Anisotropy in Microstructure and Corrosion Behavior of NiTi Alloys Produced by Laser Powder Bed Fusion

Laser powder bed fusion (LPBF) induces pronounced microstructural anisotropy in NiTi alloys, which strongly governs their corrosion behavior in physiological environments. Here, the orientation-dependent microstructure and corrosion performance of LPBF NiTi alloys were systematically investigated on the XY (perpendicular to build direction) and XZ (parallel to build direction) planes. The XY plane is dominated by polygonal B2 grains, whereas semi-quantitative XRD analysis and TEM observations indicate a relatively larger contribution of lamellar B19′ martensite on the XZ plane. Electrochemical tests in Hank’s solution (pH 3–7) reveal pronounced corrosion anisotropy. At pH 7, the XZ plane exhibits a higher charge transfer resistance (143.9 vs. 109.1 kΩ cm2) and a lower corrosion current density (0.231 vs. 0.599 μA cm−2) than the XY plane. After 72 h immersion, the Rct of the XZ plane remains approximately 31% higher than that of the XY plane at pH 7, while its apparent donor density is lower than that of the XY plane at pH 3 (7.38 × 1029 vs. 12.33 × 1029 cm−3). The superior electrochemical response of the XZ plane correlates with its denser lamellar B19′ morphology and lower passive-film donor density. Competition between interface-assisted passivation and interface-related electrochemical heterogeneity is proposed as a possible contributor to the anisotropic corrosion response.