Events Calendar

07 Jul
Swanson School of Engineering
Event Type

Defenses

Topic

Research

Target Audience

Graduate Students

Website

https://pitt.zoom.us/j/8995266392

University Unit
Department of Mechanical Engineering and Materials Science
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PhD Dissertation Proposal Presentation-Zhengwu Fang

This is a past event.

Title: 

"ATOMIC-SCALE IN SITU TEM INVESTIGATION OF STRESS-INDUCED GRAIN BOUNDARY BEHAVIORS IN FCC CRYSTALS” 

ABSTRACT:

Stress-driven GB migration is an important plastic deformation mechanism of nanocrystalline materials that has been extensively studied in the past decades. However, although progressing, the atomistic mechanisms of migration of faceted GBs and mixed tilt-twist GBs, i.e., the two general GB types, have remained largely unclear mainly due to the lack of direct experimental observation. 

In this study, in-situ high-resolution transmission electron microscopy (HRTEM) combined with molecular dynamic (MD) simulations are employed to investigate stress-driven migration behaviors of typical faceted GBs and mixed tilt-twist GBs in Au nanocrystals. The dynamic GB structural transformation arising from reversible facet transformation and GB dissociation was observed during the shear-mediated migration of faceted <110>-tilt GBs. A reversible transformation was found to occur between  and  S11(113) GB facets, and be accomplished by the coalescence and detachment of  type GB steps that mediated the GB migration. In comparison, the dissociation of   GB into S11(113) and S3(111) GBs was accomplished via the reaction of  type steps that involved the emission of partial dislocations. The loading-dependence of such transformations and the roles of GB junctions on accommodating the GB migration are also analyzed. In addition, two distinct migration behaviors were observed during the migration of a   mixed tilt-twist GB. The underlying atomistic mechanism is attributed to the activation of GB disconnections carrying Burgers vectors with different magnitudes and the opposite directions. Two lattice correspondence transformation relations, i.e., -to- and -to-are identified and attributed to the structural similarity of these atomic planes. MD simulations will be performed to further verify these lattice transformation relations. This work would enrich our understanding on the complexity of stress-induced migration of general GBs in polycrystalline materials. 

Metallic nanocrystals show unique properties such as pseudoelasticity compared to their bulk counterparts. GB-assisted pseudoelasticity in Au nanowires will be examined in this proposal, including the orientation-dependence of bending-formed GBs in nanowires, and their annihilation and migration behaviors during the deformation recovery. This study would provide important guidelines for the application of metallic nanowires in designing reliable and sustainable nano electromechanical systems (NEMS) devices.

Dial-In Information

Join Zoom Meeting: 

Link: https://pitt.zoom.us/j/8995266392 

Passcode: 070722 

Meeting ID: 899 526 6392 

Thursday, July 7 at 10:00 a.m.

Benedum Hall, Room 624

PhD Dissertation Proposal Presentation-Zhengwu Fang

Title: 

"ATOMIC-SCALE IN SITU TEM INVESTIGATION OF STRESS-INDUCED GRAIN BOUNDARY BEHAVIORS IN FCC CRYSTALS” 

ABSTRACT:

Stress-driven GB migration is an important plastic deformation mechanism of nanocrystalline materials that has been extensively studied in the past decades. However, although progressing, the atomistic mechanisms of migration of faceted GBs and mixed tilt-twist GBs, i.e., the two general GB types, have remained largely unclear mainly due to the lack of direct experimental observation. 

In this study, in-situ high-resolution transmission electron microscopy (HRTEM) combined with molecular dynamic (MD) simulations are employed to investigate stress-driven migration behaviors of typical faceted GBs and mixed tilt-twist GBs in Au nanocrystals. The dynamic GB structural transformation arising from reversible facet transformation and GB dissociation was observed during the shear-mediated migration of faceted <110>-tilt GBs. A reversible transformation was found to occur between  and  S11(113) GB facets, and be accomplished by the coalescence and detachment of  type GB steps that mediated the GB migration. In comparison, the dissociation of   GB into S11(113) and S3(111) GBs was accomplished via the reaction of  type steps that involved the emission of partial dislocations. The loading-dependence of such transformations and the roles of GB junctions on accommodating the GB migration are also analyzed. In addition, two distinct migration behaviors were observed during the migration of a   mixed tilt-twist GB. The underlying atomistic mechanism is attributed to the activation of GB disconnections carrying Burgers vectors with different magnitudes and the opposite directions. Two lattice correspondence transformation relations, i.e., -to- and -to-are identified and attributed to the structural similarity of these atomic planes. MD simulations will be performed to further verify these lattice transformation relations. This work would enrich our understanding on the complexity of stress-induced migration of general GBs in polycrystalline materials. 

Metallic nanocrystals show unique properties such as pseudoelasticity compared to their bulk counterparts. GB-assisted pseudoelasticity in Au nanowires will be examined in this proposal, including the orientation-dependence of bending-formed GBs in nanowires, and their annihilation and migration behaviors during the deformation recovery. This study would provide important guidelines for the application of metallic nanowires in designing reliable and sustainable nano electromechanical systems (NEMS) devices.

Dial-In Information

Join Zoom Meeting: 

Link: https://pitt.zoom.us/j/8995266392 

Passcode: 070722 

Meeting ID: 899 526 6392 

Thursday, July 7 at 10:00 a.m.

Benedum Hall, Room 624

Event Type

Defenses

Topic

Research

Target Audience

Graduate Students

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