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.

Keywords: Ni-Ta-Ti system, Experimental liquidus surface projection, CALPHAD method