Thermodynamic modelling of high strength, high toughness Ti alloys
Wang, Hang (2012)
Ph.D. thesis, University of Birmingham.
Titanium alloys with good combinations of toughness and strength are being designed for aerospace applications: e.g. aeroengine compressors and aircraft undercarriages. It is not an easy process to optimise the balance between toughness and strength in this system, and in practice some aspects of the physical metallurgy of titanium alloys are not very well understood, such as the B2-type ordering in titanium alloys. The aim of this work mainly concerns the development of thermodynamic models, based upon CALPHAD techniques (CALculation of PHAse Diagrams).
First-principles calculations were performed in order to provide the thermodynamic modelling with information concerning sublattice occupation. The energies of formation of different point defects in the ordered B2 phase of the Ti-Al binary system have been predicted. The dominant point defects in the sublattice of the B2-TiAl structure were determined to be either substitutional vacancies or anti-site defects, depending on the major element in the alloy. The concentrations of total and thermal point defects were estimated in the present work. The results of first-principles calculations have been adopted in the CALPHAD thermodynamic modelling.
Thermodynamic assessment of the Ti-Al-Cr-V quaternary system was carried out, neglecting substitutional vacancies. A two sublattice model for B2 phase (Al,Cr,Ti,V)0.5:(Al,Cr,Ti,V)0.5 was used and a set of self-consistent thermodynamic parameters is presented. The predicted phase equilibria and order/disorder transformation temperature are shown to be in good agreement with experimental information, both in the Ti-Al-Cr-V quaternary system and in the important binary and ternary subsystems. The modelling results were used to predict the B2-stable region, which is important for the design of titanium alloys. Next, substitutional vacancies were taken into account in a thermodynamic model of the B2 phase (Al,Ti,V,Va)0.5:(Al,Ti,V,Va)0.5 in the Ti-Al-V ternary system, taking into account the theoretical results of first-principles calculations. The temperature of the β transus in Ti-6Al-4V alloy has been predicted to aid simulation of microstructure evolution. Experiments were carried out in order to calibrate the thermodynamic models, and to investigate the effect of B2-ordering on the kinetics of α precipitation from the β matrix.
By using the obtained thermodynamic parameters, two databases of atomic mobilities were developed – one with substitutional vacancies and the other without. The calculated results from both databases fit the experimental data: concentration profiles, diffusivities and diffusion paths. Finally, microstructural kinetics associated with the β/α phase transformation in the Ti-6Al-4V alloy has been simulated using the phase-field method coupled with the CALPHAD approach. The needle-shape α structure was simulated and the mechanism of formation of Widmanstätten α structure was studied.
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