GENERATION AND HEALING OF POROSITIES IN HIGH PURITY COPPER BY HIGH-PRESSURE TORSION

Yuanshen Qi ¹ Eugen Rabkin ¹ Boris Straumal ^2,3,4

¹Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa, Israel
²Institute of Nanotechnology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany
³MISIS, National University of Science and Technology, Moscow, Russia
⁴Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Russia

Generation and elimination of porosities in metals under loading or during processing has long been an important research topic in physical and mechanical metallurgy. In recent years, with the development of severe plastic deformation techniques, percolating network of porosities acting as ultra-fast diffusion paths have been found in ultrafine-grained metals after high-pressure torsion (HPT) process, even though the combination of high hydrostatic pressure and shear deformation contributing to elimination of porosities was imposed. In this study, two 99.9995 wt.% purity copper disks, one in its bulk form and the other one with artificially drilled holes for introducing a pre-existing porosity of 4.48%, were processed by HPT. Percolating network of pores with porosity of 0.066% was found in the bulk disk after HPT, while the one with pre-drilled holes possessed 0.035% porosity after the same processing route. Systemic characterization using electron microscopies revealed that the generation of porosities was due to the heterogeneous distribution of strain which caused vacancy agglomeration and nucleation of voids. The presence of porosities promoted plastic instability and localised plastic flow and caused grain refinement.