Experimental investigation and thermodynamic optimization of the Ni–Ta–Ti system

Abstract

The liquidus surface projection of the Ni–Ta–Ti system was constructed in this work. Thirty-seven as-cast alloys were characterized using scanning electron microscope equipped with energy dispersive spectrometer (SEM/EDS) and X-ray diffraction (XRD) methods to analyze the primary phases and solidification paths of alloys. Ten primary solidification regions of bcc(Ta,Ti), fcc(Ni), NiTi, NiTi₂, NiTa₂, Ni₂Ta, Ni₃Ta, Ni₃Ti, μ and τ were determined. The crystal structure of the intermetallic phase τ was identified as the Ni₃Sn type with space group P6₃/mmc. Moreover, the thermodynamic parameters of the Ni–Ta–Ti system were optimized via the CALculation of PHAse Diagram (CALPHAD) method based on the available experimental data. In this work, NiTi₂, Ni₃Ti, NiTa₂, Ni₂Ta, Ni₃Ta and τ were modelled by two-sublattice model (Ni,Ta,Ti)_m(Ni,Ta,Ti)_n. A four-sublattice model (Ni,Ta,Ti)₁(Ta,Ti)₄(Ni,Ta,Ti)₂(Ni,Ta,Ti)₆ was adopted to describe μ. The calculated liquidus surface projection and isothermal sections with the obtained thermodynamic parameters were consistent with the experimental data.