A systematic study of interatomic potentials for mechanical behaviours of Ti-Al alloys

Intermetallic Ti-Al alloys exhibit attractive physical and mechanical properties at high temperatures relevant to turbine machinery applications. However, TiAl and Ti₃Al alloys have low ductility and low fracture toughness, as well as anomalous hardening behaviour at high temperatures. Molecular dynamics (MD) simulations can directly model critical atomistic mechanisms responsible for these intriguing properties, but the validity of the underlying empirical/semi-empirical interatomic potentials for such complex alloys is not clear. Here, we examine four interatomic potentials (two EAM and two MEAM) developed for the Ti-Al material system and identify their respective strength and weakness in modeling the plastic and fracture properties of the L1₀ γ-TiAl and D0₁₉ α₂-Ti₃Al crystal structures. We compare the lattice constants, elastic constants and their temperature-dependence, cohesive energies, stacking fault energies, surface decohesion and thermal expansion coefficients against available experimental and density functional theory data. In addition, we test the deformation behaviours of these potentials under uniaxial tensile loading conditions at a wide range of temperatures. Our results show that these interatomic potentials can accurately reproduce some but not the full scope of the material properties of the γ and α₂ crystal structures. In particular, one MEAM potential reproduces some of the properties (some of the negative Cauchy pressure and stacking fault energies) comparable to DFT-calculated data. The results here provide guidance to select the appropriate interatomic potentials for specific applications. Our study also indicates that further optimization of the parameters in the MEAM formalism may lead to better interatomic potentials for the Ti-Al system.