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 3
Difference in the deformation of coarse-grained and nano grains
Text book,
Office hour, by appointment
Department of Materials Science and Engineering 1

2. Dislocation source in coarse-grained metals: Frank-Read source
F = tb
Dislocation velocity should be similar under the same stress
Department of Materials Science and Engineering 2

3. Twinning mechanisms in CG metal: Pole mechanism
Due to the screw dislocation, the partial moves to the next slip plane after rotating around the screw for 360o
Other mechanisms:
3 layer twinning: Mahajan & Chen, Acta Metall. 21, 1353 (1973).
Prismatic Glide mechanism: Venables, Phil. Mag. 6, 379 (1961)
Double pole mechanism:Niewczas & Saada, Phil. Mag. 82, 167(2002).
Department of Materials Science and Engineering 3

4. Nanocrystalline grains often are dislocation free
Define Nanocrystalline and nanostructured mater.
Dislocation and microstructure variation with grain size in Ti
Department of Materials Science and Engineering 4

5. What Do We Know about Deformation Physics in Nanomaterials?
Grain rotation and grain boundary sliding at finest grain size (e.g. <10nm)
Molecular Dynamics simulations
Swygenhoven et al, Phys. Rev. B, 60, 22 (1999).
Schiotz et al, Nature, 391, 561 (1998).
Yamakov et al, Nature Materials, 3, 43 (2004).
Experimental Evidence
Shan et al, Science, 305, 654 (2004).
Ke et al, Nanostructured Materials, 5, 689 (1995).
Liao et al, Appl. Phys. Lett. 88, (2006 ), our own work
Note: the critical grain may change from metal to metal.

6. Molecular Dynamics simulations
At a few tens nanometers, partial dislocation emission and deformation twinning dominate
Molecular Dynamics simulations
Swygenhoven, Science, 296, 66 (2002).
Schiotz et al, Nature, 391, 561 (2002)
Yamakov et al, Nature Materials, 1, 43 (2002).
Nano Al
Partial dislocations emitted from grain boundaries
3 deformation twinning mechanisms in nano Al

7. Our experimental work provides first and direct experimental evidence to support the MD simulation results in nanomaterials
Appl. Phys. Lett. 84, 592 (2004)
Partials emission from grain boundaries and twinning
Nano Cu
Twinning mechanisms 1:
homogeneous twinning inside a nano Al grain
Appl. Phys. Lett. 83, 632 (2003)

8. Twinning mechanisms 2 in nanomateials: twinning by partials from grain boundaries
Twin formed by successive partial dislocation emission from a grain boundary
Chen et al, Science, 300, 1275 (2003).
Nano-Al film/indented
Appl. Phys. Lett. 83, 5062 (2003).
Cryo-milled nano-Al powder

9. Twinning mechanisms 3 in nanomaterials: Grain boundary splitting/migration
MD simulation
Yamakov et al, Nature Materials, 1, 45 (2002).
Appl. Phys. Lett. 83, 5062 (2003)

10. Fivefold Twins in nanomaterials
Five-fold twins were usually grown in nano-particles obtained by vapor deposition, electrodeposition, etc
HPTed Cu (RT)
This five-fold twin was formed via a sequential twinning by partials emitted from grain/twin boundaries
Appl. Phys. Lett. 84, 592 (2004).

11. Partials Multiply at Grain Boundary to Form a Twin (nanomaterials)
Most twins are formed this way
One partial is needed on each plane to form a twin
It is statistically impossible to have a partial on each plane
Zhu, Wu, Liao, et al, APL, . 95, (2009).

12. Partial dislocations on the grain boundary
GPFE curves are not applicable to nanomaterials with non-equilibrium grain boundaries
Non-equilibrium grain boundaries
Wu, Zhu, APL, 89, (2006). Our work
Huang, Zhu, Jiang, Lowe, Acta Mat, 49, 1497 (2001).
Department of Materials Science and Engineering 12

13. Mechanism for Partial to Multiply on Grain Boundary
b  b1 + b2
b2 glide out, b1 cross slip to the next plane
b1  b + (-b2)
-b2 stays on the grain boundary and
b repeats the above process
Appl. Phys. Lett. 95, (2009).

14. Mechanism for Partial to Multiply on Grain Boundary
b  b1 + b2
b2 glide out, b1 cross slip to the next plane
b1  + b’ + (-b3)
-b3 stays on the grain boundary
b’ dissociates
b’  b1 + b3
b3 glide out, b1 cross slip to the next plane
b1  + b + (-b2)
b2 and b3 are emitted alternatively
b2 + b3  b1

15. Homework (due in one week)
Lecture 3: 5, 8
Department of Materials Science and Engineering 15