittle mg24y5 phase also increases with the increase of Y content, and the crack is easy to propagate along these eutectic phases during tension. Therefore, although the grain size of the alloy is refined, the tensile strength of the alloy is almost kept at 198 MPa, while the elongation decreases from 16.5% to 10.1%.