Thesis defense of Daniel Bernal Rodríguez
Thesis defense of Daniel Bernal Rodríguez
Thesis defense of Daniel Bernal Rodríguez
Title of the thesis: "Hierarchical microstructure design and cast processing route of a modified TNM gamma TiAl alloy.". Obtained the SOBRESALIENTE CUM LAUDE qualification.
- Title of the thesis: "Hierarchical microstructure design and cast processing route of a modified TNM gamma TiAl alloy."
- Court:
- President: Dra. Dª. María Teresa Pérez Prado
- Vocal: Dr. D. Rui Yang
- Vocal: Dra. Dª. Inmaculada López Galilea
- Vocal: Dr. D. Joseba Iñaki Madariaga Rodríguez
- Secretary: Dra. Dª. Nuria Herrero Dorca
Abstract
Gamma titanium aluminides following the B-solidifying pathway, are intermetallic alloys that have the potential to replace heavier materials in high temperature structural applications, such as aerospace and aeronautics. They meet the requirements for being used in the newly designed turbines that have to withstand higher demanding conditions, allowing improved efficiency, as well as pollution and noise reduction. Indeed, y-TiAl are an attractive option to replace Ni-base superalloys due to their lower density, good mechanical properties at elevated temperatures and good oxidation resistance.
Among them, the third generation of y-TiAl alloy TNM is a promising candidate for utilization in the low-pressure turbines (LPT) of aircraft. However, its manufacturing route is time-consuming, involving casting/HIPing plus forging and multiple HT to achieve the required properties. In this regard, alloying elements play a key role in this process, hence boron is often added to refine the grain size in order to improve mechanical properties. However, the formation mechanism of the different types of borides in cast TiAl alloys is not yet clearly understood.
In light of this gap, this study seeks to design a modified TNM alloy with superior properties and define a suitable processing window. This was accomplished by designing a simplified processing route and correlating the chemical composition and solidification cooling rate of cast TiAl alloys, with the type of boride precipitated and the resulting microstructure.
The study reveals that simpler processing namely casting/HIPing plus a single-step heat treatment achieve a microstructure appropriate for structural applications. Regarding microstructure developing, the work also shows that boron contents below a critical value and cooling rates during solidification above a key value promote the formation of detrimental ribbon borides. While boron contents above a critical value and cooling rates during solidification below a key value promote the formation of a refined microstructure through the precipitation of the beneficial boride. Finally, a novel integrated HIP-heat-treatment processing route is presented to achieve an optimized creep resistance modified TNM alloy.