The structural, mechanical and thermodynamic properties of the orthorhombic TMA1 (TM=Ti, Y, Zr and Hf) aluminides from first-principles calculations

The orthorhombic ZrAl-type aluminides are attractive high temperature materials due to the excellent oxidation resistance and high strength. However, the structural and mechanical properties of the other orthorhombic ZrAl-type aluminides are unknown until now. In this work, the structural, elastic modulus, hardness and thermodynamic properties of the orthorhombic TMAl aluminides are studied by the first-principles calculations. Four TMAl aluminides: TiAl, YAl, ZrAl and HfAl are considered. The result shows that three new TMAl aluminides: YAl, ZrAl and HfAl are firstly predicted based on the phonon dispersion. In four TMAl aluminides, the calculated bulk modulus and elastic stiffness of HfAl are larger than the other TMAl aluminides. However, the Vickers hardness of TiAl is 16.3 GPa, which is bigger than the other TMAl aluminides. Naturally, the high mechanical properties of the orthorhombic TMAl aluminides is attributed to the strong cohesive force of TM-Al-TM layered structure, which is demonstrated by the TM-Al and Al-Al bonds. In addition, the Debye temperature of TiAl is 489.2 K, which is larger than the other TMAl aluminides. The thermal stability of the orthorhombic TMAl aluminides is determined by the vibration of Al and Al-Al bond.