# Influence of Noble Metals on the Microstructure and Properties of Biodegradable Mg-Nd-Zr Alloy
## 3 Results and Discussion
### Quantitative Metallographic Analysis  
The microscopic observation revealed the presence of lamellar and spherical intermetallic phases in the studied alloys. Lamellar intermetallic compounds were located predominantly along the grain boundaries, and spherical ones were located in the center of the grain. Spherical intermetallic phases can serve as additional crystallization centers and contribute to the refinement of grains and structural components of the alloy. Metallographic analysis showed that the volume fraction of the intermetallic phase increases with increasing content of alloying elements in the alloy (Table 2). When 0.05...0.10% modifiers were added to the alloy, the volume fraction of spherical intermetallic compounds increased to a greater extent compared to lamellar ones. A further increase in the alloying content (up to 0.2%) led to a slight increase in the volume fraction of spherical inclusions located inside the grain and an intensive increase in the number of lamellar inclusions. Analysis of the distribution of intermetallides by size showed that lamellar ones were predominated in the original alloy, most of which were 4-15 um in size. Spherical intermetallides were mainly 2.0-7.9 um in size. Additives of Au and Ag contributed to the refinement of the intermetallic phase. The number of intermetallides smaller than 2 um increased, but the number of coarse ones (>11.6 um) decreased. With an Au and Ag content of 0.05...0.1 wt.%, there was an intensive increase in the volume percentage of spherical intermetallides located inside the grain, the grain was refined and the plasticity of the alloys increased. A further increase in the content of elements in the alloy to 0.2 wt.% increased the volume percentage of lamellar and spherical intermetallic compounds and increased the tensile strength. Thus, gold and silver, positively changing the morphology and topology of intermetallic compounds, refined the macro- and micrograins in the alloy and contributed to an increase in its mechanical properties. At the same time, the highest indicators of strength and ductility of the alloy were achieved with complex modification with noble metals (Table 3). IMS containing Ag and Au presumably possess higher melting temperatures than Mg. During cooling of the melt, IMSc with Ag and Au solidify first in the form of spherical particles. They act as crystallization sites refining the grains and improving mechanical properties. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{**Modifier**} & \multirow{2}{*}{
\begin{tabular}{c} **Content,** \\ **wt.\%** \\ \end{tabular} } & \multicolumn{6}{c}{**Distribution of Intermetallides (V-10-3, vol. \%)**} \\ \cline{2-9}  & & & & & **by Size Groups, \(\mu\)m** & & \\ \cline{2-9}  & & **<2** & **2.0...3.9** & **4.0...7.9** & **8.0...11.5** & **11.6...15.0** & **15.1...19.0** & **Total** \\ \hline No alloying with Au/Ag & 7/0 & 19/53 & 37/29 & 31/11 & 31/12 & 19/0 & 144/105 \\ \hline \multirow{3}{*}{Au} & 0.05 & 40/1 & 82/75 & 21/43 & 11/25 & 31/0 & 19/1 & 204/145 \\  & 0.10 & 65/2 & 91/155 & 43/43 & 11/19 & 19/1 & 7/1 & 236/221 \\  & 0.20 & 76/79 & 160/129 & 42/25 & 6/7 & 18/1 & 0/1 & 302/242 \\ \hline \multirow{3}{*}{Ag} & 0.05 & 47/1 & 48/110 & 31/43 & 25/37 & 35/7 & 19/1 & 205/199 \\  & 0.10 & 91/19 & 29/116 & 47/44 & 19/32 & 19/7 & 16/1 & 221/219 \\  & 0.20 & 147/62 & 25/116 & 73/34 & 19/26 & 11/7 & 5/0 & 280/245 \\ \hline
0.1 wt.% Au + 0.1 wt.% Ag & 143/112 & 111/119 & 69/71 & 9/14 & 0/0 & 0/0 & 232/316 \\ \hline \end{tabular} Note: the numerator—the volume percentage of lamellar intermetallides, the denominator—the volume percentage of spherical intermetallides.  
\end{table} 
Table 2: Volume fraction of intermetallic compounds (V) and their distribution by size in the alloy of the Mg-Nd-Zr system depending on Au and Ag contents. The dependence of UTS on the content of Au and Ag can be expressed by the following regression:  
\[UTS=119.7\cdot x+233.4;\ R^{2}=0.88,\]  
where  
\(x\)--modifier content, wt.%.