Abstract
Phase relations in the ternary system Al–Ni–Ti have been experimentally established for the isothermal section at 900°C for concentrations 0.1⩽xAl⩽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 melting of proper elemental powder blends. The existence of four ternary compounds, τ1 to τ4, is confirmed, however, in contrast to earlier investigations at significantly different compositions and with different shape of the homogeneity regions. This is particularly true for the phase regions of τ3-Al3NiTi2 with the MgZn2-type structure ranging from Al30Ni28Ti42 (composition lowest in Al) to Al50Ni16Ti34 (composition richest in Al) and for τ2-Al2NiTi. The complex atom site substitution mechanism in τ3 changing from Ti/Al exchange at Al-poor compositions towards Ni/Al replacement for the Al-rich part was monitored in detail by quantitative X-ray powder diffraction techniques (Rietveld analyses). In contrast to earlier reports, claiming a two-phase region Ni{AlxTi1-x}2+τ3, we observed two closely adjoining three-phase equilibria: α2-AlTi3+Ni{AlxTi1-x}2+ τ4-AlNi2Ti and α2-AlTi3+τ3-Al2NiTi2+τ4-AlNi2Ti. The earlier reported “homogeneous phase at Al23Ni26Ti51′” was shown by high resolution microprobe and X-ray diffraction measurements to be an extremely fine-grained eutectic. The experimental results are in fine agreement with the thermodynamic calculation.
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