Occurrence and origin of non-martensitic acicular artifacts on NiTi

Acicular structures on NiTi are often interpreted as martensite or martensitic surface relief. These structures are identified as artifacts and form during etching using hydrofluoric acid based etching solutions. Their origin is clarified in the present work. After standard metallographic preparation, the acicular structures cannot be distinguished from bulk material by standard analysis, e.g. energy dispersive X-ray analysis and Auger electron spectroscopy. By investigating samples after etching, but previous to cleaning with water, rod-like crystals on the NiTi surface were detected. The crystals are identified as NiTiF6⋅6H2O by X-ray diffraction. During etching, the crystals develop on the surface and locally prevent contact of etching solution and NiTi base material. Further etching solely removes NiTi at adjacent uncovered areas. During the final cleaning with water the crystals completely dissolve and the surface relief with a shape according to that of the dissolved crystals remains. The surface relief does not represent the bulk microstructure.