Events Calendar

06 Feb
Swanson School of Engineering
Event Type

Defenses

Target Audience

Faculty, Graduate Students, Postdocs

University Unit
Department of Mechanical Engineering and Materials Science
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PhD Dissertation Defense, Sixue Zheng

This is a past event.

Title: 

"In situ TEM investigation on mechanical behaviors of metallic nanocrystals at the atomic scale 

ABSTRACT: 

  

Metallic nanocrystals with superior physical and mechanical properties are envisioned as promising building blocks in stretchable and flexible electronics. In a practical application, the mechanical stability of the nanoscale metals is of vital importance for the reliability of the electronic devices. In this study, in situ nanomechanical testing under high-resolution transmission electron microscopy (HRTEM) was employed to investigate the atomic-scale mechanical behaviors of metallic nanocrystals.  

It is found that, instead of local thickening commonly reported thus far, abnormal thinning events consecutively occur in silver nanocrystals, where preexisting dislocations and crystal slip serve as stimuli to activate surface atom diffusion. As the width of silver nanocrystal reduces to a critical size, tensile-fracture-like failure occurs, caused by surface atom diffusion. Different from the silver nanocrystals, pure displacive mechanism is dominant in platinum nanocrystals, resulting in continuous thickening during compression. 

The timely- and atomic-resolved observations show that sequential partial dislocation slip in varying slip systems and the decomposition of high-energy grain boundary account for the fivefold twin formation in a nanoscale gold single crystal under bending as well as the reversible formation and dissolution of fivefold twin in the gold nanocrystal with preexisting twin under tension and shearing. In addition, the complex stress state in the neck area facilitates fivefold twin formation in a bi-twinned gold nanocrystal, disobeying Schmid law. 

Finally, a TEM-based in situ high-temperature nanomechanical testing method is proposed based on the Joule heating effect produced by the electric current through the metallic nanocrystals. By this method, deformation twinning, phase transformation and perfect dislocation slip are observed to sequentially occur in the tungsten nanocrystal during tensile loading at elevated temperature. The activated deformation modes in tungsten nanocrystals are related to the loading orientation and the experimental temperature. 

Monday, February 6 at 11:00 a.m.

Benedum Hall, 611
3700 O'Hara Street, Pittsburgh, PA 15261

PhD Dissertation Defense, Sixue Zheng

Title: 

"In situ TEM investigation on mechanical behaviors of metallic nanocrystals at the atomic scale 

ABSTRACT: 

  

Metallic nanocrystals with superior physical and mechanical properties are envisioned as promising building blocks in stretchable and flexible electronics. In a practical application, the mechanical stability of the nanoscale metals is of vital importance for the reliability of the electronic devices. In this study, in situ nanomechanical testing under high-resolution transmission electron microscopy (HRTEM) was employed to investigate the atomic-scale mechanical behaviors of metallic nanocrystals.  

It is found that, instead of local thickening commonly reported thus far, abnormal thinning events consecutively occur in silver nanocrystals, where preexisting dislocations and crystal slip serve as stimuli to activate surface atom diffusion. As the width of silver nanocrystal reduces to a critical size, tensile-fracture-like failure occurs, caused by surface atom diffusion. Different from the silver nanocrystals, pure displacive mechanism is dominant in platinum nanocrystals, resulting in continuous thickening during compression. 

The timely- and atomic-resolved observations show that sequential partial dislocation slip in varying slip systems and the decomposition of high-energy grain boundary account for the fivefold twin formation in a nanoscale gold single crystal under bending as well as the reversible formation and dissolution of fivefold twin in the gold nanocrystal with preexisting twin under tension and shearing. In addition, the complex stress state in the neck area facilitates fivefold twin formation in a bi-twinned gold nanocrystal, disobeying Schmid law. 

Finally, a TEM-based in situ high-temperature nanomechanical testing method is proposed based on the Joule heating effect produced by the electric current through the metallic nanocrystals. By this method, deformation twinning, phase transformation and perfect dislocation slip are observed to sequentially occur in the tungsten nanocrystal during tensile loading at elevated temperature. The activated deformation modes in tungsten nanocrystals are related to the loading orientation and the experimental temperature. 

Monday, February 6 at 11:00 a.m.

Benedum Hall, 611
3700 O'Hara Street, Pittsburgh, PA 15261

Event Type

Defenses

Target Audience

Faculty, Graduate Students, Postdocs

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