Lightweight and high-temperature performance are important indicators to promote the development of the aviation industry. Al-Li alloys with low density, high specific strength, excellent low temperature performance and good corrosion resistance are an ideal aerospace structural material. Ni-based and Co-based superalloys with superior high-temperature properties are key materials for aero-engine turbine blades. However, with the continuous development of the aviation industry, higher requirements have been placed on the performance of these materials. Therefore, designing materials with better performance is the focus of current research in the aviation field.

Understanding how the presence and stability of phases in a given material influences its mechanical properties is critical to designing novel materials. Systematic analysis of stable and metastable phases in alloy systems by experiments is expensive and time-consuming because slow kinetics may hinder the convergence towards thermodynamic equilibrium. Thus, in this thesis, the stability of the different phases in Al-Li, Ni-Al and Co-Al alloys were studied by first-principles calculations in combination with statistical mechanics principles. The obtained thermodynamic information can provide theoretical support for the design of novel multicomposition alloy.

Firstly, the whole solid-state region of the Al-Li phase diagram was predicted by first-principles calculations and statistical mechanics, including the effect of configurational disorder and lattice vibrations. The formation enthalpy of different configurations at different temperatures was accurately predicted by means of cluster expansions that were fitted from first-principles calculations. The vibrational contribution was determined from the bond length vs. bond stiffness relationships for each type of chemical bond within and the Gibbs free energy of each phase as a function of temperature was obtained from Monte Carlo simulations. The predicted phase diagram was in excellent agreement with the currently accepted experimental one in terms of the stable (AlLi, Al2Li3, AlLi2, Al4Li9) and metastable (Al3Li) phases. In addition, it provided accurate information about the gap between Al3Li and AlLi solvus lines. The Ni-Al phase diagram was also predicted using the same methodology. The contribution of magnetic enthalpy was also included in the Gibbs free energy of each phase in this case. The computed phase diagram was in excellent agreement with the experimentally accepted phase diagram and provided information about the phase boundary between AlNi3 and Ni below 300K, which was unknown before.

Finally, the phase diagram of the Co-Al system was predicted. The magnetic entropic contribution was also included through the Heisenberg Hamiltonian. The computed phase diagram was compared with the currently accepted experimental phase diagram and the different contributions to the stability of each phase were analyzed independently. The potential
of this strategy to predict phase diagrams of magnetic systems was clearly established. Overall, the methodology developed in this thesis shows that accurate phase diagrams of alloys of technological interest can be predicted from first-principles calculations and statistical mechanics principles.