Instructor: Yuntian Zhu 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 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

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?

Early attempts to improve the ductility always sacrifice the strength

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

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

fcc systems

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, 2280-2283 (2006).

Before Tensile testing Second phase particles, low dislocation density Zhao, Liao, Cheng, Ma and Zhu, Advanced Mater. 18, 2280-2283 (2006).

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

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.

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)

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)

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.

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, 081903 (2008).

hcp systems

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

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

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

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

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

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

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

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

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

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

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

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

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