Network of Porosity Formed in Ultrafine-Grained Copper Produced by Equal Channel Angular Pressing

Radiotracer experiments on diffusion of ${}^{63}\mathrm{Ni}$ and ${}^{86}\mathrm{Rb}$ in severely deformed commercially pure copper (8 passes of equal channel angular pressing) reveal unambiguously the existence of ultrafast transport paths. A fraction of these paths remains in the material even after complete recrystallization. Scanning electron microscopy and focused ion beam techniques are applied. Deep grooves are found which are related to original high-energy interfaces. In-depth sectioning near corresponding triple junctions reveals clearly multiple microvoids or microcracks caused by the severe deformation. Long-range tracer penetration over tens of micrometers proves that these submicrometer-large defects are connected by highly diffusive paths and that they appear with significant frequency.

Figure 1

Microstructure of UFG pure Cu in the as-prepared state (a) and after annealing at 423 K for 15 h (b).

Figure 2

Penetration profiles measured for diffusion of ${}^{63}\mathrm{Ni}$ in UFG Cu at 471 (circles) and 498 K (squares) in comparison with penetration of ${}^{63}\mathrm{Ni}$ in coarse grain copper (stars) and ${}^{86}\mathrm{Rb}$ (triangles) in UFG Cu at 293 K (a). The counting rates for Ni diffusion at 498 K are multiplied by factor of 10 for a convenient presentation. In (b) the reproducibility of separation of the ${}^{63}\mathrm{Ni}$ signal from the total ${}^{63}\mathrm{Ni}$ plus ${}^{86}\mathrm{Rb}$ spectrum is demonstrated for different counting dates.

Figure 3

In-depth analysis of polished and chemically etched surface of UFG Cu. (a) and (b) represent successive sections cut by FIB. The scale bars represent $2\mu \mathrm{m}$.

Figure 4

SEM image of UFG Cu annealed in a high vacuum chamber at 673 K for 33 h. The thermal grooves corresponding to nonequilibrium GBs are seen. The regions $I$ and $II$ are sectioned in depth by FIB (Figs. 5, 6).

Figure 5

Sequence of sections of the area $I$ in Fig. 4 performed by FIB starting before the given TJ (a), at the TJ position (b), and behind the TJ (c),(d). The scale bars correspond to $1\mu \mathrm{m}$.

Figure 6

Sequence of sections of the area $II$ in Fig. 4 performed by FIB. The scale bars are $1\mu \mathrm{m}$.