NC State University Department of Materials Science and Engineering 1

MSE 791: Mechanical Properties of Nanostructured Materials Module 3: Fundamental Physics and Materials Design

Instructor: Yuntian Zhu Office: 308 RBII Ph: 513-0559

ytzhu@ncsu.edu

Lecture 9 Utilizing deformation mechanisms to design

nanostructured materials for superior mechanical properties

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Only a few nanostructured materials show good ductility

The yielding strength is normalized by the yield strength of a material’s coarse-grained counterpart

Issue: How do we obtain high ductility in nanostructured materials?

Nanostructured materials have much higher strength than their coarse-grained counterparts

Koch, Scripta Mater. 49 (2003) 657 Zhu & Liao, Nature Mat., 3 (2004) 351.

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Early attempts to improve the

ductility always sacrifice the

strength

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Design Nanomaterials with high strength and High ductility

The Issue:

How do we increase the ductility without trading off the strength?

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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

The Key to high ductility:

Restore strain hardening

Tensile curves

Compression

CG Ti

nano Ti

dσdε

≥σ

Considère's Criterion for Necking:

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fcc systems

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Two-Phase Alloy: Introducing second-phase particles to trap dislocations

Zhao, Liao, Cheng, Ma and Zhu, Advanced Mater. 18, 2280-2283 (2006).

Materials: Age hardening 7075 Al alloy Processing: Cryo-rolling to produce nanostructure, then age hardening

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Before Tensile testing Second phase particles, low dislocation density

Zhao, Liao, Cheng, Ma and Zhu, Advanced Mater. 18, 2280-2283 (2006).

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After tensile testing High density of dislocations

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• 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

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Strategy III: use preexisting twin to restore strain hardening capability

Single-Phase Alloy with Medium Stacking Fault Energy: Introduce pre-existing twins

Lu, Shen, Chen, Qian, Lu, Science, 304 (2004) 422.

Electrodeposited Cu After tensile deformation

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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: 10.1038, Sept. 2010.

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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, 2949-2953 (2006)

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Deformation at liquid nitrogen temperature to introduce deformation twins: Mechanical Behavior

Zhao, Bingert, Liao, Cui, Han, Sergueeva, Mukherjee, Valiev, Langdon, Zhu, Adv. Mater. 18, 2949-2953 (2006)

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Strategy IV: High-angle grain boundary and low dislocation density yield higher ductility in Cu

Zhao, Bingert, Zhu, Liao, Valiev, Horita, Langdon, Zhou, Lavernia, APL, 92, 081903 (2008).

ECAP HPT+CR

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hcp systems

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Twinning as a function of Grain Size for Mg-alloys

Not much age hardening is observed in nanostructured Mg Alloys either

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Strategies for enhancing ductility without sacrificing strength

•  Second phase particles•  Solute clusters•  Twins•  Stacking faults (new possibility, need

verification)•  High-angle grain boundaries