## 4 Results and Discussion
### Microstructural Evolution  
A metallurgical microscope was used to observe the microstructure of the OFHC-Cu and Mo jets obtained from the recovery experiment to analyze the microstructural evolution of the two materials during the jet-formation process.  
Figure 13 shows the microstructural morphology of OFHC-Cu before and after jet formation ((a): microstructure of the OFHC-Cu liner; (b-d): microstructural morphology of different parts of the slug of the jet; (e,f): microstructural morphology of the jet). The analysis revealed that compared with that before jet formation, OFHC-Cu decreased considerably in the crystal grain size and exhibited notable microstructural flow characteristics after the formation of a jet. This phenomenon suggests that driven by explosive loading, the liner material underwent dynamic recrystallization, during which the initial coarse crystals were crushed and reorganized, resulting in the formation of a large number of equiaxed fine crystals.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{4}{c}{**Model Parameters**} & \multirow{2}{*}{\(T_{j}\) **(K)**} & \multirow{2}{*}{\(\begin{array}{c}\text{{Jet Strength}}\\ \text{(MPa)}\end{array}\)} \\ \cline{2-2} \cline{5-6}  & \(A\) **(MPa)** & & & & & & \\ \cline{1-1} \cline{5-6}  & 90 & 1.09 & 300 & 1356 & 873 & 144.32 \\ Mo [32] & 441.25 & 0.1774 & 226 & 3660 & 2826 & 286.66 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Jet strength estimates.  
Figure 12: Flow-stress distribution of the jets.  
A comparison of Figure 13b-f revealed that the crystal grains in the OFHC-Cu slug ranged from 1.3 to 8.51 \(\upmu\)m in size, with the average size of 3.4 \(\upmu\)m, whereas the crystal grains in the OFHC-Cu jet particles ranged from 0.84 to 3.6 \(\upmu\)m in size, with the average size of 2.4 \(\upmu\)m. The jet exhibited a smaller grain size and contained a larger number of fine crystals than the slug, suggesting more significant deformation and a higher degree of dynamic recrystallization in the material in the jet than in the slug. Moreover, the deformed OFHC-Cu structure contained a large number of concentrated deformation zones (Figure 13c,e,f), around which the structure exhibited a smaller grain size (Figure 13c).  
Figure 14a shows the microstructure of the Mo liner; Figure 14b,c show the microstructural morphology of different parts of the slug; and Figure 14d-f show the microstructural morphology of the jet. The analysis revealed that similar to OFHC-Cu, Mo also underwent dynamic recrystallization during the jet-formation process. The slug exhibited the  
Figure 13: Microstructural morphology of OFHC-Cu. (**a**) Initial structure; (**b–d**) Slug structure; (**e,f**) Jet structure.  
average crystal grain size of 23.4 \(\upmu\)m. The material in the jet underwent a higher degree of recrystallization, with the average grain size of 7.8 \(\upmu\)m. Compared with OFHC-Cu, the fully recrystallized Mo jet exhibited a larger crystal grain size, resulting in a relatively low coordinated PD capacity. A large number of macroscopic cracks (10-20 \(\upmu\)m in width) formed due to the concentrated deformation were found in the microstructure of the Mo slug and jet (Figure 14b,d).