Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


Dissertation Jingyi Zhao 1012.pdf (3.97 MB)

ETD Abstract Container

Abstract Header

Relating Grain Boundaries to the Mechanical Properties of Polycrystalline Material: Gradient Nanocrystalline Material and Electro-Plasticity

Zhao, Jingyi, Zhao
http://rave.ohiolink.edu/etdc/view?acc_num=akron153296020243128

Abstract Details

2018, Doctor of Philosophy, University of Akron, Mechanical Engineering.
Grain boundary is one of the most important interfaces that hinder dislocation motion in polycrystalline materials. Knowledge of how grain boundary affects plastic deformation is crucial for both the material design and manufacturing. Grain boundary has the multi-dimensional features, i.e. the grain boundary network topography (~microns) and grain boundary interfacial properties (~nm). Those features can dramatically influence the mechanical properties of material. In this work, the gradient nanocrystalline material and electro-plasticity are investigated, and the effects of grain boundary characteristics on these plasticity phenomena were studied. In the first part of the dissertation, the gradient nanocrystalline (GNC) coppers were produced by Ultrasonic Nanocrystallization Surface Modification (UNSM), and their mechanical properties were measured using tensile tests. It shows that the GNC copper possesses better yield strength and decent ductility compared with the homogeneous polycrystalline copper. A theoretic model, which considers the dislocation interactions in the heterogeneous grain boundary network, was proposed to analyze and explain the superior mechanical properties of GNC material. Our model shows that the yield strength of GNC material can be estimated using the root mean square (RMS), instead of rule of mixture (ROM), of the yield strengths of coarse-grained (CG) layer and gradient-structure (GS) layer. The higher yield strength in GNC material is a result of the dislocation interaction between CG layer and GS layer. On the other hand, the dislocation interaction also induces the dislocation migration from CG layer to GS layer, leading to a transition from dislocation-starvation to dislocation-strengthening. As a result, the dislocation accumulation capacity can be recovered and provide extra strengthening hardening to accommodate large deformation. Our work provides new physical understandings to the unique strengthening mechanisms of GNC materials. In the second part of the dissertation, the electro-plasticity in electric-assisted formation was investigated and the role of grain boundary was examined. During electric-assisted formation (EAF), grain boundary interacts with both electric flows and dislocations. The interaction between grain boundary and dislocation, under the effect of electric current, is important to the physical understanding of electro-plasticity. To simplify this question, we have decomposed the work into two steps. Firstly, a multi-scale numerical model was proposed to simulate the current density distribution in a nanocrystalline metal subjected to electropulsing. The electric current density obtained from the macro scale model was used as the boundary condition for the nano/micro scale model. The electric current density distribution, considering the material heterogeneity of grain boundaries, was predicted by solving the Maxwell’s electromagnetic equations. The simulation results suggested that heterogeneous current density rises as a result of the heterogeneity in the electromagnetic properties at grain boundaries. Due to this current density heterogeneity, grain rotation and selective heating may occur under the effect of electric current. In the second step, enlightened by our simulation work, the electric assisted tensile tests, which have the similar root mean square current densities and, thus, the same temperature rises, were investigated by conducting electric-assisted tensile test. The short duration (100 µs) and high frequency (120 to 800 Hz) pulsed current were used. It shows that, there exists a threshold value of the peak current density, where the electro-pulsing with higher peak current density can reduce more flow stress than direct current. The electric-induced grain rotation, which was suggested in our simulation works, was proved by our experiments as well. The electric-induced grain rotation can reduce the dislocation accumulation around grain boundary and reduce the flow stress eventually. The threshold peak current density is closely related to the electric-induced grain rotation. The results can shed some lights on process design of EAF for an optimized efficiency in reducing the flow stress. This dissertation gives two specific examples of how grain boundary characteristics affect the polycrystalline plasticity. New physical understanding of the grain boundary effects on plastic deformation can be attained, providing fundamental principles for material design and advanced manufacturing.
Chang Ye (Committee Chair)
Yalin Dong (Advisor)
Xiaosheng Gao (Committee Member)
Guoxiang Wang (Committee Member)
Shiller Paul (Committee Member)
En Chen (Committee Member)
110 p.
Materials Science; Mechanical Engineering
Grain Boundary; Dislocation; Gradient Nanocrystalline Material; Electro-Plasticity

Recommended Citations

Citations

  • Zhao, Zhao, J. (2018). Relating Grain Boundaries to the Mechanical Properties of Polycrystalline Material: Gradient Nanocrystalline Material and Electro-Plasticity [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron153296020243128

    APA Style (7th edition)

  • Zhao, Zhao, Jingyi. Relating Grain Boundaries to the Mechanical Properties of Polycrystalline Material: Gradient Nanocrystalline Material and Electro-Plasticity . 2018. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron153296020243128.

    MLA Style (8th edition)

  • Zhao, Zhao, Jingyi. "Relating Grain Boundaries to the Mechanical Properties of Polycrystalline Material: Gradient Nanocrystalline Material and Electro-Plasticity ." Doctoral dissertation, University of Akron, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=akron153296020243128

    Chicago Manual of Style (17th edition)

akron153296020243128
368
© 2018, all rights reserved.
This open access ETD is published by University of Akron and OhioLINK.