# Influence of Noble Metals on the Microstructure and Properties of Biodegradable Mg-Nd-Zr Alloy
## 3 Results and Discussion
### Mechanical Properties after Biocorrosion  
The biocorrosion was studied by keeping the tensile testing samples (diameters of the head and working part were 10 mm and 5 mm, respectively) in Gelofusine(r) for 1, 2 and 3 months, followed by tensile testing of the corroded samples. Visual inspection of such samples after tensile testing showed the presence of multiple sites of corrosion initiation in the form of craters with the presence of crumbling white deposits from biodegradation products (Figure 4). Microstructural study of cross sections of the heads of tensile samples has shown that the depth of the corrosion damage craters after 3 months of aging in Gelofusine(r) for all variants of the alloy was approximately at the same level and amounted to \(\sim\)2.0 mm (Figure 5a). At the same time, corrosion damage spread throughout the body of grains of the \(\delta\)-solid solution and strengthening phases (Figure 5b). \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Modifier Content, wt.%**} & \multicolumn{3}{c}{**Mechanical Properties**} \\ \cline{2-3}  & **UTS, MPa** & **TE, \%** \\ \hline - & 232 & 3.6 \\
0.05\% Au & 236 & 4.0 \\
0.10\% Au & 250 & 5.7 \\
0.20\% Au & 254 & 4.9 \\
0.05\% Ag & 239 & 4.7 \\
0.10\% Ag & 252 & 5.9 \\
0.20\% Ag & 256 & 5.4 \\
0.10\% Au + 0.10\% Ag & 262 & 6.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mechanical properties of the Mg-based alloy depending on Ag and Au contents (after heat treatment). Figure 4: Samples for tensile tests after exposure to Gelofusine® for (**a**) 1 month; (**b**) 2 months; (**c**) 3 months. The results of mechanical tests of the studied samples after biodegradation have shown a loss of strength of the alloy with various modification options. The lowest level of strength after biocorrosion was observed in the magnesium alloy without modification. Silver and gold contributed to increasing the tensile strength of the magnesium alloy. The samples with complex modifications of Au and Ag had the highest properties after 3 months of aging in Gelofusine(r) (Table 4). The conducted studies showed the possibility of successfully modifying Mg-based alloys with noble metals in amounts from 0.05 to 0.2 wt.%. An analysis of data from the literature on the use of silver and gold in the manufacture of implants showed that the experiments were mainly related to coating or use in pure form [33; 34]. Other studies have shown the positive effect of silver on the properties of a Mg-based alloys. Modification of a Mg-Nd-Zr alloys with silver and gold, described in this article, makes it possible to increase the volume fraction of intermetallic phases by \(\sim\)1.5 times, shifting them towards smaller size groups, with the simultaneous formation of spherical intermetallic compounds located in the center of the grains and acting as additional crystallization centers. The obtained results as to the refinement of the microstructure of the alloy are consistent with studies carried out by other authors [35]. The studies carried out showed the possibility of reducing the grain size and significant refinement of the structural components due to the modification carried out. In addition, the dependence of the separate and joint influence of noble metals on the mechanical properties of a magnesium alloy has been established. Thus, separate modification of \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Modifier Content,** & \multicolumn{3}{c}{**UTS after Immersion in Gelofusine®, MPa**} \\ \cline{2-4}
**wt.\%** & **1 Month** & **2 Months** & **3 Months** \\ \hline - & 232 & 170 & 150 \\
0.05\% Au & 236 & 200 & 190 \\
0.10\% Au & 250 & 205 & 200 \\
0.20\% Au & 254 & 195 & 185 \\
0.05\% Ag & 239 & 201 & 200 \\
0.10\% Ag & 252 & 208 & 205 \\
0.20\% Ag & 256 & 205 & 202 \\
0.10\% Au + 0.10\% Ag & 262 & 215 & 210 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mechanical properties of the Mg-based alloy depending on Ag and Au contents after different times of immersion in Gelofusine®.