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 7
Utilizing deformation mechanisms to design nanostructured materials for superior mechanical properties
Text book,
Office hour, by appointment
Department of Materials Science and Engineering 1

2. Only a few nanostructured materials show good ductility
The yielding strength is normalized by the yield strength of a material’s coarse-grained counterpart
Nanostructured materials have much higher strength than their coarse-grained counterparts
Koch, Scripta Mater. 49 (2003) 657
Zhu & Liao, Nature Mat., 3 (2004) 351.
Issue: How do we obtain high ductility in nanostructured materials?

3. Early attempts to improve the ductility always sacrifice the strength

4. Design Nanomaterials with high strength and High ductility
The Issue: How do we increase the ductility without trading off the strength?

5. What affects the ductility of nano/UFG materials?
Low strain hardening
low ductility
Lack of dislocation accumulation
Strain rate sensitivity
Nano fcc metals has higher strain rate sensitivity → high ductility
Nano bcc metals have lower strain rate sensitivity
The effect of low strain hardening dominates
Low ductility
Compression
CG Ti
nano Ti
Tensile curves
For example, Prof. Ma in John Hopkin’s University proposed 8 strategies. However, these strategies always sacrifice the strength.
Therefore, it is a tough issue.
Considère's Criterion for Necking:
The Key to high ductility:
Restore strain hardening

6. fcc systems

7. Two-Phase Alloy: Introducing second-phase particles to trap dislocations
Materials: Age hardening 7075 Al alloy
Processing: Cryo-rolling to produce nanostructure, then age hardening
In the following, I’ll present 4 strategies to simultaneously increase the strength and ductility of nanostructured materials.
Before I go further, I’d like define nanostructured materials as those with structural features such as grains, subgrains and dislocation cells smaller than 100 nm. This definition is not as strict as nanocrystalline materials, and has been widely used in my field.
These 4 strategies increase the strain hardening rate without sacrificing the strength.
Each strategy can be applied to a specific type of metals or alloys.
Zhao, Liao, Cheng, Ma and Zhu, Advanced Mater. 18, (2006).

8. Before Tensile testing
Second phase particles, low dislocation density
Zhao, Liao, Cheng, Ma and Zhu, Advanced Mater. 18, (2006).

10. Other potential methods to introduce second phase particles
Second phase particles can be used to simultaneously improve the strength and ductility
Density
Particle size
Other potential methods to introduce second phase particles
Powder consolidation
Formation of inter-metallic compound during ball-milling

11. Strategy III: use preexisting twin to restore strain hardening capability
Single-Phase Alloy with Medium Stacking Fault Energy: Introduce pre-existing twins
Electrodeposited Cu
After tensile deformation
Lu, Shen, Chen, Qian, Lu, Science, 304 (2004) 422.

12. Deformation at liquid nitrogen temperature to introduce deformation twins: Processing and structure
Zhao, Bingert, Liao, Cui, Han, Sergueeva, Mukherjee, Valiev, Langdon, Zhu, Adv. Mater. 18, (2006)

13. Deformation at liquid nitrogen temperature to introduce deformation twins: Mechanical Behavior
Zhao, Bingert, Liao, Cui, Han, Sergueeva, Mukherjee, Valiev, Langdon, Zhu, Adv. Mater. 18, (2006)

14. Nanostructural Hierarchy in Al-age hardening alloy
High density of dislocations
Subnanometer clusters in grain interior
Nanometer-sized solute clusters (4 nm, 4x18 nm )
Nanometer grains (~26 nm)
Nature Comm. DOI: , Sept

15. Strategy IV: High-angle grain boundary and low dislocation density yield higher ductility in Cu
HPT+CR
ECAP
Zhao, Bingert, Zhu, Liao, Valiev, Horita, Langdon, Zhou, Lavernia, APL, 92, (2008).

16. hcp systems

17. Twinning as a function of Grain Size for Mg-alloys
Not much age hardening is observed in nanostructured Mg Alloys either

18. Wang et al., Materials Research Letters, 2, 61(2013)

19. Materials Research Letters, 2, 61(2013). Our own work

20. Ultra-Strong Mg Alloys vis Nano-Spaced Stacking Faults
Materials Research Letters, 2, 61(2013). Our own work

21. Strategies for enhancing ductility without sacrificing strength
Second phase particles
Solute clusters
Twins
Stacking faults (new possibility, need verification)
High-angle grain boundaries

22. Graded Strutures Bamboo Bone
Nature evolution produces gradient structure in biological systems

23. Structural Gradient PNAS, 111 (20), 7197 (2014). Our own work
Materials Research Letters, 3, 184(2014). Our own work

24. Unique mechanical properties of gradient structured metals
Most twins are formed this way
A combination of strength and ductility that is not accessible to homogeneous materials
PNAS, 111 (20), 7197 (2014). Our own work
Materials Research Letters, 3, 184(2014). Our own work

25. Synergetic strengthening: 1+1 > 2
PNAS, 111 (20), 7197 (2014). Our own work
Materials Research Letters, 3, 184(2014). Our own work

26. What happens under tensile strain
PNAS, 111 (20), 7197 (2014). Our own work
Materials Research Letters, 3, 184(2014). Our own work

27. The ultimate challenge:
Nanostructured
Coarse grained
The ultimate challenge:
Ductility
Strength

28. Heterogeneous Lamella Structure (Ti)
Wu, et al, PNAS. 111, 7197(2014). Our work

29. Future direction
Heterogeneous structures will be the next hot research area after nanomaterials era
New Idea: Softer phase embedded in hard phase to constrain the deformation of the softer phase for higher back stress
New Strategy: Make materials “frustrated”
Mechanical incompatability during deformation