1. Instructor: Yuntian Zhu
MSE 791: Mechanical Properties of Nanostructured Materials Module 3: Fundamental Physics and Materials Design
Instructor: Yuntian Zhu
Office: 308 RBII
Ph:
Lecture 6
Deformation Mechanisms of Nanocrystalline bcc metals
Text book,
Office hour, by appointment
Department of Materials Science and Engineering 1

2. Deformation mechanisms in nano bcc metals has not been well studied
It is expected to be different
from those in nano fcc metals

3. Strain rate sensitivity
m is the strain rate sensitivity
Higher m can lead to higher ductility
For superplastic deformation: m > 0.3
Department of Materials Science and Engineering 3

4. Some Difference in Strain Rate Sensitivity between bcc and fcc metals
What caused the difference?
Wei, J. Mater. Sci. 42, 1709 (2007).

5. We need to check some fundamental backgrounds
What caused the difference?
We need to check some fundamental backgrounds

6. Difference in Strain Rate Sensitivity between bcc and fcc metals: Coarse grains
The strain rate sensitivity of bcc metals are
an order of magnitude higher
than that of fcc metals
This is caused by the difference in dislocation structure

7. Difference in dislocation structure between bcc and fcc metals: Coarse Grain
Edge dislocations dissociate into two partials on the same slip plane
Screw dislocations dissociate onto three slip planes
fcc
Both screw and 60o dislocations dissociate into two partials on the same slip plane

8. Dislocation structure is determined by crystal structure
Could be (110), (011) and planes
fcc
bcc
Difference in dislocation type caused the difference in strain rate sensitivity
BC = Ba +aC
BC = Bd + dC
Screw dislocation structure in bcc

9. Screw dislocations moves by double-kink nucleation in bcc metals
Edge dislocation kinks glide fast and disappear
Screw dislocations move by double-kink nucleation and motions
Deformation controlled by the double–kink nucleation, which is thermally activated
This leads to high strain rate sensitivity in coarse-grained bcc metals
Wei, J. Mater. Sci. 42, 1709 (2007).

10. What caused the strain rate sensitivity decrease in bcc metals
What caused the strain rate sensitivity decrease in bcc metals? Current Theory
For bcc metals, deformation is controlled by the nucleation double-kinks
When the flow stress is very high, v* decreases to nearly constant
Flow stress increases with decreasing grain size, hence smaller m
Theory is simple and easy to understand.
The implicit assumption: The deformation mechanisms remains controlled by double-kinks from coarse-grained bcc metals to nano bcc metals
Is this correct?

11. HPT-processed Mo Distribution of dislocation density 1.0 x1014 m-2
The dislocation density was calculated as the number of dislocations per unit area in high-resolution TEM (HRTEM) micrographs.
Grain size effect on the density of screw dislocations, which was calculated as the total length of screw dislocation segments per unit volume.
Edge dislocation: the number of dislocations per unit area;
Screw dislocation: the total length of screw dislocation segments per unit volume;
Cheng et al, Mater. Res. Lett. 11 11

12. Grain Size Effects on SRS
Dislocations distribution
Cheng et al, Mater. Res. Lett. 12 12

13. Grain Size Effects on SRS
SRS vs grain size
Cheng et al, Mater. Res. Lett. 13 13

14. Some Difference in Strain Rate Sensitivity between bcc and fcc metals
What caused the difference?
Wei, J. Mater. Sci. 42, 1709 (2007).

15. Conclusion
The high strain rate sensitivity in coarse grained bcc metals is caused by the double-kink mechanisms for screw dislocation slip
The low strain rate sensitivity in nano bcc metals is caused by edge and mixed dislocations
In very small grain sizes, strain rate sensitivity increases again due to grain boundary mediated processes 15 15