Titanium-based intermetallics as next-generation aerospace materials

Abstract

The central aim of the work presented in this thesis is to investigate the effect of Nb substitution by IVB, VB, and VIB elements of the periodic table, Al substitution by IIIA elements of the periodic table, and quaternary alloying of Ti2AlNb intermetallics, to enhance its phase stability, mechanical and thermal properties using ab-initio methods based on Density Functional Theory (DFT). In the last decade, there have been consistent efforts to study the effect of substitution of Nb in Ti2AlNb intermetallics on its phase stability, mechanical and thermal properties. As Ti-based alloys are potential next-generation aerospace materials owing to their high specific strength, low density, high creep, and oxidation resistance it is crucial to enhance their mechanical and thermal properties. The advancement in computational methodologies has now made it possible to predict various properties of materials without actually fabricating them, thus saving both cost and time of research. The 0 K studies of the Ti2AlNb intermetallics reveal that the O phase is most stable followed by the 2 and B2 phases, respectively. The densities of the Ti2AlNb intermetallics are approximately half of the Ni-based intermetallics. Further, the elastic moduli of the Ti2AlNb intermetallic are considerably low as compared to the nickel-aluminides. Substitution of Nb with IVB, VB and VIB elements in Ti2AlNb intermetallics show that the Ti2AlMo system possesses the best combination of phase stability, mechanical and thermal properties among all Ti2AlX (X= Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) systems. The substation of Al by IIIA elements in Ti2AlNb and Ti2AlMo reveal that the Boron enhances the phase stability, mechanical and thermal properties in Ti2YX (Y = B, Al, Ga, In, Tl; X= Nb, Mo) systems. Further, the Ti2YMo have better stability, mechanical and thermal properties as compared to the Ti2YNb systems. The quaternary alloying of Ti2AlNb elements show that IVB elements prefer to substitute Ti atoms, while VB and VI B elements prefer to substitute the Nb atoms in both B2 and O phases. However, V shows different preferences in the B2 and O phases. Further, all the quaternary improves the mechanical and thermal properties of the Ti2AlNb intermetallics. Nevertheless, the quaternary elements that increase the hardness in the B2 phase decrease the same in the O phase and vice versa. The present calculations are crucial to provide a track in developing the Ti-based intermetallics as a potential substitute for Ni-based alloys in the next generation aerospace applications making modern flights safer and more economical.