## 3 Results and Discussion
### Microstructure  
The metallurgical structure of ZK60 and ZK60-La obtained by optical microscopy are shown in Figure 7. Both ZK60 and ZK60-La consisted of dendrite grains and intermetallics. The average grain size was 13.1 \(\upmu\)m for ZK60, 10.2 \(\upmu\)m for ZK60-0.5 La, 7.6 \(\upmu\)m for ZK60-1.0 La, 6.3 \(\upmu\)m for ZK60-1.5 La, and 5.1 \(\upmu\)m for ZK60-2.0 La, respectively. The results showed that alloying La to the ZK60 contributed to grain refinement. The related refinement mechanism was as follows: During solidification, the solute La atoms possessed strong segregation ability and formed constitutional undercooling in a diffusion layer. The constitutional undercooling could reduce the diffusion rate of solute atoms, which restricted grain growth and promoted the primary Mg matrix nucleation. Moreover, the La-containing intermetallic could form at grain boundary, which could further prevent the grain growth.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Point**} & **Mg** & **Zn** & **La** & **C** & **O** & **Ca** & **P** \\ \cline{2-7}  & **at\%** & **at\%** & **at\%** & **at\%** & **at\%** & **at\%** & **at\%** & **at\%** \\ \hline
1 & 14.72 & 4.62 & - & 17.90 & 58.37 & 2.51 & 1.88 \\
2 & 7.15 & 0.89 & 0.30 & 18.72 & 61.34 & 6.61 & 4.99 \\
3 & 8.07 & 1.12 & 0.45 & 17.32 & 62.69 & 5.79 & 4.56 \\
4 & 6.50 & 3.79 & 0.21 & 21.42 & 21.42 & 2.90 & 2.25 \\
5 & 10.05 & 6.25 & 0.84 & 21.49 & 58.20 & 1.61 & 1.56 \\ \hline \hline \end{tabular}
\end{table}
Table 2: EDS results of the degradation products indicated by the red point in Figure 6c.  
Figure 7: The metallurgical structure of **(a)** ZK60; **(b)** ZK60-0.5 La; **(c)** ZK60-1.0 La; **(d)** ZK60-1.5 La; **(e)** ZK60-2.0 La; EDS results of the secondary phase indicated by the points; **(f)** point A in Figure 7a; **(g)** point B in Figure 7c; **(h)** point C in Figure 7e; **(i)** XRD spectra of the samples.  
The distribution of the intermetallic phase was analyzed by SEM and EDS. The ZK60 constituted of the \(\alpha\)-Mg phase and intermetallic phase. The intermetallic phase in ZK60 was sparsely distributed along grain boundary with an island-like structure. Some intermetallic phase with short-bar shape appeared after alloying 0.5 wt % or 1.0 wt % La and increased with the La content increasing. The intermetallic phase with short-bar shape was widened and gradually connected to form a semi-continuous network as La content reached 1.5 wt % or 2.0 wt %. The EDS results revealed that the phase with the island-like shape (point A in Figure 7a) was constituted of Mg (89.41 at. %) and Zn (10.59 at. %) (Figure 6f), and the phase with short-bar shape (point B in Figure 7c) was constituted of Mg (91.27 at. %), Zn (7.60 at. %) and La (1.13 at. %) (Figure 7g). The phase with semi-continuous network shape (point C in Figure 7e) was constituted of Mg (88.53 at. %), Zn (9.38 at. %) and La (2.09 at. %) (Figure 7h). The XRD spectra of the samples were presented in Figure 7i. Combined with the EDS results, the phase with island-like shape was suggested to be MgZn, the phase with short-bar shape and phase with semi-continuous network shape was suggested to be the MgZnLa phase.