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

The Al-Ni-Ti phase diagram has been thermodynamically assessed and a consis tent set of thermodynamic functions has been developed. The thermodynamic m odeling is based on an experimental investigation of the phase equilibria i n the composition range of 0.1 less than or equal to x(Al) less than or equ al to 0.7. Alloys were prepared by argon-arc or vacuum-electron beam meltin g of elemental powder blends. X-ray powder diffraction, metallography, SEM and EMPA-techniques were employed to analyze the samples in the as-cast sta te as well as after annealing at 800, 900 and 1000 degrees C. The existence of the four ternary compounds, tau(1) to tau(4), has been confirmed, altho ugh homogeneity regions differ significantly from reports in the literature . The homogeneous phase, previously claimed at "Al23Ni26Ti51", is gen shown by high resolution microprobe and X-ray diffraction measurements to be an extremely fine-grained eutectic structure. The congruent melting behavior o f tau(4) = AlNi2Ti is confirmed, but, in contrast to earlier reports, prima ry crystallization and congruent melting have been observed for tau(1) = Al 13Ni2Ti5 and tau(3) = Al3NiTi2. In contrast to earlier assessments, tau(1), tau(2) and tau(3) are experimentally found to be stable at 800, 900 and 10 00 degrees C. The thermodynamic modeling of the ternary phases a and a is d one with simplified sublattice models, considering their crystal structure and homogeneity ranges. The sublattice model for tau(4) is taken from an ea rlier asessment of the nickel-rich ternary phase equilibria. The present as sessment covers the entire composition range. An application to the solidif ication behavior of ternary alloys is also exemplified. (C) 1999 Elsevier S cience Ltd. All rights reserved.