Phase stability and cohesive properties of Ti–Zn intermetallics: First-principles calculations and experimental results

Abstract

The total energies and equilibrium cohesive properties of 48 intermetallics in the Ti–Zn system are calculated employing electronic density-functional theory (DFT), ultrasoft pseudopotentials and the generalized gradient approximation for the exchange-correlation energy. Selected experiments are performed to determine the heat of formation by calorimetry and to measure the isotropic elastic moduli by pulse-echo method. In alloys containing 50 at.% or less Ti, where it was possible to synthesize nearly single phase specimens, the calculated heats of formation agree within a few kJ/mol of the calorimetry data, while the calculated isotropic elastic constants agree within 5% of the experimental values. The demonstrated level of agreement suggests high accuracy for the calculated cohesive properties of structures where there is no measured data. For the stable intermetallics, the calculated zero-temperature lattice parameters agree to within ±1% of experimental data at ambient temperature. In two complex phases, TiZn 16 and Ti 3Zn 22, the unit cell parameters obtained from ab initio calculations show an excellent agreement with those obtained by rigorous structural analysis of X-ray diffraction data. For all structures considered, we provide optimized unit cell geometries. Also, for most structures we provide zero-temperature bulk moduli and their pressure derivatives, as defined by the equation of state. In Ti-rich alloys, sluggish reaction kinetics between Ti and Zn powder and phase diagram constraints pose significant problems, thus hindering accurate measurement of heat of formation by direct reaction calorimetry. Therefore, the calculation of such a quantity, and other cohesive properties as well, of solid phases in the entire composition range by ab initio methods remains an attractive viable option. The bonding between Ti and Zn is discussed based on the analyses of calculated densities of states and charge densities; these analyses suggest that bonding in Ti–Zn intermetallics shares many features in common with transition-metal aluminides due, in particular, to the presence of pronounced hybridization between Ti d and Zn p electronic states.