Electron diffuse-scattering fluctuation and correlation analysis in complex face-centered-cubic alloys via four-dimensional scanning transmission electron microscopy

Kung, Po-Cheng

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https://hdl.handle.net/2142/129358 Copy

Description

Title

Electron diffuse-scattering fluctuation and correlation analysis in complex face-centered-cubic alloys via four-dimensional scanning transmission electron microscopy

Author(s)

Kung, Po-Cheng

Issue Date

2024-12-19

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

Zuo, Jian-Min

Doctoral Committee Chair(s)

Zuo, Jian-Min

Committee Member(s)

• Krogstad, Jessica A
• Bellon, Pascal
• Stubbins, James F
• Ertekin, Elif
• Shoemaker, Daniel P

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Chemical short-range ordering
• Electron diffuse scattering
• 4D-STEM
• Directional partial ordering
• Thermal diffuse scattering

Abstract

In recent years, there has been a surge of interest in chemically complex single-phase face-centered-cubic (FCC) alloys, such as high/medium entropy alloys (H/MEA) and austenitic steels. This interest stems from their exceptional combination of high strength and toughness over a broad temperature range. A key factor believed to significantly influence the mechanical properties of these alloys is the formation of chemical short-range ordering (CSRO). The nature of CSRO dictates its interaction with dislocations, potentially leading to either material strengthening or weakening. While this variability presents challenges in predicting mechanical properties, it also offers an unique opportunity to fine-tune the performance of these alloys through controlled manipulation of CSRO behavior. Consequently, gaining a comprehensive understanding of CSRO characteristics and their evolution under various processing conditions is essential for the effective design and optimization of complex FCC alloys. The investigation of CSRO behaviors in alloys frequently relies on the analysis of diffuse-scattering signals recorded by electron diffraction, which provide valuable insights into the structural characteristics of ordering, including direction and periodicity. Among the various electron diffraction techniques available, four-dimensional scanning transmission electron microscopy (4D-STEM) has emerged as a particularly powerful tool for studying CSRO in alloys. This technique’s unique combination of high spatial resolution and large field of view enables the examination of diffuse-scattering signals originating from individual nanoscale ordered domains, as well as facilitating statistical analysis of these signals across the entire mapping area. Consequently, 4D-STEM offers a comprehensive understanding of CSRO behaviors in alloys. However, 4D-STEM data inherently contains additional diffraction signals, including significantly stronger Bragg peaks and other forms of diffuse scattering, which can complicate the analysis of CSRO-specific diffuse scattering. To address this challenge, it is crucial to develop robust methodologies for differentiating CSRO diffuse-scattering signals from other sources within 4D-STEM data, thereby enabling the extraction of critical structural information pertaining to CSRO in alloys. This dissertation presents a novel analytical approach for examining CSRO diffuse-scattering signals in 4D-STEM data, which I have termed the 4D-STEM diffuse-scattering fluctuation and correlation analysis. This innovative method capitalizes on the wealth of diffraction information contained within 4D-STEM datasets, employing statistical tools to elucidate the characteristic behavior of observed electron diffuse-scattering signals. The fluctuation analysis component facilitates the identification of spatially fluctuating diffuse-scattering signals that are indicative of nanoscale heterogeneous domains. Complementing this, the correlation analysis enables the detection of strongly correlated diffuse-scattering signals, which collectively form the characteristic diffuse-scattering pattern associated with a heterogeneous feature in the alloy. By systematically acquiring and analyzing these characteristic diffuse-scattering patterns along multiple zone axes, it becomes possible to extract comprehensive structural information about the feature in question. This multi-faceted approach ultimately allows for a definitive determination of whether the observed feature represents CSRO or another type of heterogeneous structure within the alloy. The CSRO behaviors of a (CrCoNi)93Al4Ti2Nb MEA were investigated using 4D-STEM diffuse-scattering fluctuation and correlation analysis. In the solution-treated MEA, two distinct sets of diffuse-scattering signals were identified: the 1/3 {422} and 1/2 {311} signals, and the {110} peaks. Fluctuation analysis revealed that the 1/3 {422} and 1/2 {311} signals originate from both the matrix and heterogeneous nanoscale domains. Correlation analysis further demonstrated that the heterogeneous-domain 1/3 {422} and 1/2 {311} signals stem from {111} planar defects, while their homogeneous-matrix counterparts were attributed to thermal diffuse scattering (TDS). Notably, neither of these signals were linked to CSRO. In contrast, the {110} diffuse-scattering peaks were associated with heterogeneous ordered domains. The correlation analysis uncovered that the ordering structure exhibits L12 CSRO with directional partial ordering, characterized by individual domains ordering along one of the three {100} directions. The 4D-STEM diffuse-scattering fluctuation and correlation analysis was employed to investigate the thermal evolution of local ordering in the (CrCoNi)93Al4Ti2Nb MEA. Fluctuation analyses of alloys subjected to various annealing temperatures revealed that the degree of ordering in heterogeneous CSRO domains increases with annealing temperature. Correlation analysis demonstrated that during the initial stages of the ordering process, the ordering predominantly progresses along the original ordering direction of each directionally partially ordered domain, thus preserving the directional partial ordering. Upon reaching an annealing temperature of 700◦C, the ordering structure transforms into the ideal L12 structure, losing the directionality of partial ordering and reestablishing cubic symmetry. Concurrently, a distinct ordering process was observed within the matrix, characterized by the development of densely distributed nanometer/sub-nanometer scale ordered domains exhibiting L12 long-range/medium-range ordering. Notably, this matrix ordering process was found to be independent of and exhibited faster kinetics compared to the ordering process in the heterogeneous CSRO domains. This study also explores the influence of TDS on CSRO diffuse-scattering analysis. TDS signals, ubiquitous in typical electron diffraction patterns, pose a potential challenge by overlapping with CSRO diffuse-scattering signals, thereby complicating the analysis. The overlap issue is particularly evident in the 1/3 {422} and 1/2 {311} diffuse-scattering signals, which have been attributed to both CSRO and TDS in previous studies. Through the application of kinematic TDS theory in pure nickel, this study offers valuable insights into the nature of TDS signals, with findings that can be generalized to complex FCC alloys. Armed with this enhanced understanding of TDS signals, the study demonstrates a strategic approach to mitigate the impact of TDS on CSRO diffuse-scattering analysis. This approach involves the careful selection of zone axes where TDS and CSRO signals are spatially distinct, thereby minimizing signal interference and enhancing the accuracy of CSRO analysis.

Graduation Semester

2025-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129358

Copyright and License Information

Copyright 2025 Po-Cheng Kung

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