1. Glass-Like Behavior in General Grain Boundary During Migration
Hao Zhang1, David J. Srolovitz1,2
1 Princeton University
2 Yeshiva University
Jack F. Douglas, James A. Warren
National Institute of Standards and Technology

2. Are General Grain Boundaries Glassy?
General Boundaries
Exclude low angle, low S and coherent twin grain boundaries
Structure
“Amorphous-cement” model suggested that the metal grains in cast iron were “cemented” together by a thin layer of ‘amorphous’ material (Rosenhain and Ewen, J I Met ,1913)
The RDF suggests liquid like structure at high T (Wolf, Phys Rev Lett , 1996; Curr Opin Solid St M , 2001; Acta Mater. 53 1, 2005 )
Others show partial crystalline structure (Gleiter, Phys Rev B , 1987; Appl Phys Lett , 1987; Van Swygenhoven , Phys Rev B , 2000 )
Dynamics
Grain boundary viscosity (Ashby, Surf Sci , 1972 )
Grain boundary migration and diffusion suggests structural transition temperature (Wolf, Acta Mater. 53 1, 2005 )
self-diffusion in the grain-boundary suggested that the diffusion mechanism is similar to that in bulk metallic glasses (Mishin, J Mater Sci , 2005 )

3. Simulation Details (001) q Molecular dynamics in NVT ensemble
EAM-type (Voter-Chen) potential for Ni
[010] tilt general grain boundary with q=40.23º
Periodic boundary conditions in x and y
One grain boundary & two free surfaces
Fixed strain, xx and yy
Source of driving force is the elastic energy difference due to crystal anisotropy
Driving force is constant during simulation q
(001) X Z Y

4. Grain Boundary Migration
Grain boundary migration tends to be continuous at high temperature, while shows “intermittent” at lower temperature
The waiting period becomes longer as temperature decreasing

5. Mobility vs. T – Arrhenius?OR
Temperature dependence of grain boundary mobility can be nicely fitted into Vogel-Fulcher Form, which is commonly used in super-cooled liquid system
T0 denotes the temperature that mobility disappears

6. Catch Strings and Determine their Length
The atom is treated as mobile if
Find string pair among mobile atoms using
The Weight-averaged mean string length:

7. “Typical” Strings

8. String-like Motion Within Grain Boundary
String-like cooperative motion within grain boundary is significant at low temperature
The fraction of non-trivial strings in the mobile atoms can be over 40% at 780K

9. String Length vs. Temperature
String length distribution function P(n) follows exp(-n/<n>)
S grain boundaries have shorter strings, therefore they are less frustrated than general grain boundaries
String length increases as temperature decreasing, similar behavior is found in supercooled liquids

10. “Intermittent” Migration Behavior

11. Movie X Y Z X Z Y

12. Migration Mechanism at Low TGB
Stage I
Steps GB GB
Stage II
Grain boundary migration at low T is associated with nucleation of steps/terrace

13. Further Observations
“Selected” migration region can be best described by Arrhenius law
The activation energy is about 0.37 eV (smaller than the apparent activation energy)

14. Grain Boundary Migration Model
Overall Migration t
GB Position t1 t2 L
Since the migration region follows Arrhenius
Mention the simple check at High temperature K, Q=0.37eV

15. Conclusion
Temperature dependence of Grain boundary migration in general tilt boundaries is found to be described by Vogel-Fulcher relation, which is characteristic in glass-forming liquid
String-like atomic motion in grain boundaries is similar to those in liquid system
It is reasonable to believe that string-like cooperative motion dominates the rate of grain boundary migration at low T
The migration model suggests grain boundary migration is controlled by different atomistic mechanisms. The waiting period is associated with the nucleation of steps.