Plastic Strain Recovery for Nano-crystalline Cu

For traditional materials, as we are familiar with, their stress-strain behavior has elastic and plastic regions. When a specimen deforms into plastic region, the plastic deformation will stay as a constant vs. time. However, for the nanocrystalline Cu films we fabricated with grain size of about 40 nm, a phenomena called “plastic strain recovery” or “healing” is found. Our group found this phenomena independently and at the same time, Saif’s group in IL also found the similar phenomena for nanocrystalline Au and Al.
The mechanisms count for this phenomena has not been decided yet, although dislocation migration is considered to be the No. 1 candidate.

• Demonstration of the phenomena
  

1. Microscopy image of film right after test
(Wrinkles indicate plastic strain at this time)

2. Microscopy image of film after healing
(flat surface indicates plastic strain has recovered)

On one healed film, we also found clear evidence of dislocation activities, slip band, grain-boundary sliding...
The following SEM image shows the film has slip bands along the typical angles where max. shear stress exists