An ab initio molecular dynamics study on the structural and electronic properties of AlB2, TiB2 and (Alx,Ti(1−x))B2 in Al-Ti-B master alloys

The structural and electronic properties of AlB2, TiB2 and (Alx,Ti(1−x))B2 diborides in Al-Ti-B master alloys are investigated by ab initio molecular dynamics calculations at high temperature. It is found that the dominant bonding states of AlB2 and TiB2 at 1073 K are ionic and covalent, respectively. The larger linear thermal expansion coefficient of AlB2 than that of TiB2 results from their different bonding nature. The original ionic or covalent bonding states of Al or Ti with B atoms is reversed to be covalent or ionic to suit the whole bonding environment of the diborides when Al or Ti atoms acts as the solute atoms replacing Ti or Al atoms to form (Alx,Ti(1−x))B2 duplex diborides. The inter-layer cohesion in (Alx,Ti(1−x))B2 diborides is weakened owing to the weakened bonding and enhanced anti-bonding of Al-B and Ti-B bonds. The thermal stability of (Alx,Ti(1−x))B2 duplex diborides compared with AlB2 and TiB2 are elucidated from thermodynamic considerations. The higher formation energy of (Alx,Ti(1−x))B2 duplex diborides is a barrier for restricting the transformation from AlB2 to TiB2.