Thermal-Induced Phase Transformation Behavior of NiTiNb Hypoeutectic, Eutectic, and Hypereutectic Alloys

Abstract

In the present work, three NiTiNb alloys with nominal compositions of Ni50–x/2Ti50–x/2Nbx (x = 18, 20 and 22, at.%) were prepared. The microstructure, thermal-induced phase transformation, and phase components of NiTiNb hypoeutectic, eutectic, and hypereutectic alloys were investigated by scanning electron microscopy coupled with energy dispersive spectrometer (SEM/EDS), differential scanning calorimetry (DSC), and X-ray diffraction (XRD), respectively. Two peaks occurred in the DSC curves of Ni41Ti41Nb18 solution at 750 and 850 °C. The different microstructures of NiTiNb hypoeutectic, eutectic and hypereutectic alloys, the occurrence of two peaks in the DSC curves, and the characteristic of martensite transformation temperature were analyzed and discussed. The results show that different Nb content leads to different microstructures of Ni50–x/2Ti50–x/2-Nbx alloys. Moreover, the difference of Ni/Ti ratio between the primary NiTi matrix and NiTi in eutectic structure strongly influenced the phase transformation behavior that the DSC curves of Ni41Ti41Nb18 alloy show two peaks. Furthermore, under the same solution treatment, the Ni40Ti40Nb20 alloy has a lower Ms than the other two alloys. The Ms has a tendency to decrease with an increase in solution temperature.