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

摘要

The Al-Ni-Ti phase diagram has beenthermodynamically assessed and a consistent set ofthermodynamic functions has been developed. Thethermodynamic modeling is based on an experimentalinvestigation of the phase equilibria in the compositionrange of 0.1 ≤XA1≤0.7. Alloys were prepared by argon-arcor vacuum-electron beam melting of elemental powderblends. X-ray powder diffraction, metallography, SEM andEMPA-techniques were employed to analyze the samples inthe as-cast state as well as after annealing at 800, 900and 1000℃. The existence of the four ternary compounds,τ1 to τ4, has been confirmed, although homogeneityregions differ significantly from reports in theliterature. The homogeneous phase, previously claimed at“A123Ni26Ti51”, is shown by high resolution microprobeand X-ray diffraction measurements to be an extremelyfine-grained eutectic structure. The congruent meltingbehavior of τ4=AlNi2Ti is confirmed, but, in contrast toearlier reports, primary crystallization and congruentmelting have been observed for τ1=Al13Ni2Ti5 and τ3=Al3NiTi2. In contrast to earlier assessments, τ1, τ2 and τ3 are experimentally found to be stable at 800,900 and 1000℃. The thermodynamic modeling of the ternaryphases τ2 and τ3 is done with simplified sublatticemodels, considering their crystal structure andhomogeneity ranges. The sublattice model for τ4 is takenfrom an earlier assessment of the nickel-rich ternaryphase equilibria. The present assessment covers theentire composition range. An application to thesolidification behavior of ternary alloys is also exemplified.