Microstructure and Superior Corrosion Resistance of an In-Situ Synthesized NiTi-Based Intermetallic Coating via Laser Melting Deposition

A nickel-titanium (NiTi)-based intermetallic coating was in-situ synthesized on a Ti-6Al-4V (TC4) substrate via laser melting deposition (LMD) using Ni-20Cr and TC4 powders. Scanning electron microscopy, X-ray diffraction, a digital microhardness tester and an electrochemical analyzer were used to evaluate the microstructure, Vicker's microhardness and electrochemical corrosion resistance of the intermetallic coating. Results indicate that the microstructure of the intermetallic coating is composed of NiTi₂, NiTi and Ni₃Ti. The measured microhardness achieved is as high as ~850 HV_0.2, ~2.5 times larger than that of the TC4 alloy, which can be attributed to the solid solution strengthening of Al and Cr, dispersion strengthening of the intermetallic compounds, and grain refinement strengthening from the rapid cooling of LMD. During the electrochemical corrosion of 3.5% NaCl solution, a large amount of Ti ions were released from the intermetallic coating surface and reacted with Cl^- ions to form [TiCl₆]² with an increase in corrosion voltage. In further hydrolysis reactions, TiO₂ formation occurred when the ratio of [TiCl₆]^2- reached a critical value. The in-situ synthesized intermetallic coating can achieve a superior corrosion resistance compared to that of the TC4 alloy.