Influences of xTi/ xAl on atomic states, lattice constants and potential-energy planes of ordered FCC TiAl-type alloys

Abstract

Based on the framework of systematic science of alloys, the basic information about states, volumes and potential energies of characteristic atoms in FCC Ti–Al system has been determined. For ordered FCC TiAl type alloys, the functions of the atomic states, atomic potential energies, cohesive energies, atomic volumes, the lattice constants and axis ratio c/ a′ varying with concentrations have also been established. The extreme points of curves v∼ x Al, a∼ x Al and c/ a′∼ x Al are all at the x Al=70%. When x Ti/ x Al=1, ψ (FCC TiAl) =ψ 8 Ti +ψ 4 Al = [Ar](3d n ) 0.125 (3d c ) 2.7125 (4s c ) 0.55 (4s f ) 0.6125+ [Ne](3s c ) 1.789 (3p c ) 0.75 (3s f +3p f ) 0.454 , calculated lattice constants a′=0.39954 nm, c=0.40777 nm, c/ a′=1.0206, which are in good agreement with experimental values. When x Ti/ x Al>1, ψ 8 Ti atoms decrease and ψ 7 Ti , ψ 6 Ti … atoms appear in succession with the increase of x Ti; ψ 4 Al atoms decrease and ψ 3 Al , ψ 2 Al … atoms appear one after another. Because the influence of the change of Ti atomic states exceeds that of the change of Al atomic states, rich Ti-FCC TiAl alloys have larger atomic volumes, larger lattice constants, c/ a′ much closer to unity and comparatively good ductility, though the stability of FCC lattice is reduced. When x Ti/ x Al<1, with the increase of x Al, the atomic states ψ 8 Ti and ψ 4 Al changes in the direction opposite to the case with x Ti/ x Al>1. Thus, ordered rich Al-FCC TiAl alloys have smaller atomic volumes, smaller lattice constants, larger c/ a′ which depart further from unity and poorer ductility, and there is an increase of stability for the FCC lattice.