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Herein, we systematically investigate the corrosion behaviour of a wrought Ti-6Al-3Nb-2Zr-1Mo (Ti80) alloy in artificial seawater with various fluoride ion (F−) concentrations and pH values using electrochemical, static immersion measurements and surface characterizations. Results of electrochemical tests manifest that increasing the F− concentration and/or reducing pH value can deteriorate the corrosion performance of Ti80 alloy due to the dissolution of the compact passive film into the porous passive film. Besides, the electrochemical parameters obtained from polarization curves indicate that the F− concentration has few effects on the cathodic reaction but noticeable promoting effects on the anodic process, whereas the increase of pH plays a significant role in inhibiting both cathodic and anodic processes. In addition, surface characterizations reveal the preferential dissolution of the equiaxed α phase compared to the intergranular β phase, resulting from its relatively low Volta potential. We also interpret the F− concentrations and pH values dependent corrosion behaviour based on the mixed potential theory and illustrate the corrosion mechanisms associated with F− in artificial seawater. We believe that the insights into the F− concentration and pH value dependent corrosion behaviour can provide practical guidelines for the design and development of Ti alloys with high-corrosion resistance.

In this study, corrosion behaviours of Ti-6Al-3Nb-2Zr-1Mo-xC (x = 0, 0.05, 0.10 and 0.15 wt%) alloys in 5 M HCl solution are investigated. Widmanstatten microstructure, characterized by the obvious prior-β grain boundary, the inside of which consists of lamellar α-phase and intergranular retained β-phase without TiC precipitates, can be identified for all alloys. Electrochemical and immersion measurements signify that C alloying significantly improves the corrosion resistance of Ti-6Al-3Nb-2Zr-1Mo alloy; the corrosion rate dramatically reduces to 0.38 mm·year−1 when C content increases to 0.15 wt%, nearly one-third that of Ti-6Al-3Nb-2Zr-1Mo alloy. Besides, electrochemical parameters demonstrate that the addition of C has few effects on the cathodic hydrogen evolution reaction but noticeable inhibitive effects on the anodic corrosion. Surface characterizations reveal that the addition of C can efficiently alleviate the corrosion degree of prior-β grain boundaries and the lamellar α-phase is preferentially corroded, compared to intergranular retained β-phase.

Compared with the common additive manufacturing techniques, the vacuum operating environment and high deposition efficiency of wire-based vacuum additive manufacturing are more expective in the preparation of NiTi shape memory alloys. In the present work, the strategy of interlaminar residence was adopted to deposit the dense wall-structure NiTi shape memory alloy on the commercially pure titanium substrate using the electron beam freeform fabrication (EBF3) technique. The principle of phase constitution, microstructure evolution, martensitic transformation behavior, mechanical properties, and the functional response was studied in detail. The stable regions of as-deposited NiTi alloy were composed of B2 austenite at ambient temperature, and a small amount of B19′ martensite also was observed. Meanwhile, the nano-scaled Ti4Ni2Ox precipitate existed in the interdendritic and grain interior of the EBF3-fabricated NiTi alloy. The stable regions of EBF3-fabricated NiTi alloy presented a single reversible martensitic transformation (B2 ↔ B19′) during the cooling and heating process. In addition, the solidification characterization, such as microstructure evolution and crystallographic orientation in the building direction and horizon direction, brought about certain differences in the mechanical properties and functional response, such as tensile strength, elongation, superelasticity, and damping capacity. The corresponding intrinsic mechanism has been discussed. Our work also provides a promising method for the flexible manufacture of dense shape memory alloys with large scale and complex geometry, which is beneficial to expanding their application fields.