High-temperature deformation behavior of TiNi-Nb hypoeutectic alloy

The hot deformation behavior of a TiNi–Nb hypoeutectic alloy (Ti–16Ni–5Nb) was investigated via isothermal compression tests in the temperature range of 700−900 °C and strain rate range of 0.01−10 s−1. The effect of the deformation parameters on the deformation mechanism and microstructural evolution was characterized by means of scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and differential scanning calorimetry. The results revealed that the as-cast TiNi–Nb hypoeutectic alloy consisted of NiTi2 intermetallic, α-Ti + NiTi2 lamellar eutectic microstructure, and transformed β-Ti. With increasing strain, the flow stress increased sharply to a peak value due to the work-hardening effect during the initial stage of deformation. The stress then decreased continuously owing to dynamic softening, reaching a steady state, which exhibited the typical features of dynamic recrystallization (DRX). With deformation, the NiTi2 phase transformed from a continuous network-type structure to a discontinuous dispersed structure. Flow localization bands induced the formation of microcracks as well as flow softening below the β transition temperature (Tβ), and DRX was typically observed above Tβ, especially at high temperatures. In addition, the thermal deformation activation energy increased initially and then decreased in the range of 229.05−261.71 kJ/mol. A developed constitutive model was established to predict the flow stress under high temperature deformation conditions. The correlation between the experimental data and predicted values of the flow stress confirmed the capability of the model to accurately predict the flow stress of the TiNi–Nb alloy.