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

Abstract

Radiotracer experiments on diffusion of 63Ni and 86Rb 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.

Microstructure evolution during severe plastic deformation

“…In an earlier work [34], radiotracer measurements revealed unambiguously that percolating porosity develops even in ductile copper under certain conditions of severe plastic deformation.…”

“…As the maximum shear stress that develops in the material during ECAP of 250 MPa (see above) is greater than the shear yield stress of 144 MPa, when ECAP is carried out without the application of a back-stress, percolated micro-cavities are likely to be formed. This prediction is borne out by experiments [29,34]. Now, when a back-pressure of 200 MPa is applied, again using Mohr's circle construction and von Mises yield criterion, one calculates the maximum shear stress that develops during ECAP as τ max = (2−0.2−1.5)/2 GPa = 0.150 GPa or 150 MPa, which is very close to (actually slightly more than) the shear yield stress of the ECAP material of 144 MPa.…”

“…The most important and intriguing finding of these studies is the presence of (percolating) porosity, which is formed even in such highly ductile metals like ECAP-deformed pure copper [29]. This percolating internal porosity, which is reported in certain well-documented cases such as room-temperature ECAP-deformed pure Cu [34] and Cu-based alloys [35] or HPT processed pure Cu [36] at room temperature, represents the fastest diffusion paths in an otherwise dense nanostructured material. Although the volume fraction of this porosity is extremely low, about a ppm, its presence in the given cases could be unambiguously established by the radiotracer technique due to the extreme sensitivity of the method and the volume-averaged information gained.…”

Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena

“…5 for ECAP Ni. The zipper contrast at the majority 2 An important difference is the absence of so-called percolated porosity in SPD Ni, which was discovered in ECAP Cu [127] and Cu-based alloys [128]. This porosity represents an extremely fast transport path toping the above introduced list of the short-circuit paths for diffusion in UFG materials.…”

Revealing interfacial diffusion kinetics in ultra-fine-laminated Ni with low-angle grain boundaries

“…In earlier works it was shown that this method reveals unambiguously the existence of interconnected porosity, e.g. in SPD Cu and Cu-based alloys [16,17]). As shown in Figure 2(a), the specific radioactivity values of 63 Ni decreases sharply to the background value ( < 3 × 10 −2 Bq mg −1 ) at a depth of ∼ 3 μm in the as-DPD sample (RT_1) and the DPD sample annealed at 385 K for 95 h (RT_2).…”