Underlying deformation behavior of medium/high entropy alloys using advanced electron imaging and diffraction

Yin, Kaijun

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https://hdl.handle.net/2142/132753 Copy

Description

Title

Underlying deformation behavior of medium/high entropy alloys using advanced electron imaging and diffraction

Author(s)

Yin, Kaijun

Issue Date

2025-11-11

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

Zuo, Jian-Min

Doctoral Committee Chair(s)

Bellon, Pascal

Committee Member(s)

• Cao, Qing
• Stinville, Jean-Charles

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)

• Medium/High entropy alloys
• Deformation behavior
• Electron imaging and diffraction

Abstract

Medium and high entropy alloys (M/HEAs), formed in a large region of compositional space that are little explored, have demonstrated unusual combinations of strength, ductility, and damage tolerance. Their complex chemistries create rich defect energy landscapes and multiple pathways for mechanical deformation, yet the link between local structure and deformation mechanisms remains incompletely resolved in M/HEAs. This thesis investigates the local structure and deformation mechanisms in selected M/HEAs using advanced transmission electron microscopy (TEM) methods, including scanning electron nanodiffraction (SEND), also known as 4D scanning TEM or 4D-STEM. We use 4D-STEM to acquire position-resolved nanodiffraction patterns at the nanometer spatial resolution. The diffraction dataset are data-mined to extract local strain, orientation, and phase information. Taking advantage of recent advances in fast detectors, we develop a cepstral analysis workflow for cepstral STEM to improve planar faults identification. The 4D-STEM analysis is combined with conventional diffraction contrast TEM analysis of dislocations, and atomic resolution STEM imaging of local defects, including planar faults. Three representative M/HEA systems are studied in this thesis. The first system is the MEA CrCoNi with the face-centered cubic (FCC) crystal structure. Under tensile loading, CrCoNi develops extensive planar faults that includes stacking faults, deformation twins, and stress-induced local HCP transformation. For this system, We compare the samples of as-cast water-quenched (WQ) CrCoNi, which exhibits predominantly L11 chemical short-range order (CSRO), with the heat-treated (HT) CrCoNi, which shows predominantly L12 CSRO from the prior study. Our results show that L12 CSRO in the HT sample restricts dislocation cross-slips and suppresses the HCP phase formation, thus stabilizing the FCC matrix. Experimental and simulated stacking-fault widths distribution show that heat treatment raises the mean stacking-fault energy (SFE) from slightly negative to positive and reduces the fraction of very low-SFE regions. Atomistic simulations which include the effects of local CSRO also reproduce this SFE increase and provide additional insight about the fault-energy landscape. Using the measured SFE distributions as input, kinematic Monte Carlo (kMC) simulations then capture the ensuing microstructural evolution. Thus, the SFE adjustments from short-range order rebalance the planar slip, faulting, and twinning, providing a route to tailor the mechanical response. These findings identify statistical fault energetics and CSRO tuning as complementary controls for understanding and engineering deformation in HEAs. The second system comprises several refractory HEAs (RHEAs) with a primary body-centered cubic (BCC) crystal structure. Electron diffraction reveals that in the as-cast HfNb0.3TiZr and HfTa0.5TiZr alloys, nanoscale ω phases with additional orthorhombic O’ phase are observed in the BCC matrix; whereas in HfNbTa0.5TiZr and HfNbTaTiZr, single-phase BCC is observed. All four samples exhibit ductility at room temperature. In-situ neutron and TEM analysis show that under compression, the BCC phase locally transforms to HCP phase in the HfNb0.3TiZr and HfTa0.5TiZr alloys with the ω/O’ phase; while HfNbTa0.5TiZr and HfNbTaTiZr deforms through dislocations motions. Consistently, the HfNb0.3TiZr and HfTa0.5TiZr alloys exhibit a mid-strain uptick in work-hardening rate, indicating transformation-assisted strengthening, while the HfNbTa0.5TiZr and HfNbTaTiZr alloys display a typical BCC response with strong initial hardening followed by recovery-dominated, monotonically decaying hardening. These results suggest that ultrafine and transformable ω phases can trigger transformation-induced plasticity (TRIP) and add strengthening to HfNb0.3TiZr and HfTa0.5TiZr alloys. Meanwhile, the single-phase BCC HfNbTa0.5TiZr and HfNbTaTiZr alloys can also achieve great ductility when mobile screw dislocations disperse strain by kink-pair glide while edge dislocations interact strongly with forest and solute fields to sustain work hardening, thereby supporting uniform elongation. The third HEA system studied here is the eutectic alloy AlCoCrFeNi2.1. This sample solidifies into alternating lamellae of hard BCC with uniformly dispersed B2 precipitates, and ductile FCC phases. Mechanical testing on this material demonstrated that the nano-lamellar architecture is associated with a much improved low-cycle fatigue performance. In-situ neutron diffraction shows that during cyclic loading, the FCC phase begins to yield before the BCC phase, and the applied load is clearly partitioned between the two phases. TEM characterization of deformed samples at high strain amplitudes shows lamellar breakdown into irregular segments, dislocation cell formation, and B2 clustering at the interface of FCC/BCC lamellae. Together, these results clarify the strengthening mechanisms behind the exceptional fatigue performance of additive manufacturing (AM) EHEAs and support their use in structural applications. In summary, across multiple HEAs systems, we correlated the microstructures and their deformation mechanisms by integrating (S)TEM and advanced 4D-STEM characterization with materials processing, mechanical testing, in-situ neutron diffraction and theory support. This study highlights the complex mechanisms behind structure-induced deformation in M/HEAs and facilitates the manufacturing of high quality HEAs with enhanced microstructure control.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132753

Copyright and License Information

Copyright 2025 Kaijun Yin

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