The ternary system Al–Ni–Ti Part II: thermodynamic assessment and experimental investigation of polythermal phase equilibria

Abstract

The Al–Ni–Ti phase diagram has been thermodynamically assessed and a consistent set of thermodynamic functions has been developed. The thermodynamic modeling is based on an experimental investigation of the phase equilibria in the composition range of 0.1⩽x Al⩽0.7. Alloys were prepared by argon-arc or vacuum-electron beam melting of elemental powder blends. X-ray powder diffraction, metallography, SEM and EMPA-techniques were employed to analyze the samples in the as-cast state as well as after annealing at 800, 900 and 1000°C. The existence of the four ternary compounds, τ 1 to τ 4, has been confirmed, although homogeneity regions differ significantly from reports in the literature. The homogeneous phase, previously claimed at “Al 23Ni 26Ti 51”, is shown by high resolution microprobe and X-ray diffraction measurements to be an extremely fine-grained eutectic structure. The congruent melting behavior of τ 4=AlNi 2Ti is confirmed, but, in contrast to earlier reports, primary crystallization and congruent melting have been observed for τ 1=Al 13Ni 2Ti 5 and τ 3=Al 3NiTi 2. In contrast to earlier assessments, τ 1, τ 2 and τ 3 are experimentally found to be stable at 800, 900 and 1000°C. The thermodynamic modeling of the ternary phases τ 2 and τ 3 is done with simplified sublattice models, considering their crystal structure and homogeneity ranges. The sublattice model for τ 4 is taken from an earlier asessment of the nickel-rich ternary phase equilibria. The present assessment covers the entire composition range. An application to the solidification behavior of ternary alloys is also exemplified.