Abstract
The effect of Nb content on the microstructure, thermal characteristics, compressive behavior, and hardness of the as-cast 55Ni-(45-x) Ti-xNb (0-20 wt.%) shape memory alloy was systematically examined. The alloys were prepared using a vacuum arc melting furnace. X-ray diffraction, Differential scanning calorimetry, optical microscopy, Energy-dispersive X-ray spectroscopy, and Scanning electron microscopy were employed for characterization. The results demonstrated that NiTi alloys' microstructure exhibited dendritic, rod-shaped grains with a heterogeneous eutectic phase. While for Nb- containing alloys, the majority of NiTi phases changed into a spherical shape with few black eutectic phases. The NiTi alloys’ average grain size was 163 μm. With an increase in Nb content, the average grain size dropped to about 25 μm. Both forward and reverse transformation curves related to the thermally induced phase changes of all alloys showed single-step transitions. By adding Nb to the binary Ni-Ti alloy, both of the onset temperatures for the reverse and the martensitic transformations were decreased. The average hardness decreased from 263 to a range of 200 to 212 HV as a result of adding from 5 to 20 wt.% Nb. The compressive strength increases from 2625 to range of 3126 to 3333 N/mm² with the addition of 5 to 20 wt.% Nb. This study provides a clear relationship between alloy composition, phase evolution, and mechanical performance, unlike the previous studies where they tend to focus on either the binary NiTi system or a single isolated property, new insights are presented. The findings can contribute in designing advanced smart materials.