Advances in electron imaging and diffraction of material defects and deformation mechanisms

Hsiao, Haw-Wen

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https://hdl.handle.net/2142/110684 Copy

Description

Title

Advances in electron imaging and diffraction of material defects and deformation mechanisms

Author(s)

Hsiao, Haw-Wen

Issue Date

2021-04-21

Director of Research (if dissertation) or Advisor (if thesis)

Zuo, Jian-Min

Doctoral Committee Chair(s)

Zuo, Jian-Min

Committee Member(s)

• Bellon, Pascal
• Huang, Pinshane Y.
• Maass, Robert

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• In-situ TEM
• STEM
• nanocrystalline materials
• high entropy alloys
• mechanical properties

Abstract

The mechanical properties of metals are strongly determined by their microstructure and internal defects. Thus, a fundamental understanding of the structure-property relationship is the cornerstone of metals research to realize specific mechanical properties for specific structures. The prerequisite is the knowledge regarding the structure, or more specifically the designed microstructure, and the associated structural response during deformation. To achieve this goal, a robust structure characterization approach is developed in this work based on the combination of various ex-situ transmission electron microscopy (TEM) and in-situ TEM straining techniques. The combination facilitates data mining and interpretation of microscopy dataset, providing comprehensive details of the interplay between microstructure, defects, and mechanical properties. We apply our approach to the study of two types of materials with high structural complexity: high entropy alloys (HEAs) and nanocrystalline (NC) materials. HEAs belong to a new class of emerging alloys that have attracted tremendous interest in the materials research community for their unique structural and mechanical properties. Unlike conventional alloys with a primary base element(s) and minor additions of secondary elements, a HEA has multiple elements, where every element is a principal component in a concentrated solid solution. The resulted chemical disorder and atomic misfits lead to a distorted crystal lattice. However, the characterization of disorder is very challenging. Here, we unravel the structural and chemical natures of disorder in a Al0.1CrCoFeNi HEA by using advanced scanning TEM techniques. We quantitatively reveal the non-randomness of chemical disorder and lattice distortion in a concentrated solid solution. The chemical inhomogeneity varies with locations, leading to inhomogeneous and directional lattice distortion clusters. This suggests additional strengthening from such inhomogeneity along with solution hardening as a unique aspect of HEAs. NC materials are characterized by their reduced grain size and high-density grain boundaries. This provides high mechanical strength according to the Hall-Petch relationship up to a limit. The nanostructure also presents distinctive deformation behaviors. To understand the underlying deformation mechanisms, it is crucial to resolve the nanostructural evolution during deformation. However, the fine nanostructure also presents a major challenge for characterization. We have developed advanced in-situ techniques to monitor nanostructure and defects evolution during deformation. In addition, we perform scanning electron nanobeam diffraction (SEND) before testing to acquire local structural information and help analyze in-situ microscopy data. In the case of NC ZrN, the plastic flow is demonstrated to be carried out by intermittent granular activities. The activities are quantitatively followed via electron imaging and diffraction and correlated with the mechanical response of the material. Furthermore, we discovered embryonic shear bands formed by cooperative granular activities at the early stage of deformation, leading to the final shear failure under compressive stress. In the case of NC molybdenum, we characterize dislocation development and interactions with grain boundaries by electron imaging. Multiple mechanisms are operated to carry out plastic flow, including dislocation hardening, grain boundary deformation, intermittent inter- and intra-granular dislocation avalanches. Both edge and screw dislocation and actively involved. The above findings provide better understandings of these complex high-strength materials and demonstrate a robust and quantitative characterization approach.

Graduation Semester

2021-05

Type of Resource

Thesis

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Copyright 2021 Haw-Wen Hsiao

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