Atomic states, potential energies, volumes, stability and brittleness of ordered FCC TiAl2 type alloys

In this paper, the framework of the systematic science of alloys (SSA) is presented. It has been proved that according to the basic information of sequences of characteristic crystals in the FCC Ti-Al system, not only the states, potential energies and volumes of atoms at various lattice points, and average atomic states, average atomic potential energies, average atomic volumes and lattice constants of the cells, as well as cohesive energies, heats of formation, bulk moduli and Debye temperatures for h- and γ-TiAl2 compounds can be calculated, but also the compositional variations of the atomic states, atomic potential energies, atomic volumes, lattice constants, cohesive energies, heat of formation, bulk moduli and Debye temperatures of the ordered FCC TiAl2 type alloys and their Ti- and Al-components can be calculated by the SSA framework. The average atomic states of h- and γ-TiAl2 compounds are, respectively, ψ(h-TiAl2)=1/6ψ10,1Ti+1/6ψ10,2Ti+1/2ψ8,4Al+1/6ψ6,6Al and ψ(γ-TiAl2)=1/3ψ10,1Ti+1/3ψ8,6Al+1/3ψ6,6Al.At low temperature, the h-TiAl2 compound with lower potential energy and smaller volume is more stable than the γ-TiAl2 compound, because the ψ10,2Ti and the ψ8,4Al atoms have lower potential energies and smaller volumes than the corresponding ψ10,1Ti and ψ8,6Al atoms. But at high temperature, the γ-TiAl2 compound with larger vibrational entropy and larger mixing entropy due to containing ψ8,6Al- and more the same state ψ10,1Ti-atoms with higher potential energy may be more stable than h-TiAl2 compound. The calculated lattice constants of γ-TiAl2 compounds are in good agreement with experimental ones. The relationships of brittleness with atomic states and potential energy wave planes for FCC TiAl2 compounds have been analysed.