The student Juan José Trujillo Tadeo obtained an EXCELLENT CUM LAUDE grade with International Doctorate Mention

The student Juan José Trujillo Tadeo obtained an EXCELLENT CUM LAUDE grade with International Doctorate Mention

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• Thesis title: Design and characterization of lightweight non-equiatomic Al-Zn-Mg-Si Medium Entropy Alloys

Court:

• Presidency: María Teresa Guraya Díez (UPV/EHU)
• Vocal: Zigor Azpilgain Balerdi (Mondragon Unibertsitatea)
• Vocal: José Victoria Hernández (Helmholtz-Zentrum Hereon)
• Vocal: Iban Vicario Gómez (Tecnalia Research and Innovation)
• Secretary: Pablo García Michelena (Mondragon Unibertsitatea)

Abstract:

The development of lightweight materials for the transportation and aerospace industry is a crucial challenge today. Reducing weight without compromising structural properties and without significantly increasing costs is essential to mitigate fuel consumption and greenhouse gas emissions. While alloys of aluminum, magnesium, titanium, and beryllium, with densities ranging from 1.74 to 4.43 g/cm3, are commonly used, their limited properties and costs have driven the search for new alloys.

High entropy alloys (HEAs), complex composition alloys (CCAs), and multiple element alloys (MPEAs) have gained attention due to their unique combination of properties. By exploring the central region of phase diagrams with multiple components, these alloys offer high strength, ductility, superconductivity, catalytic activity, corrosion resistance, and radiation tolerance.

A novel subcategory, lightweight high-entropy alloys, incorporates light metallic elements to reduce weight while maintaining exceptional mechanical properties. Leveraging the high-entropy effect, these alloys promote the formation of solid solution phases instead of complex intermetallic phases. This innovative approach opens up new possibilities for developing advanced materials that outperform some established ones, showing great potential.

However, these new lightweight high-entropy alloys exhibit high compression strength and hardness but often have little to no ductility. Therefore, this thesis focuses on designing lightweight alloys applying the high-entropy concept to select elements that can form an alloy with a high FCC solid solution, a crystal structure known for its high ductility, to address this issue.

Furthermore, a thorough understanding of the physical and mechanical properties of these alloys is sought to evaluate their potential in future applications. To achieve this, chemical compositions have been defined to ensure good compatibility, high solubility, and melting at intermediate temperatures to form a high FCC solid solution. In-depth research into phase transformations in the designed alloy provides profound insights.

From this perspective, compression properties were evaluated, and two techniques were applied to optimize and enhance mechanical properties, with a particular focus on achieving ductility. For the first technique, chemical composition was adjusted, and modifying elements like Sr and Sb were used, followed by heat treatments for phase modification, aiming to improve mechanical properties with special attention to ductility.

For the second technique, two manufacturing methods, Near Solidus Forming and Directional Solidification, followed by heat treatments, were explored to change the microstructure and improve strength properties and achieve ductility through another approach. The results obtained from these two approaches, both chemical and process-related, have shown good compression strength properties. However, ductility has not been achieved through either of these methods. It is confirmed that the designed alloy is not suitable for applications requiring ductility. Nevertheless, it might find other applications where high hardness, compression strength, or wear resistance are needed.