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 μm‐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 μm. These twin grain boundaries (GBs) are faceted. The geometry of the GB facets is analyzed using Σ3 coincidence site lattice (CSL). The Σ3 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 Σ3 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): Teff = 920 ± 50 K. In other words, the HPT at RT results in Σ3 GB facets as if the sample is annealed at Teff = 920 ± 50 K. After high pressure torsion (HPT) of high‐purity copper, it contains faceted twin grain boundaries (GBs). In the equilibrium, such facets appear at elevated temperature. Thus, the HPT at room temperature results in GB facets as if the sample is annealed at Teff = 920 ± 50 K.