3.1. As-Cast and as-Homogenized Microstructures
Figure 1 shows the optical micrographs of the as-cast ZM31-xY alloys. It is found that the as-cast microstructures are mainly made up of α-Mg matrix and a mass of intermetallic compounds, which are distributed along the dendritic boundaries. Compared with Y-free ZM31 alloy, the addition of Y element results in significant changes in the microstructure. When the Y content is 0.3%, the eutectic compounds increase and the dendrites are refined. When the Y content reaches 0.7%, the fish bone network compounds begin to appear. When the Y content increases to 1.5%, all the eutectic compounds appear as the fish bone networks. When the Y content is 3%, new lamellar structure compounds appear in the matrix. When the Y content is 5% and 10%, all the fish bone eutectic compounds are replaced by the lamellar shape and dendrites are further refined. Generally speaking, adding Y element is useful to decrease the dendrite size, change microstructure and influence volume fraction of the second phases.
To further study the phase transformation of the test alloys, the phases of the as-cast ZM31-xY alloys are analyzed by XRD (
Figure 2) and corresponding phase analysis results are summarized in
Table 3. The phase composition of ZM31-xY alloys mainly includes α-Mg, Mn, Mg
7Zn
3, I-phase, W-phase and LPSO phase. As indicated in
Figure 2, it is easy to see that second phases change with the Zn/Y mole ratio in the as-cast test alloys. According to the analysis results of the optical microstructure, it can be preliminary inferred that the eutectic compound in the ZM31 alloy is the Mg
7Zn
3 phase, the second phases in ZM31-0.3Y alloy are the mixture of Mg
7Zn
3 and I-phase, the fish bone network phase is a W-phase and the phase with lamellar structure is the LPSO phase.
Figure 3 reveals the typical SEM images of the as-cast ZM31-xY alloys. It is further confirmed that with the increase of the Y content, the volume fraction of the second phases increase and the dendritic structure is refined. From ZM31 alloy to ZM31-10Y alloy, the second phase structures changes from a gray white coarse structure (marked as B), fine strip-like structure (marked as C and D), and network structure (marked as E) to a gray block structure (marked as F). Combining XRD (
Figure 2) and EDS analysis results (
Table 4), it can be concluded that A is α-Mg matrix, B with a gray white coarse structure is identified as Mg
7Zn
3, C and D are I-phase, E shown in network structure is W-phase and F is LPSO phase. In addition, there are some bright white granular particles for the ZM31-10Y alloy in
Figure 3g, which have been confirmed as RE-rich particles in many other studies [
21,
22].
Figure 4 reveals the DSC curves of the as-cast ZM31-xY alloys. The important temperature parameters extracted from the figure are summarized in
Table 5. Three endothermic peaks are observed on the curve, and their temperatures are approximately 525 °C (peak1), 540 °C (peak 2) and 620 °C (peak3), respectively. According to the Mg-Zn-Y ternary phase diagram and the analysis results of previous studies [
23,
24], we can infer that the temperature of peak 1 is related to the dissolution of the W-phase, the temperature of peak 2 is related to the dissolution of the LPSO phase and the temperature of peak 3 is related to the melting of the test alloys. By comparing peak 2 and peak 3, it can be seen that the LPSO phase has a higher dissolution temperature, which indicates that it may have better thermal stability than the W-phase. With the increasing Y content, peak 3 is shown to decrease evidently, which indicates that the formation of RE phase is conducive to improve the castability of the alloys. In addition, no obvious peak for I-phase transformation is found in
Figure 4, which may be due to too little I-phase being detected.
It is worth noting that W-phase observed in the as-cast investigated alloys is a network structure. From previous research, Xu et al. has found that the formation of network W-phase will result in the deterioration of mechanical properties of the Mg-Zn-Y alloy [
25]. Therefore, for the purpose of reducing the adverse effect of the network-like W-phase, the W-phase is dissolved as much as possible during the homogenization treatment. According to the DSC results, the melting temperatures of the W-phase and LPSO phase are both higher than 500 °C, which indicates that for the ZM31- (0.7–10) Y alloys with W-phase and LPSO phase, the homogenization treatment temperature is predicted to be higher than that of the ZM31- (0–0.3) Y alloys with only the Mg
7Zn
3 phase and I-phase. Therefore, ZM31-1.5Y and ZM31-3Y alloys are used as research objects, and two homogenization treatment temperatures of 420 °C/12 h and 500 °C/16 h are performed.
Figure 5 shows the SEM images of as-homogenized ZM31-1.5Y and ZM31-3Y alloys under different treatment temperatures. For the ZM31-1.5Y alloy, the second phase is mainly composed of the W-phase. Comparing
Figure 5a,b, it is easy to find that the W-phase still maintains a network structure after heat treatment at 420 °C, while the W-phase is significantly dissolved and diffused after homogenization treatment at 500 °C. For the ZM31-3Y alloy, the second phase mainly consists of the W-phase and LPSO phase. Comparing
Figure 5c,d, the W-phase has the same experimental results, but the structure of the LPSO phase has not changed even at 500 °C homogenization treatment. This result provides a good reference for the design of the homogenization process for the test alloys, which means that a higher homogenization temperature should be used for the alloys containing W-phase. In this way, it is possible to better transform the network-like W-phase to the particle phase during the extrusion process, and to achieve a dispersion strengthening effect. Therefore, different homogenization processes are performed for the test alloys containing different Y contents, that is, ZM31 and ZM31-0.3Y alloys: 420 °C/12 h, other alloys: 500 °C/16 h.
Figure 6 exhibits the optical images of the as-homogenized ZM31-xY alloys. Compared with
Figure 1, the volume fraction of the residual phases after the homogenization treatment is significantly reduced, and the distribution of the second phases becomes thinner and discontinuous.
Figure 7 presents the SEM images of the as-homogenized ZM31-xY alloys, and
Table 6 lists the corresponding EDS results for second phases. According to EDS results, A, B, C, D, E, F and G are identified as α-Mg matrix, I-phase, W-phase, W-phase, LPSO phase, Y-rich phase and LPSO phase, respectively. Compared with the SEM results of the as-cast alloys (
Figure 3), it can be found that the volume fraction of the Mg
7Zn
3 phase decreases dramatically, the I-phase becomes discontinuous and the W-phase also transforms from the network structure to a discontinuous honeycomb-like network. However, the LPSO phase still remains the block structure, and the result is the same as that shown in
Figure 5.
In addition to the morphology of the second phases, there is an interesting phenomenon that as the Y content increases, many cuboid-shape Y-rich partials still remain in alloys and precipitate further. The possible reason is that Y content is already much larger than the required value. From the previously analyzed SEM results (
Figure 3e–g) and the XRD results of as-homogenized ZM31-xY alloys containing the LPSO phase (
Figure 8), it can be seen that as the Y content increases, the volume fraction of LPSO phase in the experimental alloys increases. When the Y content is equal to 5%, the Zn/Y ratio is lower than the transition threshold of the RE phase, which hinders the further growth of the LPSO phase. As can be clearly seen in
Figure 7g, compared to the as-extruded ZM31-5Y alloy, the number of bright white square particles in the as-extruded ZM31-10Y alloy increases.
3.2. As-Extruded Microstructure
Figure 9 reveals the SEM images of the as-extruded ZM31-xY alloys, the observation direction is parallel to the extrusion direction. After the hot extrusion, the second phases are broken and distributed at the grain boundary. In
Figure 9a, the grain size of the as-extruded ZM31 alloy has changed greatly after the extrusion due to the occurrence of the dynamic recrystallization (DRX) during the hot extrusion. When the content of Y is equal to 0.3%, 0.7% and 1.5%, the average grain size of the alloys decreases to about 4.5 μm, 2.5 μm and 1.5 μm, respectively. Moreover, it is also clear that there are many small bright white particles parallel to the extrusion direction, which are the second phases that we have been observed before (I-phase and eutectic W-phase). During the homogenization treatment, the I-phase and W-phase are effectively decomposed and dissolved, so for the alloys with a Y content not higher than 1.5%, they can be successfully extruded at low temperatures (350 °C) However, with the increase of the Y content, the LPSO phase is formed, and it is difficult for the alloys to be successfully extruded at low temperatures, and higher temperatures (480 °C) are required for extrusion processing. From
Figure 9e–f, it can be seen that the grain size of the alloys with the high Y content (≥3%) becomes larger than that of the low Y content (≤1.5%), and the average grain size is increased to 5 μm, 8.3 μm and 10.5 μm, respectively. There are mainly two reasons, one is that the appearance of the LPSO phase accelerates the DRX, and the other is that the alloys containing the LPSO phase require a longer extrusion time during the process, providing the grain more time to develop. In the previous studies, it has been reported that the existence of the LPSO phase strongly accelerates the refinement of recrystallized grains of the alloy during the extrusion at higher temperature, and the LPSO phase itself also acts directly as a strong reinforcement in the alloy resulting in higher extrusion temperature [
26,
27].
Figure 9e–f shows the microstructure of the alloys extruded at a higher temperature of 480 °C, the microstructure of the LPSO phase does not change much after the extrusion process, and still distributes at the grain boundary with the grey block, which is consistence with the results of
Figure 5.
To further study the second phase in the test alloys, TEM microstructure observation is carried out on the extruded ZM31-3Y alloy.
Figure 10a shows a bright-field (BF) TEM micrograph of the second phase, which is identified as the W-phase with an FCC structure by the SAED pattern. In
Figure 9b, it can be seen that a lot of gray second phases have a different direction. These phases are LPSO phases with a crystal distance of 1.8 nm, which are consistent with the results of the LPSO phase in 14H reported in the literature. Many studies have reported that 18H is unstable and will change to 14H at 480 °C [
25,
28,
29]. Hence, it can be inferred that the LPSO phase is 14H after high-temperature homogenization and extrusion. In addition to the second phase, many black particle phases are dispersed in the alloy matrix, as shown in
Figure 10d. According to the results of EDX analysis (
Figure 10e–f), the black particle phase are the pure Mn phases. It is well known that Mn addition can significantly refine the grain size of the Mg alloy, and it is dispersed in the matrix as a dispersed particle phase, which helps to improve the mechanical properties of the alloy [
19,
20]. In this work, the Mn phase has been further evidenced in this form.