# Microstructural, Electrochemical and In Vitro Analysis of Mg-0.5 Ca-xGd Biodegradable Alloys
## 3 Results and Discussions  
### Microstructural Analysis  
The morphological and microstructural analysis of Mg-0.5 Ca-xGd alloys by light microscopy is presented in Figure 1. The microstructure typically exhibits aspects of as-melted metallic alloys with Mg\({}_{2}\)Ca tabular intermetallic structures and particles of Mg\({}_{5}\)Gd which are proportionally equally distributed. The enhancement with Gd shows the growth of round particles with a separation trend of somewhat uniformly distributed black dots [48]. The structural appearance of the SEM images of the said specimens is shown in Figure 2 and the existence of an intermetallic stage is noticeable. Gd-based compounds are usually acicular and round in shape and are observed to be whiter than the Mg2 Ca tabular intermetallic compounds. Gd has a slightly low influence on the size and shape of Mg grains, confirmed by literature studies [49; 50] on Mg-based alloys with a Gd content between 1 wt.% and 4 wt.%.  
Figure 1: Optical images of samples: (**a**) Mg–0.5 Ca–0.5 Gd; (**b**) Mg–0.5 Ca–1 Gd; (**c**) Mg–0.5 Ca–1.5 Gd; (**d**) Mg–0.5 Ca–2 Gd; (**e**) Mg–0.5 Ca–3 Gd.  
Figure 2: SEM images of the samples: (**a**) Mg–0.5 Ca–0.5 Gd; (**b**) Mg–0.5 Ca–1 Gd; (**c**) Mg–0.5 Ca–1.5 Gd; (**d**) Mg–0.5 Ca–2 Gd; (**e**) Mg–0.5 Ca–3 Gd.  
The chemical composition of the Mg-0.5 Ca-xGd alloys was investigated by energy dispersive spectroscopy and is shown in Figure 3. Cross-sectional areas of the specimens were analyzed in 10 separate zones and the mean values are given.  
The structural morphology investigation emphasizes homogeneous elements and the growth of peculiar phases, rated in a \(\alpha\)-Mg-based solid solution (\(\alpha\)-Mg), Mg\({}_{2}\)Ca and Mg\({}_{5}\)Gd. The Mg\({}_{2}\)Ca complex element is situated at the limits of the \(\alpha\)-Mg grains, realizing a film-coated eutectic with \(\alpha\)-Mg. X-ray diffraction peaks of the experimentally Mg-0.5 Ca-xGd alloys are given in Figure 4. From the XRD analysis shown in Figure 4, the Mg, Mg2 Ca and Mg5 Gd phases were detected in the X-ray diffraction peaks of the experimentally Mg-0.5 Ca-xGd alloys. \(\alpha\)-Mg (ICDD-PDF-International Center for Diffraction Data-PDF: 01-071-9399) has been identified at different 20 values (30.24\({}^{\circ}\), 35.77\({}^{\circ}\), 46.82\({}^{\circ}\), 56.34\({}^{\circ}\) and 77.62\({}^{\circ}\)) as the main structure with a hexagonal crystalline phase. Moreover, the existence of Mg\({}_{2}\)Ca (ICDD-PDF: 01-073-5122) was shown at 20 = 34.86\({}^{\circ}\), 67.42\({}^{\circ}\) and 96.88\({}^{\circ}\), and Mg\({}_{5}\)Gd (ICDD-PDF: 01-071-9618) in a cubic system at 20 = 29.46\({}^{\circ}\), 61.15\({}^{\circ}\) and 87.64\({}^{\circ}\). As the Gd percentage gradually increased, the diffraction peak of Mg5 Gd intensified. The lattice parameters of the Mg-0.5 Ca-xGd alloy phases are presented in Table 4.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Compound** & **Space** & **Crystal** & **a** & **b** & **c** & **\(\alpha\)** & \(\beta\)** & \(\gamma\)** & **Cell Volume** & **RIR** \\  & **Group** & **System** & **(Å)** & **(Å)** & **(Å)** & **(Å)** & **(°)** & **(°)** & **(°)** & **(°)** & **(10\({}^{6}\)\(\mu\)m\({}^{2}\))** & **RIR** \\ \hline Mg & P63/mmc & Hexagonal & 3.2089 & 3.2089 & 5.2101 & 90 & 90 & 120 & 46.46 & 4.01 \\ \hline Mg\({}_{2}\)Ca & P63/mmc & Hexagonal & 6.2250 & 6.2250 & 10.1800 & 90 & 90 & 120 & 341.63 & 2.30 \\ \hline Mg\({}_{5}\)Gd & F43m & Cubic & 3.9500 & 3.9500 & 3.9500 & 90 & 90 & 90 & 61.63 & 18.05 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Lattice parameters of Mg–0.5 Ca–xGd alloy phases.  
Figure 3: Average elemental composition obtained by energy-dispersive spectroscopy (EDS) analysis (SD Mg: \(\pm\)0.27, \(\pm\)0.45, \(\pm\)0.26, \(\pm\)0.36, \(\pm\)0.38; SD Ca: \(\pm\)0.11, \(\pm\)0.08, \(\pm\)0.11, \(\pm\)0.12, \(\pm\)0.12; SD Gd: \(\pm\)0.13, \(\pm\)0.18, \(\pm\)0.16, \(\pm\)0.26, \(\pm\)0.41).