Atom order and thermodynamic properties of the ternary Laves phase Ti(TiyNixAl1–x–y)2

Abstract

Abstract Combined refinement of neutron powder, X-ray single crystal and powder diffraction data allows precise evaluation of symmetry and atom site occupation for the MgZn2-type Laves phase. For compositions Ti(Ni1–x,Alx)2 titanium atoms were found to fully occupy the 4f site, whilst Ni- and Al-atoms randomly share the 2a and 6h sub-lattice sites of space group P63/mmc. At higher Ti-concentrations, Ti-atoms tend to enter the 2a and 6h site. Compositional dependences of the site occupations were used to explain the curved shape of the homogeneity region of the Laves phase. Investigation of the phase relations around the Ti-rich side of the ternary Laves phase was based on LOM, EPMA and X-ray diffraction experiments on argon arc-melted bulk alloys, which were annealed at 1000 °C in vacuum-sealed silica capsules for 100 h. The Ti-rich end of the homogeneity region of the ternary Laves phase at 1000 °C was derived from EMPA to be at Ti0.463Ni0.273Al0.264. Heat of formation data, derived from adiabatic drop calorimetry, were Δ298 H 0 = –57.9 ± 3.5 kJ/mol for Ti0.34Ni0.18Al0.48and –61.6 ± 3.3 kJ/mol for Ti0.36Ni0.24Al0.40. For the ab initio density functional theory applications a large number of structural models were investigated in order to calculate the concentration dependent heats of formation, structural stabilities, lattice parameters, bulk moduli and site occupancies, which are in good agreement with experiment. The theoretical analysis revealed that there are no Ni—Ni nearest neighbours when the concentration of Ni is smaller than that of Al. With increasing Al concentration, Al starts to fill 2a sites, and then continues occupying 6h sites. In compounds with Ti concentration larger than 33.3 at% the 4f sites are fully occupied by Ti, and the excessive Ti prefers 6h sites. The Al concentration of the Al-rich end of the homogeneity region of the ternary Laves phase was predicted to be 57 at%.