Composition and Crystalline Structure of Ternary Phases in the Ta–Ni–Al System

Abstract

The composition and crystal structure of compounds produced by self-propagating high-temperature synthesis (SHS) from the 5Ta–2Ni–3Al (at %) powder mixture followed by vacuum remelting at 3000°C are studied. The SHS product contains the following phases: TaNiAl (Laves τ1 phase), NiAl, Ni2Al3, and Ta. Its microstructure includes Ta85Ni7Al8, Ta52Ni20Al28, and Ta53Ni25Al22 ternary phases according to elemental analysis data. Reflections belonging to no known ternary phases in the Ta–Ni–Al system under consideration are revealed in the X-ray diffraction pattern of the remelted material. Based on the homological approach, it is found that these reflections belong to three phases with the structural types W6Fe7 ( $$R\bar {3}m$$ ), Ti2Ni ( $$Fd\bar {3}m$$ ), and Ta3Al ( $${{P{{4}_{2}}} \mathord{\left/ {\vphantom {{P{{4}_{2}}} {mnm}}} \right. \kern-0em} {mnm}}$$ ). They are identified as reflections of three compounds, Ta6.5Ni6.5, Ti2Ni, and Ta2.84Al0.91, with unit-cell parameters differing from these for the same compounds with the conservation of the structural type. An increase in the unit-cell parameters of all revealed phases is noted when compared with known binary intermetallic compounds. This can be associated with the presence of Al atoms in the crystal lattice from the Ta6.5Ni6.5 phase and Al and Ta atoms in the phase with the Ti2Ni structural type. Phases Ta6.5Ni6.5 and Ti2Ni phases are identified as Ta6Ni6Al and Ta2Ni0.5Al0.5 by X-ray structural analysis and crystal-chemical modeling, and their structural type, composition, and unit-cell parameters are determined. The structure and composition are refined by the full-profile analysis, and the unit-cell parameters of the phases and their quantitative ratio in the material are determined. The phase composition of the material is as follows, wt %: 47 Ta6Ni6Al, 16 Ta2Ni0.5Al0.5, and 37 Ta3Al.