Influence of carbon on energetics, electronic structure, and mechanical properties of TiAl alloys

We present first-principles calculations of carbon-doped TiAl alloys. The effect of carbon on the structural, electronic, and elastic behavior of the gamma phase (L1(0) structure) of TiAl is investigated. The calculated enthalpy of formation at zero temperature indicates that carbon atoms favor to occupy rather interstitial than substitutional positions. The computed solubility of carbon in the stoichiometric gamma phase is very low, in agreement with experimental findings. However, it is significantly enhanced for the Ti-rich alloy and when located inside Ti-6 octahedra. Mechanical properties such as Cauchy pressure, elastic anisotropy, Young's modulus, as well as Pugh and Poisson ratios of stoichiometric and off-stoichiometric compositions are analyzed as a function of carbon concentration and its location. As a general trend, we obtain that below a concentration of 3 at.%, carbon plays a minor role in changing the ductile behavior of gamma-TiAl. A slight increase in ductility is found in the Ti-rich gamma alpha phase if either located in the Ti-plane (Ti4Al2 octahedral site) or in a Ti-6 octahedra.