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In Situ Tem Study on the Nanomechanical Behaviors of Metallic Nanowires

Wang, Jiangwei (2015) In Situ Tem Study on the Nanomechanical Behaviors of Metallic Nanowires. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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In this dissertation, the relationships between structure-mechanical properties-deformation mechanisms in face-centered cubic (FCC) and body-centered cubic (BCC) metallic nanowires have been studied using in situ TEM nanomechanical testing.

It is well known that smaller is stronger and the widely-observed size effect is believed to arise from dislocation interactions inside nanocrystals. However, the interaction mechanisms in small-volume nanocrystals remain unexplored. Here, it was found that surface-nucleated dislocations can strongly interact inside the confined volume of Au nanowires, which led to a new type of dislocation-originated stacking fault tetrahedra (SFT), at variance to the widely-believed vacancy-originated SFT. The atomic-scale processes of nucleation, migration and annihilation of dislocation-originated SFT were revealed, shedding new light onto the strain hardening and size effect in small-volume materials.

Although nanoscale twinning is an effective mean to enhance the strength of metals, twin-size effect on the deformation and failure of nanotwinned metals remains largely unexplored, especially at the minimum twin size. Here, a new type of size effect in nanotwinned Au nanowires is presented. As twin size reaches the angstrom-scale, Au nanowires exhibit a remarkable ductile-to-brittle transition that is governed by the heterogeneous-to-homogeneous dislocation nucleation transition. Quantitative measurements show that approaching such a nanotwin size limit gives rise to the ultra-high strength in Au nanowires, close to the ideal strength limit of perfect Au.

Twinning is a fundamental deformation mode that competes against dislocation slip in crystals. At the nanoscale, higher stresses are required for plastic deformation than that in bulk, which favors twinning over dislocation slip. Indeed, deformation twinning has been well-documented in FCC nanocrystals. However, it remains unexplored in BCC nanostructures. Here, it shows that twinning is the dominant deformation mode in BCC-tungsten nanocrystals. A competition between twinning and dislocation slip occurs when changing the loading orientations, attributed to the defect growth controlled plasticity in BCC nanocrystals. Several important commonalities and differences in FCC and BCC nanocrystals deforming at nanoscale are further proposed.

The results from this dissertation advance fundamental understanding of plastic deformation in a broad class of metals and alloys, and are of technological importance for degradation control and future design of ultra-strength nanomaterials.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Wang, Jiangweijiw62@pitt.eduJIW62
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairMao, Scott X.sxm2@pitt.eduSXM2
Committee MemberGleeson, Brianbmg50@pitt.eduBMG50
Committee MemberWiezorek, Jörg M.K.wiezorek@pitt.eduWIEZOREK
Committee MemberSlaughter, William S.wss@pitt.eduWSS
Committee MemberWang, Guofengguw8@pitt.eduGUW8
Committee MemberLi, Guangyonggul6@pitt.eduGUL6
Date: 28 January 2015
Date Type: Publication
Defense Date: 23 September 2014
Approval Date: 28 January 2015
Submission Date: 11 November 2014
Access Restriction: 2 year -- Restrict access to University of Pittsburgh for a period of 2 years.
Number of Pages: 163
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Mechanical Engineering
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: Nanomechanics; Deformation; Mechanical Properties; Nanowires; In situ TEM; Structure-properties relationship.
Date Deposited: 28 Jan 2015 21:42
Last Modified: 28 Jan 2017 06:15


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