The ternary system Al-Ni-Ti Part I: Isothermal section at 900 degrees C; Experimental investigation and thermodynamic calculation

Phase relations in the ternary system Al-Ni-Ti have been experimentally est ablished for the isothermal section at 900 degrees C for concentrations 0.1 lessthan or equal to x(Al) less than or equal to 0.7. The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA-techniques on about 40 ternary alloys, prepared by argon-arc or vacuum-electron beam m elting of proper elemental powder blends. The existence of four ternary com pounds,tau(1) to tau(4), is confirmed, however, in contrast to earlier inve stigations at significantly different compositions and with differentshape of the homogeneity regions. This is particularly true for the phase regions of tau(3)-Al3NiTi2 with the MgZn2-type structure ranging from Al30Ni28Ti42 (composition lowest in Al) to Al50Ni16Ti34 (composition richest in Al) and for tau(2)-Al2NiTi. The complex atom site substitution mechanism in tau(3) changing from Ti/Al exchange at Al-poor compositions towards Ni/Al replace ment for the Al-rich part was monitored in detail by quantitative X-ray pow der diffraction techniques (Rietveld analyses). In contrast to earlier repo rts, claiming a two-phase region Ni{AlxTi1-x}(2) + tau(3) we observed two c losely adjoining three-phase equilibria: alpha(2)-AlTi3 + Ni{AlxTi1-x}(2) tau(4)-AlNi2Ti and alpha(2)-AlTi3 + tau(3)-Al2NiTi2 + tau(4)-AlNi2Ti. The earlier reported"homogeneous phase at Al23Ni26Ti51'" was shown by high reso lution microprobe and X-ray diffraction measurements to be an extremely fin e-grained eutectic. The experimental results are in fine agreementwith the thermodynamic calculation. (C) 1999 Elsevier Science Ltd. All rightsreserve d.