| Author: | Sun, Yixuan |
| Title: | Development of high-performance eutectic high-entropy alloys via laser additive manufacturing |
| Advisors: | Wang, Chunjin (ISE) Chen, Zibin (ISE) Cheung, C. F. Benny (ISE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Department: | Department of Industrial and Systems Engineering |
| Pages: | 241 pages : color illustrations |
| Language: | English |
| Abstract: | AlCoCrFeNi₂.₁ eutectic high-entropy alloys (EHEAs) demonstrate exceptional room-temperature strength-ductility balance through their dual FCC(L1₂) and B2 phases, yet face severe performance degradation between 400-600 °C, including rapid strength loss and poor wear resistance, that blocks their adoption in critical structural applications such as aerospace components and high-temperature tooling. This intermediate-temperature weakness stems from interfacial instability and phase evolution that can be addressed through compositional design strategies. The research tackles this bottleneck by combining additive manufacturing (AM) processing advantages with systematic Ti micro-alloying to achieve reliable intermediate-temperature performance, while simultaneously investigating the complete manufacturability characteristics, including machinability and surface integrity. Chapter 1 provides a comprehensive overview of high-entropy alloy development, classification, and distinctive characteristics, with particular focus on EHEAs and their dual-phase microstructures. The Objectives and Significance of this thesis are outlined, clearly defining the goal of improving the strength-ductility balance for AlCoCrFeNi₂.₁ HEA in the 400-600 °C range while balancing service performance and manufacturability. The chapter concludes with the Organization of the Thesis, providing a roadmap for systematic investigation from fundamental understanding to practical implementation. Chapter 2 systematically reviews the current state of research in AM-EHEA, identifying critical knowledge gaps, including data scarcity in the intermediate-temperature (400-600 °C) range and poor mechanical and tribological behavior with underlying unclear mechanisms. The literature review reveals that AM offers unique advantages for micro-alloying strategies, enabling precise compositional control and novel microstructure not achievable through conventional processing routes. Chapter 3 details the experimental approach using Laser Energy Net-Shaping (LENS) processing with controlled laser energy density and various Ti content additions (2.5 at.% and 4.68 at.%). Process optimization yields near-full density builds (>99.8%) with defect-free microstructures, while comprehensive testing protocols for tensile, tribological, and microstructural characterization. Chapter 4 develops fundamental design rules by systematically varying processing parameters and varying Ti content (x = 0, 0.15, 0.30). The work establishes a baseline duplex FCC(L1₂)/B2 microstructure with 2-6 μm grains, achieving room-temperature strength of 1.3 GPa with 10-16% elongation. Valence electron concentration (VEC) mapping defines safe composition limits (7.2-7.6) and constrains Ti content to ≤4.68 at.% to prevent continuous brittle B2 formation. Chapter 5 demonstrates that EHEA-Ti0.15 micro-alloying (2.5 at.%) stabilizes tensile performance across the problematic 400-600 °C range. At 400 °C, ultimate tensile strength reaches 1.36 GPa with a yield strength of 0.89 GPa, while 600 °C testing maintains 1.03 GPa strength with up to 17.7% elongation. Mechanistic analysis reveals enhanced B2 chemical ordering with temperature-responsive nanoprecipitation, optimized microstructure morphology that reduces lattice mismatch, doubles the strain-hardening behavior, and delays interfacial failure. Chapter 6 explores tribological behavior using EHEA-Ti0.3 (4.68 at.%) compositions under dry sliding conditions from room temperature to 600 °C. Results show inverse temperature dependence with friction coefficients decreasing from 0.75 to 0.51 and wear rates dropping by an order of magnitude (to ~10⁻⁵ mm³/(N·m)) at 600 °C. Cross-sectional analysis attributes this improvement to stress-distributing high-angle grain boundary networks, protective Cr₂O₃/Al₂O₃ tribofilm formation, and in-situ HCP nanophases that maintain hardness. Chapter 7 connects microstructural features to machinability through systematic micro-milling studies comparing EHEA-Ti0.15 and EHEA-Ti0.3 compositions. While cutting forces increase 60-100% with Ti content due to higher flow stress, EHEA-Ti0.3 delivers superior surface integrity with only 49.3% roughness increase after 150 mm of milling compared to 906.1% degradation for EHEA-Ti0.15. Johnson-Cook modeling establishes safe operating windows for precision milling applications. Chapter 8 presents the overall conclusions and future perspective: we conclude that the research successfully addresses the identified gaps in intermediate-temperature (400-600 °C) performance through systematic Ti micro-alloying and microstructural evolution mechanism investigation, establishing quantitative design guidelines for AM-EHEA development. We also propose that future work should focus on extending the compositional design framework to EHEA systems and exploring multi-scale modeling approaches to refine and predict performance for wide-temperature-range applications. |
| Rights: | All rights reserved |
| Access: | open access |
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