Electronic structure, phase stability, and chemical ordering of the ω phase in a Ti3 Al2 X (X=Nb,V) alloy

The phase stability of B2 Ti3 Al2 X (X=Nb or V) and slightly rearranged atomic structures is examined by first-principles calculations. The ground-state energy calculations show instability in some of the Ti3 Al2 X configurations against the ω structure type of atomic displacement. We use electronic density of states and Mulliken population analysis to understand the hybridization between the atoms and the electronic origin of the stability or instability of each system. In order to estimate the strength of each bond, the heats of formation for several compounds are calculated. We find that the strength of the transition metal-Al bond increases from V to Nb to Ti, with Ti-V and Ti-Nb being weakly unstable. By examining several atomic configurations, it is shown that the stability of each structure is directly related to the number of Ti-Al bonds in each configuration. It is confirmed that the formation of the ω phase in Ti3 Al2 X is a combined displacive-replacive transformation. The crystal structure parameters, such as lattice constants and bulk modulus, are calculated and compared with available experimental data.