We observed very high thermal stability (after annealing at 600 °C for 1 h) of the near-surface layers of ultrafine grains in the 99.9995 wt% (5N5) purity Cu processed by high-pressure torsion. We attributed this extraordinary thermal stability of ultrafine-grained microstructure to the nitrogen-filled nano-sized pores that pinned the grain boundaries. Our study reveals that a combination of severe shear strain and surface attrition causes the capture of N2 molecules in the near-surface layer of the deformed metal from the ambient atmosphere. This phenomenon may play an important role in formation of percolating porosity in metals processed by severe plastic deformation.
- Electron energy loss spectroscopy (EELS)
- Grain boundary migration
- High-pressure torsion
- Thermal stability