Role of strain gradient on the formation of nanocrystalline structure produced by severe plastic deformation

In the present study, the formation of nanocrystalline structure by high speed drilling was investigated. Several micrometers thick nanocrystalline layer was observed at the top surface of drilled hole. Nanocrystalline layers showed high hardness and good thermal stability. The estimation of necessary dislocation density to produce nanocrystalline structure and the comparison between the estimated and measured strain gradient near the drilled hole surface was conducted. It is proposed that deformation with a large strain gradient is an important condition to produce nanocrystalline structure.