| Author: | ZHANG, Yongyun |
| Title: | Microstructure and properties of refractory high entropy alloys fabricated by laser additive manufacturing |
| Advisors: | Chan, K. C. (ISE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Industrial and Systems Engineering |
| Pages: | xvi, 215 pages : color illustrations |
| Language: | English |
| Abstract: | Refractory high-entropy alloys (RHEAs), made with refractory elements, offer excellent strength, ductility, wear resistance, and high-temperature stability, making them potential for industrial applications. However, traditional fabrication methods, however, struggle with the direct fabrication of RHEAs for industrial applications. Laser additive manufacturing (LAM) offers a modern solution that efficiently melts refractory metals, refines grains, and adjusts compositions. Despite this, LAM-fabricated RHEAs often have defects, resulting in poor mechanical properties across temperature ranges. Therefore, improving the properties of RHEAs across a large-temperature range via LAM remains a critical challenge. Understanding the mechanical properties, especially tensile properties, of LAM-fabricated RHEAs is crucial for their potential engineering applications. In our work, we fabricated a Ti42Hf21Nb21V16 (T42) RHEA through the Laser Engineered Net Shaping (LENS) method, which exhibits a high yield strength (over one gigapascal) and a significant high tensile strain (around 22.5%), outperforming the as-cast counterpart. The unique nanostructure and doped interstitial atoms produced after the LENS procedure provide an alternative solution to the long-standing issue of the strength-ductility trade-off in RHEAs. The tensile properties of the LENS-fabricated T42 alloys under elevated temperatures were further investigated, showing good tensile strength stability over a broad temperature spectrum. The T42 alloy has large local lattice strain, and elastic moduli stability across different temperatures, demonstrating good tensile strength across a broad temperature range. This approach of inducing lattice distortions and maintaining stable elastic constants offers a new way to produce RHEAs capable of high-temperature performance. The LENS-fabricated T42 RHEA has also demonstrated superior wear resistance at elevated temperatures. The oxidation process of the alloy was examined. It was found that protective oxide nanolayers were formed in the early stage, which then developed into a polycrystalline oxide coating under stress. This significantly reduced wear rates from 2.69 × 10-4 mm3/(N·m) at room temperature to just 6.90 × 10-7 mm3/(N·m) at 600°C. These findings show that using LAM to fabricate RHEAs with enhanced high-temperature wear resistance is promising for creating durable coatings at elevated temperatures. However, the T42 alloy experiences significant oxide formation when the temperature is increased to 800 °C, reducing its mechanical strength and wear resistance. To address this, an in-situ alloying technique with Al was applied to the T42 alloy through LENS, resulting in an Al-doped RHEA with enhanced strength and ductility at high temperatures compared to the T42 counterpart. Al induces even larger lattice distortion, and bolsters strength, but above 600°C, the yield strength decreases due to a softening effect from a reduced elastic modulus. The addition of Al improves the plasticity of the alloy by forming a dense oxide layer that blocks oxygen diffusion. This approach of in-situ alloying via LENS offers a solution for developing RHEAs with superior high-temperature capabilities, whilst maintaining robust mechanical properties under elevated temperatures. This research presents significant advancements in the study of RHEAs fabricated through the LENS approach. Our work successfully fabricates RHEAs with superior performance across a range of temperatures. We have overcome the tensile strength-ductility trade-off in our T42 alloy at room temperature. Furthermore, we provided insights into its high-temperature behaviors relating to the lattice distortions or the oxidation sequence. By employing in-situ alloying, we have further improved the mechanical properties of the fabricated alloy. These findings not only deepen our understanding of RHEAs under diverse conditions but also pave the way for the development of advanced materials, unlocking new possibilities for their industrial applications. |
| Rights: | All rights reserved |
| Access: | open access |
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