Effects of Cryogenic treatment on the Microstructure and Mechanical Behavior of pre-strained CoCrFeMnNi alloy

다국어 초록 (Multilingual Abstract)

High-entropy alloys (HEAs) represent a paradigm shift in alloy design, with the equiatomic CoCrFeMnNi (Cantor) alloy emerging as a benchmark material exhibiting exceptional mechanical properties across wide temperature ranges. This study systematically investigates the microstructural evolution and mechanical behavior of the Cantor alloy under various processing conditions, with emphasis on achieving enhanced strength-ductility synergy. The research encompasses two primary investigations: (1) the impact of cyclic deep cryogenic treatment (DCT) on pre-strained samples, revealing progressive hardness enhancement through dislocation network refinement and nano-twin formation; (2) thermo-mechanical processing combining pre-compression, annealing, and cryo-quenching to achieve optimized mechanical properties. Microstructural analysis via electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction (XRD), and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) reveals that the single-phase FCC structure remains stable under all processing conditions. Post-processing techniques significantly enhance yield strength from ~200 MPa in the as-cast state to values exceeding 700 MPa, while maintaining substantial ductility through effective stress distribution and defect interaction. The dominant deformation mechanisms, dislocation glide at room temperature and nano-twinning at cryogenic temperatures, are promoted by the alloy's relatively low stacking fault energy (20– 35 mJ/m2). Cyclic DCT induces dislocation density increases up to 5.9 × 1014 m⁻2, while thermo-mechanical processing produces refined grain structures (67.9 μm) enriched with annealing twins that further restrict dislocation motion. These findings establish that sophisticated thermo-mechanical processing strategies effectively engineer the Cantor alloy's microstructure to achieve superior strength-ductility balance.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1. Emergence and importance of HEAs 1
• 1.2. Historical background of Cantor alloy2
• 1.3. Microstructure, Properties and Strengthening Mechanism of Cantor alloy 4
• 1.4. Mechanical Limitations and Strengthening Strategies of the Cantor Alloy 6
• 1.5. Grain Refinement Strategies in As-Cast High Entropy Alloys 7
• 1.6. Post-Processing and Thermo-Mechanical Treatments for Enhancing Cantor Alloy
• Performance 8
• Chapter 2. Literature review 11
• 2.1. Development of mechanical properties of the Cantor alloy system 11
• 2.2. Mechanical properties of cantor 14
• 2.2.1. Room temperature 14
• 2.2.2. Cryogenic Temperature 18
• 2.3. Influences of Alloying Elements and Phase Structure on the Mechanical Properties of
• the Cantor Alloy 22
• 2.4. Deformation mechanism in Cantor alloy 26
• 2.4.1. Dislocation Systems 26
• 2.4.2. Twinning systems 31
• 2.4.3. Stacking Faults 36
• Chapter 3. Impact of cyclic cryogenic treatment on the hardening behavior of pre-strained
• CoCrFeMnNi alloy 38
• 3.1. Introduction 38
• 3.2. Materials & Methodology 40
• 3.3. Results & Discussion 43
• 3.4. Conclusion 52
• Chapter 4. Impact of thermo-mechanical processing on the microstructure and mechanical
• properties of Cantor alloy 53
• 4.1. Introduction 53
• 4.2. Materials and Methodology 55
• 4.3. Results & Discussion 56
• Chapter 5. Conclusions and Future Works 66
• References 68
• viii