Faceting of Twin Grain Boundaries in High-Purity Copper Subjected to High Pressure Torsion

The microstructure of high-purity 5N5 copper processed by high pressure torsion (HPT) is studied. Close to the top and bottom surfaces of HPT disk, the 2-10 mu m-thick ultrafine-grained layers with equiaxial grains and grain size of about 150 nm are formed. This grain size is typical for HPT of copper and its alloys. However, the remaining bulk layer in the HPT disk contained mainly elongated intersecting twins with high aspect ratio and length of up to 1 mu m. These twin grain boundaries (GBs) are faceted. The geometry of the GB facets is analyzed using sigma 3 coincidence site lattice (CSL). The sigma 3 twins after HPT contained (100)(CSL), (110)(CSL), (010)(CSL), and non-CSL 9R facets, but not (120)(CSL) and (130)(CSL) facets. Earlier, the stability diagram for the sigma 3 GB facets is experimentally constructed for the same 5N5 high-purity copper. The comparison of the data with this diagram allows to estimate for the first time the effective temperature of pure copper during HPT processing at room temperature (RT): T-eff = 920 +/- 50 K. In other words, the HPT at RT results in sigma 3 GB facets as if the sample is annealed at T-eff = 920 +/- 50 K.