Microstructure and mechanical properties of rheo squeeze casting alloy

Microstructures of as-cast RSC-Mg–0.43Nd–xY–0.08Zn–0.11Zr alloys: (a) x=0; (b) x=0.12.
Microstructures of as-cast Mg–0.43Nd –xY–0.08Zn–0.11Zr (at. %) alloys: (a) x=0; (b)x=0.03; (c) x=0.06; (d) x=0.12. The inset in (b), (c) and (d) are the corresponding high-magnification images by SEM.
XRD patterns of as-cast RSC-Mg–0.43Nd–xY–0.08Zn–0.11Zr alloys

Figure 1 shows the microstructure of as cast rheo squeeze casting Mg – 0.43nd – XY – 0.08zn – 0.11zr (x = 0, 0.12). Compare figure 1a with figure 2a, It can be seen from Fig. 1b and Fig. 2d that the grain size of rheo squeeze casting alloy is reduced compared with that of metal mold casting alloy: the alloy without y is reduced from 39 μ m to 36 μ m; while for the alloy with 0.12% y content, the grain size is smaller (smaller than that of metal mold casting alloy) due to the large plastic deformation between primary particles 30 μ m), which is difficult to distinguish from secondary primary particles. In addition, the needle like mg12nd phase in the grain of the 0.12% y alloy was not found in the rheo squeeze casting alloy, which proves once again that the solid solubility of Nd in α – Mg increases when it solidifies under pressure. Figure 3 shows the XRD pattern of the alloy. It can be seen from the figure that the phase composition of rheo squeeze casting alloy is the same as that of metal mold casting alloy. The alloy without y is mainly composed of α – Mg and mg12nd phases, and the alloy with 0.12% y content is mainly composed of α – Mg, mg12nd and mg24y5 phases.

Microstructures of T4-treated RSC-Mg–0.43Nd–xY–0.08Zn–0.11Zr alloys: (a) x=0; (b)x=0.12.
XRD patterns of T4-treated RSC-Mg–0.43Nd–xY–0.08Zn–0.11Zr alloys

Figure 4 shows the microstructure of T4 rheo squeeze casting Mg – 0.43nd – XY – 0.08zn – 0.11zr (x = 0, 0.12). It can be seen from the figure that the second phase in the structure is almost completely dissolved into the α – Mg matrix after solution treatment, and the same result can be obtained from XRD. Compared with the metal mold casting alloy, the grain size of the alloy without y in rheo squeeze casting decreases from 70 μ m (Fig. 5a) to 53 μ m (Fig. 4a), and the grain size of the alloy with 0.12% y content decreases from 49 μ m (Fig. 6-4d) to 40 μ m (Fig. 4b).

Microstructures of T6-treated RSC-Mg–0.43Nd–xY–0.08Zn–0.11Zr alloys: (a) x=0; (b) x=0.12.
Optical micrographs of the solution-treated Mg–0.43Nd –xY–0.08Zn–0.11Zr alloys. (a) x=0; (b) x=0.03; (c) x=0.06; (d) x=0.12.

Figure 6 shows the microstructure of T6 rheo squeeze casting Mg – 0.43nd – XY – 0.08zn – 0.11zr (x = 0, 0.12) alloy. It can be seen from the figure that after T6 treatment, some Zr containing particles in the two alloys are located in the interior of the particles and some at the interface of the particles. The black rod-shaped zn2zr3 phase also precipitates in the particles, which is the same as that in the metal mold casting alloy. The addition of Y promotes the atomic segregation in the aging process of the alloy. The amount of zn2zr3 phase in the alloy with 0.12% y content is significantly more than that in the alloy without Y Content (compared with FIG. 6b and a). In addition, compared with the metal mold casting alloy, the particle size of rheo squeeze casting alloy is significantly reduced (compared with FIG. 6a and Fig. 7A, FIG. 7b and Fig. 7d). The gold with 0.12% Y Content in T6 state was characterized by TEM (Fig. 8). According to the dark field image (Fig. 8a) and diffraction pattern (Fig. 8b), the precipitated phase of the alloy is mainly β′.

TEM dark field image and corresponding SAED pattern with B//[0111] of the T6-treated RSC- Mg–0.43Nd–0.12Y–0.08Zn–0.11Zr alloy
Optical micrograph of the T6 treated Mg–0.43Nd –xY–0.08Zn–0.11Zr alloys. (a) x=0; (b) x=0.03; (c) x=0.06; (d) x=0.12

Figure 9 shows the mechanical properties of as cast and T6 rheo squeeze cast Mg – 0.43nd – 0.08zn – 0.11zr alloy and Mg – 0.43nd – 0.12y – 0.08zn – 0.11zr alloy. It can be seen from the figure that the yield strength and tensile strength of as cast and T6 alloy with 0.12y content are better than those without y content, but the elongation difference is not significant. The yield strength, tensile strength and elongation of the alloy with 0.12% Y Content in T6 state can reach 168 MPa, 329 MPa and 8.7% respectively. According to the research results in the previous sections of this chapter, the high mechanical properties of the alloy with 0.12% y content are mainly due to the combined effects of grain refinement (compared with FIG. 1a and B, FIG. 5a and b), solid solution strengthening, zn2zr3 phase and β′ phase strengthening. In addition, according to the research results in section 5.42, the main reason why the mechanical properties of rheo squeeze casting alloy are better than those of metal mold casting alloy is the effect of grain refinement (compared with FIG. 6a and Fig. 2A, FIG. 6b and Fig. 2b).

Mechanical properties of RSC- Mg–0.43Nd–xY–0.08Zn–0.11Zr (x=0, 0.12)alloys
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