**3. Results**

### *3.1. Microstructure and Composition*

SEM images of Mg-Zn-Gd alloys with different content of Gd after holding for 40 min are displayed in Figure 1. The content of Gd in the Mg-Zn-Gd alloys seen in Figure 1a–c is 3 at. %, 4 at. %, and 5 at. %, respectively. It can be clearly seen that the phase morphologies of the alloys changed significantly. When the content of Gd is 3 at. % in the Mg(70-x)Zn30-Gdx alloy (Figure 1a), three main phases are included in the alloy: a light grey pentapetaloid phase, a black punctate phase, and a dark grey matrix phase. According to the XRD patterns shown in Figure 2, these phases may be judged to be I-phase, α-Mg phase, Mg7Zn3 phase, respectively. This is consistent with the report in the literature by Gröbner et al. [17]. The TEM images of the light grey pentapetaloid phase (shown in Figure 5a,b) also appear to be a typical quasicrystal diffraction spot, which can also prove that the light grey pentapetaloid phase is an icosahedral quasicrystal (I-phase).

**Figure 1.** Mg-Zn-Gd alloys with different Gd content after holding for 40 min: (**a**) Mg67Zn30Gd3; (**b**) Mg66Zn30Gd4; (**c**) Mg65Zn30Gd5; (**d**) locally enlarged image of (b).

Figure 1b is an SEM image of the Mg66Zn30Gd4 alloy and Figure 1d is a local enlargement of Figure 1b. When the content of Gd reaches 4 at. %, the major phases except the black phase α-Mg and dark grey phase Mg7Zn3 are the light grey symmetrical rod phase and white punctate phase, which can be clearly seen from Figure 1d. The distribution of the symmetrical rod phase is disorderly and its length is 20 μm to 50 μm. The white punctate phase is dispersed in the alloy with a small volume. In the Mg65Zn30Gd5 alloy (Figure 1c), the other three phases do not change a lot but the light grey phase appears irregularly shaped, which is quite different from the symmetrical rod phase.

As shown in the XRD patterns (Figure 2), when the content of Gd is 3 at. %, the peaks of the I-phase appear in the pattern of Figure 2a. However, the I-phase peaks are not detected in patterns of alloys with 4 at. % and 5 at. % Gd. On the contrary, in Figure 2b,c, phase Gd-Zn is found and there are obvious peaks in the places where 2θ is 23, 24.2, 36, 38, and 40.5, and these peaks are not the diffraction peaks of the other three phases. Thus, it can be inferred that these peaks may be the diffraction peaks of the symmetrical rod phase. Hence, deducing from Figures 1 and 2, the symmetrical rod phase is unlikely to be the icosahedral phase of quasicrystals.

**Figure 2.** XRD patterns of di fferent Mg-Zn-Gd alloys: (**a**) Mg67Zn30Gd3; (**b**) Mg66Zn30Gd4; (**c**) Mg65Zn30Gd5.

Figure 3 is a mapping scanning analysis of the main morphologies of the Mg66Zn30Gd4 alloy. It is obvious that the white punctate phase appears black in Mg and bright in the Zn and Gd graphs. This means that the white punctate phase contains almost no Mg element but embodies Zn and Gd, which is a kind of Gd-Zn alloy. By analyzing Figures 2 and 3, it can be approximately determined that the white punctate phase is the Gd-Zn phase. The white punctate phase Gd-Zn is mostly distributed over the symmetrical rod phase. Hence, the formation of this phase maybe due to the increase of Gd content, which means that the excess Gd element precipitates during the solidification of the alloy and reacts with Zn to form the Gd-Zn phase. In addition, the light grey symmetrical rod phase has bright colors in all three scanning images, meaning that the phase can be ascertained as a ternary phase containing Mg, Zn, and Gd elements.

To further research the phase composition, EDS analysis of the pentapetaloid I-phase and symmetrical rod phase was carried out. As shown in Figure 4, points 1 and 2 are the constituents of the pentapetaloid I-phase and the symmetrical rod phase, respectively. The atomic composition of the pentapetaloid I-phase is 65.82 at. % Mg, 29.22 at. % Zn, and 4.96 at. % Gd. Thus, it can be seen that the ratio of Zn/Gd in the pentapetaloid I-phase is 5.89, which is very close to the theoretical value Mg3Zn6Gd1 [18] of the quasicrystalline phase in the Mg-Zn-Re system. The existence of the quintic rotational symmetry axis in Figure 5b and the EDS energy spectrum analysis further prove that the determination of the quintic petal phase as a quasicrystal phase is correct. The atomic composition of the symmetrical rod phase is 77.52 at. % Mg, 18.59 at. % Zn, and 3.89 at. % Gd, and the ratio of Zn/Gd is approximately 4.8. This is much less than the theoretical value of quasicrystalline.

Moreover, the TEM images of the symmetrical rod phase are shown in Figure 5c,d. It is obvious that the selected-area electron di ffraction spots are very complex and di fferent from the typical di ffraction spots pattern (Figure 5b) of quasicrystalline; it may include various phases judging by the signed rectangles of di fferent colors, but it does not include the quasicrystal phase. Jiang et al. [19] have reported that the ratio of Zn/Gd in the W-phase, which is common in Mg-Zn-Re alloys and is similar to the quasicrystalline phase, is about 1.5. As per the previous analysis, the Zn/Gd ratio of the symmetrical rod phase is 4.8, which is not only far from 1.5 ( the ratio of W-phase ), but also di fferent from 6 ( the ratio of I-phase ). Therefore, by analyzing the SEM, XRD, EDS, and TEM graphics of the symmetrical

rod phase, the composition of this phase is similar to that of quasicrystals, but its structure does not seem to have the characteristics of typical quasicrystals.

**Figure 3.** Mapping scanning analysis of the Mg66Zn30Gd4 alloy.


**Figure 4.** EDS analysis of different phases: (1) pentapetaloid phase; (2) symmetrical rod phase.

**Figure 5.** TEM images of different phases: (**<sup>a</sup>**,**b**) pentapetaloid phase; (**<sup>c</sup>**,**d**) symmetrical rod phase.

### *3.2. Morphological Evolution*

In order to investigate the formation and stability of the symmetrical rod phase, samples of the Mg66Zn30Gd4 alloy with different holding time at 720 ◦C were selected for research. As shown in Figure 6a, when the holding time is 5 min, most of the phases in the alloy are strip-like lamellar eutectic phases with a longitudinal midline running through the structure; meanwhile, there are some black α-Mg phases and white dotted Gd-Zn phases, which show that the GdZn phase is easy to form in the solidification of the alloy. After being held for 10 min (Figure 6b), the strip-like lamellar eutectic phases were less than before, and the white dotted phases did not increase significantly; however, the symmetrical rod phases had preliminarily formed. In the blue area of Figure 6c, the symmetrical rod phases had basically formed, and in the red area, it was half rod phase and half lamellar eutectic structure. Hence, it can be inferred that the symmetrical rod phase evolved gradually over time from the lamellar eutectic structure with the longitudinal midline as the demarcation line. When the holding time was 15 min, the evolution of the symmetrical rod phases had finished, and there was no lamellar eutectic structure which remained (Figure 6d). In Figure 6e, the size of the symmetrical rod phase increased to 40–70 μm but the morphologies essentially remained unchanged, which reflected the good stability of the symmetrical rod phase.

From Figure 7, it can be seen that the XRD diffraction patterns of the alloys after holding for 5, 10, and 15 min are basically the same and they all contain three identical known phases: Mg7Zn3, Gd-Zn and α-Mg. At the same time, peaks appear at positions 23, 24.2, 36, and 40.5 2θ, which is consistent with Figure 2b. Based on the analysis of Figures 6 and 7, it can be inferred that the symmetrical rod phase appeared early in the alloy, and gradually evolved from the lamellar network structure to the symmetrical rod phase structure.

**Figure 6.** SEM images of the Mg66Zn30Gd4 alloy after different holding times at 720 ◦C: (**a**) 5 min; (**b**) 10 min; (**c**) magnification of (b); (**d**) 15 min; (**e**) 80 min.

**Figure 7.** XRD patterns of the Mg66Zn30Gd4 alloy after different holding times at 720 ◦C: (**a**) 5 min; (**b**) 10 min; (**c**) 15 min.

### *3.3. Thermal Stability*

In addition, thermodynamic and microhardness tests of the petaloid quasicrystal phase (I-phase) and the symmetrical rod phase were carried out. Figure 8a shows the DSC analysis curves of the

Mg67Zn30Gd3 and Mg66Zn30Gd4 alloys. It can be seen that when the temperature reaches 345 ◦C, both lines have endothermic peaks, which is due to the melting of Mg7Zn3. Another endothermic peak of the Mg67Zn30Gd3 alloy appears at 417 ◦C, and according to the reports of Zhang et al. [16], this is the melting peak of the petal-like quasicrystal phase; meanwhile, the melting peak of the symmetrical rod phase in the Mg66Zn30Gd4 alloy appears at 453 ◦C, which means that the symmetrical rod phase may have a better thermal stability than the petal-like quasicrystalline phase. In order to verify the above hypothesis, samples of the Mg67Zn30Gd3 and Mg66Zn30Gd4 alloys were heat-treated at 430 ◦C for study. The results shown in Figure 8b,c indicate that the morphology of the quasicrystalline phase changed dramatically as a result of melting and that the petal-like morphology became a lamellar network structure; however, the morphology of the symmetrical rod phase remained stable in the main and only a few lamellar eutectic structures occurred in the interior. The microhardness of the quasicrystalline and symmetrical rod phases before and after heat treatment were studied; the results show that the microhardness of the symmetrical rod phase did not decrease obviously but that that of the quasicrystalline phase decreased a lot. All the above analyses prove that the symmetrical rod phase has a better thermal stability than the petal-like quasicrystalline phase.

**Figure 8.** (**a**) Di fferential scanning calorimetry (DSC) analysis curves of the di fferent Mg-Zn-Gd alloys: 1-Mg67Zn30Gd3, 2-Mg66Zn30Gd4; (**b**,**<sup>c</sup>**) SEM images of Mg67Zn30Gd3 and Mg66Zn30Gd4 after heat treatment at 430 ◦C, respectively; (**d**) microhardness of the quasicrystalline and symmetrical rod phases before and after heat treatment.
