Impact of Zirconium Content on Phase Structure and Corrosion Properties of NiTi-Based Shape Memory Alloys

In this study, a pre-alloyed NiTi alloy was alloyed with 1, 3, 5, and 15% Zirconium (Zr) by mechanical alloying (MA), and the effect of Zr on the microstructure, corrosion behavior, and phase transformation temperatures of the pre-alloyed NiTi alloy was examined. The microstructural properties of the produced NiTi-Zr alloys were characterized by SEM + EDS, XRD analysis, microhardness tests, and electrochemical corrosion experiments. As a result, with increasing Zr addition, brass-shaped NiTi-Zr intermetallics were formed at the grain boundaries in the NiTi alloy with 1 and 3% Zr alloy, it was observed that it was completely dispersed in the matrix structure in the NiTi alloy with 5% Zr alloy. However, in the NiTi alloy with 15% Zr, NiTi-Zr intermetallics in needle-like morphology (martensitic) were determined in the matrix structure. The formation of NiTi and NiTi-Zr intermetallics due to Zr increased microhardness. Corrosion resistance also increases depending on the amount of Zr. The lowest corrosion resistance was obtained in the NiTi alloy (0.0239 mm/year), and the highest corrosion resistance was obtained in the NiTi alloy containing 15% Zr (0.0016 mm/year). During cooling, the austenite-martensite transformation temperature dropped by around 20 degrees C due to the 15% Zr added to the NiTi alloy.