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

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⩽ x Al⩽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, τ 1 to τ 4, 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 τ 3-Al 3NiTi 2 with the MgZn 2-type structure ranging from Al 30Ni 28Ti 42 (composition lowest in Al) to Al 50Ni 16Ti 34 (composition richest in Al) and for τ 2-Al 2NiTi. The complex atom site substitution mechanism in τ 3 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{Al x Ti 1- x } 2+τ 3, we observed two closely adjoining three-phase equilibria: α 2-AlTi 3+Ni{Al x Ti 1- x } 2+ τ 4-AlNi 2Ti and α 2-AlTi 3+τ 3-Al 2NiTi 2+τ 4-AlNi 2Ti. The earlier reported “homogeneous phase at Al 23Ni 26Ti 51′” 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.