Effect of Ta Content on the Microstructure and Properties of NiTiTa Functional Coatings In Situ Synthesized by Directed Energy Deposition

In this study, surface alloying technology based on Gas Tungsten Arc Welding (GTAW) was used to synthesize in situ NiTiTa coatings on a NiTi substrate using commercially pure Ta foils. The influence of different Ta contents (0.91, 1.42, and 2.91 at.%) on the microstructure, phase formation, hardness, corrosion resistance, and X-ray visibility of the prepared coatings were systematically studied. These results show that the NiTiTa coatings fabricated by GTAW were free of microcracks with good surface quality and superior adhesion to the NiTi substrate. The NiTiTa coatings are mainly composed of columnar austenitic NiTi (B2), and martensitic NiTi (B19') with (Ti, Ta)2Ni precipitating at the grain boundaries. The proportion of B19' martensite and the Ta content dissolved in the NiTi matrix increases with the increasing addition of Ta. In addition, β-Ta appeared in the coating formed with 1.42 at.% Ta and precipitated abundantly when the Ta amount was increased to 2.91 at.%. Changes in phase composition and secondary phases lead to a decrease in the material nanohardness. To simulate the body fluid environment, corrosion tests were conducted in Hank's solution at a rate of 0.5 mV/s. Electrochemical tests show that the NiTiTa coatings exhibit superior corrosion resistance, where the corrosion potential, Ecorr, increased with increasing Ta content. The enhanced X-ray visibility of the newly formed coatings was also revealed. This work provides a cost-effective method for in situ synthesis of NiTiTa coatings on NiTi alloys, highlighting its potential for improving the corrosion resistance and X-ray visibility of NiTi shape memory alloys.