The ternary system Al-Ni-Ti Part I: Isothermal section at 900°C; Experimental investigation and thermodynamic calculation
Résumé
Phase relations in the ternary system Al±Ni±Ti have been experimentally established for the isothermal section at 900°C for concentrations 0.1 4 x Al 4 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, t 1 to t 4 , is con®rmed, 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 t 3 -Al 3 NiTi 2 with the MgZn 2-type structure ranging from Al 30 Ni 28 Ti 42 (composition lowest in Al) to Al 50 Ni 16 Ti 34 (composition richest in Al) and for t 2 -Al 2 NiTi. The complex atom site substitution mechanism in t 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 +t 3 , we observed two closely adjoining three-phase equilibria: a 2 -AlTi 3 +Ni{Al x Ti 1-x } 2 + t 4 -AlNi 2 Ti and a 2 -AlTi 3 +t 3 -Al 2 NiTi 2 +t 4 -AlNi 2 Ti. The earlier reported ``homogeneous phase at Al 23 Ni 26 Ti 51 0 '' 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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