Atomic states, potential energies, volumes, stability, and brittleness of ordered FCC TiAl 3-type alloys

Abstract

It has been proved that according to the basic information of sequences of characteristic atoms in the FCC Ti–Al lattice system, not only the states, potential energies, volumes of atoms at the 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 and bulk moduli for D0 22–, L1 2– and C–TiAl 3 compounds can be calculated, but also the compositional variations of the atomic states, atomic potential energies, atomic volumes, lattice constants, cohesive energies, and heats of formation of the ordered FCC TiAl 3 type and disordered FCC Ti x Al (1− x ) alloys and their Ti- and Al-components can be calculated in the framework of the systematic science of alloys. The average atomic state of the FCC TiAl 3 compound consisting of ψ 12 Ti and ψ 8 Al characteristic atoms is 0.25[Ar](3d n) 0.006 (3d c) 2.974 (4s c) 0.5085(4s f) 0.5115+0.75[Ne](3s c) 1.847(3p c) 0.568(3s f) 0.585. The weak-bonding ψ 8 Al atoms play a determining role in forming the FCC structure. The strong-bonding ψ 12 Ti atoms play a determining role in causing brittleness. The D0 22–TiAl 3 compound is slightly more stable than the L1 2– and the C–TiAl 3 compounds. The calculated lattice constants and heat of formation ( a = 0.3856 nm , c = 0.8622 nm , Δ H = - 37.07 kJ / mol ) of the D0 22–TiAl 3 compound are in excellent agreement with experimental values. The relationships of brittleness with atomic states, potential energy wave planes, bond networks, ratio x Ti/ x Al, ordering degree and grain boundary for ordered FCC TiAl 3-type alloys have been analysed.